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Princeton, NJ, United States

Princeton University is a private Ivy League research university in Princeton, New Jersey. Founded in 1746 in Elizabeth as the College of New Jersey, Princeton was the fourth chartered institution of higher education in the American colonies and thus one of the nine Colonial Colleges established before the American Revolution. The institution moved to Newark in 1747, then to the current site nine years later, where it was renamed Princeton University in 1896. The present-day College of New Jersey in nearby Ewing Township, New Jersey, is an unrelated institution. Princeton had close ties to the Presbyterian Church, but has never been affiliated with any denomination and today imposes no religious requirements on its students.Princeton provides undergraduate and graduate instruction in the humanities, social science, natural science, and engineering. It offers professional degrees through the Woodrow Wilson School of Public and International Affairs, the School of Engineering and Applied Science, the School of Architecture and the Bendheim Center for Finance. The University has tied with the Institute for Advanced Study, Princeton Theological Seminary, and the Westminster Choir College of Rider University. By endowment per student, Princeton is the wealthiest school in the United States.Princeton has been associated with 37 Nobel laureates, 17 National Medal of Science winners, two Abel Prize winners, eight Fields Medalists , nine Turing Award laureates, three National Humanities Medal recipients and 204 Rhodes Scholars. Wikipedia.


Riehl C.,Princeton University
Current Biology | Year: 2010

How do cooperatively breeding groups resist invasion by parasitic "cheaters," which dump their eggs in the communal nest but provide no parental care [1,2]? Here I show that Greater Anis (Crotophaga major), Neotropical cuckoos that nest in social groups containing several breeding females [3], use a simple rule based on the timing of laying to recognize and reject eggs laid by extragroup parasites. I experimentally confirmed that Greater Anis cannot recognize parasitic eggs based on the appearance of host egg phenotypes or on the number of eggs in the clutch. However, they can discriminate between freshly laid eggs and those that have already been incubated, and they accordingly eject asynchronous eggs. This mechanism is reliable in naturally parasitized nests, because group members typically lay their eggs in tight synchrony, whereas the majority of parasitic eggs are laid several days later. Rejection of asynchronous eggs therefore provides a rare empirical example of a complex, group-level behavior that arises through relatively simple "rules of thumb" without requiring advanced cognitive mechanisms such as learning, counting, or individual recognition. © 2010 Elsevier Ltd. All rights reserved.


G-quadruplex DNA is a four-stranded DNA structure formed by non-Watson-Crick base pairing between stacked sets of four guanines. Many possible functions have been proposed for this structure, but its in vivo role in the cell is still largely unresolved. We carried out a genome-wide survey of the evolutionary conservation of regions with the potential to form G-quadruplex DNA structures (G4 DNA motifs) across seven yeast species. We found that G4 DNA motifs were significantly more conserved than expected by chance, and the nucleotide-level conservation patterns suggested that the motif conservation was the result of the formation of G4 DNA structures. We characterized the association of conserved and non-conserved G4 DNA motifs in Saccharomyces cerevisiae with more than 40 known genome features and gene classes. Our comprehensive, integrated evolutionary and functional analysis confirmed the previously observed associations of G4 DNA motifs with promoter regions and the rDNA, and it identified several previously unrecognized associations of G4 DNA motifs with genomic features, such as mitotic and meiotic double-strand break sites (DSBs). Conserved G4 DNA motifs maintained strong associations with promoters and the rDNA, but not with DSBs. We also performed the first analysis of G4 DNA motifs in the mitochondria, and surprisingly found a tenfold higher concentration of the motifs in the AT-rich yeast mitochondrial DNA than in nuclear DNA. The evolutionary conservation of the G4 DNA motif and its association with specific genome features supports the hypothesis that G4 DNA has in vivo functions that are under evolutionary constraint.


Kim P.,Princeton University | Abkarian M.,CNRS Charles Coulomb Laboratory | Stone H.A.,Korea Advanced Institute of Science and Technology
Nature Materials | Year: 2011

Mechanical instabilities that cause periodic wrinkling during compression of layered materials find applications in stretchable electronics and microfabrication, but can also limit an application™s performance owing to delamination or cracking under loading and surface inhomogeneities during swelling. In particular, because of curvature localization, finite deformations can cause wrinkles to evolve into folds. The wrinkle-to-fold transition has been documented in several systems, mostly under uniaxial stress. However, the nucleation, the spatial structure and the dynamics of the invasion of folds in two-dimensional stress configurations remain elusive. Here, using a two-layer polymeric system under biaxial compressive stress, we show that a repetitive wrinkle-to-fold transition generates a hierarchical network of folds during reorganization of the stress field. The folds delineate individual domains, and each domain subdivides into smaller ones over multiple generations. By modifying the boundary conditions and geometry, we demonstrate control over the final network morphology. The ideas introduced here should find application in the many situations where stress impacts two-dimensional pattern formation. © 2011 Macmillan Publishers Limited. All rights reserved.


Hopfield J.J.,Princeton University
Annual Review of Condensed Matter Physics | Year: 2014

Subfields of physics are born, expand, and develop in intellectual scope, then can spawn new offspring by subdividing, can disappear by being absorbed in new definitions of the fields of physics, or may merely decline in vigor and membership. Textbooks, seminar programs, graduate courses, and the chosen structure of industrial laboratories all contributed to making solid state physics a vibrant subfield for 30 years, to ultimately disappear into regroupings with names such as condensed matter, materials science, biological physics, complexity, and quantum optics. This review traces the trajectory of the subfield solid state physics through the experiences of the author in relationship to major university departments and Bell Labs, with digressions into how he became a physicist, physics education, and choosing research problems. © Copyright 2014 by Annual Reviews. All rights reserved.


Braverman M.,Princeton University | Moitra A.,Institute for Advanced Study
Proceedings of the Annual ACM Symposium on Theory of Computing | Year: 2013

We prove an unconditional lower bound that any linear pro- gram that achieves an O(n1-ε) approximation for clique has size 2 Ω(n-ε). There has been considerable recent interest in proving unconditional lower bounds against any linear pro- gram. Fiorini et al. [13] proved that there is no polyno- mial sized linear program for traveling salesman. Braun et al. [7] proved that there is no polynomial sized O(n 1/2-ε)- approximate linear program for clique. Here we prove an optimal and unconditional lower bound against linear pro- grams for clique that matches Håstad's [15] celebrated hard- ness result. Interestingly, the techniques used to prove such lower bounds have closely followed the progression of tech- niques used in communication complexity. Here we develop an information theoretic framework to approach these ques- tions, and we use it to prove our main result. Also we resolve a related question: How many bits of communication are needed to get ε-advantage over random guessing for disjointness? Kalyanasundaram and Schnitger [18] proved that a protocol that gets constant advantage re- quires (n) bits of communication. This result in conjunc- tion with amplification implies that any protocol that gets ε-advantage requires Ω(ε2n) bits of communication. Here we improve this bound to Ω (εn), which is optimal for any > 0. Copyright 2013 ACM.


Nelson J.,Institute for Advanced Study | Nguyen H.L.,Princeton University
Proceedings of the Annual ACM Symposium on Theory of Computing | Year: 2013

We give near-tight lower bounds for the sparsity required in several dimensionality reducing linear maps. First, consider the Johnson-Lindenstrauss (JL) lemma which states that for any set of n vectors in Rd there is an A ∈ Rm×d with m = O(ε-2 log n) such that mapping by A preserves the pairwise Euclidean distances up to a 1 ε factor1. We show there exists a set of n vectors such that any such A with at most s non-zero entries per column must have s = Ω(ε-1 log n/ log(1/ε)) if m < Ω(n/ log(1/ε)). This improves the lower bound of Ω(min{ε-2; ε-1√ log m d}) by [Dasgupta- Kumar-Sarlós, STOC 2010], which only held against the stronger property of distributional JL, and only against a certain restricted class of distributions. Meanwhile our lower bound is against the JL lemma itself, with no restrictions. Our lower bound matches the sparse JL upper bound of [Kane-Nelson, SODA 2012] up to an O(log(1/ε)) factor. Next, we show that any m×n matrix with the k-restricted isometry property (RIP) with constant distortion must haveΩ (k log(n/k)) non-zeroes per column if m = O(k log(n/k)), the optimal number of rows for RIP, and k < n/ polylog n. This improves the previous lower bound of (min{k; n/m}) by [Chandar, 2010] and shows that for most k it is impossible to have a sparse RIP matrix with an optimal number of rows. Both lower bounds above also offer a tradeoff between sparsity and the number of rows. Lastly, we show that any oblivious distribution over subspace embedding matrices with 1 non-zero per column and preserving distances in a d dimensional-subspace up to a constant factor must have at least Ω(d2) rows. This matches an upper bound in [Nelson-Nguyen, arXiv abs/1211.1002] and shows the impossibility of obtaining the best of both of constructions in that work, namely 1 non-zero per column and d · polylog d rows. Copyright 2013 ACM.


Johnson-Laird P.N.,Princeton University | Johnson-Laird P.N.,New York University | Khemlani S.S.,U.S. Navy | Goodwin G.P.,University of Pennsylvania
Trends in Cognitive Sciences | Year: 2015

This review addresses the long-standing puzzle of how logic and probability fit together in human reasoning. Many cognitive scientists argue that conventional logic cannot underlie deductions, because it never requires valid conclusions to be withdrawn - not even if they are false; it treats conditional assertions implausibly; and it yields many vapid, although valid, conclusions. A new paradigm of probability logic allows conclusions to be withdrawn and treats conditionals more plausibly, although it does not address the problem of vapidity. The theory of mental models solves all of these problems. It explains how people reason about probabilities and postulates that the machinery for reasoning is itself probabilistic. Recent investigations accordingly suggest a way to integrate probability and deduction. © 2015 Elsevier Ltd.


Hsiang S.M.,Princeton University | Hsiang S.M.,National Bureau of Economic Research | Hsiang S.M.,University of California at Berkeley | Burke M.,University of California at Berkeley | And 2 more authors.
Science | Year: 2013

A rapidly growing body of research examines whether human conflict can be affected by climatic changes. Drawing from archaeology, criminology, economics, geography, history, political science, and psychology, we assemble and analyze the 60 most rigorous quantitative studies and document, for the first time, a striking convergence of results. We find strong causal evidence linking climatic events to human conflict across a range of spatial and temporal scales and across all major regions of the world. The magnitude of climate's influence is substantial: for each one standard deviation (1σ) change in climate toward warmer temperatures or more extreme rainfall, median estimates indicate that the frequency of interpersonal violence rises 4% and the frequency of intergroup conflict rises 14%. Because locations throughout the inhabited world are expected to warm 2σ to 4σ by 2050, amplified rates of human conflict could represent a large and critical impact of anthropogenic climate change.


Sun X.R.,Princeton University
Nature communications | Year: 2013

The use of genetically encodable calcium indicator proteins to monitor neuronal activity is hampered by slow response times and a narrow Ca(2+)-sensitive range. Here we identify three performance-limiting features of GCaMP3, a popular genetically encodable calcium indicator protein. First, we find that affinity is regulated by the calmodulin domain's Ca(2+)-chelating residues. Second, we find that off-responses to Ca(2+) are rate-limited by dissociation of the RS20 domain from calmodulin's hydrophobic pocket. Third, we find that on-responses are limited by fast binding to the N-lobe at high Ca(2+) and by slow binding to the C-lobe at lower Ca(2+). We develop Fast-GCaMPs, which have up to 20-fold accelerated off-responses and show that they have a 200-fold range of K(D), allowing coexpression of multiple variants to span an expanded range of Ca(2+) concentrations. Finally, we show that Fast-GCaMPs track natural song in Drosophila auditory neurons and generate rapid responses in mammalian neurons, supporting the utility of our approach.


Discussions of "big data" in medicine often revolve around gene sequencing and biosamples. It is perhaps less recognized that administrative data in the form of vital records, hospital discharge abstracts, insurance claims, and other routinely collected data also offer the potential for using information from hundreds of thousands, if not millions, of people to answer important questions. However, the increasing ease with which such data may be used and reused has increased concerns about privacy and informed consent. Addressing these concerns without creating insurmountable barriers to the use of such data for research is essential if we are to avoid a "missed opportunity" in pediatrics research. Copyright © 2013 by the American Academy of Pediatrics.


Metzger B.D.,Princeton University
Monthly Notices of the Royal Astronomical Society | Year: 2010

A small fraction of core-collapse supernovae (SNe) show evidence that the outgoing blast wave has encountered a substantial mass 1-10 M of circumstellar matter (CSM) at radii 102-103 au, much more than can nominally be explained by pre-explosion stellar winds. In extreme cases, this interaction may power the most luminous, optically energetic SNe yet discovered. Interpretations for the origin of the CSM have thus far centred on explosive eruptions from the star just years-decades prior to the core collapse. Here, we consider an alternative possibility that the inferred CSM is a relic disc left over from stellar birth. We investigate this hypothesis by calculating the evolution of proto-stellar discs around massive stars following their early embedded phase using a self-similar accretion model. We identify a brief initial gravitationally unstable ('gravito-turbulent') phase, followed by a much longer period of irradiation-supported accretion during which less effective non-gravitational forms of angular momentum transport dominate. Although external influences, such as the presence of a wide binary companion, may preclude disc survival in many systems, we find that massive ( 1-10 M) discs can preferentially survive around the most massive stars. Reasons for this perhaps counterintuitive result include (1) the shorter stellar lifetimes and (2) large photoevaporation radii ( 103 au) of very massive stars; (3) suppression of the magnetorotational instability due to the shielding from external sources of ionization and (4) relative invulnerability of massive discs to lower mass stellar collisions and luminous blue variable eruptions. Furthermore, discs with radii 102-103 au are picked out by the physics of the embedded stage of accretion and the requisite conditions for subsequent disc survival. The disc mass, radius and scaleheight at core collapse typically result in an 10 per cent efficiency for converting the kinetic energy of the exploding star into radiation, potentially producing a total electromagnetic output of 1050-1051 erg. We identify two regimes of disc-supernova interaction, which are distinguished by whether the shocked disc radiates its thermal energy before being engulfed by the expanding SN ejecta. This dichotomy may explain the difference between very luminous supernova which show narrow H line emission and those which show no direct evidence for hydrogen-rich CSM interaction. Because very luminous SNe are rare, testing the relic disc model requires constraining the presence of long-lived discs around a small fraction of very massive stars. © 2010 The Author. Journal compilation © 2010 RAS.


Zhang F.,Georgia Institute of Technology | Leonard N.E.,Princeton University
IEEE Transactions on Automatic Control | Year: 2010

Autonomous mobile sensor networks are employed to measure large-scale environmental fields. Yet an optimal strategy for mission design addressing both the cooperative motion control and the cooperative sensing is still an open problem. We develop strategies for multiple sensor platforms to explore a noisy scalar field in the plane. Our method consists of three parts. First, we design provably convergent cooperative Kalman filters that apply to general cooperative exploration missions. Second, we present a novel method to determine the shape of the platform formation to minimize error in the estimates and design a cooperative formation control law to asymptotically achieve the optimal formation shape. Third, we use the cooperative filter estimates in a provably convergent motion control law that drives the center of the platform formation to move along level curves of the field. This control law can be replaced by control laws enabling other cooperative exploration motion, such as gradient climbing, without changing the cooperative filters and the cooperative formation control laws. Performance is demonstrated on simulated underwater platforms in simulated ocean fields. © 2010 IEEE.


Stone J.M.,Princeton University | Gardiner T.A.,Sandia National Laboratories
Astrophysical Journal, Supplement Series | Year: 2010

We describe the implementation of the shearing box approximation for the study of the dynamics of accretion disks in the Athena magnetohydrodynamic (MHD) code. Second-order Crank-Nicholson time differencing is used for the Coriolis and tidal gravity source terms that appear in the momentum equation for accuracy and stability. We show that this approach conserves energy for epicyclic oscillations in hydrodynamic flows to round-off error. In the energy equation, the tidal gravity source terms are differenced as the gradient of an effective potential in a way that guarantees that total energy (including the gravitational potential energy) is also conserved to round-off error. We introduce an orbital advection algorithm for MHD based on constrained transport to preserve the divergence-free constraint on the magnetic field. This algorithm removes the orbital velocity from the time step constraint, and makes the truncation error more uniform in radial position. Modifications to the shearing box boundary conditions applied at the radial boundaries are necessary to conserve the total vertical magnetic flux. In principle, similar corrections are also required to conserve mass, momentum, and energy; however in practice we find that the orbital advection method conserves these quantities to better than 0.03% over hundreds of orbits. The algorithms have been applied to studies of the nonlinear regime of the MRI in very wide (up to 32 scale heights) horizontal domains. © 2010 The American Astronomical Society.


Dryer F.L.,Princeton University
Proceedings of the Combustion Institute | Year: 2015

Internal combustion engines running on liquid fuels will remain the dominant prime movers for road and air transportation for decades, probably for most of this century. The world's appetite for liquid transportation fuels derived from petroleum and other fossil resources is already immense, will grow, will at some future time become economically unsustainable, and will become infeasible only in the very long term. The ongoing process of augmenting and eventually replacing petroleum-derived fuels with liquid alternative fuels must necessarily involve approaches that result in comparatively much lower net carbon cycle emissions from the transportation sector, most likely through a combination of carbon sequestration and renewable fuel production. The successful growth and establishment of a sustainable, profitable alternative fuels industry will be best facilitated by approaches that integrate alternative products into petroleum-derived fuel streams (i.e., gasolines, diesel, and jet fuels) and consider synergistic evolution of and integration with prevailing refining and liquid fuel distribution infrastructures. The emergence of low temperature combustion strategies, particularly those implementing dual fuel methods to achieve Reaction Controlled Compression Ignition (RCCI), offers the potential to significantly improve operating efficiency and reduce emissions with minimal aftertreatment. For all advanced combustion engine technologies, but especially for RCCI, a clear understanding of fuel property influences on combustion behaviors will be important to achieving projected engine performance and emissions. To achieve the benefits projected by emerging engine technologies, the properties of petroleum-derived fuels themselves must be modified over time, but the effects of blending candidate alternative fuels with these conventional fuels must also be understood. Predicting the coupled physical and chemical property effects of real fuels on energy conversion system performance and emissions is a daunting problem, even for petroleum-derived real fuels, since each is composed of several hundred to thousands of individual chemical species typically belonging to one of a few organic classes (e.g., n-paraffins, iso-paraffins, cyclo-paraffins, olefins, aromatics). For specific combustion applications, it is often the global combustion response to variations in the composition of fuel mixtures - inclusive of those occurring by blending petroleum-derived fuel with alternative fuel candidates - that is of interest for fuel property optimization. This paper presents an overview of tools used for evaluating and emulating combustion-relevant properties of real fuels and alternative fuel candidates. New analytical and statistical methods can provide important insights as to how the ensembles of distinct molecular structures found in a given fuel mixture contribute to the physical and chemical kinetic properties that govern its combustion in energy conversion processes. Such tools can in turn assist in screening candidate alternative fuels for more detailed study. © 2014 The Combustion Institute.


Conroy C.,University of California at Santa Cruz | Graves G.J.,Princeton University | Van Dokkum P.G.,Yale University
Astrophysical Journal | Year: 2014

The stellar populations of galaxies hold vital clues to their formation histories. In this paper we present results based on modeling stacked spectra of early-type galaxies drawn from the Sloan Digital Sky Survey as a function of velocity dispersion, σ, from 90 km s-1 to 300 km s -1. The spectra are of extremely high quality, with typical signal-to-noise ratio of 1000 Å-1, and a wavelength coverage of 4000 Å -8800 Å. Our population synthesis model includes variation in 16 elements from C to Ba, a two-component star formation history, the shift in effective temperature, Δ T eff, of the stars with respect to a solar metallicity isochrone, and the stellar initial mass function, among other parameters. In our approach we fit the full optical spectra rather than a select number of spectral indices and are able to, for the first time, measure the abundances of the elements V, Cr, Mn, Co, and Ni from the integrated light of distant galaxies. Our main results are as follows: (1) light-weighted stellar ages range from 6-12 Gyr from low to high σ; (2) [Fe/H] varies by less than 0.1 dex across the entire sample; (3) Mg closely tracks O, and both increase from ≈0.0 at low σ to ∼0.25 at high σ; Si and Ti show a shallower rise with σ, and Ca tracks Fe rather than O; (4) the iron peak elements V, Cr, Mn, and Ni track Fe, while Co tracks O, suggesting that Co forms primarily in massive stars; (5) C and N track O over the full sample and [C/Fe] and [N/Fe] exceed 0.2 at high σ; and (6) the variation in Δ T eff with total metallicity closely follows theoretical predictions based on stellar evolution theory. This last result is significant because it implies that we are robustly solving not only for the detailed abundance patterns but also the detailed temperature distributions (i.e., isochrones) of the stars in these galaxies. A variety of tests reveal that the systematic uncertainties in our measurements are probably 0.05 dex or less. Our derived [Mg/Fe] and [O/Fe] abundance ratios are 0.05-0.1 dex lower than most previous determinations. Under the conventional interpretation that the variation in these ratios is due to star formation timescale variations, our results suggest longer star formation timescales for massive early-type galaxies than previous studies. Detailed chemical evolution models are necessary in order to translate the abundance ratio distributions of these galaxies into constraints on their formation histories. Alternatively, these data may provide useful constraints on the nucleosynthetic pathways for elements whose production is not well understood. © 2014. The American Astronomical Society. All rights reserved.


Cen R.,Princeton University
Astrophysical Journal Letters | Year: 2011

Utilizing state-of-the-art adaptive mesh refinement cosmological hydrodynamic simulations with ultra-high resolution (114 h -1pc) and a large sample size (≥3300 galaxies of stellar mass ≥109 M ⊙), we show how the stellar light of Lyman break galaxies at z = 2 is distributed between optical/ultraviolet (UV) and far-infrared (FIR) bands. With a single scalar parameter for dust obscuration we can simultaneously reproduce the observed UV luminosity function for the entire range (3-100 M ⊙yr-1) and extant FIR luminosity function at the bright end (≥20 M ⊙yr-1). We quantify that galaxies more massive or having higher star formation rate (SFR) tend to have larger amounts of dust obscuration mostly due to a trend in column density and in a minor part due to a mass (or SFR)-metallicity relation. It is predicted that the FIR luminosity function in the range SFR = 1-100 M ⊙yr-1 is a power law with a slope of about-1.7. We further predict that there is a "galaxy desert" at SFRFIR < 0.02(SFRUV/10 M ⊙ yr-1)2.1 M ⊙ yr-1 in the SFRUV - SFRFIR plane. Detailed distributions of SFRFIR at a fixed SFRUV are presented. Upcoming observations by the Atacama Large Millimeter Array should test this model. If confirmed, it validates the predictions of the standard cold dark matter model and has important implications on the intrinsic SFR function of galaxies at high redshift. © 2011. The American Astronomical Society. All rights reserved..


In developed nations that rely on multiple, competing health insurers-for example, Switzerland and Germany-the prices for health care services and products are subject to uniform price schedules that are either set by government or negotiated on a regional basis between associations of health insurers and associations of providers of health care. In the United States, some states-notably Maryland-have used such all-payer systems for hospitals only. Elsewhere in the United States, prices are negotiated between individual payers and providers. This situation has resulted in an opaque system in which payers with market power force weaker payers to cover disproportionate shares of providers' fixed costs-a phenomenon sometimes termed cost shifting-or providers simply succeed in charging higher prices when they can. In this article I propose that this price-discriminatory system be replaced over time by an all-payer system as a means to better control costs and ensure equitable payment. © 2011 Project HOPE-The People-to-People Health Foundation, Inc.


Choptuik M.W.,University of British Columbia | Pretorius F.,Princeton University
Physical Review Letters | Year: 2010

We present results from numerical solution of the Einstein field equations describing the head-on collision of two solitons boosted to ultrarelativistic energies. We show, for the first time, that at sufficiently high energies the collision leads to black hole formation, consistent with hoop-conjecture arguments. This implies that the nonlinear gravitational interaction between the kinetic energy of the solitons causes gravitational collapse, and that arguments for black hole formation in super-Planck scale particle collisions are robust. © 2010 The American Physical Society.


Scully M.O.,Texas A&M University | Scully M.O.,Princeton University
Physical Review Letters | Year: 2010

The fundamental limit to photovoltaic efficiency is widely thought to be radiative recombination which balances radiative absorption. We here show that it is possible to break detailed balance via quantum coherence, as in the case of lasing without inversion and the photo-Carnot quantum heat engine. This yields, in principle, a quantum limit to photovoltaic operation which can exceed the classical one. The present work is in complete accord with the laws of thermodynamics. © 2010 The American Physical Society.


Isseroff L.Y.,Princeton University | Carter E.A.,Andlinger Center for Energy and the Environment
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We show that a "one-shot" GW approach (denoted G 0W 0) can accurately calculate the photoemission/inverse- photoemission properties of Cu 2O. As the results of any perturbative method are heavily dependent on the reference state, the appropriate reference Hamiltonian for G 0W 0 is identified by evaluating the performance of density-functional-theory-based input wave functions and eigenvalues generated with selected exchange-correlation functionals. It is shown that a reference Hamiltonian employing the hybrid Heyd-Scuseria-Ernzerhof functional used in conjunction with G 0W 0 produces an accurate photoemission/inverse-photoemission band gap and photoemission spectrum whose character is then further analyzed. The physical origin of why a hybrid functional is required for the zeroth-order wave function is discussed, giving insight into the unique electronic structure of Cu 2O in comparison to other transition-metal oxides. © 2012 American Physical Society.


Wright J.S.,Tsinghua University | Fueglistaler S.,Princeton University
Atmospheric Chemistry and Physics | Year: 2013

We present the time mean heat budgets of the tropical upper troposphere (UT) and lower stratosphere (LS) as simulated by five reanalysis models: the Modern-Era Retrospective Analysis for Research and Applications (MERRA), European Reanalysis (ERA-Interim), Climate Forecast System Reanalysis (CFSR), Japanese 25-yr Reanalysis and Japan Meteorological Agency Climate Data Assimilation System (JRA-25/JCDAS), and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis 1. The simulated diabatic heat budget in the tropical UTLS differs significantly from model to model, with substantial implications for representations of transport and mixing. Large differences are apparent both in the net heat budget and in all comparable individual components, including latent heating, heating due to radiative transfer, and heating due to parameterised vertical mixing. We describe and discuss the most pronounced differences. Discrepancies in latent heating reflect continuing difficulties in representing moist convection in models. Although these discrepancies may be expected, their magnitude is still disturbing. We pay particular attention to discrepancies in radiative heating (which may be surprising given the strength of observational constraints on temperature and tropospheric water vapour) and discrepancies in heating due to turbulent mixing (which have received comparatively little attention). The largest differences in radiative heating in the tropical UTLS are attributable to differences in cloud radiative heating, but important systematic differences are present even in the absence of clouds. Local maxima in heating and cooling due to parameterised turbulent mixing occur in the vicinity of the tropical tropopause. © Author(s) 2013.


Schiro M.,Princeton University
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Whether a small quantum mechanical system is able to equilibrate with its environment once an external local perturbation drives it out of thermal equilibrium is a central question which cuts across many different fields of science. Here we consider such a problem for a correlated quantum impurity coupled to a fermionic reservoir and driven out of equilibrium by local quantum quenches such as those recently realized in optical absorption experiments on single quantum dots. We argue that equilibration in this problem is deeply connected to the occurrence of the Kondo effect at low energy and that a highly nontrivial dynamical behavior may emerge whenever a local quantum critical point intrudes between a conventional Kondo screened phase and a Kondo unscreened one. We discuss this issue in the context of the Anderson impurity model coupled to a pseudogap reservoir by using a correlated time-dependent variational wave function that is able to qualitatively describe this physics © 2012 American Physical Society.


Rafikov R.R.,Princeton University
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

It was recently proposed that metal-rich white dwarfs (WDs) accrete their metals from compact discs of debris found to exist around more than a dozen of them. At the same time, elemental abundances measured in atmospheres of some WDs imply vigorous metal accretion at rates up to 10 11gs -1, far in excess of what can be supplied solely by Poynting-Robertson drag acting on such discs of debris. To explain this observation we propose a model, in which rapid transport of metals from the disc on to the WD naturally results from interaction between this particulate disc and a spatially coexisting disc of metallic gas. The latter is fed by evaporation of debris particles at the sublimation radius located at several tens of WD radii. Because of pressure support the gaseous disc orbits the WD slower than the particulate disc. Resultant azimuthal drift between them at speed ≲1 m s -1 causes aerodynamic drag on the disc of solids and drives inward migration of its constituent particles. Upon reaching the sublimation radius, particles evaporate, enhancing the density of the metallic gaseous disc and leading to positive feedback. Under favourable circumstances (low viscosity in the disc of metallic gas and efficient aerodynamic coupling between the discs) a system evolves in a runaway fashion, destroying the discs of debris on time-scale of ~10 5yr, and giving rise to high metal accretion rates up to gs -1, in agreement with observations. © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Khoury J.,University of Pennsylvania | Steinhardt P.J.,Princeton University
Physical Review Letters | Year: 2010

The Universe can be made flat and smooth by undergoing a phase of ultraslow (ekpyrotic) contraction, a condition achievable with a single, canonical scalar field and conventional general relativity. It has been argued, though, that generating scale-invariant density perturbations requires at least two scalar fields and a two-step process that first produces entropy fluctuations and then converts them to curvature perturbations. In this Letter we identify a loophole in the argument and introduce an ekpyrotic model based on a single, canonical scalar field that generates nearly scale-invariant curvature fluctuations through a purely "adiabatic mechanism" in which the background evolution is a dynamical attractor. The resulting spectrum can be slightly red with distinctive non-Gaussian fluctuations. © 2010 The American Physical Society.


Wyart M.,Princeton University | Wyart M.,New York University
Physical Review Letters | Year: 2010

An approximate relation between the vibrational entropy and the mean square displacement of the particles is derived. Using observations of the short-time dynamics in liquids of various fragility, it is argued that (i) if the crystal entropy is significantly smaller than the liquid entropy at Tg, the extrapolation of the vibrational entropy leads to the correlation TK T0, where TK is the Kauzmann temperature and T0 is the temperature extracted from the Vogel-Fulcher fit of the viscosity, and (ii) the jump in specific heat associated with vibrational entropy is very small for strong liquids, and increases with fragility. The analysis suggests that these correlations stem from the stiffening of the Boson peak under cooling, underlying the importance of this phenomenon on the dynamical arrest. © 2010 The American Physical Society.


Rafikov R.R.,Princeton University
Astrophysical Journal Letters | Year: 2011

Recent discoveries of compact (sizes ≲R⊙) debris disks around more than a dozen metal-rich white dwarfs (WDs) suggest that pollution of these stars with metals may be caused by accretion of high-Z material from the disk. But the mechanism responsible for efficient transfer of mass from a particulate disk to the WD atmosphere has not yet been identified. Here we demonstrate that radiation of the WD can effectively drive accretion of matter through the disk toward the sublimation radius (located at several tens of WD radii), where particles evaporate, feeding a disk of metal gas accreting onto the WD. We show that, contrary to some previous claims, Poynting-Robertson (PR) drag on the debris disk is effective at providing metal accretion rate ṀPR ∼ 108 g s-1 and higher, scaling quadratically with WD effective temperature. We compare our results with observations and show that, as expected, no WD hosting a particulate debris disk shows evidence of metal accretion rate below that produced by the PR drag. Existence of WDs accreting metals at rates significantly higher than ṀPR suggests that another mechanism in addition to the PR drag drives accretion of high-Z elements in these systems. © 2011. The American Astronomical Society. All rights reserved.


