Kavli Institute for Cosmology
Kavli Institute for Cosmology
News Article | May 10, 2017
The origins of space and time are among the most mysterious and contentious topics in science. Our February 2017 article “Pop Goes the Universe” argues against the dominant idea that the early cosmos underwent an extremely rapid expansion called inflation. Its authors instead advocate for another scenario—that our universe began not with a bang but with a bounce from a previously contracting cosmos. In the letter below, a group of 33 physicists who study inflationary cosmology respond to that article. It is followed by a reply from the authors (an extended version of their reply can be found here). In “Pop Goes the Universe,” by Anna Ijjas, Paul J. Steinhardt and Abraham Loeb, the authors (hereafter “IS&L”) make the case for a bouncing cosmology, as was proposed by Steinhardt and others in 2001. They close by making the extraordinary claim that inflationary cosmology “cannot be evaluated using the scientific method” and go on to assert that some scientists who accept inflation have proposed “discarding one of [science’s] defining properties: empirical testability,” thereby “promoting the idea of some kind of nonempirical science.” We have no idea what scientists they are referring to. We disagree with a number of statements in their article, but in this letter, we will focus on our categorical disagreement with these statements about the testability of inflation. There is no disputing the fact that inflation has become the dominant paradigm in cosmology. Many scientists from around the world have been hard at work for years investigating models of cosmic inflation and comparing these predictions with empirical observations. According to the high-energy physics database INSPIRE, there are now more than 14,000 papers in the scientific literature, written by over 9,000 distinct scientists, that use the word “inflation” or “inflationary” in their titles or abstracts. By claiming that inflationary cosmology lies outside the scientific method, IS&L are dismissing the research of not only all the authors of this letter but also that of a substantial contingent of the scientific community. Moreover, as the work of several major, international collaborations has made clear, inflation is not only testable, but it has been subjected to a significant number of tests and so far has passed every one. Inflation is not a unique theory but rather a class of models based on similar principles. Of course, nobody believes that all these models are correct, so the relevant question is whether there exists at least one model of inflation that seems well motivated, in terms of the underlying particle physics assumptions, and that correctly describes the measurable properties of our universe. This is very similar to the early steps in the development of the Standard Model of particle physics, when a variety of quantum field theory models were explored in search of one that fit all the experiments. Although there is in principle a wide space of inflationary models to examine, there is a very simple class of inflationary models (technically, “single-field slow-roll” models) that all give very similar predictions for most observable quantities—predictions that were clearly enunciated decades ago. These “standard” inflationary models form a well-defined class that has been studied extensively. (IS&L have expressed strong opinions about what they consider to be the simplest models within this class, but simplicity is subjective, and we see no reason to restrict attention to such a narrow subclass.) Some of the standard inflationary models have now been ruled out by precise empirical data, and this is part of the desirable process of using observation to thin out the set of viable models. But many models in this class continue to be very successful empirically. The standard inflationary models predict that the universe should have a critical mass density (that is, it should be geometrically flat), and they also predict the statistical properties of the faint ripples that we detect in the cosmic microwave background (CMB). First, the ripples should be nearly “scale-invariant,” meaning that they have nearly the same intensity at all angular scales. Second, the ripples should be “adiabatic,” meaning that the perturbations are the same in all components: the ordinary matter, radiation and dark matter all fluctuate together. Third, they should be “Gaussian,” which is a statement about the statistical patterns of relatively bright and dark regions. Fourth and finally, the models also make predictions for the patterns of polarization in the CMB, which can be divided into two classes, called E-modes and B-modes. The predictions for the E-modes are very similar for all standard inflationary models, whereas the levels of B-modes, which are a measure of gravitational radiation in the early universe, vary significantly within the class of standard models. The remarkable fact is that, starting with the results of the Cosmic Background Explorer (COBE) satellite in 1992, numerous experiments have confirmed that these predictions (along with several others too technical to discuss here) accurately describe our universe. The average mass density of the universe has now been measured to an accuracy of about half of a percent, and it agrees perfectly with the prediction of inflation. (When inflation was first proposed, the average mass density was uncertain by at least a factor of three, so this is an impressive success.) The ripples of the CMB have been measured carefully by two more satellite experiments, the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite, as well as many ground- and balloon-based experiments—all confirming that the primordial fluctuations are indeed nearly scale-invariant and very accurately adiabatic and Gaussian, precisely as predicted (ahead of time) by standard models of inflation. The B-modes of polarization have not yet been seen, which is consistent with many, though not all, of the standard models, and the E-modes are found to agree with the predictions. In 2016 the Planck satellite team (a collaboration of about 260 authors) summarized its conclusions by saying that “the Planck results offer powerful evidence in favour of simple inflationary models.” So if inflation is untestable, as IS&L would have us believe, why have there been so many tests of it and with such remarkable success? While the successes of inflationary models are unmistakable, IS&L nonetheless make the claim that inflation is untestable. (We are bewildered by IS&L’s assertion that the dramatic observational successes of inflation should be discounted while they