Miller N.,Princeton University | Gerlai R.,University of Toronto
PLoS ONE | Year: 2012

Animal groups on the move can take different configurations. For example, groups of fish can either be 'shoals' or 'schools': shoals are simply aggregations of individuals; schools are shoals exhibiting polarized, synchronized motion. Here we demonstrate that polarization distributions of groups of zebrafish (Danio rerio) are bimodal, showing two distinct modes of collective motion corresponding to the definitions of shoaling and schooling. Other features of the group's motion also vary consistently between the two modes: zebrafish schools are faster and less dense than zebrafish shoals. Habituation to an environment can also alter the proportion of time zebrafish groups spend schooling or shoaling. Models of collective motion suggest that the degree and stability of group polarization increases with the group's density. Examining zebrafish groups of different sizes from 5 to 50, we show that larger groups are less polarized than smaller groups. Decreased fearfulness in larger groups may function similarly to habituation, causing them to spend more time shoaling than schooling, contrary to most models' predictions. © 2012 Miller, Gerlai.


MacIejko J.,Princeton University
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We revisit the problem of a single quantum impurity on the edge of a two-dimensional time-reversal invariant topological insulator and show that the zero-temperature phase diagram contains a large local moment region for antiferromagnetic Kondo coupling, which was missed by previous poor man's scaling treatments. The combination of an exact solution at the so-called decoupling point and a renormalization group analysis à la Anderson-Yuval-Hamann allows us to access the regime of strong electron-electron interactions on the edge and strong Kondo coupling. We apply similar methods to the problem of a regular one-dimensional array of quantum impurities interacting with the edge liquid. When the edge electrons are at half-filling with respect to the impurity lattice, the system remains gapless unless the Luttinger parameter of the edge is less than 1/2 in which case two-particle backscattering effects drive the system to a gapped phase with long-range Ising antiferromagnetic order. This is in marked contrast with the gapped disordered ground state of the ordinary half-filled one-dimensional Kondo lattice. © 2012 American Physical Society.


Burrows A.,Princeton University
Reviews of Modern Physics | Year: 2013

Core-collapse theory brings together many facets of high-energy and nuclear astrophysics and the numerical arts to present theorists with one of the most important, yet frustrating, astronomical questions: "What is the mechanism of core-collapse supernova explosions?" A review of all the physics and the 50-year history involved would soon bury the reader in minutiae that could easily obscure the essential elements of the phenomenon, as we understand it today. Moreover, much remains to be discovered and explained, and a complicated review of an unresolved subject in flux could grow stale fast. Therefore, this paper describes various important facts and perspectives that may have escaped the attention of those interested in this puzzle. Furthermore, an attempt to describe the modern theory's physical underpinnings and a brief summary of the current state of play are given. In the process, a few myths that have crept into modern discourse are identified. However, there is much more to do and humility in the face of this age-old challenge is clearly the most prudent stance as its eventual resolution is sought. © 2013 American Physical Society.


Chen X.,Stevens Institute of Technology | Xu S.,Stevens Institute of Technology | Yao N.,Princeton University | Shi Y.,Stevens Institute of Technology
Nano Letters | Year: 2010

Energy harvesting technologies that are engineered to miniature sizes, while still increasing the power delivered to wireless electronics, 1,2 portable devices, stretchable electronics, 3 and implantable biosensors, 4,5 are strongly desired. Piezoelectric nanowire- and nanofiber-based generators have potential uses for powering such devices through a conversion of mechanical energy into electrical energy. 6 However, the piezoelectric voltage constant of the semiconductor piezoelectric nanowires in the recently reported piezoelectric nanogenerators 7-12is lower than that of lead zirconate titanate (PZT) nanomaterials. Here we report a piezoelectric nanogenerator based on PZT nanofibers. The PZT nanofibers, with a diameter and length of approximately 60 nm and 500 μm, were aligned on interdigitated electrodes of platinum fine wires and packaged using a soft polymer on a silicon substrate. The measured output voltage and power under periodic stress application to the soft polymer was 1.63 V and 0.03 μW, respectively. © 2010 American Chemical Society.


Bouchet F.,University of Lyon | Venaille A.,Princeton University
Physics Reports | Year: 2012

The theoretical study of the self-organization of two-dimensional and geophysical turbulent flows is addressed based on statistical mechanics methods. This review is a self-contained presentation of classical and recent works on this subject; from the statistical mechanics basis of the theory up to applications to Jupiter's troposphere and ocean vortices and jets. Emphasize has been placed on examples with available analytical treatment in order to favor better understanding of the physics and dynamics.After a brief presentation of the 2D Euler and quasi-geostrophic equations, the specificity of two-dimensional and geophysical turbulence is emphasized. The equilibrium microcanonical measure is built from the Liouville theorem. Important statistical mechanics concepts (large deviations and mean field approach) and thermodynamic concepts (ensemble inequivalence and negative heat capacity) are briefly explained and described.On this theoretical basis, we predict the output of the long time evolution of complex turbulent flows as statistical equilibria. This is applied to make quantitative models of two-dimensional turbulence, the Great Red Spot and other Jovian vortices, ocean jets like the Gulf-Stream, and ocean vortices. A detailed comparison between these statistical equilibria and real flow observations is provided.We also present recent results for non-equilibrium situations, for the studies of either the relaxation towards equilibrium or non-equilibrium steady states. In this last case, forces and dissipation are in a statistical balance; fluxes of conserved quantity characterize the system and microcanonical or other equilibrium measures no longer describe the system. © 2012 Elsevier B.V.


Schenkel B.A.,Princeton University
Journal of Climate | Year: 2016

The present study provides a climatology of multiple tropical cyclone (TC) events (MTCEs) and the potential environmental factors responsible for triggering MTCEs in the North Atlantic (NATL), eastern North Pacific (EPAC), and western North Pacific (WPAC). While single TC events (STCEs) occur more frequently than MTCEs in each basin, a substantial fraction (34%-57%) of all TCs within each basin occur during MTCEs. Comparison of the total monthly number of MTCEs and STCEs reveals significant correlations (0.79 ≤R ≤0.90), while nonsignificant correlations exist between the annual number of MTCEs and STCEs. New TCs that form during MTCEs occur in the eastern main development region east of the STCE formation location in the NATL and EPAC, while new TC formation locations are spread evenly throughout the WPAC during both MTCEs and STCEs. The spatiotemporal separation between TCs during MTCEs is consistent among basins with median zonal distances between TCs of ~(1640-2010) km and median temporal separation between TC formation of 3.00-3.25 days. Composites of EPAC MTCEs suggest the existence of significantly stronger large-scale intraseasonal anomalies compared to STCEs, which may favor EPAC MTCE occurrence. Eastward zonal group velocities and the agreement of the zonal wavelength of TC-induced Rossby waves with the observed zonal distance between TCs suggests that Rossby wave radiation may contribute to a substantial fraction of MTCEs in all basins. These results suggest remarkable similarity in MTCE characteristics among basins, while potentially indicating that the large-scale environment is preconditioned for EPAC MTCE occurrence. © 2016 American Meteorological Society.


Rykaczewski R.R.,Princeton University | Dunne J.P.,National Oceanic and Atmospheric Administration
Geophysical Research Letters | Year: 2010

A leading hypothesis relating productivity with climate variability in the California Current Ecosystem (CCE) describes an alternation between warmer, well-stratified periods of low productivity and cooler periods of high productivity. This empirical relationship suggests that productivity will decline with global warming. Here, we explore the response of productivity to future climate change in the CCE using an earth system model. This model projects increases in nitrate supply and productivity in the CCE during the 21st century despite increases in stratification and limited change in wind-driven upwelling. We attribute the increased nitrate supply to enrichment of deep source waters entering the CCE resulting from decreased ventilation of the North Pacific. Decreases in dissolved-oxygen concentration and increasing acidification accompany projected increases in nitrate. This analysis illustrates that anthropogenic climate change may be unlike past variability; empirical relationships based on historical observations may be inappropriate for projecting ecosystem responses to future climate change. © 2010 by the American Geophysical Union.


Cen R.,Princeton University
Astrophysical Journal Letters | Year: 2014

While the Sersic profile family provides adequate fits for the surface brightness profiles of observed galaxies, its physical origin is unknown. We show that if the cosmological density field is seeded by random Gaussian fluctuations, as in the standard cold dark matter model, galaxies with steep central profiles have simultaneously extended envelopes of shallow profiles in the outskirts, whereas galaxies with shallow central profiles are accompanied by steep density profiles in the outskirts. These properties are in accord with those of the Sersic profile family. Moreover, galaxies with steep central profiles form their central regions in smaller denser subunits that possibly merge subsequently, which naturally leads to the formation of bulges. In contrast, galaxies with shallow central profiles form their central regions in a coherent fashion without significant substructure, a necessary condition for disk galaxy formation. Thus, the scenario is self-consistent with respect to the correlation between observed galaxy morphology and the Sersic index. We further predict that clusters of galaxies should display a similar trend, which should be verifiable observationally. © 2014. The American Astronomical Society. All rights reserved.


Cen R.,Princeton University
Astrophysical Journal Letters | Year: 2014

A new physical framework for the emergence of the Hubble sequence is outlined, based on novel analyses performed to quantify the evolution of cold streams of a large sample of galaxies from a state-of-the-art ultra-high resolution, large-scale adaptive mesh-refinement hydrodynamic simulation in a fully cosmological setting. It is found that the following three key physical variables of galactic cold inflows crossing the virial sphere substantially decrease with decreasing redshift: the number of streams N90 that make up 90% of concurrent inflow mass flux, average inflow rate per stream and mean (mass flux weighted) gas density in the streams ngas. Another key variable, the stream dimensionless angular momentum parameter λ, is found to instead increase with decreasing redshift. Assimilating these trends and others naturally leads to a physically coherent scenario for the emergence of the Hubble sequence, including the following expectations: (1) the predominance of a mixture of disproportionately small irregular and complex disk galaxies at z ≥ 2 when most galaxies have multiple concurrent streams, (2) the beginning of the appearance of flocculent spirals at z ∼ 1-2 when the number of concurrent streams are about 2-3, (3) the grand-design spiral galaxies appear at z ≤ 1 when galaxies with only one major cold stream significantly emerge. These expected general trends are in good accord with observations. Early-type galaxies are those that have entered a perennial state of zero cold gas stream, with their abundance increasing with decreasing redshift. © 2014. The American Astronomical Society. All rights reserved.


Papenfort K.,Princeton University | Vogel J.,University of Wurzburg
Frontiers in cellular and infection microbiology | Year: 2014

Enteric pathogens often cycle between virulent and saprophytic lifestyles. To endure these frequent changes in nutrient availability and composition bacteria possess an arsenal of regulatory and metabolic genes allowing rapid adaptation and high flexibility. While numerous proteins have been characterized with regard to metabolic control in pathogenic bacteria, small non-coding RNAs have emerged as additional regulators of metabolism. Recent advances in sequencing technology have vastly increased the number of candidate regulatory RNAs and several of them have been found to act at the interface of bacterial metabolism and virulence factor expression. Importantly, studying these riboregulators has not only provided insight into their metabolic control functions but also revealed new mechanisms of post-transcriptional gene control. This review will focus on the recent advances in this area of host-microbe interaction and discuss how regulatory small RNAs may help coordinate metabolism and virulence of enteric pathogens.


Huff E.M.,Ohio State University | Graves G.J.,Princeton University
Astrophysical Journal Letters | Year: 2014

We describe a new method for measuring galaxy magnification due to weak gravitational lensing. Our method makes use of a tight scaling relation between galaxy properties that are modified by gravitational lensing, such as apparent size, and other properties that are not, such as surface brightness. In particular, we use a version of the well-known fundamental plane relation for early-type galaxies. This modified "photometric fundamental plane" uses only photometric galaxy properties, eliminating the need for spectroscopic data. We present the first detection of magnification using this method by applying it to photometric catalogs from the Sloan Digital Sky Survey. This analysis shows that the derived magnification signal is within a factor of three of that available from conventional methods using gravitational shear. We suppress the dominant sources of systematic error and discuss modest improvements that may further enhance the lensing signal-to-noise available with this method. Moreover, some of the dominant sources of systematic error are substantially different from those of shear-based techniques. With this new technique, magnification becomes a useful measurement tool for the coming era of large ground-based surveys intending to measure gravitational lensing. © 2014. The American Astronomical Society. All rights reserved.


In response to virus infection, cells can alter protein expression to modify cellular functions and limit viral replication. To examine host protein expression during infection with human cytomegalovirus (HCMV), an enveloped DNA virus, we performed a semiquantitative, temporal analysis of the cell surface proteome in infected fibroblasts. We determined that resident low density lipoprotein related receptor 1 (LRP1), a plasma membrane receptor that regulates lipid metabolism, is elevated early after HCMV infection, resulting in decreased intracellular cholesterol. siRNA knockdown or antibody-mediated inhibition of LRP1 increased intracellular cholesterol and concomitantly increased the infectious virus yield. Virions produced under these conditions contained elevated cholesterol, resulting in increased infectivity. Depleting cholesterol from virions reduced their infectivity by blocking fusion of the virion envelope with the cell membrane. Thus, LRP1 restricts HCMV infectivity by controlling the availability of cholesterol for the virion envelope, and increased LRP1 expression is likely a defense response to infection. Copyright © 2012 Elsevier Inc. All rights reserved.


Murthy M.,Princeton University
Development | Year: 2010

Cellularization of the Drosophila embryo is the process by which a syncytium of ∼6000 nuclei is subdivided into discrete cells. In order to individualize the cells, massive membrane addition needs to occur by a process that is not fully understood. The exocyst complex is required for some, but not all, forms of exocytosis and plays a role in directing vesicles to appropriate domains of the plasma membrane. Sec5 is a central component of this complex and we here report the isolation of a new allele of sec5 that has a temperature-sensitive phenotype. Using this allele, we investigated whether the exocyst complex is required for cellularization. Embryos from germline clones of the sec5ts1 allele progress normally through cycle 13. At cellularization, however, cleavage furrows do not invaginate between nuclei and consequently cells do not form. A zygotically translated membrane protein, Neurotactin, is not inserted into the plasma membrane and instead accumulates in cytoplasmic puncta. During cellularization, Sec5 becomes concentrated at the apical end of the lateral membranes, which is likely to be the major site of membrane addition. Subsequently, Sec5 concentrates at the sub-apical complex, indicating a role for Sec5 in the polarized epithelium. Thus, the exocyst is necessary for, and is likely to direct, the polarized addition of new membrane during this form of cytokinesis.


Bai X.-N.,Princeton University
Astrophysical Journal | Year: 2011

Non-ideal MHD effects play an important role in the gas dynamics in protoplanetary disks (PPDs). This paper addresses the influence of non-ideal MHD effects on the magnetorotational instability (MRI) and angular momentum transport in PPDs using the most up-to-date results from numerical simulations. We perform chemistry calculations using a complex reaction network with standard prescriptions for X-ray and cosmic-ray ionizations. We first show that whether or not grains are included, the recombination time is at least one order of magnitude less than the orbital time within five disk scale heights, justifying the validity of local ionization equilibrium and strong coupling limit in PPDs. The full conductivity tensor at different disk radii and heights is evaluated, with the MRI active region determined by requiring that (1) the Ohmic Elsasser number Λ be greater than 1 and (2) the ratio of gas to magnetic pressure β be greater than βmin(Am) as identified in the recent study by Bai & Stone, where Am is the Elsasser number for ambipolar diffusion. With full flexibility as to the magnetic field strength, we provide a general framework for estimating the MRI-driven accretion rate and the magnetic field strength in the MRI active layer. We find that the MRI active layer always exists at any disk radius as long as the magnetic field in PPDs is sufficiently weak. However, the optimistically predicted in the inner disk (r = 1-10 AU) appears insufficient to account for the observed range of accretion rates in PPDs (around 10-8 M⊙yr-1) even in the grain-free calculation, and the presence of solar abundance sub-micron grains further reduces by one to two orders of magnitude. Moreover, we find that the predicted increases with radius in the inner disk where accretion is layered, which would lead to runaway mass accumulation if disk accretion is solely driven by the MRI. Our results suggest that stronger sources of ionization and/or additional mechanisms such as magnetized wind are needed to explain the observed accretion rates in PPDs. In contrast, our predicted is on the order of 10-9 M⊙yr-1 in the outer disk, consistent with the observed accretion rates in transitional disks. © 2011. The American Astronomical Society. All rights reserved.


Starikovskiy A.,Princeton University
Plasma Sources Science and Technology | Year: 2013

The dynamics of pulsed nanosecond discharge development in liquid water, ethanol and hexane were investigated experimentally. High-voltage pulses with durations of 20 and 60 ns and amplitudes of 6-60 kV were used for discharge initiation. It is shown that the dynamics of discharge formation fundamentally differ between liquids with low and high dielectric permittivity coefficients. In water (high permittivity), two phases were observed in the process of discharge development. The first phase is connected with electrostriction compression of the media near the needle tip and the formation of a rarefaction wave in the surrounding liquid. The second phase (the discharge phase) has a pronounced start delay, which depends on the voltage of the high-voltage electrode. Thus, at low voltages, the pulse length is insufficient for the initiation of the discharge, and the process consists of the first phase only, i.e. the formation of an electrostriction rarefaction wave. At higher voltages, the discharge start delay time decreases rapidly, and the discharge commences simultaneously with the formation of hydrodynamic perturbations by the electrostriction forces present in the media. Shadowgraphic laser visualization of the process demonstrates the transition from a pure hydrodynamic density perturbation in the rarefaction wave to a developed streamer-leader process with a strong energy release in the channels and the formation of strong shock waves around the channels. Unlike in water, the first phase is essentially non-existent in liquids with low dielectric permittivity coefficients because of the small electrostriction forces and the low intensity of the rarefaction wave that is formed. The second phase in the process (discharge) begins at significantly higher voltages on the high-voltage electrode, immediately leading to the long branched structure of the streamer-leader flash. The difference in the nanosecond discharge development in liquid dielectrics may be explained by the formation of micro-discontinuities in the media during the electrostriction compression/rarefaction stage in liquids with high dielectric permittivities. © 2013 IOP Publishing Ltd.


A skillful decadal prediction that foretells varying regional climate conditions over seasonal-interannual to multidecadal time scales is of societal significance. However, predictions initialized from the climateobserving system tend to drift away from observed states toward the imperfect model climate because of the model biases arising from imperfect model equations, numeric schemes, and physical parameterizations, as well as the errors in the values of model parameters. Here, a simple coupled model that simulates the fundamental features of the real climate system and a "twin" experiment framework are designed to study the impact of initialization and parameter optimization on decadal predictions. One model simulation is treated as "truth" and sampled to produce "observations" that are assimilated into other simulations to produce observation-estimated states and parameters. The degree to which the model forecasts based on different estimates recover the truth is an assessment of the impact of coupled initial shocks and parameter optimization on climate predictions of interests. The results show that the coupled model initialization through coupled data assimilation in which all coupled model components are coherently adjusted by observations minimizes the initial coupling shocks that reduce the forecast errors on seasonal-interannual time scales. Model parameter optimization with observations effectively mitigates the model bias, thus constraining the model drift in long time-scale predictions. The coupled model state-parameter optimization greatly enhances the model predictability. While valid "atmospheric" forecasts are extended 5 times, the decadal predictability of the "deep ocean" is almost doubled. The coherence of optimized model parameters and states is critical to improve the long time-scale predictions. © 2011 American Meteorological Society.


Basso B.,Princeton University | Rej A.,Institute for Advanced Study
Nuclear Physics B | Year: 2013

In this paper we study the integrability of a family of models with U(1)×SU(N) symmetry. They admit fermionic and bosonic formulations related through bosonization and subsequent T-duality. The fermionic theory is just the CP N-1 sigma model coupled to a self-interacting massless fermion, while the bosonic one defines a one-parameter deformation of the O(2N) sigma model. For N=2 the latter model is equivalent to the integrable deformation of the O(4) sigma model discovered by Wiegmann. At higher values of N we find that integrability is more sporadic and requires a fine-tuning of the parameters of the theory. A special case of our study is the N=4 model, which was found to describe the AdS 4×CP 3 string theory in the Alday-Maldacena decoupling limit. In this case we propose a set of asymptotic Bethe ansatz equations for the energy spectrum. © 2012.


Mesinger A.,Princeton University
Monthly Notices of the Royal Astronomical Society | Year: 2010

It is generally taken for granted that reionization has completed by z = 6, due to the detection of flux in the Lyα forest at redshifts z < 6. However, since reionization is expected to be highly inhomogeneous, much of the spectra pass through just the ionized component of the intergalactic medium (IGM) even for non-negligible values of the volume-weighted mean neutral hydrogen fraction, We study the expected signature of an incomplete reionization at z ∼ 5 -6, using very large-scale (2 Gpc) seminumeric simulations. We find that ruling out an incomplete reionization is difficult at these redshifts since: (1) quasars reside in biased regions of the ionization field, with fewer surrounding H i patches than implied by the global mean, ; this bias extends tens of comoving megaparsecs for ; (2) absorption from the residual neutral hydrogen inside the ionized IGM generally dominates over the absorption from the remaining H i regions, allowing them to effectively 'hide' among the many dark spectral patches and (3) modelling the Lyα forest and its redshift evolution even in just the ionized IGM is very difficult, and nearly impossible to do a priori. We propose using the fraction of pixels which are dark as a simple, nearly model-independent upper limit on Alternately, the size distribution of regions with no detectable flux (dark gaps) can be used to place a more model-dependent constraint. Either way, the current sample of quasars is statistically insufficient to constrain at z ∼ 6 to even the 10 per cent level. At z ∼ 5, where there are more available sightlines and the forest is less dark, constraining might be possible given a large dynamic range from very deep spectra and/or the Lyβ forest. We conclude with the caution against overinterpreting the observations. There is currently no direct evidence that reionization was complete by z ∼ 5 -6. © 2010 The Author. Journal compilation © 2010 RAS.


Andrews-Hanna J.C.,Colorado School of Mines | Lewis K.W.,Princeton University
Journal of Geophysical Research E: Planets | Year: 2011

Mars was warmer and wetter during the early to middle Noachian, before a hydrologic and climatic transition in the late Noachian led to a decrease in erosion rates, a change in valley network morphology, and a geochemical shift from phyllosilicate to sulfate formation that culminated in the formation of widespread sulfate-rich sedimentary deposits in Meridiani Planum and the surrounding Arabia Terra region. This secular evolution was overprinted by episodic and periodic variability, as recorded in the fluvial record, sedimentary layering, and erosional discontinuities. We investigate the temporal evolution of Martian groundwater hydrology during the Noachian and early Hesperian epochs using global-scale hydrological models. The results suggest that the more active hydrological cycle in the Noachian was a result of a greater total water inventory, causing a saturated near-surface and high precipitation rates. The late Noachian hydrologic, climatic, and geochemical transition can be explained by a fundamental shift in the hydrological regime driven by a net loss of water due to impact and solar wind erosion of the atmosphere. Following this transition, the water table retreated deep beneath the surface, except in isolated regions of focused groundwater upwelling and evaporation, producing the playa evaporites in Meridiani Planum and Arabia Terra. This long-term evolution was modulated by shorter-term climate forcing in the form of periodic and chaotic variations in the orbital parameters of Mars, resulting in changes in the volume of water sequestered in the polar caps and cryosphere. This shorter-term forcing can explain the observed periodic and bundled sedimentary layering, erosional unconformities, and evidence for a fluctuating water table at Meridiani Planum. Copyright 2011 by the American Geophysical Union.


Miles R.B.,Princeton University
Progress in Aerospace Sciences | Year: 2015

Since 2000 there has been a revolution in diagnostics of high-speed air flows. The foundations for this revolution were laid over the past few decades, but with the development of new short pulse and pulse burst laser technologies, higher laser powers and higher pulse energies, new high-speed cameras, better laser control and improved detection and laser delivery methodologies, many very effective new capabilities have been developed. Newly developed methods for molecular tagging velocimetry provide high fidelity visualization of transport properties and may be extended to simultaneous temperature measurements. Rapid field imaging with frequency tunable pulse burst lasers shows instantaneous flow structure and complex boundary and mixing interactions. Extending these pulse burst concepts to swept volumetric imaging is very promising for full volumetric data collection. Fast wavelength modulation spectroscopy follows real-time flow variation, and three-dimensional particle imaging extends particle imaging velocimetry to volumetric data acquisition. © 2014 Elsevier Ltd. All rights reserved.


Cen R.,Princeton University
Astrophysical Journal Letters | Year: 2014

We study the evolution of angular momenta of M * = 10 10-1012 M⊙galaxies utilizing large-scale ultra-high resolution cosmological hydrodynamic simulations and find that the spin of the stellar component changes direction frequently because of interactions with nearby systems, such as major mergers, minor mergers, significant gas inflows, and torques. The rate and nature of change of spin direction cannot be accounted for by large-scale tidal torques, because the rates of the latter fall short by orders of magnitude and because the apparent random swings of the spin direction are inconsistent with the alignment by linear density field. The implications for galaxy formation as well as the intrinsic alignment of galaxies are profound. Assuming the large-scale tidal field is the sole alignment agent, a new picture emerging is that intrinsic alignment of galaxies would be a balance between slow large-scale coherent torquing and fast spin reorientation by local interactions. What is still open is whether other processes, such as feeding galaxies with gas and stars along filaments or sheets, introduce coherence for spin directions of galaxies along the respective structures. © 2014. The American Astronomical Society. All rights reserved..


Cen R.,Princeton University
Astrophysical Journal Letters | Year: 2014

We report on the discovery of a relation between the number of star formation (SF) peaks per unit time, νpeak, and the size of the temporal smoothing window function, Δt, used to define the peaks: νpeak∝Δt 1-φ (φ ∼ 1.618). This relation holds over the range of Δt = 10-1000 Myr that can be reliably computed here, using a large sample of galaxies obtained from a state-of-the-art cosmological hydrodynamic simulation. This means that the temporal distribution of SF peaks in galaxies as a population is fractal with a Hausdorff fractal dimension equal to φ-1. This finding reveals, for the first time, that the superficially chaotic process of galaxy formation is underlined by temporal self-organization up to at least one gigayear. It is tempting to suggest that, given the known existence of spatial fractals (such as the power-law two-point function of galaxies), there is a joint spatio-temporal self-organization in galaxy formation. From an observational perspective, it will be urgent to devise diagnostics to probe the SF histories of galaxies with good temporal resolution to facilitate a test of this prediction. If confirmed, it would provide unambiguous evidence for a new picture of galaxy formation that is interaction driven, cooperative, and coherent in and between time and space. Unravelling its origin may hold the key to understanding galaxy formation. © 2014. The American Astronomical Society. All rights reserved.


Chaudhury K.N.,Princeton University
IEEE Signal Processing Letters | Year: 2013

Recently, it was demonstrated in , that the robustness of the classical Non-Local Means (NLM) algorithm can be improved by incorporating ℓp (0 < p \leq 2) regression into the NLM framework. This general optimization framework, called Non-Local Patch Regression (NLPR), contains NLM as a special case. Denoising results on synthetic and natural images show that NLPR consistently performs better than NLM beyond a moderate noise level, and significantly so when p is close to zero. An iteratively reweighted least-squares (IRLS) algorithm was proposed for solving the regression problem in NLPR, where the NLM output was used to initialize the iterations. Based on exhaustive numerical experiments, we observe that the IRLS algorithm is globally convergent (for arbitrary initialization) in the convex regime 1 p 2, and locally convergent (e.g., fails rarely using NLM initialization) in the non-convex regime 0 < p < 1. In this letter, we study the cost associated with the IRLS updates, and this, along with the framework of bounded optimization, is used to analyze the convergence of the algorithm. © 1994-2012 IEEE.


Klebanov I.R.,Princeton University
International Journal of Modern Physics A | Year: 2010

We provide a brief introduction to the ABJM theory, the level k U(N) × U(N) superconformal Chern-Simons matter theory which has been conjectured to describe N coincident M2-branes. We discuss its dual formulation in terms of M-theory on AdS 4 × S 7/ k and review some of the evidence in favor of the conjecture. We end with a brief discussion of the important role played by the monopole operators. © 2010 World Scientific Publishing Company.


Uzdensky D.A.,University of Colorado at Boulder | Spitkovsky A.,Princeton University
Astrophysical Journal | Year: 2014

The magnetosphere of a rotating pulsar naturally develops a current sheet (CS) beyond the light cylinder (LC). Magnetic reconnection in this CS inevitably dissipates a nontrivial fraction of the pulsar spin-down power within a few LC radii. We develop a basic physical picture of reconnection in this environment and discuss its implications for the observed pulsed gamma-ray emission. We argue that reconnection proceeds in the plasmoid-dominated regime, via a hierarchical chain of multiple secondary islands/flux ropes. The inter-plasmoid reconnection layers are subject to strong synchrotron cooling, leading to significant plasma compression. Using the conditions of pressure balance across these current layers, the balance between the heating by magnetic energy dissipation and synchrotron cooling, and Ampere's law, we obtain simple estimates for key parameters of the layers - temperature, density, and layer thickness. In the comoving frame of the relativistic pulsar wind just outside of the equatorial CS, these basic parameters are uniquely determined by the strength of the reconnecting upstream magnetic field. For the case of the Crab pulsar, we find them to be of order 10 GeV, 1013 cm-3, and 10 cm, respectively. After accounting for the bulk Doppler boosting due to the pulsar wind, the synchrotron and inverse-Compton emission from the reconnecting CS can explain the observed pulsed high-energy (GeV) and very high energy (∼100 GeV) radiation, respectively. Also, we suggest that the rapid relative motions of the secondary plasmoids in the hierarchical chain may contribute to the production of the pulsar radio emission. © 2014. The American Astronomical Society. All rights reserved.


An analysis of more than 3000 galaxies resolved at better than 114 h -1 pc at z = 0.62 in a "LAOZI" cosmological adaptive mesh refinement hydrodynamic simulation is performed and insights are gained on star formation quenching and color migration. The vast majority of red galaxies are found to be within three virial radii of a larger galaxy at the onset of quenching, when the specific star formation rate experiences the sharpest decline to fall below ∼10-2-10-1 Gyr-1 (depending on the redshift). Thus, we shall call this mechanism "environment quenching," which encompasses satellite quenching. Two physical processes are largely responsible: Ram pressure stripping first disconnects the galaxy from the cold gas supply on large scales, followed by a longer period of cold gas starvation taking place in a high velocity-dispersion environment, in which during the early part of the process, the existing dense cold gas in the central region (≤10 kpc) is consumed by in situ star formation. On average, quenching is found to be more efficient (i.e., a larger fraction of galaxies being quenched) but not faster (i.e., the duration being weakly dependent on the environment) in a denser environment. Throughout this quenching period and the ensuing one in the red sequence, galaxies follow nearly vertical tracks in the color-stellar mass diagram. In contrast, individual galaxies of all masses grow most of their stellar masses in the blue cloud, prior to the onset of quenching, and progressively more massive blue galaxies with already relatively older mean stellar ages continue to enter the red sequence. Consequently, correlations among observables of red galaxies-such as the age-mass relation-are largely inherited from their blue progenitors at the onset of quenching. While the color makeup of the entire galaxy population strongly depends on the environment, which is a direct result of environment quenching, physical properties of blue galaxies as a subpopulation show little dependence on the environment. A variety of predictions from the simulation are shown to be in accordance with extant observations. © 2014. The American Astronomical Society. All rights reserved.


Tomida K.,Princeton University
Astrophysical Journal | Year: 2014

Among many physical processes involved in star formation, radiation transfer is one of the key processes because it dominantly controls the thermodynamics. Because metallicities control opacities, they are one of the important environmental parameters that affect star formation processes. In this work, I investigate protostellar collapse in solar-metallicity and low-metallicity (Z = 0.1 Z) environments using three-dimensional radiation hydrodynamic and magnetohydrodynamic simulations. Because radiation cooling in high-density gas is more effective in low-metallicity environments, first cores are colder and have lower entropies. As a result, first cores are smaller, less massive, and have shorter lifetimes in low-metallicity clouds. Therefore, first cores would be less likely to be found in low-metallicity star forming clouds. This also implies that first cores tend to be more gravitationally unstable and susceptible to fragmentation. The evolution and structure of protostellar cores formed after the second collapse weakly depend on metallicities in the spherical and magnetized models, despite the large difference in the metallicities. Because this is due to the change of the heat capacity by dissociation and ionization of hydrogen, it is a general consequence of the second collapse as long as the effects of radiation cooling are not very large during the second collapse. On the other hand, the effects of different metallicities are more significant in the rotating models without magnetic fields, because they evolve slower than other models and therefore are more affected by radiation cooling. © 2014. The American Astronomical Society. All rights reserved.


Subramaniam A.B.,Harvard University | Guidotti G.,Harvard University | Manoharan V.N.,Harvard University | Stone H.A.,Princeton University
Nature Materials | Year: 2013

Hydrated networks of glycans (polysaccharides) - in the form of cell walls, periplasms or gel-like matrices - are ubiquitously present adjacent to cellular plasma membranes. Yet, despite their abundance, the function of glycans in the extracellular milieu is largely unknown. Here we show that the spatial configuration of glycans controls the phase behaviour of multiphase model lipid membranes: inhomogeneous glycan networks stabilize large lipid domains at the characteristic length scale of the network, whereas homogeneous networks suppress macroscopic lipid phase separation. We also find that glycan-patterned phase separation is thermally reversible - thus indicating that the effect is thermodynamic rather than kinetic - and that phase patterning probably results from a preferential interaction of glycans with ordered lipid phases. These findings have implications for membrane-mediated transport processes, potentially rationalize long-standing observations that differentiate the behaviour of native and model membranes and may indicate an intimate coupling between cellular lipidomes and glycomes. © 2013 Macmillan Publishers Limited.


Dobson A.P.,Princeton University | Dobson A.P.,Santa Fe Institute
PLoS Biology | Year: 2014

Since their introduction in 1995 and 1996, wolves have had effects on Yellowstone that ripple across the entire structure of the food web that defines biodiversity in the Northern Rockies ecosystem. Ecological interpretations of the wolves have generated a significant amount of debate about the relative strength of top-down versus bottom-up forces in determining herbivore and vegetation abundance in Yellowstone. Debates such as this are central to the resolution of broader debates about the role of natural enemies and climate as forces that structure food webs and modify ecosystem function. Ecologists need to significantly raise the profile of these discussions; understanding the forces that structure food webs and determine species abundance and the supply of ecosystem services is one of the central scientific questions for this century; its complexity will require new minds, new mathematics, and significant, consistent funding. © 2014 Andy P.


Buschman T.J.,Princeton University | Miller E.K.,Massachusetts Institute of Technology
Philosophical Transactions of the Royal Society B: Biological Sciences | Year: 2014

We review the neural mechanisms that support top-down control of behaviour and suggest that goal-directed behaviour uses two systems that work in concert. A basal ganglia-centred system quickly learns simple, fixed goal-directed behaviours whileaprefrontal cortex-centred system gradually learns more complex (abstract or long-term) goal-directed behaviours. Interactions between these two systems allow top-down control mechanisms to learn how to direct behaviour towards a goal but also how to guide behaviour when faced with a novel situation. © 2014 The Author(s) Published by the Royal Society. All rights reserved.


Muralidharan S.,Princeton University | Haataja M.,Prism Technology
Physical Review Letters | Year: 2010

Bulk-immiscible binary systems often form stress-induced miscible alloy phases when deposited on a substrate. Both alloying and surface dislocation formation lead to the decrease of the elastic strain energy, and the competition between these two strain-relaxation mechanisms gives rise to the emergence of pseudomorphic compositional nanoscale domains, often coexisting with a partially coherent single phase. In this work, we develop a phase-field crystal model for compositional patterning in monolayer aggregates of binary metallic systems. We first demonstrate that the model naturally incorporates the competition between alloying and misfit dislocations, and quantify the effects of misfit and line tension on equilibrium domain size. Then, we quantitatively relate the parameters of the phase-field crystal model to a specific system, CoAg/Ru(0001), and demonstrate that the simulations capture experimentally observed morphologies. © 2010 The American Physical Society.


Abdeljawad F.,Princeton University | Haataja M.,Prism Technology
Physical Review Letters | Year: 2010

At low temperatures, monolithic bulk metallic glasses (BMGs) exhibit high strength and large elasticity limits. On the other hand, BMGs lack overall ductility due to highly localized deformation mechanisms. Recent experimental findings suggest that the problem of catastrophic failure by shear band propagation in BMGs can be mitigated by tailoring microstructural features at different length scales to promote more homogeneous plastic deformation. Herein, based on a continuum approach, we present a quantitative analysis of the effects of microstructure on the deformation behavior of monolithic BMGs and BMG composites. In particular, simulations highlight the importance of short-ranged structural correlations on ductility in monolithic BMGs and demonstrate that particle size controls the ductility of BMG composites. In broader terms, our results provide new avenues for further improvements to the mechanical properties of BMGs. © 2010 The American Physical Society.


Konovalova A.,Princeton University | Sogaard-Andersen L.,Max Planck Institute for Terrestrial Microbiology | Kroos L.,Michigan State University
FEMS Microbiology Reviews | Year: 2014

Bacteria use proteases to control three types of events temporally and spatially during the processes of morphological development. These events are the destruction of regulatory proteins, activation of regulatory proteins, and production of signals. While some of these events are entirely cytoplasmic, others involve intramembrane proteolysis of a substrate, transmembrane signaling, or secretion. In some cases, multiple proteolytic events are organized into pathways, for example turnover of a regulatory protein activates a protease that generates a signal. We review well-studied and emerging examples and identify recurring themes and important questions for future research. We focus primarily on paradigms learned from studies of model organisms, but we note connections to regulated proteolytic events that govern bacterial adaptation, biofilm formation and disassembly, and pathogenesis. Bacteria use regulated proteolysis to destroy or activate regulatory proteins, and to produce signals, in order to temporally and spatially control processes of morphological development. © 2013 Federation of European Microbiological Societies.


Rafikov R.R.,Princeton University
Astrophysical Journal | Year: 2011

The issue of giant planet formation by core accretion (CA) far from the central star is rather controversial because the growth of a massive solid core necessary for triggering the gas runaway can take longer than the lifetime of the protoplanetary disk. In this work, we assess the range of separations at which CA may operate by (1) allowing for an arbitrary (physically meaningful) rate of planetesimal accretion by the core and (2) properly taking into account the dependence of the critical mass for the gas runaway on the planetesimal accretion luminosity. This self-consistent approach distinguishes our work from similar studies in which only a specific planetesimal accretion regime was explored and/or the critical core mass was fixed at some arbitrary level. We demonstrate that the largest separation at which the gas runaway can occur within 3 Myr corresponds to the surface density of solids in the disk ≳0.1 g cm-2 and is 40-50 AU in the minimum mass solar nebula. This limiting separation is achieved when the planetesimal accretion proceeds at the fastest possible rate, even though the high associated accretion luminosity increases the critical core mass, delaying the onset of gas runaway. Our constraints are independent of the mass of the central star and vary only weakly with the core density and its atmospheric opacity. We also discuss various factors that can strengthen or weaken our limits on the operation of CA. © 2011. The American Astronomical Society.


Powell W.B.,Princeton University
INFORMS Journal on Computing | Year: 2010

We consider the problem of optimizing over time hundreds or thousands of discrete entities that may be characterized by relatively complex attributes, in the presence of different forms of uncertainty. Such problems arise in a range of operational settings such as transportation and logistics, where the entities may be aircraft, locomotives, containers, or people. These problems can be formulated using dynamic programming but encounter the widely cited "curse of dimensionality." Even deterministic formulations of these problems can produce math programs with millions of rows, far beyond anything being solved today. This paper shows how we can combine concepts from artificial intelligence and operations research to produce practical solution methods that scale to industrial-strength problems. Throughout, we emphasize concepts, techniques, and notation from artificial intelligence and operations research to show how these fields can be brought together for complex stochastic, dynamic problems. © 2010 INFORMS.


Pais D.,Princeton University
PloS one | Year: 2013

We present a dynamical systems analysis of a decision-making mechanism inspired by collective choice in house-hunting honeybee swarms, revealing the crucial role of cross-inhibitory 'stop-signalling' in improving the decision-making capabilities. We show that strength of cross-inhibition is a decision-parameter influencing how decisions depend both on the difference in value and on the mean value of the alternatives; this is in contrast to many previous mechanistic models of decision-making, which are typically sensitive to decision accuracy rather than the value of the option chosen. The strength of cross-inhibition determines when deadlock over similarly valued alternatives is maintained or broken, as a function of the mean value; thus, changes in cross-inhibition strength allow adaptive time-dependent decision-making strategies. Cross-inhibition also tunes the minimum difference between alternatives required for reliable discrimination, in a manner similar to Weber's law of just-noticeable difference. Finally, cross-inhibition tunes the speed-accuracy trade-off realised when differences in the values of the alternatives are sufficiently large to matter. We propose that the model, and the significant role of the values of the alternatives, may describe other decision-making systems, including intracellular regulatory circuits, and simple neural circuits, and may provide guidance in the design of decision-making algorithms for artificial systems, particularly those functioning without centralised control.


Keller G.,Princeton University
Special Paper of the Geological Society of America | Year: 2014

The recent discovery of the direct link between Deccan volcanism and the end-Cretaceous mass extinction also links volcanism to the late Maastrichtian rapid global warming, high environmental stress, and the delayed recovery in the early Danian. In comparison, three decades of research on the Chicxulub impact have failed to account for long-term climatic and environmental changes or prove a coincidence with the mass extinction. A review of Deccan volcanism and the best age estimate for the Chicxulub impact provides a new perspective on the causes for the end-Cretaceous mass extinction and supports an integrated Deccan-Chicxulub scenario. This scenario takes into consideration climate warming and cooling, sea-level changes, erosion, weathering, ocean acidification, high-stress environments with opportunistic species blooms, the mass extinction, and delayed postextinction recovery. The crisis began in C29r (upper CF2 to lower CF1) with rapid global warming of 4°C in the oceans and 8°C on land, commonly attributed to Deccan phase 2 eruptions. The Chicxulub impact occurred during this warm event (about 100-150 k.y. before the mass extinction) based on the stratigraphically oldest impact spherule layer in NE Mexico, Texas, and Yucatan crater core Yaxcopoil-1. It likely exacerbated climate warming and may have intensified Deccan eruptions. The reworked spherule layers at the base of the sandstone complex in NE Mexico and Texas were deposited in the upper half of CF1, ∼50-80 k.y. before the Cretaceous-Tertiary (K-T) boundary. This sandstone complex, commonly interpreted as impact tsunami deposits of K-T boundary age, was deposited during climate cooling, low sea level, and intensified currents, leading to erosion of nearshore areas (including Chicxulub impact spherules), transport, and redeposition via submarine channels into deeper waters. Renewed climate warming during the last ∼50 k.y. of the Maastrichtian correlates with at least four rapid, massive volcanic eruptions known as the longest lava flows on Earth that ended with the mass extinction, probably due to runaway effects. The kill mechanism was likely ocean acidification resulting in the carbonate crisis commonly considered to be the primary cause for four of the five Phanerozoic mass extinctions. © 2014 The Geological Society of America. All rights reserved.


Iknayan K.J.,University of California at Berkeley | Tingley M.W.,Princeton University | Furnas B.J.,University of California at Berkeley | Beissinger S.R.,University of California at Berkeley
Trends in Ecology and Evolution | Year: 2014

Estimates of species richness and diversity are central to community and macroecology and are frequently used in conservation planning. Commonly used diversity metrics account for undetected species primarily by controlling for sampling effort. Yet the probability of detecting an individual can vary among species, observers, survey methods, and sites. We review emerging methods to estimate alpha, beta, gamma, and metacommunity diversity through hierarchical multispecies occupancy models (MSOMs) and multispecies abundance models (MSAMs) that explicitly incorporate observation error in the detection process for species or individuals. We examine advantages, limitations, and assumptions of these detection-based hierarchical models for estimating species diversity. Accounting for imperfect detection using these approaches has influenced conclusions of comparative community studies and creates new opportunities for testing theory. © 2013 Elsevier Ltd.


Disalle B.F.,Princeton University | Disalle B.F.,Carnegie Mellon University | Bernhard S.,Carnegie Mellon University
Journal of the American Chemical Society | Year: 2011

A series of novel Ir(III) luminophores containing pendant pyridyl moieties that allow for adsorption onto metal surfaces has been synthesized. These new photosensitizer compounds have been evaluated for their efficacy in hydrogen-producing photoreactions. The new complexes are shown to significantly outperform the control photosensitizers without adsorbing moieties. © 2011 American Chemical Society.


Li A.,University of Missouri | Draine B.T.,Princeton University
Astrophysical Journal Letters | Year: 2012

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to coal- or kerogen-like organic nanoparticles with a mixed aromatic-aliphatic structure. However, we show in this Letter that this hypothesis is inconsistent with observations. We estimate the aliphatic fraction of the UIE carriers based on the observed intensities of the 3.4 μm and 6.85 μm emission features by attributing them exclusively to aliphatic C-H stretch and aliphatic C-H deformation vibrational modes, respectively. We derive the fraction of carbon atoms in aliphatic form to be <15%. We conclude that the UIE emitters are predominantly aromatic, with aliphatic material at most a minor part of the UIE carriers. The PAH model is consistent with astronomical observations and PAHs dominate the strong UIE bands. © 2012. The American Astronomical Society. All rights reserved..


Li S.,Princeton University
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2011

We present a 1.488 approximation algorithm for the metric uncapacitated facility location (UFL) problem. Previously the best algorithm was due to Byrka [1]. By linearly combining two algorithms A1(γf) for γf ≈ 1.6774 and the (1.11,1.78)-approximation algorithm A2 proposed by Jain, Mahdian and Saberi [8], Byrka gave a 1.5 approximation algorithm for the UFL problem. We show that if γf is randomly selected from some distribution, the approximation ratio can be improved to 1.488. Our algorithm cuts the gap with the 1.463 approximability lower bound by almost 1/3. © 2011 Springer-Verlag.


Greene J.E.,Princeton University
Nature Communications | Year: 2012

Bridging the gap between the approximately ten solar mass 'stellar mass' black holes and the 'supermassive' black holes of millions to billions of solar masses are the elusive 'intermediate-mass' black holes. Their discovery is key to understanding whether supermassive black holes can grow from stellar-mass black holes or whether a more exotic process accelerated their growth soon after the Big Bang. Currently, tentative evidence suggests that the progenitors of supermassive black holes were formed as ∼10 4 -10 5 M ⊙ black holes via the direct collapse of gas. Ongoing searches for intermediate-mass black holes at galaxy centres will help shed light on this formation mechanism. © 2012 Macmillan Publishers Limited. All rights reserved.


Papenfort K.,Princeton University | Papenfort K.,Ludwig Maximilians University of Munich | Vanderpool C.K.,University of Illinois at Urbana - Champaign
FEMS Microbiology Reviews | Year: 2015

Bacterial small regulatory RNAs (sRNAs) are commonly known to repress gene expression by base pairing to target mRNAs. In many cases, sRNAs base pair with and sequester mRNA ribosome-binding sites, resulting in translational repression and accelerated transcript decay. In contrast, a growing number of examples of translational activation and mRNA stabilization by sRNAs have now been documented. A given sRNA often employs a conserved region to interact with and regulate both repressed and activated targets. However, the mechanisms underlying activation differ substantially from repression. Base pairing resulting in target activation can involve sRNA interactions with the 5' untranslated region (UTR), the coding sequence or the 3' UTR of the target mRNAs. Frequently, the activities of protein factors such as cellular ribonucleases and the RNA chaperone Hfq are required for activation. Bacterial sRNAs, including those that function as activators, frequently control stress response pathways or virulence-associated functions required for immediate responses to changing environments. This review aims to summarize recent advances in knowledge regarding target mRNA activation by bacterial sRNAs, highlighting the molecular mechanisms and biological relevance of regulation. © 2015 FEMS.


An elegantly simple and probably ancient molecular mechanism of allostery is described for the Escherichia coli arginine repressor ArgR, the master feedback regulator of transcription in L-arginine metabolism. Molecular dynamics simulations with ArgRC, the hexameric domain that binds L-arginine with negative cooperativity, reveal that conserved arginine and aspartate residues in each ligand-binding pocket promote rotational oscillation of apoArgRC trimers by engagement and release of hydrogen-bonded salt bridges. Binding of exogenous L-arginine displaces resident arginine residues and arrests oscillation, shifting the equilibrium quaternary ensemble and promoting motions that maintain the configurational entropy of the system. A single L-arg ligand is necessary and sufficient to arrest oscillation, and enables formation of a cooperative hydrogen-bond network at the subunit interface. The results are used to construct a free-energy reaction coordinate that accounts for the negative cooperativity and distinctive thermodynamic signature of L-arginine binding detected by calorimetry. The symmetry of the hexamer is maintained as each ligand binds, despite the conceptual asymmetry of partially-liganded states. The results thus offer the first opportunity to describe in structural and thermodynamic terms the symmetric relaxed state predicted by the concerted allostery model of Monod, Wyman, and Changeux, revealing that this state is achieved by exploiting the dynamics of the assembly and the distributed nature of its cohesive free energy. The ArgR example reveals that symmetry can be maintained even when binding sites fill sequentially due to negative cooperativity, which was not anticipated by the Monod, Wyman, and Changeux model. The molecular mechanism identified here neither specifies nor requires a pathway for transmission of the allosteric signal through the protein, and it suggests the possibility that binding of free amino acids was an early innovation in the evolution of allostery.


Goyal S.,Princeton University
PLoS computational biology | Year: 2010

Recent evidence suggests that the metabolism of some organisms, such as Escherichia coli, is remarkably efficient, producing close to the maximum amount of biomass per unit of nutrient consumed. This observation raises the question of what regulatory mechanisms enable such efficiency. Here, we propose that simple product-feedback inhibition by itself is capable of leading to such optimality. We analyze several representative metabolic modules--starting from a linear pathway and advancing to a bidirectional pathway and metabolic cycle, and finally to integration of two different nutrient inputs. In each case, our mathematical analysis shows that product-feedback inhibition is not only homeostatic but also, with appropriate feedback connections, can minimize futile cycling and optimize fluxes. However, the effectiveness of simple product-feedback inhibition comes at the cost of high levels of some metabolite pools, potentially associated with toxicity and osmotic imbalance. These large metabolite pool sizes can be restricted if feedback inhibition is ultrasensitive. Indeed, the multi-layer regulation of metabolism by control of enzyme expression, enzyme covalent modification, and allostery is expected to result in such ultrasensitive feedbacks. To experimentally test whether the qualitative predictions from our analysis of feedback inhibition apply to metabolic modules beyond linear pathways, we examine the case of nitrogen assimilation in E. coli, which involves both nutrient integration and a metabolic cycle. We find that the feedback regulation scheme suggested by our mathematical analysis closely aligns with the actual regulation of the network and is sufficient to explain much of the dynamical behavior of relevant metabolite pool sizes in nutrient-switching experiments.


Zakamska N.L.,Johns Hopkins University | Greene J.E.,Princeton University
Monthly Notices of the Royal Astronomical Society | Year: 2014

We analyse Sloan Digital Sky Survey spectra of 568 obscured luminous quasars. The [OIII] λ5007 Å emission line shows blueshifts and blue excess, indicating that some of the narrowline gas is undergoing an organized outflow. The velocity width containing 90 per cent of line power ranges from 370 to 4780 km s-1, suggesting outflow velocities up to ~2000 km s-1, and is strongly correlated with the radio luminosity among the radio-quiet quasars. We propose that radio emission in radio-quiet quasars is due to relativistic particles accelerated in the shocks within the quasar-driven outflows; star formation in quasar hosts is insufficient to explain the observed radio emission. The median radio luminosity of the sample of vLv[1.4 GHz] = 1040 erg s-1 suggests a median kinetic luminosity of the quasar-driven wind of Lwind = 3 × 1044 erg s-1, or about 4 per cent of the estimated median bolometric luminosity Lbol = 8 × 1045 erg s-1. Furthermore, the velocity width of [OIII] is positively correlated with mid-infrared luminosity, which suggests that outflows are ultimately driven by the radiative output of the quasar. Emission lines characteristic of shocks in quasi-neutral medium increase with the velocity of the outflow, which we take as evidence of quasar-driven winds propagating into the interstellar medium of the host galaxy. Quasar feedback appears to operate above the threshold luminosity of Lbol ~ 3 × 1045 erg s-1. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Plant hydraulics mediate terrestrial woody plant productivity, influencing global water, carbon, and biogeochemical cycles, as well as ecosystem vulnerability to drought and climate change. While inter-specific differences in hydraulic traits are widely documented, intra-specific hydraulic variability is less well known and is important for predicting climate change impacts. Here, I present a conceptual framework for this intra-specific hydraulic trait variability, reviewing the mechanisms that drive variability and the consequences for vegetation response to climate change. I performed a meta-analysis on published studies (n = 33) of intra-specific variation in a prominent hydraulic trait - water potential at which 50% stem conductivity is lost (P50) - and compared this variation to inter-specific variability within genera and plant functional types used by a dynamic global vegetation model. I found that intra-specific variability is of ecologically relevant magnitudes, equivalent to c. 33% of the inter-specific variability within a genus, and is larger in angiosperms than gymnosperms, although the limited number of studies highlights that more research is greatly needed. Furthermore, plant functional types were poorly situated to capture key differences in hydraulic traits across species, indicating a need to approach prediction of drought impacts from a trait-based, rather than functional type-based perspective. © 2014 New Phytologist Trust.


The Greater Ani (Crotophaga major) is a neotropical cuckoo in which several females lay eggs in a single nest. Group members synchronize egg laying and compete for reproduction by ejecting early-laid eggs from the communal nest. Eggs are large (∼17% of female body mass) and vary greatly in size. I assessed the effects of egg size, hatching asynchrony, and position in the laying order on the survival and growth of nestlings to test the hypothesis that females invest more resources in eggs that are likely to give rise to surviving offspring. The egg's position in the female's laying sequence was a significant predictor of egg mass, with first-laid eggs and last-laid eggs consistently smaller than those in the middle of the clutch. Females that initiated laying in the communal nest almost always lost their first-laid eggs, and these females also exhibited the most extreme variation in egg mass. Nestlings from last-laid eggs were more likely to hatch asynchronously and starve before fledging, and the mass of last-laid eggs decreased as the degree of hatching asynchrony increased. Ejection and starvation probabilities were not affected by egg mass; therefore, low survivorship of first-laid and last-laid eggs was due solely to their position in the laying order, not to their smaller size. These data suggest that individual females allocate more resources to eggs that are likely to survive to fledging and that reproductive competition among communally breeding females explains much of the variation in egg size in this species. © The Author 2010.


Austin R.H.,Princeton University
Physical Biology | Year: 2014

Political correctness urges us to state how wonderful it is to work with biologists and how, just as the lion will someday lie down with the lamb, so will interdisciplinary work, where biologists and physicists are mixed together in light, airy buildings designed to force socialization, give rise to wonderful new science. But it has been said that the only drive in human nature stronger than the sex drive is the drive to censor and suppress, and so I claim that it is OK for physicists and biologists to maintain a wary distance from each other, so that neither one censors or suppresses the wild ideas of the other. © 2014 IOP Publishing Ltd.


Martin A.C.,Princeton University
Developmental Biology | Year: 2010

Embryonic development involves global changes in tissue shape and architecture that are driven by cell shape changes and rearrangements within cohesive cell sheets. Morphogenetic changes at the cell and tissue level require that cells generate forces and that these forces are transmitted between the cells of a coherent tissue. Contractile forces generated by the actin-myosin cytoskeleton are critical for morphogenesis, but the cellular and molecular mechanisms of contraction have been elusive for many cell shape changes and movements. Recent studies that have combined live imaging with computational and biophysical approaches have provided new insights into how contractile forces are generated and coordinated between cells and tissues. In this review, we discuss our current understanding of the mechanical forces that shape cells, tissues, and embryos, emphasizing the different modes of actomyosin contraction that generate various temporal and spatial patterns of force generation. © 2009 Elsevier Inc.


Vanderbei R.J.,Princeton University
Mathematical Programming Computation | Year: 2012

Many interesting and fundamentally practical optimization problems, ranging from optics, to signal processing, to radar and acoustics, involve constraints on the Fourier transform of a function. It is well-known that the fast Fourier transform (fft) is a recursive algorithm that can dramatically improve the efficiency for computing the discrete Fourier transform. However, because it is recursive, it is difficult to embed into a linear optimization problem. In this paper, we explain the main idea behind the fast Fourier transform and show how to adapt it in such a manner as to make it encodable as constraints in an optimization problem. We demonstrate a real-world problem from the field of high-contrast imaging. On this problem, dramatic improvements are translated to an ability to solve problems with a much finer grid of discretized points. As we shall show, in general, the "fast Fourier" version of the optimization constraints produces a larger but sparser constraint matrix and therefore one can think of the fast Fourier transform as a method of sparsifying the constraints in an optimization problem, which is usually a good thing. © 2012 Springer and Mathematical Optimization Society.


Leonard N.E.,Princeton University
Annual Reviews in Control | Year: 2014

Systematic design of decentralized feedback for coordinated control of multi-agent systems has much to gain from the rigorous examination of the nonlinear dynamics of collective animal behavior. Animals in groups, from bird flocks to fish schools, employ decentralized strategies and have limitations on sensing, computation, and actuation. Yet, at the level of the group, they are known to manage a variety of challenging tasks quickly, accurately, robustly and adaptively in an uncertain and changing environment. In this paper we review recent work on models and methods for studying the mechanisms of collective migration and collective decision-making in high-performing animal groups. Through bifurcation analysis we prove systematically how behavior depends on parameters that model the system and the environment. These connections lay the foundations for proving systematic control design methodologies that endow engineered multi-agent systems with the remarkable features of animal group dynamics. © 2014 Elsevier Ltd. All rights reserved.


Korennykh A.,Princeton University | Walter P.,Howard Hughes Medical Institute | Walter P.,University of California at San Francisco
Annual Review of Cell and Developmental Biology | Year: 2012

The unfolded protein response (UPR) is a network of intracellular signaling pathways that maintain the protein-folding capacity of the endoplasmic reticulum (ER) in eukaryotic cells. Dedicated molecular sensors embedded in the ER membrane detect incompletely folded or unfolded proteins in the ER lumen and activate a transcriptional program that increases the abundance of the ER according to need. In metazoans the UPR additionally regulates translation and thus relieves unfolded protein load by globally reducing protein synthesis. If homeostasis in the ER cannot be reestablished, the metazoan UPR switches from the prosurvival to the apoptotic mode. The UPR involves a complex, coordinated action of many genes that is controlled by one ER-embedded sensor, Ire1, in yeasts, and three sensors, Ire1, PERK, and ATF6, in higher eukaryotes, including human. We discuss the emerging molecular understanding of the UPR and focus on the structural biology of Ire1 and PERK, the two recently crystallized UPR sensors. Copyright © 2012 by Annual Reviews. All rights reserved.


The incorporation of polar functional groups into polyolefins can significantly alter the adhesion, barrier and surface properties, dyeability, printability, and compatibility of the resulting "functional polyolefin". Thus, the development of methods for the controlled synthesis of functional polyolefins from industrially relevant monomers holds the potential to expand the range of applications available to this already ubiquitous class of materials. In this Perspective, recent advances in transition-metal-catalyzed functional polyolefin synthesis will be reviewed. A common thread among the innovations discussed here is the perturbation of catalyst function by tailored design of the chelating ancillary ligand, aided in many cases by improved mechanistic understanding. Specific topics discussed here include rare examples of catalyst control over the regio- and stereochemistry of polar monomer insertion by phosphine-sulfonato palladium complexes (Drent-type), rate acceleration of insertion polymerization by binuclear cooperativity using salicylaldiminato nickel complexes (Grubbs-type), and formation of linear copolymers of ethylene and polar vinyl monomers using a cationic palladium catalyst ligated by a bisphosphine monoxide (BPMO) that contrasts the typical polymer microstructures formed by other cationic group 10 catalysts ligated by an α-diimine (Brookhart-type). © 2014 American Chemical Society.


Currie J.,Princeton University | Rossin-Slater M.,Columbia University
Journal of Health Economics | Year: 2013

A growing literature suggests that stressful events in pregnancy can have negative effects on birth outcomes. Some of the estimates in this literature may be affected by small samples, omitted variables, endogenous mobility in response to disasters, and errors in the measurement of gestation, as well as by a mechanical correlation between longer gestation and the probability of having been exposed. We use millions of individual birth records to examine the effects of exposure to hurricanes during pregnancy, and the sensitivity of the estimates to these econometric problems. We find that exposure to a hurricane during pregnancy increases the probability of abnormal conditions of the newborn such as being on a ventilator more than 30. min and meconium aspiration syndrome (MAS). Although we are able to reproduce previous estimates of effects on birth weight and gestation, our results suggest that measured effects of stressful events on these outcomes are sensitive to specification and it is preferable to use more sensitive indicators of newborn health. © 2013 Elsevier B.V..


Rushlow C.A.,New York University | Shvartsman S.Y.,Princeton University
Current Opinion in Genetics and Development | Year: 2012

Dorsoventral pattern of . Drosophila embryo is specified by the nuclear localization gradient of the transcription factor Dorsal. Genetic and genomic studies of this morphogen gradient provided important insights into spatial control of gene expression in development. Recent live imaging experiments revealed hitherto unappreciated dynamics of the Dorsal gradient and posed new questions about the mechanisms of its transcriptional interpretation. Some of these questions can be answered by models in which the morphogenetic capacity of the Dorsal gradient is potentiated by spatially uniform factors, such as Zelda, a transcription factor that plays a key role in the activation of zygotic transcription. Combinatorial effects of uniform and graded factors play an important role in the transcriptional and signaling cascades initiated by Dorsal and may explain differential positioning of gene expression borders by other morphogen gradients. © 2012 Elsevier Ltd.


Riquelme M.A.,University of California at Berkeley | Spitkovsky A.,Princeton University
Astrophysical Journal | Year: 2011

Electron acceleration to non-thermal, ultra-relativistic energies (∼10-100 TeV) is revealed by radio and X-ray observations of shocks in young supernova remnants (SNRs). The diffusive shock acceleration (DSA) mechanism is usually invoked to explain this acceleration, but the way in which electrons are initially energized or "injected" into this acceleration process starting from thermal energies is an unresolved problem. In this paper we study the initial acceleration of electrons in non-relativistic shocks from first principles, using two- and three-dimensional particle-in-cell (PIC) plasma simulations. We systematically explore the space of shock parameters (the Alfvénic Mach number, MA , the shock velocity, v sh, the angle between the upstream magnetic field and the shock normal, θBn, and the ion to electron mass ratio, mi /me ). We find that significant non-thermal acceleration occurs due to the growth of oblique whistler waves in the foot of quasi-perpendicular shocks. This acceleration strongly depends on using fairly large numerical mass ratios, mi /me , which may explain why it had not been observed in previous PIC simulations of this problem. The obtained electron energy distributions show power-law tails with spectral indices up to α ∼ 3-4. The maximum energies of the accelerated particles are consistent with the electron Larmor radii being comparable to that of the ions, indicating potential injection into the subsequent DSA process. This injection mechanism, however, requires the shock waves to have fairly low Alfénic Mach numbers, MA ≲ 20, which is consistent with the theoretical conditions for the growth of whistler waves in the shock foot (MA ≲ (mi/me)1/2). Thus, if the whistler mechanism is the only robust electron injection process at work in SNR shocks, then SNRs that display non-thermal emission must have significantly amplified upstream magnetic fields. Such field amplification is likely achieved by the escaping cosmic rays, so electron and proton acceleration in SNR shocks must be interconnected. © 2011. The American Astronomical Society. All rights reserved..


Bai X.-N.,Princeton University
Astrophysical Journal | Year: 2011

Tiny grains such as polycyclic aromatic hydrocarbons (PAHs) have been thought to dramatically reduce the coupling between the gas and magnetic fields in weakly ionized gas such as in protoplanetary disks (PPDs) because they provide a tremendous surface area to recombine free electrons. The presence of tiny grains in PPDs thus raises the question of whether the magnetorotational instability (MRI) is able to drive rapid accretion consistent with observations. Charged tiny grains have similar conduction properties as ions, whose presence leads to qualitatively new behaviors in the conductivity tensor, characterized by , where ne and denote the number densities of free electrons and all other charged species, respectively. In particular, Ohmic conductivity becomes dominated by charged grains rather than by electrons when exceeds about 103, and Hall and ambipolar diffusion (AD) coefficients are reduced by a factor of in the AD-dominated regime relative to that in the Ohmic regime. Applying the methodology of Bai, we find that in PPDs, when PAHs are sufficiently abundant (≳ 10-9 per H2 molecule), there exists a transition radius r trans of about 10-20AU, beyond which the MRI active layer extends to the disk midplane. At r < r trans, the optimistically predicted MRI-driven accretion rate is one to two orders of magnitude smaller than that in the grain-free case, which is too small compared with the observed rates, but is in general no smaller than the predicted with solar-abundance 0.1 μm grains. At r > r trans, we find that, remarkably, the predicted exceeds the grain-free case due to a net reduction of AD by charged tiny grains and reaches a few times 10-8 M ⊙yr-1. This is sufficient to account for the observed in transitional disks. Larger grains (≳ 0.1 μm) are too massive to reach such high abundance as tiny grains and to facilitate the accretion process. © 2011. The American Astronomical Society. All rights reserved..


Shin I.,Princeton University | Carter E.A.,Andlinger Center for Energy and the Environment
International Journal of Plasticity | Year: 2014

The strength and ductility of metals are governed by the motion of dislocations, which is quantified by the Peierls stress (σp). We use orbital-free density functional theory (OFDFT) to characterize the motion of 13〈112̄0〉 dislocations on the basal {0001} and prismatic {11̄00} planes in hexagonal-close-packed magnesium (Mg) in order to understand its deformation mechanisms. We predict σp values of edge dislocations on the basal and prismatic planes to be 0.6 and 35.4 MPa, respectively. The presence of stable stacking faults only on the basal plane produces partial dislocation splitting, which significantly lowers σp for basal dislocations. Our atomic scale simulations reveal that dislocation mobility is strongly correlated with the number of core atoms moving collectively. OFDFT σp results are in excellent agreement with experiments (∼0.5 and 39.2 MPa), further validating OFDFT as an independent and predictive tool for simulating plastic behavior in main group metals at the mesoscale with first principles' accuracy. © 2014 Elsevier Ltd. All rights reserved.


Sergienko O.V.,Princeton University
Journal of Glaciology | Year: 2013

Subglacial lakes beneath ice streams of Antarctica and supraglacial lakes observed on the flanks of the Greenland ice sheet may seem to be unrelated. The former derive their water from energy dissipation associated with basal friction, the latter from atmospherically driven surface melting. However, using numerical models of ice and water flow, it is shown here that they share a common relationship to basal conditions that implies that surface lakes (or depressions that could host lakes under warmer atmospheric conditions) and basal lakes might exist in tandem.


Draine B.T.,Princeton University
Astrophysical Journal | Year: 2011

Radiation pressure acting on gas and dust causes HII regions to have central densities that are lower than the density near the ionized boundary. HII regions in static equilibrium comprise a family of similarity solutions with three parameters: β, γ, and the product Q 0 n rms; β characterizes the stellar spectrum, γ characterizes the dust/gas ratio, Q 0 is the stellar ionizing output (photons/s), and n rms is the rms density within the ionized region. Adopting standard values for β and γ, varying Q 0 n rms generates a one-parameter family of density profiles, ranging from nearly uniform density (small Q 0 n rms) to shell-like (large Q 0 n rms). When Q 0 n rms ≳ 1052 cm-3 s-1, dusty HII regions have conspicuous central cavities, even if no stellar wind is present. For given β, γ, and Q 0 n rms, a fourth quantity, which can be Q 0, determines the overall size and density of the HII region. Examples of density and emissivity profiles are given. We show how quantities of interest - such as the peak-to-central emission measure ratio, the rms-to-mean density ratio, the edge-to-rms density ratio, and the fraction of the ionizing photons absorbed by the gas - depend on β, γ, and Q 0 n rms. For dusty HII regions, compression of the gas and dust into an ionized shell results in a substantial increase in the fraction of the stellar photons that actually ionize H (relative to a uniform-density HII region with the same dust/gas ratio and density n = n rms). We discuss the extent to which radial drift of dust grains in HII regions can alter the dust-to-gas ratio. The applicability of these solutions to real HII regions is discussed. © 2011. The American Astronomical Society. All rights reserved.


Riehl C.,Princeton University | Frederickson M.E.,University of Toronto
Philosophical Transactions of the Royal Society B: Biological Sciences | Year: 2016

Cheaters—genotypes that gain a selective advantage by taking the benefits of the social contributions of others while avoiding the costs of cooperating—are thought to pose a major threat to the evolutionary stability of cooperative societies. In order for cheaters to undermine cooperation, cheating must be an adaptive strategy: cheaters must have higher fitness than cooperators, and their behaviour must reduce the fitness of their cooperative partners. It is frequently suggested that cheating is not adaptive because cooperators have evolved mechanisms to punish these behaviours, thereby reducing the fitness of selfish individuals. However, a simpler hypothesis is that such societies arise precisely because cooperative strategies have been favoured over selfish ones—hence, behaviours that have been interpreted as ‘cheating’ may not actually result in increased fitness, even when they go unpunished. Here, we review the empirical evidence for cheating behaviours in animal societies, including cooperatively breeding vertebrates and social insects, and we ask whether such behaviours are primarily limited by punishment. Our review suggests that both cheating and punishment are probably rarer than often supposed. Uncooperative individuals typically have lower, not higher, fitness than cooperators; and when evidence suggests that cheating may be adaptive, it is often limited by frequency-dependent selection rather than by punishment. When apparently punitive behaviours do occur, it remains an open question whether they evolved in order to limit cheating, or whether they arose before the evolution of cooperation. © 2016 The Author(s) Published by the Royal Society. All rights reserved.


Chazelle B.,Princeton University
Communications of the ACM | Year: 2012

Algorithms offer a rich, expressive language for modelers of biological and social systems. They lay the grounds for numerical simulations and, crucially, provide a powerful framework for their analysis. The new area of natural algorithms may reprise in the life sciences the role differential equations have long played in the physical sciences. For this to happen, however, an algorithmic calculus is needed. We discuss what this program entails in the context of influence systems, a broad family of multiagent models arising in social dynamics. © 2012 ACM 0001-0782/12/12.


Chirik P.J.,Princeton University
Accounts of Chemical Research | Year: 2015

ConspectusThe hydrogenation of alkenes is one of the most impactful reactions catalyzed by homogeneous transition metal complexes finding application in the pharmaceutical, agrochemical, and commodity chemical industries. For decades, catalyst technology has relied on precious metal catalysts supported by strong field ligands to enable highly predictable two-electron redox chemistry that constitutes key bond breaking and forming steps during turnover. Alternative catalysts based on earth abundant transition metals such as iron and cobalt not only offer potential environmental and economic advantages but also provide an opportunity to explore catalysis in a new chemical space. The kinetically and thermodynamically accessible oxidation and spin states may enable new mechanistic pathways, unique substrate scope, or altogether new reactivity. This Account describes my groups efforts over the past decade to develop iron and cobalt catalysts for alkene hydrogenation. Particular emphasis is devoted to the interplay of the electronic structure of the base metal compounds and their catalytic performance. First generation, aryl-substituted pyridine(diimine) iron dinitrogen catalysts exhibited high turnover frequencies at low catalyst loadings and hydrogen pressures for the hydrogenation of unactivated terminal and disubstituted alkenes. Exploration of structure-reactivity relationships established smaller aryl substituents and more electron donating ligands resulted in improved performance. Second generation iron and cobalt catalysts where the imine donors were replaced by N-heterocyclic carbenes resulted in dramatically improved activity and enabled hydrogenation of more challenging unactivated, tri- and tetrasubstituted alkenes. Optimized cobalt catalysts have been discovered that are among the most active homogeneous hydrogenation catalysts known. Synthesis of enantiopure, C1 symmetric pyridine(diimine) cobalt complexes have enabled rare examples of highly enantioselective hydrogenation of a family of substituted styrene derivatives. Because improved hydrogenation performance was observed with more electron rich supporting ligands, phosphine cobalt(II) dialkyl complexes were synthesized and found to be active for the diastereoselective hydrogenation of various substituted alkenes. Notably, this class of catalysts was activated by hydroxyl functionality, representing a significant advance in the functional group tolerance of base metal hydrogenation catalysts. Through collaboration with Merck, enantioselective variants of these catalysts were discovered by high throughput experimentation. Catalysts for the hydrogenation of functionalized and essentially unfunctionalized alkenes have been discovered using this approach. Development of reliable, readily accessible cobalt precursors facilitated catalyst discovery and may, along with lessons learned from electronic structure studies, provide fundamental design principles for catalysis with earth abundant transition metals beyond alkene hydrogenation. © 2015 American Chemical Society.


Wellinger R.J.,Universite de Sherbrooke | Zakian V.A.,Princeton University
Genetics | Year: 2012

The mechanisms that maintain the stability of chromosome ends have broad impact on genome integrity in all eukaryotes. Budding yeast is a premier organism for telomere studies. Many fundamental concepts of telomere and telomerase function were first established in yeast and then extended to other organisms. We present a comprehensive review of yeast telomere biology that covers capping, replication, recombination, and transcription. We think of it as yeast telomeres-soup to nuts. © 2012 by the Genetics Society of America.


Lee R.F.,Princeton University
PLoS ONE | Year: 2015

In order to scientifically study the human brain's response to face-to-face social interaction, the scientific method itself needs to be reconsidered so that both quantitative observation and symbolic reasoning can be adapted to the situation where the observer is also observed. In light of the recent development of dyadic fMRI which can directly observe dyadic brain interacting in one MRI scanner, this paper aims to establish a new form of logic, dual logic, which provides a theoretical platform for deductive reasoning in a complementary dual system with emergence mechanism. Applying the dual logic in the dfMRI experimental design and data analysis, the exogenous and endogenous dual systems in the BOLD responses can be identified; the non-reciprocal responses in the dual system can be suppressed; a cerebral coordinate for reciprocal interaction can be generated. Elucidated by dual logic deductions, the cerebral coordinate for reciprocal interaction suggests: the exogenous and endogenous systems consist of the empathy network and the mentalization network respectively; the default-mode network emerges from the resting state to activation in the endogenous system during reciprocal interaction; the cingulate plays an essential role in the emergence from the exogenous system to the endogenous system. Overall, the dual logic deductions are supported by the dfMRI experimental results and are consistent with current literature. Both the theoretical framework and experimental method set the stage to formally apply the scientific method in studying complex social interaction. © 2015 Ray F. Lee.


Schwarzbauer J.E.,Princeton University | DeSimone D.W.,University of Virginia
Cold Spring Harbor Perspectives in Biology | Year: 2011

Fibronectin (FN) is a multidomain protein with the ability to bind simultaneously to cell surface receptors, collagen, proteoglycans, and other FN molecules. Many of these domains and interactions are also involved in the assembly of FN dimers into a multimeric fibrillar matrix. When, where, and how FN binds to its various partners must be controlled and coordinated during fibrillogenesis. Steps in the process of FN fibrillogenesis including FN self-association, receptor activities, and intracellular pathways have been under intense investigation for years. In this review, the domain organization of FN including the extra domains and variable region that are controlled by alternative splicing are described. We discuss how FN-FN and cell-FN interactions play essential roles in the initiation and progression of matrix assembly using complementary results from cell culture and embryonic model systems that have enhanced our understanding of this process. © 2011 Cold Spring Harbor Laboratory Press.


Harlow D.,Princeton University
Journal of High Energy Physics | Year: 2014

Abstract: In this note I elaborate on some features of a recent proposal of Papadodimas and Raju for a CFT description of the interior of a one-sided AdS black hole in a pure state. I clarify the treatment of 1/N corrections, and explain how the proposal is able to avoid some of the pitfalls that have disrupted other recent ideas. I argue however that the proposal has the uncomfortable property that states in the CFT Hilbert space do not have definite physical interpretations, unlike in ordinary quantum mechanics. I also contrast the “state-dependence” of the proposal with more familiar phenomena, arguing that, unlike in quantum mechanics, the measurement process (including the apparatus) in something like the PR proposal or its earlier manifestations cannot be described by unitary evolution. These issues render the proposal somewhat ambiguous, and it seems new ideas would be needed to make some version of it work. I close with some brief speculation on to what extent quantum mechanics should hold for the experience of an infalling observer. © 2014, The Author(s).


Silhavy T.J.,Princeton University
Cold Spring Harbor perspectives in biology | Year: 2010

The bacteria cell envelope is a complex multilayered structure that serves to protect these organisms from their unpredictable and often hostile environment. The cell envelopes of most bacteria fall into one of two major groups. Gram-negative bacteria are surrounded by a thin peptidoglycan cell wall, which itself is surrounded by an outer membrane containing lipopolysaccharide. Gram-positive bacteria lack an outer membrane but are surrounded by layers of peptidoglycan many times thicker than is found in the gram-negatives. Threading through these layers of peptidoglycan are long anionic polymers, called teichoic acids. The composition and organization of these envelope layers and recent insights into the mechanisms of cell envelope assembly are discussed.


With high-resolution (0.46 h -1kpc), large-scale, adaptive mesh-refinement Eulerian cosmological hydrodynamic simulations we compute properties of O VI and O VII absorbers from the warm-hot intergalactic medium (WHIM) at z = 0. Our new simulations are in broad agreement with previous simulations with 40% of the intergalactic medium being in the WHIM. Our simulations are in agreement with observed properties of O VI absorbers with respect to the line incidence rate and Doppler-width-column-density relation. It is found that the amount of gas in the WHIM below and above 106K is roughly equal. Strong O VI absorbers are found to be predominantly collisionally ionized. It is found that (61%, 57%, 39%) of O VI absorbers of log N(O VI) cm2 = (12.5-13, 13-14, > 14) have T < 105K. Cross correlations between galaxies and strong [N(O VI) > 1014cm -2] O VI absorbers on 100-300kpc scales are suggested as a potential differentiator between collisional ionization and photoionization models. Quantitative prediction is made for the presence of broad and shallow O VI lines that are largely missed by current observations but will be detectable by Cosmic Origins Spectrograph observations. The reported 3σ upper limit on the mean column density of coincidental O VII lines at the location of detected O VI lines by Yao etal. is above our predicted value by a factor of 2.5-4. The claimed observational detection of O VII lines by Nicastro etal., if true, is 2σ above what our simulations predict. © 2012. The American Astronomical Society. All rights reserved.


Aretakis S.,Princeton University | Aretakis S.,Institute for Advanced Study
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We prove a new type of finite time blowup for a class of semilinear wave equations on extremal black holes. The initial data can be taken to be arbitrarily close to the trivial data. The first singularity occurs along the (degenerate) future event horizon. No analogue of this instability occurs for subextremal black holes or the Minkowski spacetime. © 2013 American Physical Society.


Zhiboedov A.,Princeton University
Journal of High Energy Physics | Year: 2014

Unitary conformal field theories (CFTs) are believed to have positive (non-negative) energy correlators. Energy correlators are universal observables in higher-dimensional CFTs built out of integrated Wightman functions of the stress-energy tensor. We analyze energy correlators in parity invariant four-dimensional CFTs. The goal is to use the positivity of energy correlators to further constrain unitary CFTs. It is known that the positivity of the simplest one-point energy correlator implies that 1/3≤a/c≤31/18 where a and c are the Weyl anomaly coefficients. We use the positivity of higher point energy correlators to show that CFTs with extremal values of a/c have trivial scattering observables. More precisely, fora/c=1/3 and a/c=31/18 all energy correlators are fixed to be the ones of the free boson and the free vector theory correspondingly. Similarly, we show that the positivity and finiteness of energy correlators together imply that the three-point function of the stress tensor in a CFT cannot be proportional to the one in the theory of free boson, free fermion or free vector field. © 2014 The Author(s).


Sergienko O.V.,Princeton University
Journal of Geophysical Research: Earth Surface | Year: 2013

Recent surveys of floating ice shelves associated with Pine Island Glacier (Antarctica) and Petermann Glacier (Greenland) indicate that there are channels incised upward into their bottoms that may serve as the conduits of meltwater outflow from the sub-ice-shelf cavity. The formation of the channels, their evolution over time, and their impact on ice-shelf flow are investigated using a fully-coupled ice-shelf/sub-ice-shelf ocean model. The model simulations suggest that channels may form spontaneously in response to meltwater plume flow initiated at the grounding line if there are relatively high melt rates and if there is transverse to ice-flow variability in ice-shelf thickness. Typical channels formed in the simulations have a width of about 1-3 km and a vertical relief of about 100-200 m. Melt rates and sea-water transport in the channels are significantly higher than on the smooth flat ice bottom between the channels. The melt channels develop through melting, deformation, and advection with ice-shelf flow. Simulations suggest that both steady state and cyclic state solutions are possible depending on conditions along the lateral ice-shelf boundaries. This peculiar dynamics of the system has strong implications on the interpretation of observations. The richness of channel morphology and evolution seen in this study suggests that further observations and theoretical analysis are imperative for understanding ice-shelf behavior in warm ©2013. American Geophysical Union. All Rights Reserved.


Longuet-Higgins and Stewart (J Fluid Mech 13:481-504, 1962; Deep-Sea Res 11:529-562, 1964) and later Phillips (1977) introduced the problem of waves incident on a beach, from deep to shallow water. From the wave energy equation and the vertically integrated continuity equation, they inferred velocities to be Stokes drift plus a return current so that the vertical integral of the combined velocities was nil. As a consequence, it can be shown that velocities of the order of Stokes drift rendered the advective term in the momentum equation negligible resulting in a simple balance between the horizontal gradients of the vertically integrated elevation and wave radiation stress terms; the latter was first derived by Longuet-Higgins and Stewart. Mellor (J Phys Oceanogr 33:1978-1989, 2003a), noting that vertically integrated continuity and momentum equations were not able to deal with three-dimensional numerical or analytical ocean models, derived a vertically dependent theory of wave-circulation interaction. It has since been partially revised and the revisions are reviewed here. The theory is comprised of the conventional, three-dimensional, continuity and momentum equations plus a vertically distributed, wave radiation stress term. When applied to the problem of waves incident on a beach with essentially zero turbulence momentum mixing, velocities are very large and the simple balance between elevation and radiation stress gradients no longer prevails. However, when turbulence mixing is reinstated, the vertically dependent radiation stresses produce vertical velocity gradients which then produce turbulent mixing; as a consequence, velocities are reduced, but are still larger by an order of magnitude compared to Stokes drift. Nevertheless, the velocity reduction is sufficient so that elevation set-down obtained from a balance between elevation gradient and radiation stress gradients is nearly coincident with that obtained by the aforementioned papers. This paper includes four appendices. The first appendix demonstrates the numerical process by which Stokes drift is excluded from the turbulence stress parameterization in the momentum equation. A second appendix determines a bottom slope criterion for the application of linear wave relations to the derivation of the wave radiation stress. The third appendix explores the possibility of generalizing results by non-dimensionalization. The final appendix applies the basic theory to a problem introduced by Bennis and Ardhuin (J Phys Oceanogr 41:2008-2012, 2011). © 2013 Springer-Verlag Berlin Heidelberg.


Cen R.,Princeton University
Astrophysical Journal | Year: 2012

Using adaptive mesh refinement cosmological hydrodynamic simulations with a physically motivated supernova feedback prescription, we show that the standard cold dark matter model can account for extant observed properties of damped Lyα systems (DLAs). With detailed examination of DLAs identified for each redshift snapshot through ray tracing through the simulation volumes containing thousands of galaxies, we find the following: (1) While DLA hosts roughly trace the overall population of galaxies at all redshifts, they are always gas-rich and have tendencies of being slightly smaller and bluer. (2) The history of DLA evolution is cosmological in nature and reflects primarily the evolution of the underlying cosmic density, galaxy size, and galaxy interactions. With higher density and more interactions at high redshift the size of DLAs is a larger fraction of their virial radius. (3) The variety of DLAs at high redshift is richer with a large contribution coming from galactic aqueducts, created through close galaxy interactions. The portion of gaseous disks of galaxies where most stars reside makes a relatively small contribution to DLA incidence at z = 3-4. (4) The majority of DLAs arise in halos of mass Mh = 10 10-1012 M at z = 1.6-4, as these galaxies dominate the overall population of galaxies then. At z = 3-4, 20%-30% of DLA hosts are Lyman break galaxies (LBGs), 10%-20% are due to galaxies more massive than LBGs, and 50%-70% are from smaller galaxies. (5) Galactic winds play an indispensable role in shaping the kinematic properties of DLAs. Specifically, the high velocity width DLAs are a mixture of those arising in high-mass, high velocity dispersion halos and those arising in smaller mass systems where cold gas clouds are entrained to high velocities by galactic winds. (6) In agreement with observations, we see a weak but noticeable evolution in DLA metallicity. The metallicity distribution centers at [Z/H] = -1.5 to -1 and spans more than three decades at z = 3-4, with the peak moving to [Z/H] = -0.75 at z = 1.6 and [Z/H] = -0.5 by z = 0. (7) The star formation rate of DLA hosts is concentrated in the range 0.3-30 M yr-1 at z = 3-4, gradually shifting lower to peak at 0.5-1 M yr-1 by z = 0. (8) We predict that only 20%-30% of DLAs are within 100kpc of L* galaxies at z = 3-4, increasing to 30%-50% at z = 1.6. © 2012. The American Astronomical Society. All rights reserved.


Sourjik V.,University of Heidelberg | Wingreen N.S.,Princeton University
Current Opinion in Cell Biology | Year: 2012

Chemotaxis allows bacteria to follow gradients of nutrients and other environmental stimuli. The bacterium Escherichia coli performs chemotaxis via a run-and-tumble strategy in which sensitive temporal comparisons lead to a biased random walk, with longer runs in the preferred gradient direction. The chemotaxis network of E. coli has developed over the years into one of the most thoroughly studied model systems for signal transduction and behavior, yielding general insights into such properties of cellular networks as signal amplification, signal integration, and robustness. Despite its relative simplicity, the operation of the E. coli chemotaxis network is highly refined and evolutionarily optimized at many levels. For example, recent studies revealed that the network adjusts its signaling properties dependent on the extracellular environment, apparently to optimize chemotaxis under particular conditions. The network can even utilize potentially detrimental stochastic fluctuations in protein levels and reaction rates to maximize the chemotactic performance of the population. © 2011 Elsevier Ltd.


Feng X.,Duke University | Porporato A.,Duke University | Rodriguez-Iturbe I.,Princeton University
Nature Climate Change | Year: 2013

Climate change has altered not only the overall magnitude of rainfall but also its seasonal distribution and interannual variability worldwide. Such changes in the rainfall regimes will be most keenly felt in arid and semiarid regions, where water availability and timing are key factors controlling biogeochemical cycles, primary productivity, and the phenology of growth and reproduction, while also regulating agricultural production. Nevertheless, a comprehensive framework to understand the complex seasonal rainfall regimes across multiple timescales is still lacking. Here, we formulate a global measure of seasonality, which captures the effects of both magnitude and concentration of the rainy season, and use it to identify regions across the tropics with highly seasonal rainfall regimes. By further decomposing rainfall seasonality into its magnitude, timing and duration components, we find increases in the interannual variability of seasonality over many parts of the dry tropics, implying increasing uncertainty in the intensity, arrival and duration of seasonal rainfall over the past century. We show that such increases in rainfall variability were accompanied by shifts in its seasonal magnitude, timing and duration, thus underscoring the importance of analysing seasonal rainfall regimes in a context that is most relevant to local ecological and social processes. © 2013 Macmillan Publishers Limited. All rights reserved.


Barrett S.,Columbia University | Barrett S.,Princeton University
Journal of Environmental Economics and Management | Year: 2013

If the threshold that triggers climate catastrophe is known with certainty, and the benefits of avoiding catastrophe are high relative to the costs, treaties can easily coordinate countries' behavior so as to avoid the threshold. Where the net benefits of avoiding catastrophe are lower, treaties typically fail to help countries cooperate to avoid catastrophe, sustaining only modest cuts in emissions. These results are unaffected by uncertainty about the impact of catastrophe. By contrast, uncertainty about the catastrophic threshold normally causes coordination to collapse. Whether the probability density function has "thin" or "fat" tails makes little difference. © 2012 Elsevier Inc.


Pimentel G.L.,Princeton University
Journal of High Energy Physics | Year: 2014

It is shown that the squeezed limit of inflationary expectation values follows from reparametrization invariance of the wavefunction of the universe. This translates into a constraint on the longitudinal modes of functional derivatives of the wavefunction. Thus, the local non-Gaussianity induced by single field inflation is purely a gauge artifact. We focus on Einstein gravity in de Sitter space and single field inflation, although the formalism only relies on the diffeomorphism invariance of the theory, and thus applies to any theory of gravity. © 2014 The Authors.


Schmidt F.,Princeton University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

Primordial non-Gaussianity, in particular the coupling of modes with widely different wavelengths, can have a strong impact on the large-scale clustering of tracers through a scale-dependent bias with respect to matter. We demonstrate that the standard derivation of this non-Gaussian scale-dependent bias is in general valid only in the extreme squeezed limit of the primordial bispectrum, i.e. for clustering over very large scales. We further show how the treatment can be generalized to describe the scale-dependent bias on smaller scales, without making any assumptions on the nature of tracers apart from a dependence on the small-scale fluctuations within a finite region. If the leading scale-dependent bias Δbâ̂kα, then the first subleading term will scale as kα+2. This correction typically becomes relevant as one considers clustering over scales k 10-2h Mpc -1. © 2013 American Physical Society.


Shaevitz J.W.,Princeton University
Cold Spring Harbor perspectives in biology | Year: 2010

During the past decade, the appreciation and understanding of how bacterial cells can be organized in both space and time have been revolutionized by the identification and characterization of multiple bacterial homologs of the eukaryotic actin cytoskeleton. Some of these bacterial actins, such as the plasmid-borne ParM protein, have highly specialized functions, whereas other bacterial actins, such as the chromosomally encoded MreB protein, have been implicated in a wide array of cellular activities. In this review we cover our current understanding of the structure, assembly, function, and regulation of bacterial actins. We focus on ParM as a well-understood reductionist model and on MreB as a central organizer of multiple aspects of bacterial cell biology. We also discuss the outstanding puzzles in the field and possible directions where this fast-developing area may progress in the future.


Cen R.,Princeton University
Astrophysical Journal | Year: 2012

A new physically based model for coevolution of galaxies and supermassive black holes (SMBHs) is presented. The evolutionary track starts with an event that triggers a significant starburst in the central region of a galaxy. In this model, the main SMBH growth takes place in the post-starburst phase, fueled by recycled gas from inner bulge stars in a self-regulated fashion on a timescale that is substantially longer than 100 Myr and at a diminishing Eddington ratio with time. We argue that the SMBH cannot gorge itself during the starburst phase, despite the abundant supply of cold gas, because star formation (SF) is a preferred mode of gas consumption over accretion to the central SMBH in such an environment. We also show that feedback from SF is at least as strong as that from an active galactic nucleus (AGN); thus, if SF is in need of being quenched, AGN feedback generally does not play the primary role. The predicted relation between SMBH mass and bulge mass/velocity dispersion is consistent with observations. A clear prediction is that early-type galaxy hosts of high-Eddington-rate AGNs are expected to be light blue to green in optical color, gradually evolving to the red sequences with decreasing AGN luminosity. A suite of falsifiable predictions and implications with respect to relationships between various types of galaxies, AGNs, and others are made. For those where comparisons to extant observations are possible, the model appears to be in good standing. © 2012. The American Astronomical Society. All rights reserved.


Lee K.-G.,Princeton University
Astrophysical Journal | Year: 2012

Continuum fitting uncertainties are a major source of error in estimates of the temperature-density relation (usually parameterized as a power-law, T Δγ - 1) of the intergalactic medium through the flux probability distribution function (PDF) of the Lyα forest. Using a simple order-of-magnitude calculation, we show that few percent-level systematic errors in the placement of the quasar continuum due to, e.g., a uniform low-absorption Gunn-Peterson component could lead to errors in γ of the order of unity. This is quantified further using a simple semi-analytic model of the Lyα forest flux PDF. We find that under(over)estimates in the continuum level can lead to a lower (higher) measured value of γ. By fitting models to mock data realizations generated with current observational errors, we find that continuum errors can cause a systematic bias in the estimated temperature-density relation of 〈δ(γ)〉 -0.1, while the error is increased to σγ 0.2 compared to σγ 0.1 in the absence of continuum errors. © 2012. The American Astronomical Society. All rights reserved..


Mody K.,Princeton University | Hajian A.,University of Toronto
Astrophysical Journal | Year: 2012

We present our measurement of the "bulk flow" using the kinetic Sunyaev-Zel'dovich (kSZ) effect in the Wilkinson Microwave Anisotropy Probe (WMAP) seven-year data. As the tracer of peculiar velocities, we use Planck Early Sunyaev-Zel'dovich Detected Cluster Catalog and a compilation of X-ray-detected galaxy cluster catalogs based on ROSAT All-Sky Survey. We build a full-sky kSZ template and fit it to the WMAP data in W band. Using a Wiener filter we maximize the signal-to-noise ratio of the kSZ cluster signal in the data. We find no significant detection of the bulk flow, and our results are consistent with the ΛCDM prediction. © 2012. The American Astronomical Society. All rights reserved.


Sergienko O.V.,Princeton University
Journal of Geophysical Research: Earth Surface | Year: 2010

Disintegration of ice shelves along the Antarctic Peninsula over the past two decades has clearly demonstrated their high sensitivity to recent changes in the local thermal regime of the atmosphere and ocean and has given rise to the question of whether mechanical coupling with waves in the ocean may provide the triggering mechanism that starts collapse events. Motivated by these events, this study considers a more general question: how ocean waves affect the stress regime of floating ice, and in particular, how ocean waves can influence the creation of fractures and its fatiguing that may lead to breakup and collapse. A new treatment of ice shelf/ocean wave interaction in which the typical "thin plate" approximation is relaxed is presented here, and exact, analytic solutions describing ice shelf stresses induced by long (>60 s period) ocean waves in various idealized ice/ocean geometries are derived. The numerical calculations demonstrate that the amplitudes of the wave-induced stresses are sufficiently large to initiate top to bottom crevasse penetration through the depth of the ice shelf. The cyclic nature of the wave-induced stresses contributes to ice fatigue and damage that is also a precursor to ice shelf disintegration. Although primarily theoretical, the results of the present analysis suggest that ocean waves could be a potential trigger of ice shelf collapse as well as less dramatic, but equally important, episodic calving. Copyright 2010 by the American Geophysical Union.


Lehner L.,Perimeter Institute for Theoretical Physics | Pretorius F.,Princeton University
Annual Review of Astronomy and Astrophysics | Year: 2014

Throughout the Universe many powerful events are driven by strong gravitational effects that require general relativity to fully describe them. These include compact binary mergers, black hole accretion, and stellar collapse, where velocities can approach the speed of light and extreme gravitational fields (ΦNewt/c2≃1) mediate the interactions. Many of these processes trigger emission across a broad range of the electromagnetic spectrum. Compact binaries further source strong gravitational wave emission that could directly be detected in the near future. This feat will open up a gravitational wave window into our Universe and revolutionize our understanding of it. Describing these phenomena requires general relativity, and where dynamical effects strongly modify gravitational fields the full Einstein equations coupled to matter sources. Numerical relativity is a field within general relativity concerned with studying such scenarios that cannot be accurately modeled via perturbative or analytical calculations. In this review, we examine results obtained within this discipline, with a focus on its impact in astrophysics. Copyright © 2014 by Annual Reviews.


Law C.K.,Princeton University
AIAA Journal | Year: 2012

The state of research on developing fuel options for next-generation chemical propulsion is reviewed for aviation fuels and energetic fuels. For aviation fuels, the development is based on considerations of cost, energy security, and climate change, with Fischer-Tropsch synthetic fuels and biofuels hold potential as alternative aviation fuels. The need for basic research to develop predictive capability for the oxidative chemistry of evolving fuels in evolving engine designs is emphasized, illustrated by the intricate reaction pathways and the enormity of the reaction mechanisms involved. Recent research activities toward achieving the goal of fuel design are discussed through the development of detailed mechanisms, reduced mechanisms, and surrogate fuels. For the development of highenergy-density propellants, advances in several classes of materials are discussed, including metallized and hypergolic propellants and propellants with strained and functionalized molecules, as well as nanoparticle addition. The impact of the recent progress in chemical synthesis, materials science, and nano science on these advances is noted. Copyright © 2011 by the author.


Poor H.V.,Princeton University
IEEE Wireless Communications | Year: 2012

Wireless networking applications continue to motivate challenging problems in information theory, signal processing, and other fields. This article explores briefly four research areas, primarily involving information theoretic or inferential problems, each of which is motivated by a wireless application-layer issue. In particular, the four applications of secure file transfer, inference, real-time multimedia transmission, and social networking, are used to motivate consideration of four respective research problems involving the wireless physical layer: physical layer security in data networks, distributed inference in sensor networks, finite-blocklength capacity in multimedia networks, and connectivity in small-world networks. © 2012 IEEE.


Lloyd I.D.,Princeton University | Vecchi G.A.,National Oceanic and Atmospheric Administration
Journal of Climate | Year: 2010

The Indian Ocean exhibits strong variability on a number of time scales, including prominent intraseasonal variations in both the atmosphere and ocean. Of particular interest is the south tropical Indian Ocean thermocline ridge, a region located between 12° and 5°S, which exhibits prominent variability in sea surface temperature (SST) due to dominant winds that raise the thermocline and shoal the mixed layer. In this paper, submonthly (less than 30 day) cooling events in the thermocline ridge region are diagnosed with observations and models, and are related to large-scale conditions in the Indo-Pacific region. Observations from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) satellite were used to identify 16 cooling events in the period 1998-2007, which on average cannot be fully accounted for by air-sea enthalpy fluxes. Analysis of observations and a hierarchy of models, including two coupled global climate models (GFDL CM2.1 and GFDL CM2.4), indicates that ocean dynamical changes are important to the cooling events. For extreme cooling events (above 2.5 standard deviations), air-sea enthalpy fluxes account for approximately 50% of the SST signature, and oceanic processes cannot in general be neglected. For weaker cooling events (1.5-2.5 standard deviations), air-sea enthalpy fluxes account for a larger fraction of the SST signature. Furthermore, it is found that cooling events are preconditioned by large-scale, low-frequency changes in the coupled ocean-atmosphere system. When the thermocline is unusually shallow in the thermocline ridge region, cooling events are more likely to occur and are stronger; these large-scale conditions are more (less) likely during La Niña (El Niño/Indian Ocean dipole) events. Strong cooling events are associated with changes in atmospheric convection, which resemble the Madden-Julian oscillation, in both observations and the models. © 2010 American Meteorological Society.


Kidston J.,Princeton University | Gerber E.P.,Courant Institute of Mathematical Sciences
Geophysical Research Letters | Year: 2015

Future climate predictions by global circulation models in the Coupled Model Intercomparison Project Phase 3 (CMIP3) archive indicate that the recent poleward shift of the eddy-driven jet streams will continue throughout the 21st century. Here it is shown that differences in the projected magnitude of the trend in the Southern Hemisphere are well correlated with biases in the latitude of the jet in the simulation of 20th century climate. Furthermore, the latitude of the jet in the models' 20th century climatology is correlated with biases in the internal variability of the jet stream, as quantified by the time scale of the annular mode. Thus an equatorward bias in the position of the jet is associated with both enhanced persistence of the annular mode, and an increased poleward shift of the jet. These relationships appear to be robust throughout the year except in the austral summer, when differences in forcing, particularly stratospheric ozone, make it impossible to compare the response of one model with another. These results suggest that the fidelity of a model's simulation of the 20th century climate may be related to its fitness for climate prediction. The cause of this relationship is discussed, as well as the implications for climate change projections. Copyright © 2010 by the American Geophysical Union.


Yarom A.,Princeton University
Journal of High Energy Physics | Year: 2010

A novel technique is used to compute the bulk viscosity of high temperature holographic gauge theory plasmas with softly broken conformal symmetry. Working in a black hole background which corresponds to a non-trivial solution to the Navier-Stokes equation, and using a Ward identity for the trace of the stress-energy tensor, it is possible to obtain an analytic expression for the bulk viscosity. This can be used to verify the high temperature limit of a conjectured bound on the bulk viscosity for these theories. The bound is saturated when the conformal symmetry-breaking operator becomes marginal. © SISSA 2010.


Flowers R.M.,University of Colorado at Boulder | Schoene B.,Princeton University
Geology | Year: 2010

The timing and causes of the >1.0 km elevation gain of the southern African Plateau since Paleozoic time are widely debated. We report the first apatite and titanite (U-Th)/He thermochronometry data for southern Africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau. The study area encompasses ~1500 m of relief within Archean basement of the Barberton Greenstone Belt region of the eastern Kaapvaal craton. Titanite dates are Neoproterozoic. Apatite dates are Cretaceous, with most results clustering at ca. 100 Ma. Thermal history simulations confirm Mesozoic heating followed by accelerated cooling in mid-to Late Cretaceous time. The lower temperature sensitivity of the apatite (U-Th)/He method relative to previous thermochronometry in southern Africa allows tighter constraints on the Cenozoic thermal history than past work. The data limit Cenozoic temperatures east of the escarpment to ≤35 °C, and appear best explained by temperatures within a few degrees of the modern surface temperature. These results restrict Cenozoic unroofing to less than ~850 m, and permit negligible erosion since the Cretaceous. If substantial uplift of the southern African Plateau occurred in the Cenozoic as advocated by some workers, then it was not responsible for the majority of post-Paleozoic unroofing across the eastern escarpment. Significant Mesozoic unroofing is coincident with large igneous province activity, kimberlite magmatism, and continental rifting within and along the margins of southern Africa, compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events. © 2010 Geological Society of America.


Bubeck S.,Princeton University | Cesa-Bianchi N.,University of Milan
Foundations and Trends in Machine Learning | Year: 2012

Multi-armed bandit problems are the most basic examples of sequential decision problems with an exploration-exploitation trade-off. This is the balance between staying with the option that gave highest payoffs in the past and exploring new options that might give higher payoffs in the future. Although the study of bandit problems dates back to the 1930s, exploration-exploitation trade-offs arise in several modern applications, such as ad placement, website optimization, and packet routing. Mathematically, a multi-armed bandit is defined by the payoff process associated with each option. In this monograph, we focus on two extreme cases in which the analysis of regret is particularly simple and elegant: i.i.d. payoffs and adversarial payoffs. Besides the basic setting of finitely many actions, we also analyze some of the most important variants and extensions, such as the contextual bandit model. © 2012 S. Bubeck and N. Cesa-Bianchi.


Martinson M.L.,Princeton University
American Journal of Public Health | Year: 2012

Objectives. I systematically examined income gradients in health in the United States and England across the life span (ages birth to 80 years), separately for females and males, for a number of health conditions. Methods. Using data from the National Health and Nutrition Examination Survey for the United States (n = 36 360) and the Health Survey for England (n = 55 783), I calculated weighted prevalence rates and risk ratios by income level for the following health risk factors or conditions: obesity, hypertension, diabetes, low high-density lipoprotein cholesterol, high cholesterol ratio, heart attack or angina, stroke, and asthma. Results. In the United States and England, the income gradients in health are very similar across age, gender, and numerous health conditions, and are robust to adjustments for race/ethnicity, health behaviors, body mass index, and health insurance. Conclusions. Health disparities by income are pervasive in England as well as in the United States, despite better overall health, universal health insurance, and more generous social protection spending in England.


Stone H.A.,Princeton University
ACS Nano | Year: 2012

Ice formation on surfaces and structures produces damage and inefficiencies that negatively impact all manners of activities. Not surprisingly, for a long time, an unmet challenge has been to design materials capable of minimizing or even eliminating the formation of ice on the surface of the material. In recent years, there were significant efforts to develop such ice-phobic surfaces by building on the advances made with superhydrophobic materials since these, by definition, tend to repel water. However, a robust response includes the ability to deter the formation of ice when a substrate colder than the freezing temperature is exposed either to impacting water droplets or water vapor (i.e., frost formation). In the latter case, superhydrophobic surfaces in high humidity conditions were shown to allow significant ice accumulation. Consequently, a new design idea was needed. In this issue of ACS Nano, it is shown how a liquid-infiltrated porous solid, where the liquid strongly wets and is retained within the material, has many of the properties desired for an ice-phobic substrate. The composite material exhibits low contact angle hysteresis so only small forces are needed to provoke droplets to slide off of a cold substrate. This new slippery surface shows many characteristics required for ice-phobicity, and a method is demonstrated for applying this kind of material as a coating on aluminum. Ice may have met its match. © 2012 American Chemical Society.


Metzger B.D.,Princeton University | Berger E.,Harvard - Smithsonian Center for Astrophysics
Astrophysical Journal | Year: 2012

The final inspiral of double neutron star and neutron-star-black-hole binaries are likely to be detected by advanced networks of ground-based gravitational wave (GW) interferometers. Maximizing the science returns from such a discovery will require the identification of an electromagnetic counterpart. Here we critically evaluate and compare several possible counterparts, including short-duration gamma-ray bursts (SGRBs), "orphan" optical and radio afterglows, and day-long optical transients powered by the radioactive decay of heavy nuclei synthesized in the merger ejecta ("kilonovae"). We assess the promise of each counterpart in terms of four "Cardinal Virtues": detectability, high fraction, identifiability, and positional accuracy. Taking into account the search strategy for typical error regions of tens of square degrees, we conclude that SGRBs are the most useful to confirm the cosmic origin of a few GW events, and to test the association with neutron star mergers. However, for the more ambitious goal of localizing and obtaining redshifts for a large sample of GW events, kilonovae are instead preferred. Off-axis optical afterglows are detectable for at most tens of percent of events, while radio afterglows are promising only for energetic relativistic ejecta in a high-density medium. Our main recommendations are: (1) an all-sky gamma-ray satellite is essential for temporal coincidence detections, and for GW searches of gamma-ray-triggered events; (2) the Large Synoptic Survey Telescope should adopt a one-day cadence follow-up strategy, ideally with 0.5 hr per pointing to cover GW error regions; and (3) radio searches should focus on the relativistic case, which requires observations for a few months. © 2012. The American Astronomical Society. All rights reserved.


Aizenman M.,Princeton University | Warzel S.,TU Munich
Physical Review Letters | Year: 2011

We resolve an existing question concerning the location of the mobility edge for operators with a hopping term and a random potential on the Bethe lattice. The model has been among the earliest studied for Anderson localization, and it continues to attract attention because of analogies which have been suggested with localization issues for many particle systems. We find that extended states appear through disorder enabled resonances well beyond the energy band of the operator's hopping term. For weak disorder this includes a Lifshitz tail regime of very low density of states. © 2011 American Physical Society.


Renn S.C.P.,Reed College | Schumer M.E.,Princeton University
Animal Behaviour | Year: 2013

Many behaviours vary in response to the environment (biotic or abiotic) and therefore represent an interesting form of phenotypic plasticity. Behavioural plasticity, like other plastic traits, can evolve through genetic assimilation or accommodation. However, little is known about the nature of changes in gene expression plasticity that accompanies these evolutionary changes in phenotypic plasticity. We know that variation in gene expression level, a first-order phenotype, underlies much behavioural variation. Several studies have begun to document which genes show expression-level variation related to plastic changes in behaviour as well as evolved changes in behaviour. Advances in sequencing technology allow us to address these questions on a genomic scale. By characterizing changes in gene expression according to the concept of a norm of reaction one can describe the evolved patterns of gene expression that accompany the evolution of behavioural plasticity. Here, we describe how genomic approaches can help us understand changes in gene expression that accompany or underlie the evolution of behavioural plasticity. To do this, we provide a framework of classification for the evolved patterns of gene expression plasticity that could underlie genetic assimilation or accommodation of behaviour. We provide examples of genetic assimilation from the animal behaviour and animal physiology literature that have been, or can be, studied at a genomic level. We then describe the characteristics of an appropriate study system and briefly address experimental design using the available genomic tools in a comparative context. Studying the patterns of gene expression associated with genetic assimilation will elucidate processes by which behavioural plasticity has evolved. © 2013.


Brangwynne C.P.,Princeton University
The Journal of cell biology | Year: 2013

The coordinated growth of cells and their organelles is a fundamental and poorly understood problem, with implications for processes ranging from embryonic development to oncogenesis. Recent experiments have shed light on the cell size-dependent assembly of membrane-less cytoplasmic and nucleoplasmic structures, including ribonucleoprotein (RNP) granules and other intracellular bodies. Many of these structures behave as condensed liquid-like phases of the cytoplasm/nucleoplasm. The phase transitions that appear to govern their assembly exhibit an intrinsic dependence on cell size, and may explain the size scaling reported for a number of structures. This size scaling could, in turn, play a role in cell growth and size control.


Parashar V.,The New School | Jeffrey P.D.,Princeton University | Neiditch M.B.,The New School
PLoS Biology | Year: 2013

The large family of Gram-positive quorum-sensing receptors known as the RNPP proteins consists of receptors homologous to the Rap, NprR, PlcR, and PrgX proteins that are regulated by imported oligopeptide autoinducers. Rap proteins are phosphatases and transcriptional anti-activators, and NprR, PlcR, and PrgX proteins are DNA binding transcription factors. Despite their obvious importance, the mechanistic basis of oligopeptide receptor regulation is largely unknown. Here, we report the X-ray crystal structure of the Bacillus subtilis quorum-sensing receptor RapJ in complex with the centrally important oligopeptide autoinducer competence and sporulation factor (CSF, also termed PhrC), a member of the Phr family of quorum-sensing signals. Furthermore, we present the crystal structure of RapI. Comparison of the RapJ-PhrC, RapI, RapH-Spo0F, and RapF-ComAC crystal structures reveals the mechanistic basis of Phr activity. More specifically, when complexed with target proteins, Rap proteins consist of a C-terminal tetratricopeptide repeat (TPR) domain connected by a flexible helix-containing linker to an N-terminal 3-helix bundle. In the absence of a target protein or regulatory peptide, the Rap protein 3-helix bundle adopts different conformations. However, in the peptide-bound conformation, the Rap protein N-terminal 3-helix bundle and linker undergo a radical conformational change, form TPR-like folds, and merge with the existing C-terminal TPR domain. To our knowledge, this is the first example of conformational change-induced repeat domain expansion. Furthermore, upon Phr binding, the entire Rap protein is compressed along the TPR superhelical axis, generating new intramolecular contacts that lock the Rap protein in an inactive state. The fact that Rap proteins are conformationally flexible is surprising considering that it is accepted dogma that TPR proteins do not undergo large conformational changes. Repeat proteins are widely used as scaffolds for the development of designed affinity reagents, and we propose that Rap proteins could be used as scaffolds for engineering novel ligand-switchable affinity reagents. © 2013 Parashar et al.


Nadell C.D.,Princeton University
Proceedings. Biological sciences / The Royal Society | Year: 2013

Via strength in numbers, groups of cells can influence their environments in ways that individual cells cannot. Large-scale structural patterns and collective functions underpinning virulence, tumour growth and bacterial biofilm formation are emergent properties of coupled physical and biological processes within cell groups. Owing to the abundance of factors influencing cell group behaviour, deriving general principles about them is a daunting challenge. We argue that combining mechanistic theory with theoretical ecology and evolution provides a key strategy for clarifying how cell groups form, how they change in composition over time, and how they interact with their environments. Here, we review concepts that are critical for dissecting the complexity of cell collectives, including dimensionless parameter groups, individual-based modelling and evolutionary theory. We then use this hybrid modelling approach to provide an example analysis of the evolution of cooperative enzyme secretion in bacterial biofilms.


Ho S.-W.,University of South Australia | Verdu S.,Princeton University
IEEE Transactions on Information Theory | Year: 2010

Fano's inequality relates the error probability of guessing a finitely-valued random variable X given another random variable Y and the conditional entropy of X given Y. It is not necessarily tight when the marginal distribution of X is fixed. This paper gives a tight upper bound on the conditional entropy of X given Y in terms of the error probability and the marginal distribution of X. A new lower bound on the conditional entropy for countably infinite alphabets is also found. The relationship between the reliability criteria of vanishing error probability and vanishing conditional entropy is also discussed. A strengthened form of the Schur-concavity of entropy which holds for finite or countably infinite random variables is given. © 2006 IEEE.


Petrovich C.,Princeton University
Astrophysical Journal | Year: 2015

We study the possibility that hot Jupiters (HJs) are formed through the secular gravitational interactions between two planets in eccentric orbits with relatively low mutual inclinations (≲20°) and friction due to tides raised on the planet by the host star. We term this migration mechanism Coplanar High-eccentricity Migration (CHEM) because, like disk migration, it allows for migration to occur on the same plane in which the planets formed. CHEM can operate from the following typical initial configurations: (i) the inner planet in a circular orbit and the outer planet with an eccentricity ≳0.67 for min/mout (ain/aout)1/2 ≲ 0.3; (ii) two eccentric (≳0.5) orbits for min/mout (ain/aout)1/2 ≲ 0.16. A population synthesis study of hierarchical systems of two giant planets using the observed eccentricity distribution of giant planets shows that CHEM produces HJs with low stellar obliquities (≲30°), with a semi-major axis distribution that matches the observations, and at a rate that can account for their observed occurrence. A different mechanism is needed to create large obliquity HJs, either a different migration channel or a mechanism that tilts the star or the protoplanetary disk. CHEM predicts that HJs should have distant (a ≳ 5 AU) and massive (most likely ∼1-3 times more massive than the HJ) companions with relatively low mutual inclinations (≲20°) and moderately high eccentricities (e ∼ 0.2-0.5).


Zhu Z.,Princeton University
Astrophysical Journal | Year: 2015

I calculate the spectral energy distributions of accreting circumplanetary disks using atmospheric radiative transfer models. Circumplanetary disks only accreting at 10-10 Myr-1 around a 1 MJ planet can be brighter than the planet itself. A moderately accreting circumplanetary disk (; enough to form a 10 MJ planet within 1 Myr) around a 1 MJ planet has a maximum temperature of 2000 K, and at near-infrared wavelengths (J, H, K bands), this disk is as bright as a late-M-type brown dwarf or a 10 MJ planet with a "hot start." To use direct imaging to find the accretion disks around low-mass planets (e.g., 1 MJ ) and distinguish them from brown dwarfs or hot high-mass planets, it is crucial to obtain photometry at mid-infrared bands (L′, M, N bands) because the emission from circumplanetary disks falls off more slowly toward longer wavelengths than those of brown dwarfs or planets. If young planets have strong magnetic fields (≳100 G), fields may truncate slowly accreting circumplanetary disks () and lead to magnetospheric accretion, which can provide additional accretion signatures, such as UV/optical excess from the accretion shock and line emission. © 2015. The American Astronomical Society. All rights reserved..


Lewkowycz A.,Princeton University | Maldacena J.,Institute for Advanced Study
Journal of High Energy Physics | Year: 2014

We consider a spherical region with a heavy quark in the middle. We compute the extra entanglement entropy due to the presence of a heavy quark both in N = 4 Super Yang Mills and in the N = 6 Chern-Simons matter theory (ABJM). This is done by relating the computation to the expectation value of a circular Wilson loop and a stress tensor insertion. We also give an exact expression for the Bremsstrahlung function that determines the energy radiated by a quark in the ABJM theory. © 2014 The Author(s).


Castellana M.,Princeton University
Physical Review Letters | Year: 2014

We consider two mean-field models of structural glasses, the random energy model and the p-spin model (PSM), and we show that the finite-size fluctuations of the freezing temperature are described by extreme-value statistics (EVS) distributions, establishing an unprecedented connection between EVS and the freezing transition of structural glasses. For the random energy model, the freezing-temperature fluctuations are described by the Gumbel EVS distribution, while for the PSM the freezing temperature fluctuates according to the Tracy-Widom EVS distribution, which has been recently discovered within the theory of random matrices. For the PSM, we provide an analytical argument showing that the emergence of the Tracy-Widom distribution can be understood in terms of the statistics of glassy metastable states. © 2014 American Physical Society.


Maciejko J.,Princeton University | Chua V.,University of Texas at Austin | Fiete G.A.,University of Texas at Austin
Physical Review Letters | Year: 2014

Motivated by experimental progress in the growth of heavy transition metal oxides, we theoretically study a class of lattice models of interacting fermions with strong spin-orbit coupling. Focusing on interactions of intermediate strength, we derive a low-energy effective field theory for a fully gapped, topologically ordered, fractionalized state with an eightfold ground-state degeneracy. This state is a fermionic symmetry-enriched topological phase with particle-number conservation and time-reversal symmetry. The topological terms in the effective field theory describe a quantized magnetoelectric response and nontrivial mutual braiding statistics of dynamical extended vortex loops with emergent fermions in the bulk. We explicitly compute the expected mutual statistics in a specific model on the pyrochlore lattice within a slave-particle mean-field theory. We argue that our model also provides a possible condensed-matter realization of oblique confinement. © 2014 American Physical Society.


Moreau D.,Princeton University
Quarterly Journal of Experimental Psychology | Year: 2013

Block videogame training has consistently demonstrated transfer effects to mental rotation tasks, yet how variations in training influence performance with different stimuli remains unclear. In this study, participants took mental rotation assessments before and after a 3-week training programme based on 2D or 3D block videogames. Assessments varied in terms of dimensionality (2D or 3D) and stimulus type (polygon or body). Increases in videogame scores throughout training were correlated with mental rotation improvements. In particular, 2D training led to improvements in 2D tasks, whereas 3D training led to improvements in both 2D and 3D tasks. This effect did not depend on stimulus type, demonstrating that training can transfer to different stimuli of identical dimensionality. Interestingly, traditional gender differences in 3D mental rotation tasks vanished after 3D videogame training, highlighting the malleability of mental rotation ability given adequate training. These findings emphasize the influence of dimensionality in transfer effects and offer promising perspectives to reduce differences in mental rotation via designed training programmes. © 2013 Copyright The Experimental Psychology Society.


Sergienko O.V.,Princeton University
Journal of Glaciology | Year: 2013

Ice shelves and ice tongues are dynamically coupled to their cavities. Here we compute normal modes (eigenfrequencies and eigenfunctions) of this coupled system using a thin-plate approximation for the ice shelf and potential water flow in the ice-shelf cavity. Our results show that normal modes depend not only on the ice-shelf parameters (length, thickness, Young's modulus, etc.) but also on the cavity depth. The dominant eigenmodes are higher for ice shelves floating over deeper cavities; they are also higher for shorter ice shelves and ice tongues (<50 km long). The higheigenfrequency eigenmodes are primarily controlled by the ice flexure and have similar periods to sea swell. These results suggest that both long ocean waves with periods of 100-400 s and shorter sea swell with periods of 10-20 s can have strong impacts on relatively short ice shelves and ice tongues by exciting oscillations with their eigenfrequencies, which can lead to iceberg calving and, in some circumstances, ice-shelf disintegration.


McWilliams S.T.,Princeton University
Physical Review Letters | Year: 2013

It has been suggested that maximally spinning black holes can serve as particle accelerators, reaching arbitrarily high center-of-mass energies. Despite several objections regarding the practical achievability of such high energies, and demonstrations past and present that such large energies could never reach a distant observer, interest in this problem has remained substantial. We show that, unfortunately, a maximally spinning black hole can never serve as a probe of high energy collisions, even in principle and despite the correctness of the original diverging energy calculation. Black holes can indeed facilitate dark matter annihilation, but the most energetic photons can carry little more than the rest energy of the dark matter particles to a distant observer, and those photons are actually generated relatively far from the black hole where relativistic effects are negligible. Therefore, any strong gravitational potential could probe dark matter equally well, and an appeal to black holes for facilitating such collisions is unnecessary. © 2013 American Physical Society.


Serbyn M.,Massachusetts Institute of Technology | Papic Z.,Princeton University | Abanin D.A.,Perimeter Institute for Theoretical Physics | Abanin D.A.,Institute for Quantum Computing
Physical Review Letters | Year: 2013

Recent numerical work by Bardarson, Pollmann, and Moore revealed a slow, logarithmic in time, growth of the entanglement entropy for initial product states in a putative many-body localized phase. We show that this surprising phenomenon results from the dephasing due to exponentially small interaction-induced corrections to the eigenenergies of different states. For weak interactions, we find that the entanglement entropy grows as ξln (Vt/), where V is the interaction strength, and ξ is the single-particle localization length. The saturated value of the entanglement entropy at long times is determined by the participation ratios of the initial state over the eigenstates of the subsystem. Our work shows that the logarithmic entanglement growth is a universal phenomenon characteristic of the many-body localized phase in any number of spatial dimensions, and reveals a broad hierarchy of dephasing time scales present in such a phase. © 2013 American Physical Society.


Miller O.D.,Massachusetts Institute of Technology | Johnson S.G.,Massachusetts Institute of Technology | Rodriguez A.W.,Princeton University
Physical Review Letters | Year: 2014

We show that the near-field functionality of hyperbolic metamaterials (HMM), typically proposed for increasing the photonic local density of states (LDOS), can be achieved with thin metal films. Although HMMs have an infinite density of internally propagating plane-wave states, the external coupling to nearby emitters is severely restricted. We show analytically that properly designed thin films, of thicknesses comparable to the metal size of a hyperbolic metamaterial, yield an LDOS as high as (if not higher than) that of HMMs. We illustrate these ideas by performing exact numerical computations of the LDOS of multilayer HMMs, along with their application to the problem of maximizing near-field heat transfer, to show that single-layer thin films are suitable replacements in both cases. © 2014 American Physical Society.


Zhu Z.,University of California at Irvine | Huse D.A.,Princeton University | White S.R.,University of California at Irvine
Physical Review Letters | Year: 2013

Using the density matrix renormalization group, we investigate the S=1/2 Heisenberg model on the honeycomb lattice with first (J1) and second (J2) neighbor interactions. We are able to study long open cylinders with widths up to 12 lattice spacings. For J2/J1 near 0.3, we find an apparently paramagnetic phase, bordered by an antiferromagnetic phase for J2â‰0.26 and by a valence bond crystal for J2â‰0.36. The longest correlation length that we find in this intermediate phase is for plaquette valence bond order. This correlation length grows strongly with cylinder circumference, indicating either quantum criticality or weak plaquette valence bond order. © 2013 American Physical Society.


England J.L.,Princeton University | Haran G.,Weizmann Institute of Science
Annual Review of Physical Chemistry | Year: 2011

Protein stability often is studied in vitro through the use of urea and guanidinium chloride, chemical cosolvents that disrupt protein native structure. Much controversy still surrounds the underlying mechanism by which these molecules denature proteins. Here we review current thinking on various aspects of chemical denaturation. We begin by discussing classic models of protein folding and how the effects of denaturants may fit into this picture through their modulation of the collapse, or coil-globule transition, which typically precedes folding. Subsequently, we examine recent molecular dynamics simulations that have shed new light on the possible microscopic origins of the solvation effects brought on by denaturants. It seems likely that both denaturants operate by facilitating solvation of hydrophobic regions of proteins. Finally, we present recent single-molecule fluorescence studies of denatured proteins, the analysis of which corroborates the role of denaturants in shifting the equilibrium of the coil-globule transition. © 2011 by Annual Reviews. All rights reserved.


Glisic B.,Princeton University
Measurement Science and Technology | Year: 2011

Depending on the geometric basis of measurement (gauge length), discrete strain sensors used in structural monitoring of civil engineering structures can be considered as short-gauge sensors or long-gauge sensors. Long-gauge sensors measure average strain over the gauge lengths and are used for global monitoring of structures, in particular, those built of inhomogeneous materials. However, the strain distribution along the sensor's gauge length may be nonlinear and the measured average strain value that is commonly attributed to the midpoint of the sensor may be different from the real value of strain at that point. Consequently, excessively long sensors may feature significant errors in measurement. However, short-gauge sensors are more susceptible to other types of measurement error, most notably, error caused by discontinuities (open cracks) distributed in the monitored material. Thus an optimum gauge length is to be found. The error in average strain measurement inherent to the sensor's gauge length introduced by the strain distribution and discontinuities in the monitored material is modelled for the most common applications met in civil engineering practice. The modelling takes into account the geometric properties of the monitored structure and various load cases. Guidelines for the selection of an appropriate gauge length are proposed, and tables for measurement error estimation are presented. © 2011 IOP Publishing Ltd.


Laidre M.E.,Princeton University
Journal of Experimental Marine Biology and Ecology | Year: 2011

Hermit crabs are critically dependent upon gastropod shells for their survival and reproductive fitness. While anecdotal reports have suggested that hermit crabs may be capable of removing live gastropods from their shells to access the essential shell resource, no systematic experiments have been conducted to investigate this possibility. This paper reports experiments on both marine (Pagurus bernhardus) and terrestrial (Coenobita compressus) hermit crabs in which crabs were paired in the laboratory with the gastropods whose shells they inhabit in the field. Pairings included both shelled and naked crabs and spanned the full range of the gastropod life cycle. Neither marine nor terrestrial hermit crabs were successful at removing live gastropods from their shells. Furthermore, only a small fraction of the crabs (5.7%) were capable of accessing shells in which the gastropod had been killed in advance, with its body left intact inside the shell. Finally, although hermit crabs readily entered empty shells positioned on the surface, few crabs (14.3%) were able to access empty shells that were buried just centimeters beneath them. These results suggest that hermit crabs are constrained consumers, with the shells they seek only being accessible during a narrow time window, which begins following natural gastropod death and bodily decomposition and which typically ends when the gastropod's remnant shell has been buried by tidal forces. Further experiments are needed on more species of hermit crabs as well as fine-grained measurements of (i) the mechanical force required to pull a gastropod body from its shell and (ii) the maximum corresponding force that can be generated by different hermit crab species' chelipeds. © 2010 Elsevier B.V.


Kelly M.T.,Princeton University
Structure and Bonding | Year: 2011

There are clear advantages to using hydrogen as a fuel, but storage of hydrogen in a light-weight and compact form remains a challenge. This review highlights past breakthroughs that led to the current thinking in hydrogen storage methodology. Metal organic frameworks are discussed briefly, with greater attention to the storage of hydrogen in other molecules. Many molecular storage strategies rely on the thermal decomposition of hydrogen hetero atom bonds and formation of hydrogen to hydrogen bonds. An acid-base thought model is presented to explain observed behaviors and to guide future endeavors. © 2011 Springer Verlag Berlin Heidelberg.


Smith K.M.,Princeton University | Loverde M.,Institute for Advanced Study | Zaldarriaga M.,Institute for Advanced Study
Physical Review Letters | Year: 2011

Models of inflation in which non-Gaussianity is generated outside the horizon, such as curvaton models, generate distinctive higher-order correlation functions in the cosmic microwave background and other cosmological observables. Testing for violation of the Suyama-Yamaguchi inequality τ NL(65fNL)2, where fNL and τNL denote the amplitude of the three-point and four-point functions in certain limits, has been proposed as a way to distinguish qualitative classes of models. This inequality has been proved for a wide range of models, but only weaker versions have been proved in general. In this Letter, we give a proof that the Suyama-Yamaguchi inequality is always satisfied. We discuss scenarios in which the inequality may appear to be violated in an experiment such as Planck and how this apparent violation should be interpreted. © 2011 American Physical Society.


Li Y.,Princeton University
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We study itinerant ferromagnetism in a t2g multiorbital Hubbard system in the cubic lattice, which consists of three planar oriented orbital bands of dxy,dyz, and dzx. Electrons in each orbital band can only move within a two-dimensional plane in the three-dimensional lattice parallel to the corresponding orbital orientation. Electrons of different orbitals interact through the on-site multiorbital interactions including Hund's coupling. The strong-coupling limit is considered in which there are no doubly occupied orbitals but multiple on-site occupations are allowed. We show that in the case in which there is one and only one hole for each orbital band in each layer parallel to the orbital orientation, the ground state is a fully spin-polarized itinerant ferromagnetic state, which is unique apart from the trivial spin degeneracy. When the lattice is reduced into a single two-dimensional layer, the dzx and dyz bands become quasi-one-dimensional while the dxy band remains two-dimensional. The ground-state ferromagnetism also appears in the strong-coupling limit as a generalization of the double-exchange mechanism. Possible applications to the systems of SrRuO3 and LaAlO3/SrTiO3 interface are discussed. © 2015 American Physical Society.


Chenu A.,University of Toronto | Scholes G.D.,University of Toronto | Scholes G.D.,Princeton University
Annual Review of Physical Chemistry | Year: 2015

Ultrafast energy transfer is used to transmit electronic excitation among the many molecules in photosynthetic antenna complexes. Recent experiments and theories have highlighted the role of coherent transfer in femtosecond studies of these proteins, suggesting the need for accurate dynamical models to capture the subtle characteristics of energy transfer mechanisms. Here we discuss how to think about coherence in light harvesting and electronic energy transfer. We review the various fundamental concepts of coherence, spanning from classical phenomena to the quantum superposition, and define coherence in electronic energy transfer. We describe the current status of experimental studies on light-harvesting complexes. Insights into the microscopic process are presented to highlight how and why this is a challenging problem to elucidate. We present an overview of the applicable dynamical theories to model energy transfer in the intermediate coupling regime. © 2015 by Annual Reviews. All rights reserved.


Petrovich C.,Princeton University
Astrophysical Journal | Year: 2015

We study the steady-state orbital distributions of giant planetsmigrating through the combination of theKozai-Lidov (KL) mechanism due to a stellar companion and friction due to tides raised on the planet by the host star. We run a large set of Monte Carlo simulations that describe the secular evolution of a star-planet-star triple system including the effects from general relativistic precession, stellar and planetary spin evolution, and tides. Our simulations show that KL migration produces Hot Jupiters (HJs) with semi-major axes that are generally smaller than in the observations and they can only explain the observations if the following are both true: (1) tidal dissipation at high eccentricities is at least ∼150 times more efficient than the upper limit inferred from the Jupiter-Io interaction; (2) highly eccentric planets get tidally disrupted at distances ≳ 0.015AU. Based on the occurrence rate and semi-major axis distribution of HJs, we find that KL migration in stellar binaries can produce at most ∼20% of the observed HJs. Almost no intermediate-period (semi-major axis ∼0.1-2AU) planets are formed by this mechanism - migrating planets spend most of their lifetimes undergoing KL oscillations at large orbital separations (>2AU) or as HJs. © 2015. The American Astronomical Society. All rights reserved.


Schwarzbauer J.E.,Princeton University
Cold Spring Harbor perspectives in biology | Year: 2011

Fibronectin (FN) is a multidomain protein with the ability to bind simultaneously to cell surface receptors, collagen, proteoglycans, and other FN molecules. Many of these domains and interactions are also involved in the assembly of FN dimers into a multimeric fibrillar matrix. When, where, and how FN binds to its various partners must be controlled and coordinated during fibrillogenesis. Steps in the process of FN fibrillogenesis including FN self-association, receptor activities, and intracellular pathways have been under intense investigation for years. In this review, the domain organization of FN including the extra domains and variable region that are controlled by alternative splicing are described. We discuss how FN-FN and cell-FN interactions play essential roles in the initiation and progression of matrix assembly using complementary results from cell culture and embryonic model systems that have enhanced our understanding of this process.


Xu Y.,Princeton University
Journal of Environmental Planning and Management | Year: 2011

Using the goal of a 10% reduction of SO2 emissions in China's 11th Five-YearPlan (2006-2010) as a case study, this paper analyses how environmental goals areused in a large country with multi-level governments and insufficient rule of law.After the central government set up the goal, the mitigation burden was sharedamong local governments, and mechanisms were designed to enhance theircommitment to the goal. This paper found that in the 11th Five-Year Plan, boththe central and local governments were much more committed to SO2 mitigationthan previously, which could help to explain China's recent significant reductionof SO2 emissions. The case indicates that goals can effectively plan and manageenvironmental protection and other national government agendas. © 2011 University of Newcastle upon Tyne.


Berger B.,Massachusetts Institute of Technology | Peng J.,Massachusetts Institute of Technology | Singh M.,Princeton University
Nature Reviews Genetics | Year: 2013

High-throughput experimental technologies are generating increasingly massive and complex genomic data sets. The sheer enormity and heterogeneity of these data threaten to make the arising problems computationally infeasible. Fortunately, powerful algorithmic techniques lead to software that can answer important biomedical questions in practice. In this Review, we sample the algorithmic landscape, focusing on state-of-the-art techniques, the understanding of which will aid the bench biologist in analysing omics data. We spotlight specific examples that have facilitated and enriched analyses of sequence, transcriptomic and network data sets. © 2013 Macmillan Publishers Limited. All rights reserved.


Doughty C.E.,University of Oxford | Wolf A.,Princeton University | Malhi Y.,University of Oxford
Nature Geoscience | Year: 2013

In the late Pleistocene, 97 genera of large animals went extinct, concentrated in the Americas and Australia. These extinctions had significant effects on ecosystem structure, seed dispersal and land surface albedo. However, the impact of this dramatic extinction on ecosystem nutrient biogeochemistry, through the lateral transport of dung and bodies, has never been explored. Here we analyse this process using a novel mathematical framework that analyses this lateral transport as a diffusion-like process, and we demonstrate that large animals play a disproportionately large role in the horizontal transfer of nutrients across landscapes. For example, we estimate that the extinction of the Amazonian megafauna decreased the lateral flux of the limiting nutrient phosphorus by more than 98%, with similar, though less extreme, decreases in all continents outside of Africa. This resulted in strong decreases in phosphorus availability in eastern Amazonia away from fertile floodplains, a decline which may still be ongoing. The current P limitation in the Amazon basin may be partially a relic of an ecosystem without the functional connectivity it once had. We argue that the Pleistocene megafauna extinctions resulted in large and ongoing disruptions to terrestrial biogeochemical cycling at continental scales and increased nutrient heterogeneity globally. © 2013 Macmillan Publishers Limited. All rights reserved.


Arnaudo A.M.,University of Pennsylvania | Arnaudo A.M.,Princeton University | Garcia B.A.,University of Pennsylvania
Epigenetics and Chromatin | Year: 2013

Histone post-translational modifications (PTMs) have been linked to a variety of biological processes and disease states, thus making their characterization a critical field of study. In the last 5 years, a number of novel sites and types of modifications have been discovered, greatly expanding the histone code. Mass spectrometric methods are essential for finding and validating histone PTMs. Additionally, novel proteomic, genomic and chemical biology tools have been developed to probe PTM function. In this snapshot review, proteomic tools for PTM identification and characterization will be discussed and an overview of PTMs found in the last 5 years will be provided. © 2013 Arnaudo and Garcia; licensee BioMed Central Ltd.


Johnson-Laird P.N.,Princeton University
Proceedings of the National Academy of Sciences of the United States of America | Year: 2010

To be rational is to be able to reason. Thirty years ago psychologists believed that human reasoning depended on formal rules of inference akin to those of a logical calculus. This hypothesis ran into difficulties, which led to an alternative view: reasoning depends on envisaging the possibilities consistent with the starting point-a perception of the world, a set of assertions, a memory, or some mixture of them. We construct mental models of each distinct possibility and derive a conclusion from them. The theory predicts systematic errors in our reasoning, and the evidence corroborates this prediction. Yet, our ability to use counterexamples to refute invalid inferences provides a foundation for rationality. On this account, reasoning is a simulation of the world fleshed out with our knowledge, not a formal rearrangement of the logical skeletons of sentences.


Schroder A.,Leibniz Institute of Freshwater Ecology and Inland Fisheries | van Leeuwen A.,Princeton University | Cameron T.C.,University of Essex
Trends in Ecology and Evolution | Year: 2014

Experimental and theoretical studies show that mortality imposed on a population can counter-intuitively increase the density of a specific life-history stage or total population density. Understanding positive population-level effects of mortality is advancing, illuminating implications for population, community, and applied ecology. Reconciling theory and data, we found that the mathematical models used to study mortality effects vary in the effects predicted and mechanisms proposed. Experiments predominantly demonstrate stage-specific density increases in response to mortality. We argue that the empirical evidence supports theory based on stage-structured population models but not on unstructured models. We conclude that stage-specific positive mortality effects are likely to be common in nature and that accounting for within-population individual variation is essential for developing ecological theory. © 2014 Elsevier Ltd.


Ju Y.,Princeton University | Maruta K.,Tohoku University
Progress in Energy and Combustion Science | Year: 2011

The high energy density of hydrocarbon fuels creates a great opportunity to develop combustion based micro-power generation systems to meet increasing demands for portable power devices, micro unmanned aerial vehicles, micro-satellite thrusters, and micro chemical reactors and sensors. In this paper, the recent technological development of micro-power systems and progress in fundamental understanding of micro-scale combustion are reviewed. At first, micro-scale combustion regimes are categorized by using different physical and chemical length and time scales and the resulting non-dimensional parameters and their correlations to various combustion regimes for micro and mesoscale combustion are discussed. Secondly, the recent successful developments and technical challenges of micro-thrusters, micro internal combustion engines, and micro chemical reactors summarized. Thirdly, the underlying fundamental mechanisms and ignition and flame dynamics in micro-scale combustion are reviewed, respectively, in premixed, non-premixed, catalytic, and non-equilibrium, micro-scale combustion systems. The conventional concepts of combustion limits such as the flammability limit, quenching diameter, and flame extinction and heat recirculation are revisited. The unique thermal and chemical transport mechanisms such as flame structure interaction, radical quenching, non-equilibrium transport appearing in micro-scale combustion are discussed. New flame regimes and instabilities such as flame bifurcation, weak flames, flame cells/streets, thermal and kinetic quenching, flameless low temperature catalytic combustion, repetitive extinction and ignition, spinning flames, spiral and multi-branched flames, symmetric and asymmetric oscillating flames are discussed. Finally, an overview of future research and conclusion are made. The goal of this review is to present an overview of the development of micro-power generators by focusing more on the advance in fundamental understanding of micro-scale combustion. © 2011 Elsevier Ltd. All rights reserved.


Torquato S.,Princeton University
Annual Review of Materials Research | Year: 2010

This article reviews recent inverse techniques that we have devised to optimize the structure and macroscopic properties of heterogeneous materials such as composite materials, porous media, colloidal dispersions, and polymer blends. Optimization methods provide a systematic means of designing materials with tailored properties and microstructures for a specific application. This article focuses on two inverse problems that are solved via optimization techniques: (a) the topology optimization procedure used to design heterogeneous materials and (b) stochastic optimization methods employed to reconstruct or construct microstructures. © 2010 by Annual Reviews. All rights reserved.


Wingreen N.S.,Princeton University | Huang K.C.,Stanford University
Annual Review of Microbiology | Year: 2015

With the realization that bacteria achieve exquisite levels of spatiotemporal organization has come the challenge of discovering the underlying mechanisms. In this review, we describe three classes of such mechanisms, each of which has physical origins: the use of landmarks, the creation of higher-order structures that enable geometric sensing, and the emergence of length scales from systems of chemical reactions coupled to diffusion. We then examine the diversity of geometric cues that exist even in cells with relatively simple geometries, and end by discussing both new technologies that could drive further discovery and the implications of our current knowledge for the behavior, fitness, and evolution of bacteria. The organizational strategies described here are employed in a wide variety of systems and in species across all kingdoms of life; in many ways they provide a general blueprint for organizing the building blocks of life. © Copyright ©2015 by Annual Reviews. All rights reserved.


Legg S.,Princeton University
Journal of Physical Oceanography | Year: 2014

A series of two-dimensional numerical simulations examine the breaking of first-mode internal waves at isolated ridges, independently varying the relative height of the topography compared to the depth of the ocean h0/H0; the relative steepness of the topographic slope compared to the slope of the internal wave group velocity γ; and the Froude number of the incoming internal wave Fr0. The fraction of the incoming wave energy, which is reflected back toward deep water, transmitted beyond the ridge, and lost to dissipation and mixing, is diagnosed from the simulations. For critical slopes, with γ = 1, the fraction of incoming energy lost at the slope scales approximately like h0/H0, independent of the incoming wave Froude number. For subcritical slopes, with γ < 1, waves break and lose a substantial proportion of their energy if the maximum Froude number, estimated as Frmax = Fr0/(1-h0/H0)2, exceeds a critical value, found empirically to be about 0.3. The dissipation at subcritical slopes therefore increases as both incoming wave Froude number and topographic height increase. At critical slopes, the dissipation is enhanced along the slope facing the incoming wave. In contrast, at subcritical slopes, dissipation is small until the wave amplitude is sufficiently enhanced by the shoaling topography to exceed the critical Froude number; then large dissipation extends all the way to the surface. The results are shown to generalize to variable stratification and different topographies, including axisymmetric seamounts. The regimes for low-mode internal wave breaking at isolated critical and subcritical topography identified by these simulations provide guidance for the parameterization of the mixing due to radiated internal tides. © 2014 American Meteorological Society.


Rafikov R.R.,Princeton University
Astrophysical Journal Letters | Year: 2013

The existence of planets in binaries with relatively small separations (around 20 AU), such as α Centauri or γ Cephei, poses severe challenges to standard planet formation theories. The problem lies in the vigorous secular excitation of planetesimal eccentricities at separations of several AU, where some of the planets are found, by the massive, eccentric stellar companions. High relative velocities of planetesimals preclude their growth in mutual collisions for a wide range of sizes, from below 1 km up to several hundred km, resulting in a fragmentation barrier to planet formation. Here we show that, for the case of an axisymmetric circumstellar protoplanetary disk, the rapid apsidal precession of planetesimal orbits caused by the disk gravity acts to strongly reduce the direct secular eccentricity excitation by the companion, lowering planetesimal velocities by an order of magnitude or even more at 1 AU. By examining the details of planetesimal dynamics, we demonstrate that this effect eliminates the fragmentation barrier for in situ growth of planetesimals as small as ≲ 10 km even at separations as wide as 2.6 AU (the semimajor axis of the giant planet in HD 196885), provided that the circumstellar protoplanetary disk has a small eccentricity and is relatively massive, 0.1 M. © 2013. The American Astronomical Society. All rights reserved..


Rafikov R.R.,Princeton University
Astrophysical Journal Letters | Year: 2013

Circumbinary planetary systems recently discovered by Kepler represent an important testbed for planet formation theories. Planetesimal growth in disks around binaries has been expected to be inhibited interior to ∼10 AU by secular excitation of high relative velocities between planetesimals, leading to their collisional destruction (rather than agglomeration). Here we show that gravity of an axisymmetric gaseous circumbinary disk in which planets form drives fast precession of both the planetesimal and binary orbits, resulting in strong suppression of planetesimal eccentricities beyond 2-3 AU and making possible the growth of 1-102 km objects in this region. The precise location of the boundary of the accretion-friendly region depends on the size of the inner disk cavity cleared by the binary torques and on the disk mass (even 0.01 M⊙ disk strongly suppresses planetesimal excitation), among other things, but this zone does not extend to present orbits of Kepler circumbinary planets. The precession of the orbit of the central binary, enhanced by the mass concentration that is naturally present at the inner edge of a circumbinary disk, plays a key role in this suppression, which is a feature specific to the circumbinary planet formation. © 2013. The American Astronomical Society. All rights reserved.


Walkowicz L.M.,Princeton University | Basri G.,University of California at Berkeley | Valenti J.A.,US Space Telescope Science Institute
Astrophysical Journal, Supplement Series | Year: 2013

We present the results of numerical experiments to assess degeneracies in light curve models of starspots. Using synthetic light curves generated with the Cheetah starspot modeling code, we explore the extent to which photometric light curves constrain spot model parameters, including spot latitudes and stellar inclination. We also investigate the effects of spot parameters and differential rotation on one's ability to correctly recover rotation periods and differential rotation in the Kepler light curves. We confirm that in the absence of additional constraints on the stellar inclination, such as spectroscopic measurements of vsin i or occultations of starspots by planetary transits, the spot latitude and stellar inclination are difficult to determine uniquely from the photometry alone. We find that for models with no differential rotation, spots that appear on opposite hemispheres of the star may cause one to interpret the rotation period to be half of the true period. When differential rotation is included, the changing longitude separation between spots breaks the symmetry of the hemispheres and the correct rotation period is more likely to be found. The dominant period found via periodogram analysis is typically that of the largest spot. Even when multiple spots with periods representative of the star's differential rotation exist, if one spot dominates the light curve the signal of differential rotation may not be detectable from the periodogram alone. Starspot modeling is applicable to stars with a wider range of rotation rates than other surface imaging techniques (such as Doppler imaging), allows subtle signatures of differential rotation to be measured, and may provide valuable information on the distribution of stellar spots. However, given the inherent degeneracies and uncertainty present in starspot models, caution should be exercised in their interpretation. © 2013. The American Astronomical Society. All rights reserved.


Wu Y.,University of Pennsylvania | Verdu S.,Princeton University
IEEE Transactions on Information Theory | Year: 2012

In addition to exploring its various regularity properties, we show that the minimum mean-square error (MMSE) is a concave functional of the input-output joint distribution. In the case of additive Gaussian noise, the MMSE is shown to be weakly continuous in the input distribution and Lipschitz continuous with respect to the quadratic Wasserstein distance for peak-limited inputs. Regularity properties of mutual information are also obtained. Several applications to information theory and the central limit theorem are discussed. © 2011 IEEE.


Wang S.,Princeton University
Frontiers in bioscience (Scholar edition) | Year: 2013

Bacteria derive and maintain a variety of shapes that carry selective benefits. The shapes are usually defined by a mechanically stiff exoskeletal cell wall -- a macro-molecular network of peptidoglycan. The growth of such a network is catalyzed by transglycosylases and transpeptidases, and various cell-wall remodeling enzymes further digest and process the network. To maintain the overall cell shape, the bacterial cytoskeleton coordinates cell wall synthesis on the cellular scale. Recent studies also suggest that the mechanical properties of the bacterial cytoskeleton are important for cell wall growth. Here, we review current experiments and theories on the structure, dynamics and interactions of the bacterial cell wall and cytoskeleton, and their contributions to cell shape maintenance. We also propose future research directions that will help clarify the mystery of bacterial cell morphogenesis.


Coller H.A.,Princeton University
American Journal of Pathology | Year: 2014

Although cancer has historically been viewed as a disorder of proliferation, recent evidence has suggested that it should also be considered a metabolic disease. Growing tumors rewire their metabolic programs to meet and even exceed the bioenergetic and biosynthetic demands of continuous cell growth. The metabolic profile observed in cancer cells often includes increased consumption of glucose and glutamine, increased glycolysis, changes in the use of metabolic enzyme isoforms, and increased secretion of lactate. Oncogenes and tumor suppressors have been discovered to have roles in cancer-associated changes in metabolism as well. The metabolic profile of tumor cells has been suggested to reflect the rapid proliferative rate. Cancer-associated metabolic changes may also reveal the importance of protection against reactive oxygen species or a role for secreted lactate in the tumor microenvironment. This article reviews recent research in the field of cancer metabolism, raising the following questions: Why do cancer cells shift their metabolism in this way? Are the changes in metabolism in cancer cells a consequence of the changes in proliferation or a driver of cancer progression? Can cancer metabolism be targeted to benefit patients? © 2014 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.


Siegel M.,University of Tubingen | Siegel M.,Massachusetts Institute of Technology | Buschman T.J.,Massachusetts Institute of Technology | Buschman T.J.,Princeton University | Miller E.K.,Massachusetts Institute of Technology
Science | Year: 2015

During flexible behavior, multiple brain regions encode sensory inputs, the current task, and choices. It remains unclear how these signals evolve. We simultaneously recorded neuronal activity from six cortical regions [middle temporal area (MT), visual area four (V4), inferior temporal cortex (IT), lateral intraparietal area (LIP), prefrontal cortex (PFC), and frontal eye fields (FEF)] of monkeys reporting the color or motion of stimuli. After a transient bottom-up sweep, there was a top-down flow of sustained task information from frontoparietal to visual cortex. Sensory information flowed from visual to parietal and prefrontal cortex. Choice signals developed simultaneously in frontoparietal regions and travelled to FEF and sensory cortex. This suggests that flexible sensorimotor choices emerge in a frontoparietal network from the integration of opposite flows of sensory and task information. © 2015, American Association for the Advancement of Science. All rights reserved.


Drozdov I.K.,Princeton University
Nature Physics | Year: 2014

The hallmark of a topologically insulating state of matter in two dimensions protected by time-reversal symmetry is the existence of chiral edge modes propagating along the perimeter of the sample. Among the first systems predicted to be a two-dimensional topological insulator are bilayers of bismuth. Here we report scanning tunnelling microscopy experiments on bulk Bi crystals that show that a subset of the predicted Bi-bilayers' edge states are decoupled from the states of the substrate and provide direct spectroscopic evidence of their one-dimensional nature. Moreover, by visualizing the quantum interference of edge-mode quasi-particles in confined geometries, we demonstrate their remarkable coherent propagation along the edge with scattering properties consistent with strong suppression of backscattering as predicted for the propagating topological edge states.


Cleland K.,Princeton University
Obstetrics and Gynecology | Year: 2010

Objective: To evaluate the existing data to estimate the rate of ectopic pregnancy among emergency contraceptive pill treatment failures. Data sources: Our initial reference list was generated from a 2008 Cochrane review of emergency contraception. In August 2009, we searched Biosys Previews, the Cochrane Database of Systematic Reviews, Medline, Global Health Database, Health Source: Popline, and Wanfang Data (a Chinese database). Methods: This study included data from 136 studies, which followed a defined population of women treated one time with emergency contraceptive pills (either mifepristone or levonorgestrel) and in which the number and location of pregnancies were ascertained. Results: Data from each article were abstracted independently by two reviewers. In the studies of mifepristone, 3 of 494 (0.6%) pregnancies were ectopic; in the levonorgestrel studies, 3 of 307 (1%) were ectopic. Conclusion: The rate of ectopic pregnancy when treatment with emergency contraceptive pills fails does not exceed the rate observed in the general population. Because emergency contraceptive pills are effective in lowering the risk of pregnancy, their use will reduce the chance that an act of intercourse will result in ectopic pregnancy. © 2010 by The American College of Obstetricians and Gynecologists. Published by Lippincott Williams & Wilkins.


Verma N.,Princeton University
IEEE Transactions on Very Large Scale Integration (VLSI) Systems | Year: 2011

Computational requirements in highly energy constrained applications are driving the need for ultra-low-power processors. In such devices SRAMs pose a primary energy limitation. This paper analyzes SRAM energy in practical applications using state-of-the-art power-management techniques. The design targets and array biasing for energy minimization are developed. Compared with generic logic, these are characterized by the important difference that SRAMs generally need to retain data. This restricts the use of power-gating for leakage elimination, and thus this paper considers the application of low-leakage data-retention biasing during the idle-mode. The resulting energy tradeoffs have important distinctions, and these are analyzed in the presence of practical variation levels. © 2010 IEEE.


Ha J.Y.,Princeton University
Nature Structural and Molecular Biology | Year: 2016

The conserved oligomeric Golgi (COG) complex orchestrates vesicular trafficking to and within the Golgi apparatus. Here, we use negative-stain electron microscopy to elucidate the architecture of the hetero-octameric COG complex from Saccharomyces cerevisiae. Intact COG has an intricate shape, with four (or possibly five) flexible legs, that differs strikingly from that of the exocyst complex and appears to be well suited for vesicle capture and fusion. © 2016 Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.


Terashima Y.,Tokyo Institute of Technology | Yamazaki M.,Princeton University
Physical Review Letters | Year: 2012

We show that the smooth geometry of a hyperbolic 3-manifold emerges from a classical spin system defined on a 2D discrete lattice, and moreover, show that the process of this "dimensional oxidation" is equivalent with the dimensional reduction of a supersymmetric gauge theory from 4D to 3D. More concretely, we propose an equality between (1) the 4D superconformal index of a 4D N=1 superconformal quiver gauge theory described by a bipartite graph on T2 and (2) the partition function of a classical integrable spin chain on T2. The 2D spin system is lifted to a hyperbolic 3-manifold after the dimensional reduction and using the Higgs mechanism in the 4D gauge theory. © 2012 American Physical Society.


Papic Z.,Princeton University
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We discuss the orbital effect of a tilted magnetic field on the quantum Hall effect in parabolic quantum wells. Many-body states realized at the fractional 13 and 12 filling of the second electronic subband are studied using finite-size exact diagonalization. In both cases, we obtain the phase diagram consisting of a fractional quantum Hall fluid phase that persists for moderate tilts, and eventually undergoes a direct transition to the stripe phase. It is shown that tilting of the field probes the geometrical degree of freedom of fractional quantum Hall fluids, and can be partly related to the effect of band-mass anisotropy. © 2013 American Physical Society.


Ju Y.,Princeton University | Sun W.,Georgia Institute of Technology
Progress in Energy and Combustion Science | Year: 2015

Plasma assisted combustion is a promising technology to improve engine performance, increase lean burn flame stability, reduce emissions, and enhance low temperature fuel oxidation and processing. Over the last decade, significant progress has been made towards the applications of plasma in engines and the understanding of the fundamental chemistry and dynamic processes in plasma assisted combustion via the synergetic efforts in advanced diagnostics, combustion chemistry, flame theory, and kinetic modeling. New observations of plasma assisted ignition enhancement, ultra-lean combustion, cool flames, flameless combustion, and controllability of plasma discharge have been reported. Advances are made in the understanding of non-thermal and thermal enhancement effects, kinetic pathways of atomic O production, diagnostics of electronically and vibrationally excited species, plasma assisted combustion kinetics of sub-explosion limit ignition, plasma assisted low temperature combustion, flame regime transition of the classical ignition S-curve, dynamics of the minimum ignition energy, and the transport effect by non-equilibrium plasma discharge. These findings and advances have provided new opportunities in the development of efficient plasma discharges for practical applications and predictive, validated kinetic models and modeling tools for plasma assisted combustion at low temperature and high pressure conditions. This article is to provide a comprehensive overview of the progress and the gap in the knowledge of plasma assisted combustion in applications, chemistry, ignition and flame dynamics, experimental methods, diagnostics, kinetic modeling, and discharge control. © 2014 Elsevier Ltd. All rights reserved.


E W.,Princeton University | Vanden-Eijnden E.,Courant Institute of Mathematical Sciences
Annual Review of Physical Chemistry | Year: 2010

Transition-path theory is a theoretical framework for describing rare events in complex systems. It can also be used as a starting point for developing efficient numerical algorithms for analyzing such rare events. Here we review the basic components of transition-path theory and path-finding algorithms. We also discuss connections with the classical transition-state theory. Copyright © 2010 by Annual Reviews. All rights reserved.


Rafikov R.R.,Princeton University
Astrophysical Journal | Year: 2013

We explore properties of circumbinary disks around supermassive black hole (SMBH) binaries in centers of galaxies by reformulating standard viscous disk evolution in terms of the viscous angular momentum flux FJ . If the binary stops gas inflow and opens a cavity in the disk, then the inner disk evolves toward a constant-FJ (rather than a constant ) state. We compute disk properties in different physical regimes relevant for SMBH binaries, focusing on the gas-assisted evolution of systems starting at separations 10-4- 10-2 pc, and find the following. (1) Mass pileup at the inner disk edge caused by the tidal barrier accelerates binary inspiral. (2) Binaries can be forced to merge even by a disk with a mass below that of the secondary. (3) Torque on the binary is set non-locally, at radii far larger than the binary semi-major axis; its magnitude does not reflect disk properties in the vicinity of the binary. (4) Binary inspiral exhibits hysteresis - it depends on the past evolution of the disk. (5) The Eddington limit can be important for circumbinary disks even if they accrete at sub-Eddington rates, but only at late stages of the inspiral. (6) Gas overflow across the orbit of the secondary can be important for low secondary mass, high- systems, but mainly during the inspiral phase dominated by the gravitational wave emission. (7) Circumbinary disks emit more power and have harder spectra than constant disks; their spectra are very sensitive to the amount of overflow across the secondary orbit. © 2013. The American Astronomical Society. All rights reserved.


Cen R.,Princeton University | Zheng Z.,University of Utah
Astrophysical Journal | Year: 2013

We present a new model for the observed Lyα blobs (LABs) within the context of the standard cold dark matter model. In this model, LABs are the most massive halos with the strongest clustering (protoclusters) undergoing extreme starbursts in the high-z universe. Aided by calculations of detailed radiative transfer of Lyα photons through ultrahigh resolution (159 pc), large-scale (≥30 Mpc) adaptive mesh refinement cosmological hydrodynamic simulations with galaxy formation, this model is shown to be able to, for the first time, reproduce simultaneously the global Lyα luminosity function and the luminosity-size relation of the observed LABs. Physically, a combination of dust attenuation of Lyα photons within galaxies, clustering of galaxies, and the complex propagation of Lyα photons through the circumgalactic and intergalactic medium gives rise to the large sizes and the irregular isophotal shapes of LABs that are frequently observed. A generic and unique prediction of this model is that there should be strong far-infrared (FIR) sources within each LAB with the most luminous FIR source likely representing the gravitational center of the protocluster, not necessarily the apparent center of the Lyα emission of the LAB or the most luminous optical source. Upcoming ALMA observations should unambiguously test this prediction. If verified, LABs will provide very valuable laboratories for studying the formation of galaxies in the most overdense regions of the universe at a time when the global star formation was the most vigorous. © 2013. The American Astronomical Society. All rights reserved..


Methylation of cytosine DNA residues is a well-studied epigenetic modification with important roles in formation of heterochromatic regions of the genome, and also in tissue-specific repression of transcription. However, we recently found that the ciliate Oxytricha uses methylcytosine in a novel DNA elimination pathway important for programmed genome restructuring. Remarkably, mounting evidence suggests that methylcytosine can play a dual role in ciliates, repressing gene expression during some life-stages and directing DNA elimination in others. In this essay, I describe these recent advances in the DNA methylation field and discuss whether this unexpected novel role for methylcytosine in DNA elimination might be more widely conserved in eukaryotic biology, particularly in apoptotic pathways. © 2014 WILEY Periodicals, Inc.


Nelson E.,Princeton University
Journal of Physics: Conference Series | Year: 2012

Stochastic mechanics is an interpretation of nonrelativistic quantum mechanics in which the trajectories of the configuration, described as a Markov stochastic process, are regarded as physically real. The natural stochastic generalization of classical variational principles leads to a derivation of the Schrödinger equation. A brief review of the successes and failures of the theory is given, with references.


Maldacena J.M.,Institute for Advanced Study | Pimentel G.L.,Princeton University
Journal of High Energy Physics | Year: 2011

We consider the most general three point function for gravitational waves produced during a period of exactly de Sitter expansion. The de Sitter isometries constrain the possible shapes to only three: two preserving parity and one violating parity. These isometries imply that these correlation functions should be conformal invariant. One of the shapes is produced by the ordinary gravity action. The other shape is produced by a higher derivative correction and could be as large as the gravity contribution. The parity violating shape does not contribute to the bispectrum [1, 2], even though it is present in the wavefunction. We also introduce a spinor helicity formalism to describe de Sitter gravitational waves with circular polarization. These results also apply to correlation functions in Anti-de Sitter space. They also describe the general form of stress tensor correlation functions, in momentum space, in a three dimensional conformal field theory. Here all three shapes can arise, including the parity violating one. © SISSA 2011.


Villarini G.,Princeton University | Krajewski W.F.,University of Iowa
Surveys in Geophysics | Year: 2010

It is well acknowledged that there are large uncertainties associated with radar-based estimates of rainfall. Numerous sources of these errors are due to parameter estimation, the observational system and measurement principles, and not fully understood physical processes. Propagation of these uncertainties through all models for which radar-rainfall are used as input (e. g., hydrologic models) or as initial conditions (e. g., weather forecasting models) is necessary to enhance the understanding and interpretation of the obtained results. The aim of this paper is to provide an extensive literature review of the principal sources of error affecting single polarization radar-based rainfall estimates. These include radar miscalibration, attenuation, ground clutter and anomalous propagation, beam blockage, variability of the Z-R relation, range degradation, vertical variability of the precipitation system, vertical air motion and precipitation drift, and temporal sampling errors. Finally, the authors report some recent results from empirically-based modeling of the total radar-rainfall uncertainties. The bibliography comprises over 200 peer reviewed journal articles. © Springer Science+Business Media B.V. 2009.


Lefevre N.,Princeton University
Energy Policy | Year: 2010

Economic assessments of the welfare effects of energy insecurity are typically uncertain and fail to provide clear guidance to policy makers. As a result, governments have had little analytical support to complement expert judgment in the assessment of energy security. This is likely to be inadequate when considering multiple policy goals, and in particular the intersections between energy security and climate change mitigation policies. This paper presents an alternative approach which focuses on gauging the causes of energy insecurity as a way to assist policy making. The paper focuses on the energy security implications of fossil fuel resource concentration and distinguishes between the price and physical availability components of energy insecurity. It defines two separate indexes: the energy security price index (ESPI), based on the measure of market concentration in competitive fossil fuel markets, and the energy security physical availability index (ESPAI), based on the measure of supply flexibility in regulated markets. The paper illustrates the application of ESPI and ESPAI with two case studies-France and the United Kingdom-looking at the evolution of both indexes to 2030. © 2009 Elsevier Ltd. All rights reserved.


Trussell J.,Princeton University | Trussell J.,University of Hull
Contraception | Year: 2011

This review provides an update of previous estimates of first-year probabilities of contraceptive failure for all methods of contraception available in the United States. Estimates are provided of probabilities of failure during typical use (which includes both incorrect and inconsistent use) and during perfect use (correct and consistent use). The difference between these two probabilities reveals the consequences of imperfect use; it depends both on how unforgiving of imperfect use a method is and on how hard it is to use that method perfectly. These revisions reflect new research on contraceptive failure both during perfect use and during typical use. © 2011 Elsevier Inc. All rights reserved.


Lloyd I.D.,Princeton University | Vecchi G.A.,National Oceanic and Atmospheric Administration
Journal of Climate | Year: 2011

The influence of oceanic changes on tropical cyclone activity is investigated using observational estimates of sea surface temperature (SST), air-sea fluxes, and ocean subsurface thermal structure during the period 1998-2007. SST conditions are examined before, during, and after the passage of tropical cyclones, through Lagrangian composites along cyclone tracks across all ocean basins, with particular focus on the North Atlantic. The influence of translation speed is explored by separating tropical cyclones according to the translation speed divided by the Coriolis parameter. On average for tropical cyclones up to category 2, SST cooling becomes larger as cyclone intensity increases, peaking at 1.8 K in the North Atlantic. Beyond category 2 hurricanes, however, the cooling no longer follows an increasing monotonic relationship with intensity. In the North Atlantic, the cooling for stronger hurricanes decreases, while in other ocean basins the cyclone-induced cooling does not significantly differ from category 2 to category 5 tropical cyclones, with the exception of the South Pacific. Since the SST response is nonmonotonic, with stronger cyclones producing more cooling up to category 2, but producing less or approximately equal cooling for categories 3-5, the observations indicate that oceanic feedbacks can inhibit intensification of cyclones. This result implies that large-scale oceanic conditions are a control on tropical cyclone intensity, since they control oceanic sensitivity to atmospheric forcing. Ocean subsurface thermal data provide additional support for this dependence, showing weaker upper-ocean stratification for stronger tropical cyclones. Intensification is suppressed by strong ocean stratification since it favors large SST cooling, but the ability of tropical cyclones to intensify is less inhibited when stratification is weak and cyclone-induced SST cooling is small. Thus, after accounting for tropical cyclone translation speeds and latitudes, it is argued that reduced cooling under extreme tropical cyclones is the manifestation of the impact of oceanic conditions on the ability of tropical cyclones to intensify. © 2011 American Meteorological Society.


A skillful decadal prediction that foretells varying regional climate conditions over seasonal-interannual to multidecadal time scales is of societal significance. However, predictions initialized from the climate observing system tend to drift away from observed states towards the imperfect model climate due to model biases arising from imperfect model equations, numeric schemes and physical parameterizations, as well as the errors in the values of model parameters. Here I show how to mitigate the model bias through optimizing model parameters using observations so as to constrain the model drift in climate predictions with a simple decadal prediction model. Results show that the coupled state-parameter optimization with observations greatly enhances the predictability of the coupled model. While valid "atmospheric" forecasts are extended by more than 5 times, the decadal predictability of the "deep ocean" is almost doubled. The coherence of optimized model parameters and states is critical to improve the long time scale predictions. Copyright © 2011 by the American Geophysical Union.


Broach J.R.,Princeton University
Genetics | Year: 2012

Availability of key nutrients, such as sugars, amino acids, and nitrogen compounds, dictates the developmental programs and the growth rates of yeast cells. A number of overlapping signaling networks-those centered on Ras/protein kinase A, AMPactivated kinase, and target of rapamycin complex I, for instance-inform cells on nutrient availability and influence the cells' transcriptional, translational, posttranslational, and metabolic profiles as well as their developmental decisions. Here I review our current understanding of the structures of the networks responsible for assessing the quantity and quality of carbon and nitrogen sources. I review how these signaling pathways impinge on transcriptional, metabolic, and developmental programs to optimize survival of cells under different environmental conditions. I highlight the profound knowledge we have gained on the structure of these signaling networks but also emphasize the limits of our current understanding of the dynamics of these signaling networks. Moreover, the conservation of these pathways has allowed us to extrapolate our finding with yeast to address issues of lifespan, cancer metabolism, and growth control in more complex organisms. © 2012 by the Genetics Society of America.


Botstein D.,Princeton University
Molecular Biology of the Cell | Year: 2010

Three articles from the early years of Molecular Biology of the Cell (MBoC) have had remarkably many citations in the literature since their publication ∼10 years ago. As a coauthor of these articles and the former editor of MBoC, I was asked for possible explanations. I believe the answer lies in the unusual nature of these articles: each presents and summarizes gene expression data for nearly every gene in the yeast or human genomes. Continuing interest in the data themselves by cell biologists, rather than results or conclusions drawn by the authors, best accounts for the citation history. The flatness of the numbers of citations over time, the continuing high rate of accesses to individual Web sites set up to allow searching and display of the underlying data, and the large fraction of citations in journals focused on mathematics and computation all support the same conclusion: it's the data. © 2009 by The American Society for Cell Biology.


Little S.C.,Howard Hughes Medical Institute | Gregor T.,Princeton University
Cell | Year: 2013

In the classic picture of morphogen-mediated patterning, cells acquire the correct spatial arrangement of specified fates by reading a precisely distributed gradient of morphogen. Xiong et al. now provide evidence for an alternate strategy - cells of the zebrafish neural tube actively sort to their correct positions following disordered specification by Sonic hedgehog. © 2013 Elsevier Inc.


Mozaffari-Kermani M.,Princeton University | Azarderakhsh R.,University of Waterloo
IEEE Transactions on Industrial Electronics | Year: 2013

Lightweight block ciphers are essential for providing low-cost confidentiality to sensitive constrained applications. Nonetheless, this confidentiality does not guarantee their reliability in the presence of natural and malicious faults. In this paper, fault diagnosis schemes for the lightweight internationally standardized block cipher CLEFIA are proposed. This symmetric-key cipher is compatible with yet lighter in hardware than the Advanced Encryption Standard and enables the implementation of cryptographic functionality with low complexity and power consumption. To the best of the authors' knowledge, there has been no fault diagnosis scheme presented in the literature for the CLEFIA to date. In addition to providing fault diagnosis approaches for the linear blocks in the encryption and the decryption of the CLEFIA, error detection approaches are presented for the nonlinear S-boxes, applicable to their composite-field implementations as well as their lookup table realizations. Through fault-injection simulations, the proposed schemes are benchmarked, and it is shown that they achieve error coverage of close to 100%. Finally, both application-specific integrated circuit and field-programmable gate array implementations of the proposed error detection structures are presented to assess their efficiency and overhead. The proposed fault diagnosis architectures make the implementations of the International Organization for Standardization/International Electrotechnical Commission-standardized CLEFIA more reliable. © 1982-2012 IEEE.


Krishnamurthy V.,University of British Columbia | Vincent Poor H.,Princeton University
IEEE Signal Processing Magazine | Year: 2013

How do local agents and global decision makers interact in statistical signal processing problems where autonomous decisions need to be made? When individual agents possess limited sensing, computation, and communication capabilities, can a network of agents achieve sophisticated global behavior? Social learning and Bayesian games are natural settings for addressing these questions. This article presents an overview, novel insights, and a discussion of social learning and Bayesian games in adaptive sensing problems when agents communicate over a network. Two highly stylized examples that demonstrate to the reader the ubiquitous nature of the models, algorithms, and analysis in statistical signal processing are discussed in tutorial fashion. © 1991-2012 IEEE.


Ludwig D.S.,New Balance Foundation Obesity Prevention Center | Rouse H.L.,University of Arkansas for Medical Sciences | Currie J.,Princeton University
PLoS Medicine | Year: 2013

Background:Excessive pregnancy weight gain is associated with obesity in the offspring, but this relationship may be confounded by genetic and other shared influences. We aimed to examine the association of pregnancy weight gain with body mass index (BMI) in the offspring, using a within-family design to minimize confounding.Methods and Findings:In this population-based cohort study, we matched records of all live births in Arkansas with state-mandated data on childhood BMI collected in public schools (from August 18, 2003 to June 2, 2011). The cohort included 42,133 women who had more than one singleton pregnancy and their 91,045 offspring. We examined how differences in weight gain that occurred during two or more pregnancies for each woman predicted her children's BMI and odds ratio (OR) of being overweight or obese (BMI≥85th percentile) at a mean age of 11.9 years, using a within-family design. For every additional kg of pregnancy weight gain, childhood BMI increased by 0.0220 (95% CI 0.0134-0.0306, p<0.0001) and the OR of overweight/obesity increased by 1.007 (CI 1.003-1.012, p = 0.0008). Variations in pregnancy weight gain accounted for a 0.43 kg/m2 difference in childhood BMI. After adjustment for birth weight, the association of pregnancy weight gain with childhood BMI was attenuated but remained statistically significant (0.0143 kg/m2 per kg of pregnancy weight gain, CI 0.0057-0.0229, p = 0.0007).Conclusions:High pregnancy weight gain is associated with increased body weight of the offspring in childhood, and this effect is only partially mediated through higher birth weight. Translation of these findings to public health obesity prevention requires additional study.Please see later in the article for the Editors' Summary. © 2013 Ludwig et al.


Tavoni M.,Fondazione Eni Enrico Mattei | Socolow R.,Princeton University
Climatic Change | Year: 2013

This article introduces the Climatic Change special issue dedicated to negative emissions technologies, also known as carbon dioxide removal (CDR) from the atmosphere. CDR is the only class of mitigation options able to reduce the carbon stock in the atmosphere significantly. In this special issue CDR is explored from the perspectives of integrated assessment, technology optimization, environmental science, and political science. © 2013 The Author(s).


We present a porous medium approach to representing topography, and a new algorithm for the objective interpolation of topography, for use in ocean circulation models of fixed resolution. The representation and algorithm makes use of two concepts; impermeable thin walls and porous barriers. Impermeable thin walls allow the representation of knife-edge sub-grid-scale barriers that block lateral flow between model grid cells. Porous barriers permit the sub-grid scale geometry to modulate lateral transport as a function of elevation. We find that the porous representation and the resulting interpolated topography retains key features, such as overflow sill depths, without compromising other dynamically relevant aspects, such as mean ocean depth for a cell. The accurate representation of the ocean depth is illustrated in a simple model of a tsunami that has a cross-basin travel time very much less dependent on horizontal resolution than when using conventional topographic interpolation and representation. © 2013 Elsevier Ltd.


Gubser S.S.,Princeton University
Physical Review C - Nuclear Physics | Year: 2013

Through a complex shift of the time coordinate, a modification of Bjorken flow is introduced which interpolates between a glasma-like stress tensor at forward rapidities and Bjorken-like hydrodynamics around mid-rapidity. A Landau-like full-stopping regime is found at early times and rapidities not too large. Approximate agreement with BRAHMS data on the rapidity distribution of produced particles at top Relativistic Heavy Ion Collider (RHIC) energies can be achieved if the complex shift of the time coordinate is comparable to the inverse of the saturation scale. The form of the stress tensor follows essentially from symmetry considerations, and it can be expressed in closed form. © 2013 American Physical Society.


Starikovskiy A.,Princeton University | Aleksandrov N.,Moscow Institute of Physics and Technology
Progress in Energy and Combustion Science | Year: 2013

The use of a thermal equilibrium plasma for combustion control dates back more than a hundred years to the advent of internal combustion (IC) engines and spark ignition systems. The same principles are still applied today to achieve high efficiency in various applications. Recently, the potential use of nonequilibrium plasma for ignition and combustion control has garnered increasing interest due to the possibility of plasma-assisted approaches for ignition and flame stabilization. During the past decade, significant progress has been made toward understanding the mechanisms of plasma-chemistry interactions, energy redistribution and the nonequilibrium initiation of combustion. In addition, a wide variety of fuels have been examined using various types of discharge plasmas. Plasma application has been shown to provide additional combustion control, which is necessary for ultra-lean flames, high-speed flows, cold low-pressure conditions of high-altitude gas turbine engine (GTE) relight, detonation initiation in pulsed detonation engines (PDE) and distributed ignition control in homogeneous charge-compression ignition (HCCI) engines, among others. The present paper describes the current understanding of the nonequilibrium excitation of combustible mixtures by electrical discharges and plasma-assisted ignition and combustion. © 2012 Elsevier Ltd. All rights reserved.


Maldacena J.,Institute for Advanced Study | Zhiboedov A.,Princeton University
Classical and Quantum Gravity | Year: 2013

We consider three-dimensional conformal field theories that have a higher spin symmetry that is slightly broken. The theories have a large-N limit in the sense that the operators separate into single trace and multitrace and obey the usual large-N factorization properties. We assume that the spectrum of single trace operators is similar to the one that one obtains in the Vasiliev theories. Namely the only single trace operators are the higher spin currents plus an additional scalar. The anomalous dimensions of the higher spin currents are of the order 1/N. Using the slightly broken higher spin symmetry, we constrain the three-point functions of the theories to a leading order in N. We show that there are two families of solutions. One family can be realized as a theory of N fermions with an O(N) Chern-Simons gauge field, the other as an N bosons plus the Chern-Simons gauge field. The family of solutions is parametrized by the 't Hooft coupling. At special parity preserving points, we obtain the critical O(N) models: the Wilson-Fisher one and the Gross-Neveu one. Our analysis also fixes the on-shell three-point functions of Vasiliev's theory on AdS4 or dS4. © 2013 IOP Publishing Ltd.


Seung H.S.,Princeton University | Sumbul U.,Columbia University
Neuron | Year: 2014

We describe recent progress toward defining neuronal cell types in the mouse retina and attempt to extract lessons that may be generally useful in the mammalian brain. Achieving a comprehensive catalog of retinal cell types now appears within reach, because researchers have achieved consensus concerning two fundamental challenges. The first is accuracy-defining pure cell types rather than settling for neuronal classes that are mixtures of types. The second is completeness-developing methods guaranteed to eventually identify all cell types, as well as criteria for determining when all types have been found. Case studies illustrate how these two challenges are handled by combining state-of-the-art molecular, anatomical, and physiological techniques. Progress is also being made in observing and modeling connectivity between cell types. Scaling up to larger brain regions, such as the cortex, will require not only technical advances but also careful consideration of the challenges of accuracy and completeness. © 2014 Elsevier Inc.


Appel A.W.,Princeton University
ACM Transactions on Information and System Security | Year: 2011

Tamper-evident seals are used by many states' election officials on voting machines and ballot boxes, either to protect the computer and software from fraudulent modification or to protect paper ballots from fraudulent substitution or stuffing. Physical tamper-indicating seals can usually be easily defeated, given they way they are typically made and used; and the effectiveness of seals depends on the protocol for their application and inspection. The legitimacy of our elections may therefore depend on whether a particular state's use of seals is effective to prevent, deter, or detect election fraud. This paper is a case study of the use of seals on voting machines by the State of New Jersey. I conclude that New Jersey's protocols for the use of tamper-evident seals have been not at all effective. I conclude with a discussion of the more general problem of seals in democratic elections. © 2011 ACM.


Prevost J.H.,Princeton University
International Journal for Numerical Methods in Engineering | Year: 2014

SUMMARY: Procedures to couple reservoir and geomechanical models are reviewed. The focus is on immiscible compressible non-compositional reservoir-geomechanical models. Such models require the solution to: coupled stress, pressure, saturation and temperature equations. Although the couplings between saturation and temperature with stress and fluid pressure are 'weak' and can be adequately captured thru staggered (fixed point) iterations, the couplings between stress and pressure are 'strong' and require special procedures for accurate integration. As shown and discussed in detail in our previous works, two-way coupling (i.e., simultaneous integration) of pressure and stress equations is required if poromechanical effects are to be captured accurately. In our previous work, a Galerkin implementation of both pressure and stress equations was used with equal order interpolants.However, most (if not all) reservoir simulators use a finite volume implementation of the pressure equation. Therefore, there remain important unanswered questions related to the interface between a Galerkin vertex-centered geomechanical model with a reservoir finite volume model as such an implementation has never been attempted before. We address those issues in the following by studying the interface with both a cell-centered and a vertex-centered finite volume implementation of the pressure equation. Central to the success of the implementation is the computation of the Jacobian matrix. The elemental contribution to the coupling Jacobian matrix is computed through numerical finite differencing of the residuals. The procedure is detailed herein. In the following, in order to attempt to clear confusion, the simplest case of an isothermal fully saturated, slightly compressible system is presented in detail, and the various solution strategies, simplifications and shortcomings are identified. © 2014 John Wiley & Sons, Ltd.


Fortune H.T.,University of Pennsylvania | Sherr R.,Princeton University
Physical Review C - Nuclear Physics | Year: 2012

Simple wave functions for 11 ,12Be have been around for a long time. They have been tested against many independent processes involving (and properties of) these nuclei. All are consistent, except 11Be(d,p), where the discrepancy is a 4.7σ effect for the 2 + state and 15σ for the ground state. Here, we propose a resolution to this dilemma. © 2012 American Physical Society.


Vinyard D.J.,Rutgers University | Vinyard D.J.,Princeton University | Ananyev G.M.,Rutgers University | Charles Dismukes G.,Rutgers University
Annual Review of Biochemistry | Year: 2013

Photosystem II (PSII) uses light energy to split water into chemical products that power the planet. The stripped protons contribute to a membrane electrochemical potential before combining with the stripped electrons to make chemical bonds and releasing O2 for powering respiratory metabolisms. In this review, we provide an overview of the kinetics and thermodynamics of water oxidation that highlights the conserved performance of PSIIs across species. We discuss recent advances in our understanding of the site of water oxidation based upon the improved (1.9-Å resolution) atomic structure of the Mn4CaO5 water-oxidizing complex (WOC) within cyanobacterial PSII. We combine these insights with recent knowledge gained from studies of the biogenesis and assembly of the WOC (called photoassembly) to arrive at a proposed chemical mechanism for water oxidation. © 2013 by Annual Reviews. All rights reserved.


Leslie S.-J.,Princeton University | Gelman S.A.,University of Michigan
Cognitive Psychology | Year: 2012

Generics are sentences such as " ravens are black" and " tigers are striped" , which express generalizations concerning kinds. Quantified statements such as " all tigers are striped" or " most ravens are black" also express generalizations, but unlike generics, they specify how many members of the kind have the property in question. Recently, some theorists have proposed that generics express cognitively fundamental/default generalizations, and that quantified statements in contrast express cognitively more sophisticated generalizations (Gelman, 2010; Leslie, 2008). If this hypothesis is correct, then quantified statements may be remembered as generics. This paper presents four studies with 136 preschool children and 118 adults, demonstrating that adults and preschoolers alike tend to recall quantified statements as generics, thus supporting the hypothesis that generics express cognitively default generalizations. © 2011 Elsevier Inc.


Hirschberger M.,Princeton University
Nature Materials | Year: 2016

The Dirac and Weyl semimetals are unusual materials in which the nodes of the bulk states are protected against gap formation by crystalline symmetry. The chiral anomaly, predicted to occur in both systems, was recently observed as a negative longitudinal magnetoresistance (LMR) in Na3Bi (ref. ) and in TaAs (ref. ). An important issue is whether Weyl physics appears in a broader class of materials. We report evidence for the chiral anomaly in the half-Heusler GdPtBi. In zero field, GdPtBi is a zero-gap semiconductor with quadratic bands. In a magnetic field, the Zeeman energy leads to Weyl nodes. We have observed a large negative LMR with the field-steering properties specific to the chiral anomaly. The chiral anomaly also induces strong suppression of the thermopower. We report a detailed study of the thermoelectric response function αxx of Weyl fermions. The scheme of creating Weyl nodes from quadratic bands suggests that the chiral anomaly may be observable in a broad class of semimetals. © 2016 Nature Publishing Group


Mody A.,Princeton University
Oxford Review of Economic Policy | Year: 2013

To compensate for the inflexibility due to fixed exchange rates, the eurozone needs flexibility through a system of orderly debt restructuring. With virtually no room for macroeconomic manoeuvring since the crisis onset, fiscal austerity has been the main instrument for achieving reduction of public debt levels; but because austerity also weakens growth, public debt ratios have barely budged. Austerity has also implied continued high private debt ratios, and these debt burdens have perpetuated economic stasis. Economic theory, history, and the recent experience all call for a principled debt restructuring mechanism as an integral element of the Eurozone design. Sovereign debt should be recognized as equity (a residual claim on the sovereign), operationalized by the automatic lowering the debt burden upon the breach of contractually specified thresholds. Making debt more equity-like is also the way forward for speedy private deleveraging. This debt-equity swap principle is a needed shock absorber for the future but will also serve as the principle to deal with the overhang of 'legacy' debt. © The Authors 2013. Published by Oxford University Press.


Raghavendra P.,MSR New England | Steurer D.,Princeton University
Proceedings of the Annual ACM Symposium on Theory of Computing | Year: 2010

The edge expansion of a subset of vertices S ⊆ V in a graph G measures the fraction of edges that leave S. In a d-regular graph, the edge expansion/conductance Φ(S) of a subset S ⊆ V is defined as Φ(S) = (|E(S, V\S)|)/(d|S|). Approximating the conductance of small linear sized sets (size δ n) is a natural optimization question that is a variant of the well-studied Sparsest Cut problem. However, there are no known algorithms to even distinguish between almost complete edge expansion (Φ(S) = 1-ε), and close to 0 expansion. In this work, we investigate the connection between Graph Expansion and the Unique Games Conjecture. Specifically, we show the following: We show that a simple decision version of the problem of approximating small set expansion reduces to Unique Games. Thus if approximating edge expansion of small sets is hard, then Unique Games is hard. Alternatively, a refutation of the UGC will yield better algorithms to approximate edge expansion in graphs. This is the first non-trivial "reverse" reduction from a natural optimization problem to Unique Games. Under a slightly stronger UGC that assumes mild expansion of small sets, we show that it is UG-hard to approximate small set expansion. On instances with sufficiently good expansion of small sets, we show that Unique Games is easy by extending the techniques of [4]. Copyright 2010 ACM.


Rubin A.M.,Princeton University
Geochemistry, Geophysics, Geosystems | Year: 2011

A striking observation from both Cascadia and Japan is that the tremor associated with slow slip often migrates along strike at speeds close to 10 km/d but updip and downdip at speeds approaching 100 km/h. In this paper I adopt the view that the friction law appropriate for these regions is unknown, and I ask what constraints the observed behavior places on the friction law that must be operating. A simple relation, relying only on kinematics and elasticity, states that for a moving front the ratio of propagation speed to slip speed equals the ratio of the elastic shear modulus to the peak-to-residual stress drop at the front. Thus, larger propagation speeds require some combination of larger slip speeds and smaller peak-to-residual stress drops. As a proof of concept I design a two-state-variable friction law in which the strength drop associated with the main laterally propagating front, moving into a region that has not slipped since the last slow event, is much larger than that at the secondary fronts developing on the active slip surface. Preliminary numerical simulations demonstrate that this law can generate secondary fronts that propagate updip and downdip more than 2 orders of magnitude faster than, and even at some distance from, the main lateral front, as has been observed in Cascadia and Japan. Copyright 2011 by the American Geophysical Union.


Huang X.,Princeton University | Cumming A.,McGill University
Astrophysical Journal | Year: 2012

We present models of ohmic heating in the interiors of hot Jupiters in which we decouple the interior and the wind zone by replacing the wind zone with a boundary temperature Tiso and magnetic field Bφ0. Ohmic heating influences the contraction of gas giants in two ways: by direct heating within the convection zone and by heating outside the convection zone, which increases the effective insulation of the interior. We calculate these effects and show that internal ohmic heating is only able to slow the contraction rate of a cooling gas giant once the planet reaches a critical value of internal entropy. We determine the age of the gas giant when ohmic heating becomes important as a function of mass, Tiso, and induced B φ0. With this survey of parameter space complete, we then adopt the wind zone scalings of Menou and calculate the expected evolution of gas giants with different levels of irradiation. We find that, with this prescription of magnetic drag, it is difficult to inflate massive planets or those with strong irradiation using ohmic heating, meaning that we are unable to account for many of the observed hot Jupiter radii. This is in contrast to previous evolutionary models that assumed that a constant fraction of the irradiation is transformed into ohmic power. © 2012. The American Astronomical Society. All rights reserved..


Mezei M.,Princeton University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

I study the entanglement entropy (EE) across a deformed sphere in conformal field theories (CFTs). I show that the sphere (locally) minimizes the universal term in EE among all shapes. In the work of Allais and Mezei [Phys. Rev. D 91, 046002 (2015)] it was derived that the sphere is a local extremum, by showing that the contribution linear in the deformation parameter is absent. In this paper I demonstrate that the quadratic contribution is positive and is controlled by the coefficient of the stress tensor two-point function, CT. Such a minimization result contextualizes the fruitful relation between the EE of a sphere and the number of degrees of freedom in field theory. I work with CFTs with gravitational duals, where all higher curvature couplings are turned on. These couplings parametrize conformal structures in stress tensor n-point functions; hence I show the result for infinitely many CFT examples. © 2015 American Physical Society.


Carlotti A.,Princeton University
Astronomy and Astrophysics | Year: 2013

Context. Phase-mask coronagraphs can be seen as linear systems that spatially redistribute in the pupil plane the energy collected by the telescope. Most of the on-axis light must ideally be rejected outside the aperture so as to be blocked with a Lyot stop, while almost all of the off-axis light must go through it. The unobstructed circular apertures of off-axis telescopes make this possible but all of the major telescopes are on-axis, however, and the performance of these coronagraphs is dramatically reduced by their central obstructions. Aims. Their performance can be restored by using an additional optimally designed apodizer that changes the amplitude in the first pupil plane so that the on-axis light is rejected outside the obstructed aperture of the on-axis telescope. Methods. An apodizer is assumed to be located in a pupil plane, a phase mask in a subsequent image plane, and a Lyot stop in a reimaged pupil plane. The numerical optimization model is built by maximizing the apodizer's transmission while setting constraints on the extremum values of the electric field that the Lyot stop does not block. The coronagraphic image is compared to what a non-apodized phase mask coronagraph provides and an analysis is made of the trade-offs that exist between the transmission of the apodizer and the properties of the Lyot stop. Results. The existence of a solution and the transmission of the mask depend on the geometries of the aperture and of the Lyot stop, and on the constraints that are set on the on-axis attenuation. The system's throughput is a concave function of the Lyot stop transmission. In the case of a VLT-like aperture, optimal apodizers with a transmission of 16% to 92% associated with a four-quadrant phase mask provide contrast as low as a few 10-10 at 1 λ/D from the star. The system's maximum throughput is about 64% for an apodizer with an 88% transmission and a Lyot stop with a 69% transmission. It is shown that optimizing apodizers for a vortex phase mask requires computation times much longer than in the previous case, and no result is presented for this mask. Conclusions. It is demonstrated that apodizers can be successfully optimized to allow phase mask coronagraphs to be used with the full aperture of on-axis telescopes while delivering contrast as low as, or even lower than what they could provide by themselves with off-axis telescopes. © 2013 ESO.


Fernandez R.,Institute for Advanced Study | Metzger B.D.,Princeton University
Astrophysical Journal | Year: 2013

We explore the evolution of radiatively inefficient accretion disks in which nuclear reactions are dynamically important ("Nuclear Dominated Accretion Flows" or NuDAFs). Examples of such disks are those generated by the merger of a white dwarf with a neutron star or black hole, or by the collapse of a rotating star. Here, we present two-dimensional hydrodynamic simulations that systematically explore the effect of adding a single nuclear reaction to a viscous torus. The equation of state, anomalous shear stress, and nuclear reactions are given parametric forms. Our results point to the existence of two qualitatively different regimes of NuDAF evolution: (1) steady accretion with quiescent burning or (2) detonation of the disk. These outcomes are controlled primarily by the ratio ε of the nuclear energy released to the enthalpy at the burning radius. Disks detonate if Ψ exceeds a critical value Ψcrit ∼ 1, and if burning occurs in regions where neutrino cooling is unimportant. Thermonuclear runaways are seeded by the turbulent mixing of hot ash with cold fuel at the burning front. Disks with Ψ < Ψcrit do not explode, but instead power a persistent collimated outflow of unbound material composed primarily of ash, with a mass-loss rate that increases with Ψ. We discuss the implications of our results for supernova-like counterparts from astrophysical events in the NuDAF regime. In particular, detonations following a white dwarf-neutron star merger could account for some subluminous Type Ia supernovae, such as the class defined by SN 2002cx. © 2013. The American Astronomical Society. All rights reserved..


Jenkins E.B.,Princeton University
Astrophysical Journal | Year: 2013

When the neutral interstellar medium is exposed to extreme-ultraviolet and soft X-ray radiation, the argon atoms in it are far more susceptible to being ionized than the hydrogen atoms. We make use of this fact to determine the level of ionization in the nearby warm neutral medium. By analyzing Far-Ultraviolet Spectroscopic Explorer observations of ultraviolet spectra of 44 hot subdwarf stars a few hundred parsecs away from the Sun, we can compare column densities of Ar I to those of O I, where the relative ionization of oxygen can be used as a proxy for that of hydrogen. The measured deficiency dex below the expectation for a fully neutral medium implies that the electron density n(e) 0.04 cm -3 if n(H) = 0.5 cm-3. This amount of ionization is considerably larger than what we expect from primary photoionizations resulting from cosmic rays, the diffuse X-ray background, and X-ray emitting sources within the medium, along with the additional ionizations caused by energetic secondary photoelectrons, Auger electrons, and photons from helium recombinations. We favor an explanation that bursts of radiation created by previous, nearby supernova remnants that have faded by now may have elevated the ionization, and the gas has not yet recombined to a quiescent level. A different alternative is that the low-energy portion of the soft X-ray background is poorly shielded by the H I because it is frothy and has internal pockets of very hot, X-ray emitting gases. © 2013. The American Astronomical Society. All rights reserved.


Waggoner M.R.,Princeton University
Social Science and Medicine | Year: 2013

As medical reports over the last decade indicate that food allergies among children are on the rise, peanut allergies in particular have become a topic of intense social debate. While peanut allergies are potentially fatal, they affect very few children at the population level. Yet, peanut allergies are characterized in medical and popular literature as a rising "epidemic," and myriad and broad-based social responses have emerged to address peanut allergy risk in public spaces. This analysis compares medical literature to other textual sources, including media reports, legislation, and advocacy between 1980 and 2010 in order to examine how peanut allergies transformed from a rare medical malady into a contemporary public health problem. I argue that the peanut allergy epidemic was co-constructed through interactions between experts, publics, biomedical categories, and institutions, while social reactions to the putative epidemic expanded the sphere of surveillance and awareness of peanut allergy risk. The characterization of the peanut allergy problem as an epidemic was shaped by mobility across social sites, with both discursive and material effects. © 2013 Elsevier Ltd.


Food intake is mediated, in part, through brain pathways for motivation and reinforcement. Dysregulation of these pathways may underlay some of the behaviors exhibited by patients with eating disorders. Research using animal models of eating disorders has greatly contributed to the detailed study of potential brain mechanisms that many underlie the causes or consequences of aberrant eating behaviors. This review focuses on neurochemical evidence of reward-related brain dysfunctions obtained through animal models of binge eating, bulimia nervosa, or anorexia nervosa. The findings suggest that alterations in dopamine (DA), acetylcholine (ACh) and opioid systems in reward-related brain areas occur in response to binge eating of palatable foods. Moreover, animal models of bulimia nervosa suggest that while bingeing on palatable food releases DA, purging attenuates the release of ACh that might otherwise signal satiety. Animal models of anorexia nervosa suggest that restricted access to food enhances the reinforcing effects of DA when the animal does eat. The activity-based anorexia model suggests alterations in mesolimbic DA and serotonin occur as a result of restricted eating coupled with excessive wheel running. These findings with animal models complement data obtained through neuroimaging and pharmacotherapy studies of clinical populations. Information on the neurochemical consequences of the behaviors associated with these eating disorders will be useful in understanding these complex disorders and may inform future therapeutic approaches, as discussed here. This article is part of a Special Issue entitled 'Central Control of Food Intake'. © 2012 Elsevier Ltd. All rights reserved.


Dong S.,Institute for Advanced Study | Dong S.,Peking University | Zhu Z.,Princeton University
Astrophysical Journal | Year: 2013

We infer the period (P) and size (Rp ) distribution of Kepler transiting planet candidates with Rp ≥ 1 R ⊕ and P < 250 days hosted by solar-type stars. The planet detection efficiency is computed by using measured noise and the observed time spans of the light curves for ∼120,000 Kepler target stars. We focus on deriving the shape of planet periods and radius distribution functions. We find that for orbital periods P > 10 days, the planet frequency dNp /dlog P for "Neptune-size" planets (Rp = 4-8 R ⊕) increases with period as P 0.7 ± 0.1. In contrast, dN p /dlog P for "super-Earth-size" (2-4 R ⊕) as well as "Earth-size" (1-2 R ⊕) planets are consistent with a nearly flat distribution as a function of period (P 0.11 ± 0.05 and P -0.10 ± 0.12, respectively), and the normalizations are remarkably similar (within a factor of ∼1.5 at 50 days). Planet size distribution evolves with period, and generally the relative fractions for big planets (∼3-10 R ⊕) increase with period. The shape of the distribution function is not sensitive to changes in the selection criteria of the sample. The implied nearly flat or rising planet frequency at long periods appears to be in disagreement with the sharp decline at ∼100 days in planet frequency for low-mass planets (planet mass mp < 30 M ⊕) recently suggested by the HARPS survey. Within 250 days, the cumulative frequencies for Earth-size and super-Earth-size planets are remarkably similar (∼28% and 25%), while Neptune-size and Jupiter-size planets are ∼7% and ∼3%, respectively. A major potential uncertainty arises from the unphysical impact parameter distribution of the candidates. © 2013. The American Astronomical Society. All rights reserved.


McKay D.J.,University of NorthCarolina | Lieb J.D.,University of NorthCarolina | Lieb J.D.,Princeton University
Developmental Cell | Year: 2013

Animals have body parts made of similar cell types located at different axial positions, such as limbs. The identity and distinct morphology of each structure is often specified by the activity of different "master regulator" transcription factors. Although similarities in gene expression have been observed between body parts made of similar cell types, how regulatory information in the genome is differentially utilized to create morphologically diverse structures in development is not known. Here, we use genome-wide open chromatin profiling to show that among the Drosophila appendages, the same DNA regulatory modules are accessible throughout the genome ata given stage of development, except at the loci encoding the master regulators themselves. In addition, open chromatin profiles change over developmental time, and these changes are coordinated between different appendages. We propose that master regulators create morphologically distinct structures by differentially influencing the function of the same set of DNA regulatory modules. © 2013 Elsevier Inc.


Reines A.E.,U.S. National Radio Astronomy Observatory | Greene J.E.,Princeton University | Geha M.,Yale University
Astrophysical Journal | Year: 2013

We present a sample of 151 dwarf galaxies (108.5 ≲ M ⊙z.ast; ≲ 109.5 M ⊙) that exhibit optical spectroscopic signatures of accreting massive black holes (BHs), increasing the number of known active galaxies in this stellar-mass range by more than an order of magnitude. Utilizing data from the Sloan Digital Sky Survey Data Release 8 and stellar masses from the NASA-Sloan Atlas, we have systematically searched for active BHs in ∼25,000 emission-line galaxies with stellar masses comparable to the Magellanic Clouds and redshifts z < 0.055. Using the narrow-line [O III]/Hβ versus [N II]/Hα diagnostic diagram, we find photoionization signatures of BH accretion in 136 galaxies, a small fraction of which also exhibit broad Hα emission. For these broad-line active galactic nucleus (AGN) candidates, we estimate BH masses using standard virial techniques and find a range of 105 ≲ M ⊙BH ≲ 106 M ⊙ and a median of M ⊙BH ∼ 2 × 105 M ⊙. We also detect broad Hα in 15 galaxies that have narrow-line ratios consistent with star-forming galaxies. Follow-up observations are required to determine if these are true type 1 AGN or if the broad Hα is from stellar processes. The median absolute magnitude of the host galaxies in our active sample is Mg = -18.1 mag, which is ∼1-2 mag fainter than previous samples of AGN hosts with low-mass BHs. This work constrains the smallest galaxies that can form a massive BH, with implications for BH feedback in low-mass galaxies and the origin of the first supermassive BH seeds. © 2013. The American Astronomical Society. All rights reserved..


Cen R.,Princeton University
Astrophysical Journal | Year: 2013

Halo gas in low-z (z < 0.5) ≥0.1 L * galaxies in high-resolution, large-scale cosmological hydrodynamic simulations is examined with respect to three components: cold, warm, and hot with temperatures of <105, 105-6, and >106 K, respectively. Utilizing O VI λλ1032, 1038 absorption lines, the warm component is compared to observations, and agreement is found with respect to the galaxy-O VI line correlation, the ratio of the O VI line incidence rate in blue to red galaxies, and the amount of O VI mass in star-forming galaxies. A detailed account of the sources of warm halo gas (stellar feedback heating, gravitational shock heating, and accretion from the intergalactic medium), inflowing and outflowing warm halo gas metallicity disparities, and their dependencies on galaxy types and environment is also presented. With the warm component securely anchored, our simulations make the following additional predictions. First, cold gas is the primary component in inner regions with its mass comprising 50% of all gas within galactocentric radius r = (30, 150) kpc in (red, blue) galaxies. Second, at r > (30, 200) kpc in (red, blue) galaxies the hot component becomes the majority. Third, the warm component is a perpetual minority, with its contribution peaking at ∼30% at r = 100-300 kpc in blue galaxies and never exceeding 5% in red galaxies. The significant amount of cold gas in low-z early-type galaxies, which was found in simulations and in agreement with recent observations (Thom et al.), is intriguing, as is the dominance of hot gas at large radii in blue galaxies. © 2013. The American Astronomical Society. All rights reserved..


Shenhav A.,Princeton University
Nature neuroscience | Year: 2014

Previous theories predict that human dorsal anterior cingulate (dACC) should respond to decision difficulty. An alternative theory has been recently advanced that proposes that dACC evolved to represent the value of 'non-default', foraging behavior, calling into question its role in choice difficulty. However, this new theory does not take into account that choosing whether or not to pursue foraging-like behavior can also be more difficult than simply resorting to a default. The results of two neuroimaging experiments show that dACC is only associated with foraging value when foraging value is confounded with choice difficulty; when the two are dissociated, dACC engagement is only explained by choice difficulty, and not the value of foraging. In addition to refuting this new theory, our studies help to formalize a fundamental connection between choice difficulty and foraging-like decisions, while also prescribing a solution for a common pitfall in studies of reward-based decision making.


Macnamara B.N.,Princeton University | Hambrick D.Z.,Michigan State University | Oswald F.L.,Rice University
Psychological Science | Year: 2014

More than 20 years ago, researchers proposed that individual differences in performance in such domains as music, sports, and games largely reflect individual differences in amount of deliberate practice, which was defined as engagement in structured activities created specifically to improve performance in a domain. This view is a frequent topic of popular-science writing-but is it supported by empirical evidence? To answer this question, we conducted a meta-analysis covering all major domains in which deliberate practice has been investigated. We found that deliberate practice explained 26% of the variance in performance for games, 21% for music, 18% for sports, 4% for education, and less than 1% for professions. We conclude that deliberate practice is important, but not as important as has been argued. © The Author(s) 2014.


Brangwynne C.P.,Princeton University | Tompa P.,Vrije Universiteit Brussel | Tompa P.,Hungarian Academy of Sciences | Pappu R.V.,Washington University in St. Louis
Nature Physics | Year: 2015

Intracellular organelles are either membrane-bound vesicles or membrane-less compartments that are made up of proteins and RNA. These organelles play key biological roles, by compartmentalizing the cell to enable spatiotemporal control of biological reactions. Recent studies suggest that membrane-less intracellular compartments are multicomponent viscous liquid droplets that form via phase separation. Proteins that have an intrinsic tendency for being conformationally heterogeneous seem to be the main drivers of liquid-liquid phase separation in the cell. These findings highlight the relevance of classical concepts from the physics of polymeric phase transitions for understanding the assembly of intracellular membrane-less compartments. However, applying these concepts is challenging, given the heteropolymeric nature of protein sequences, the complex intracellular environment, and non-equilibrium features intrinsic to cells. This provides new opportunities for adapting established theories and for the emergence of new physics. © 2015 Macmillan Publishers Limited. All rights reserved.


Blei D.M.,Princeton University
Communications of the ACM | Year: 2012

Topic models are algorithms for discovering the main themes that pervade a large and otherwise unstructured collection of documents. Topic models can organize the collection according to the discovered themes. Topic modeling algorithms can be applied to massive collections of documents. It can be adapted to many kinds of data. Among other applications, they have been used to find patterns in genetic data, images, and social networks. Topic modeling algorithms do not require any prior annotations or labeling of the documents, the topics emerge from the analysis of the original texts. Latent Dirichlet allocation (LDA) is a topic model of document collections. Topic modeling algorithms generally fall into two categories, sampling-based algorithms and variational algorithms. Sampling-based algorithms attempt to collect samples from the posterior to approximate it with an empirical distribution. Variational methods are a deterministic alternative to sampling-based algorithms.


Wu Y.,University of Pennsylvania | Verdu S.,Princeton University
IEEE Transactions on Information Theory | Year: 2012

Compressed sensing deals with efficient recovery of analog signals from linear encodings. This paper presents a statistical study of compressed sensing by modeling the input signal as an i.i.d. process with known distribution. Three classes of encoders are considered, namely optimal nonlinear, optimal linear, and random linear encoders. Focusing on optimal decoders, we investigate the fundamental tradeoff between measurement rate and reconstruction fidelity gauged by error probability and noise sensitivity in the absence and presence of measurement noise, respectively. The optimal phase-transition threshold is determined as a functional of the input distribution and compared to suboptimal thresholds achieved by popular reconstruction algorithms. In particular, we show that Gaussian sensing matrices incur no penalty on the phase-transition threshold with respect to optimal nonlinear encoding. Our results also provide a rigorous justification of previous results based on replica heuristics in the weak-noise regime. © 1963-2012 IEEE.


Vosk R.,Weizmann Institute of Science | Huse D.A.,Princeton University | Altman E.,Weizmann Institute of Science
Physical Review X | Year: 2015

We formulate a theory of the many-body localization transition based on a novel real-space renormalization group (RG) approach. The results of this theory are corroborated and intuitively explained with a phenomenological effective description of the critical point and of the "badly conducting" state found near the critical point on the delocalized side. The theory leads to the following sharp predictions: (i) The delocalized state established near the transition is a Griffiths phase, which exhibits subdiffusive transport of conserved quantities and sub-ballistic spreading of entanglement. The anomalous diffusion exponent α < 1/2 vanishes continuously at the critical point. The system does thermalize in this Griffiths phase. (ii) The many-body localization transition is controlled by a new kind of infinite-randomness RG fixed point, where the broadly distributed scaling variable is closely related to the eigenstate entanglement entropy. Dynamically, the entanglement grows as ~log t at the critical point, as it does in the localized phase. (iii) In the vicinity of the critical point, the ratio of the entanglement entropy to the thermal entropy and its variance (and, in fact, all moments) are scaling functions of L/ξ, where L is the length of the system and ξ is the correlation length, which has a power-law divergence at the critical point.


Waggoner M.R.,Princeton University
Journal of Health Politics, Policy and Law | Year: 2013

Since the 1980s, maternal and child health experts have sought to redefine maternity care to include the period prior to pregnancy, essentially by expanding the concept of prenatal care to encompass the time before conception. In 2004, the Centers for Disease Control and Prevention endorsed and promoted this new definition when it launched the Preconception Health and Health Care Initiative. In arguing that prenatal care was often too little too late, a group of maternal and child health experts in the United States attempted to spur improvements in population health and address systemic problems in health care access and health disparities. By changing the terms of pregnancy risk and by using maternalism as a social policy strategy, the preconception health and health care paradigm promoted an ethic of anticipatory motherhood and conflated women's health with maternal health, sparking public debate about the potential social and clinical consequences of preconception care. This article tracks the construction of this policy idea and its ultimate potential utility in health and health policy discussions. © 2013 by Duke University Press.


Botstein D.,Princeton University
Cell | Year: 2014

The 2014 Lasker∼Koshland Special Achievement Award will be presented to Mary-Claire King, a pioneer and visionary who revolutionized the use of genetics to identify disease genes, provide insights into human evolution, and champion human rights causes. Copyright © 2014 Elsevier Inc. All rights reserved.


Wagner S.,Princeton University | Bauer S.,Johannes Kepler University
MRS Bulletin | Year: 2012

Electronics can be made on elastically stretchable skin. Such skins conform to irregularly curved surfaces and carry arrays of thin-film devices and integrated circuits. Laypeople and scientists intuitively grasp the concept of electronic skins; material scientists then ask what materials are used? and how does it work? Stretchable circuits are made of diverse materials that span more than 12 orders of magnitude in elastic modulus. We begin with a brief overview of the materials and the architecture of stretchable electronics, then we discuss stretchable substrates, encapsulation, interconnects, and the fabrication of devices and circuits. These components and techniques provide the tools for creating new concepts in biocompatible circuits that conform to and stretch with living tissue. They enable wireless energy transfer via stretchable antennas, stretchable solar cells that convert sunlight to electricity, supercapacitors, and batteries that store energy in stretchable electronic devices. We conclude with a brief outlook on the technical challenges for this revolutionary technology on its road to functional stretchable electronic systems. © 2012 Materials Research Society.


Sherwin B.D.,Princeton University | Zaldarriaga M.,Institute for Advanced Study
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

A shift of the baryon acoustic oscillation (BAO) scale to smaller values than predicted by linear theory was observed in simulations. In this paper, we try to provide an intuitive physical understanding of why this shift occurs, explaining in more pedagogical detail earlier perturbation theory calculations. We find that the shift is mainly due to the following physical effect. A measurement of the BAO scale is more sensitive to regions with long-wavelength overdensities than underdensities, because (due to nonlinear growth and bias) these overdense regions contain larger fluctuations and more tracers and hence contribute more to the total correlation function. In overdense regions the BAO scale shrinks because such regions locally behave as positively curved closed universes, and hence a smaller scale than predicted by linear theory is measured in the total correlation function. Other effects which also contribute to the shift are briefly discussed. We provide approximate analytic expressions for the nonlinear shift including a brief discussion of biased tracers and explain why reconstruction should entirely reverse the shift. Our expressions and findings are in agreement with simulation results, and confirm that nonlinear shifts should not be problematic for next-generation BAO measurements. © 2012 American Physical Society.


Oppenheimer M.,Princeton University
Climatic Change | Year: 2013

The assessment of potential impacts of climate change is progressing from taxonomies and enumeration of the magnitude of potential direct effects on individuals, societies, species, and ecosystems according to a limited number of metrics toward a more integrated approach that also encompasses the vast range of human response to experience and risk. Recent advances are both conceptual and methodological, and include analysis of some consequences of climate change that were heretofore intractable. In this article, I review a selection of these developments and represent them through a handful of illustrative cases. A key characteristic of the emerging areas of interest is a focus on understanding how human responses to direct impacts of climate change may cause important indirect and sometimes distant impacts. This realization underscores the need to develop integrated approaches for assessing and modeling impacts in an evolving socioeconomic and policy context. © 2012 The Author(s).


Lam S.H.,Princeton University
Combustion and Flame | Year: 2013

The model reduction methodology of computational singular perturbation (CSP) is enhanced for chemical kinetics problems. The leading order converged CSP-refined fast basis vectors are obtained analytically by exploiting the special format of the given reaction-specific chemical kinetics data. The leading order net reaction rates of the fast reactions (after they are exhausted) are also obtained analytically. Taking advantage of these leading order analytical results, a new model reduction strategy is advocated. The implementation of this new strategy is very straightforward, and extraction of general insights on the reaction system is also easier. © 2013 The Combustion Institute.


Ferraro S.,Princeton University | Smith K.M.,Perimeter Institute for Theoretical Physics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

Primordial non-Gaussianity of local type is known to produce a scale-dependent contribution to the galaxy bias. Several classes of multifield inflationary models predict non-Gaussian bias which is stochastic, in the sense that dark matter and halos do not trace each other perfectly on large scales. In this work, we forecast the ability of next-generation large-scale structure surveys to constrain common types of primordial non-Gaussianity like fNL, gNL and τNL using halo bias, including stochastic contributions. We provide fitting functions for statistical errors on these parameters which can be used for rapid forecasting or survey optimization. A next-generation survey with volume V=25h-3Gpc3, median redshift z=0.7 and mean bias bg=2.5 can achieve σ(fNL)=6, σ(gNL)=105 and σ(τNL)=103 if no mass information is available. If halo masses are available, we show that optimally weighting the halo field in order to reduce sample variance can achieve σ(fNL)=1.5, σ(gNL)=104 and σ(τNL)=100 if halos with mass down to Mmin=1011h-1M? are resolved, outperforming Planck by a factor of 4 on fNL and nearly an order of magnitude on gNL and τNL. Finally, we study the effect of photometric redshift errors and discuss degeneracies between different non-Gaussian parameters, as well as the impact of marginalizing Gaussian bias and shot noise. © 2015 American Physical Society.


Gould J.L.,Princeton University
Current Biology | Year: 2011

Newly hatched sea turtles exposed to artificially generated magnetic fields with parameters characteristic of two sites 3700 km apart, differing only in longitude, can distinguish the two apparent locations and orient appropriately. © 2011 Elsevier Ltd All rights reserved.


Kapnick S.B.,Princeton University | Delworth T.L.,National Oceanic and Atmospheric Administration
Journal of Climate | Year: 2013

This study assesses the ability of a newly developed high-resolution coupled model from the Geophysical Fluid Dynamics Laboratory to simulate the cold-season hydroclimate in the present climate and examines its response to climate change forcing. Output is assessed from a 280-yr control simulation that is based on 1990 atmospheric composition and an idealized 140-yr future simulation in which atmospheric carbon dioxide increases at 1% yr-1 until doubling in year 70 and then remains constant. When compared with a lowresolution model, the high-resolution model is found to better represent the geographic distribution of snow variables in the present climate. In response to idealized radiative forcing changes, both models produce similar global-scale responses in which global-mean temperature and total precipitation increase while snowfall decreases. Zonally, snowfall tends to decrease in the low to midlatitudes and increase in the mid-to high latitudes. At the regional scale, the high-and low-resolution models sometimes diverge in the sign of projected snowfall changes; the high-resolution model exhibits future increases in a few select high-altitude regions, notably the northwestern Himalaya region and small regions in the Andes and southwestern Yukon, Canada. Despite such local signals, there is an almost universal reduction in snowfall as a percent of total precipitation in both models. By using a simple multivariate model, temperature is shown to drive these trends by decreasing snowfall almost everywhere while precipitation increases snowfall in the high altitudes and midto high latitudes. Mountainous regions of snowfall increases in the high-resolution model exhibit a unique dominance of the positive contribution from precipitation over temperature. © 2013 American Meteorological Society.


Farneti R.,International Center for Theoretical Physics | Vallis G.K.,Princeton University
Journal of Climate | Year: 2013

The variability and compensation of the meridional energy transport in the atmosphere and ocean are examined with the state-of-the-art GFDL Climate Model, version 2.1 (CM2.1), and the GFDL Intermediate Complexity Coupled Model (ICCM). On decadal time scales, a high degree of compensation between the energy transport in the atmosphere (AHT) and ocean (OHT) is found in the North Atlantic. The variability of the total or planetary heat transport (PHT) is much smaller than the variability in either AHT or OHT alone, a feature referred to as "Bjerknes compensation." Natural decadal variability stems from the Atlantic meridional overturning circulation (AMOC), which leadsOHTvariability. The PHT is positively correlated with the OHT, implying that the atmosphere is compensating, but imperfectly, for variations in ocean transport. Because of the fundamental role of the AMOC in generating the decadal OHT anomalies, Bjerknes compensation is expected to be active only in coupled models with a low-frequency AMOC spectral peak. The AHT and the transport in the oceanic gyres are positively correlated because the gyre transport responds to the atmospheric winds, thereby militating against long-term variability involving the wind-driven flow. Moisture and sensible heat transports in the atmosphere are also positively correlated at decadal time scales. The authors further explore the mechanisms and degree of compensation with a simple, diffusive, two-layer energy balance model. Taken together, these results suggest that compensation can be interpreted as arising from the highly efficient nature of the meridional energy transport in the atmosphere responding to ocean variability rather than any a priori need for the top-of-atmosphere radiation budget to be fixed. © 2013 American Meteorological Society.


Bollasina M.A.,Princeton University | Ming Y.,National Oceanic and Atmospheric Administration
Climate Dynamics | Year: 2013

Most of current general circulation models (GCMs) show a remarkable positive precipitation bias over the southwestern equatorial Indian Ocean (SWEIO), which can be thought of as a westward expansion of the simulated IO convergence zone toward the coast of Africa. The bias is common to both coupled and uncoupled models, suggesting that its origin does not stem from the way boundary conditions are specified. The spatio-temporal evolution of the precipitation and associated three-dimensional atmospheric circulation biases is comprehensively characterized by comparing the GFDL AM3 atmospheric model to observations. It is shown that the oceanic bias, which develops in spring and reduces during the monsoon season, is associated to a consistent precipitation and circulation anomalous pattern over the whole Indian region. In the vertical, the areas are linked by an anomalous Hadley-type meridional circulation, whose northern branch subsides over northeastern India significantly affecting the monsoon evolution (e. g., delaying its onset). This study makes the case that the precipitation bias over the SWEIO is forced by the model excess response to the local meridional sea surface temperature (SST) gradient through enhanced near-surface meridional wind convergence. This is suggested by observational evidence and supported by AM3 sensitivity experiments. The latter show that relaxing the magnitude of the meridional SST gradient in the SWEIO can lead to a significant reduction of both local and large-scale precipitation and circulation biases. The ability of local anomalies over the SWEIO to force a large-scale remote response to the north is further supported by numerical experiments with the GFDL spectral dry dynamical core model. By imposing a realistic anomalous heating source over the SWEIO the model is able to reproduce the main dynamical features of the AM3 bias. These results indicate that improved GCM simulations of the South Asian summer monsoon could be achieved by reducing the springtime model bias over the SWEIO. Deficiencies in the atmospheric model, and in particular in the convective parameterization, are suggested to play a key role. Finally, the important mechanism controlling the simulated precipitation distribution over South Asia found here should be considered in the interpretation and attribution of regional precipitation variation under climate change. © 2012 Springer-Verlag.


Yao Y.,Harvard University | Hoffman A.J.,University of Notre Dame | Gmachl C.F.,Princeton University
Nature Photonics | Year: 2012

Mid-infrared quantum cascade lasers are semiconductor injection lasers whose active core implements a multiple-quantum-well structure. Relying on a designed staircase of intersubband transitions allows free choice of emission wavelength and, in contrast with diode lasers, a low transparency point that is similar to a classical, atomic four-level laser system. In recent years, this design flexibility has expanded the achievable wavelength range of quantum cascade lasers to ∼3-25 μm and the terahertz regime, and provided exemplary improvements in overall performance. Quantum cascade lasers are rapidly becoming practical mid-infrared sources for a variety of applications such as trace-chemical sensing, health monitoring and infrared countermeasures. In this Review we focus on the two major areas of recent improvement: power and power efficiency, and spectral performance. © 2012 Macmillan Publishers Limited. All rights reserved.


Peebles P.J.E.,Princeton University
Annual Review of Astronomy and Astrophysics | Year: 2012

The modest science of cosmology I encountered a half century ago has grown into big science. I comment on steps in this development I think I understand because I was there or, in some cases, wish I had been. Wonderful insights-or lucky guesses-and elegant deductions from measurements were accompanied by the usual mix of unlucky guesses and disregard of unwelcome evidence. I say usual because I suspect the course of development of any other natural science is similarly erratic. An example in cosmology is Einstein's homogeneous Universe, which was largely accepted as a working hypothesis when there was scant evidence and seriously challenged after we had a reasonable case for homogeneity. Similar mixes of insight and inattention led to the eventual identification of the 3K microwave background, the demonstration that large-scale structure grew by the gravitational instability of the expanding Universe, and the completion of a tight network of cosmological tests. A half century ago, we had little idea what would become of what we were doing in cosmology. We have a better picture now, but I expect there to be more surprises. Copyright © 2012 by Annual Reviews.


Hardt M.,Princeton University | Talwar K.,Microsoft
Proceedings of the Annual ACM Symposium on Theory of Computing | Year: 2010

We consider the noise complexity of differentially private mechanisms in the setting where the user asks d linear queries f:ℝn → ℝ non-adaptively. Here, the database is represented by a vector in ℝn and proximity between databases is measured in the ℓ1-metric. We show that the noise complexity is determined by two geometric parameters associated with the set of queries. We use this connection to give tight upper and lower bounds on the noise complexity for any d ≤ n. We show that for d random linear queries of sensitivity 1, it is necessary and sufficient to add ℓ2-error Θ(min {d√d/∈, d√(log (n/d))/∈}) to achieve ∈-differential privacy. Assuming the truth of a deep conjecture from convex geometry, known as the Hyperplane conjecture, we can extend our results to arbitrary linear queries giving nearly matching upper and lower bounds. Our bound translates to error O(min {d/∈,√(d log(n/d)/∈}) per answer. The best previous upper bound (Laplacian mechanism) gives a bound of O(min (d/∈,√n/∈)) per answer, while the best known lower bound was Ω(√d/∈). In contrast, our lower bound is strong enough to separate the concept of differential privacy from the notion of approximate differential privacy where an upper bound of O(√d/∈) can be achieved. © 2010 ACM.