accuse the advocates of inflation of abandoning empirical science!) They contend, for example, that inflation is untestable because its predictions can be changed by varying the shape of the inflationary energy density curve or the initial conditions. But the testability of a theory in no way requires that all its predictions be independent of the choice of parameters. If such parameter independence were required, then we would also have to question the status of the Standard Model, with its empirically determined particle content and 19 or more empirically determined parameters. An important point is that standard inflationary models could have failed any of the empirical tests described above, but they did not. IS&L write about how “a failing theory gets increasingly immunized against experiment by attempts to patch it,” insinuating that this has something to do with inflation. But despite IS&L’s rhetoric, it is standard practice in empirical science to modify a theory as new data come to light, as, for example, the Standard Model has been modified to account for newly discovered quarks and leptons. For inflationary cosmology, meanwhile, there has so far been no need to go beyond the class of standard inflationary models. IS&L also assert that inflation is untestable because it leads to eternal inflation and a multiverse. Yet although the possibility of a multiverse is an active area of study, this possibility in no way interferes with the empirical testability of inflation. If the multiverse picture is valid, then the Standard Model would be properly understood as a description of the physics in our visible universe, and similarly the models of inflation that are being refined by current observations would describe the ways inflation can happen in our particular part of the universe. Both theories would remain squarely within the domain of empirical science. Scientists would still be able to compare newly obtained data—from astrophysical observations and particle physics experiments—with precise, quantitative predictions of specific inflationary and particle physics models. Note that this issue is separate from the loftier goal of developing a theoretical framework that can predict, without the use of observational data, the specific models of particle physics and inflation that should be expected to describe our visible universe. Like any scientific theory, inflation need not address all conceivable questions. Inflationary models, like all scientific theories, rest on a set of assumptions, and to understand those assumptions we might need to appeal to some deeper theory. This, however, does not undermine the success of inflationary models. The situation is similar to the standard hot big bang cosmology: the fact that it left several questions unresolved, such as the near-critical mass density and the origin of structure (which are solved elegantly by inflation), does not undermine its many successful predictions, including its prediction of the relative abundances of light chemical elements. The fact that our knowledge of the universe is still incomplete is absolutely no reason to ignore the impressive empirical success of the standard inflationary models. During the more than 35 years of its existence, inflationary theory has gradually become the main cosmological paradigm describing the early stages of the evolution of the universe and the formation of its large-scale structure. No one claims that inflation has become certain; scientific theories don’t get proved the way mathematical theorems do, but as time passes, the successful ones become better and better established by improved experimental tests and theoretical advances. This has happened with inflation. Progress continues, supported by the enthusiastic efforts of many scientists who have chosen to participate in this vibrant branch of cosmology. Empirical science is alive and well! »Click here to jump to the authors’ reply Alan H. Guth Victor F. Weisskopf Professor of Physics, Massachusetts Institute of Technology http://web.mit.edu/physics/people/faculty/guth_alan.html David I. Kaiser Germeshausen Professor of the History of Science and Professor of Physics, Massachusetts Institute of Technology http://web.mit.edu/physics/people/faculty/kaiser_david.html Andrei D. Linde Harald Trap Friis Professor of Physics, Stanford University https://physics.stanford.edu/people/faculty/andrei-linde Yasunori Nomura Professor of Physics and Director, Berkeley Center for Theoretical Physics, University of California, Berkeley http://physics.berkeley.edu/people/faculty/yasunori-nomura Charles L. Bennett Bloomberg Distinguished Professor and Alumni Centennial Professor of Physics and Astronomy, Johns Hopkins University Principal Investigator, Wilkinson Microwave Anisotropy Probe (WMAP) mission Deputy Principal Investigator and Science Working Group member, Cosmic Background Explorer (COBE) mission http://physics-astronomy.jhu.edu/directory/charles-l-bennett/ J. Richard Bond University Professor, University of Toronto and Director, Canadian Institute for Advanced Research Cosmology and Gravity Program, Canadian Institute for Theoretical Astrophysics Member of the Planck collaboration http://www.cita.utoronto.ca/~bond/ François Bouchet Director of Research, Institut d’Astrophysique de Paris, CNRS and Sorbonne Université-UPMC Deputy Principal Investigator, Planck satellite HFI (High Frequency Instrument) Consortium and Member, Planck Science Team http://savoirs.ens.fr/conferencier.php?id=145 Sean Carroll Research Professor of Physics, California Institute of Technology http://www.astro.caltech.edu/people/faculty/Sean_Carroll.html George Efstathiou Professor of Astrophysics, Kavli Institute for Cosmology, University of Cambridge Member, Planck Science Team http://www.ast.cam.ac.uk/~gpe/ Stephen Hawking Lucasian Professor of Mathematics (Emeritus) and Dennis Stanton Avery and Sally Tsui Wong-Avery Director of Research, Department of Applied Mathematics and Theoretical Physics, University of Cambridge http://www.damtp.cam.ac.uk/people/s.w.hawking/ Renata Kallosh Professor of Physics, Stanford University https://physics.stanford.edu/people/faculty/renata-kallosh Eiichiro Komatsu Director of the Department of Physical Cosmology, Max-Planck-Institute für Astrophysik, Garching Member, Wilkinson Microwave Anisotropy Probe (WMAP) collaboration http://wwwmpa.mpa-garching.mpg.de/~komatsu/ Lawrence Krauss Foundation Professor in the School of Earth and Space Exploration and Department of Physics, and Director, The Origins Project at Arizona State University http://krauss.faculty.asu.edu David H. Lyth Professor of Physics (Emeritus), Lancaster University http://www.lancaster.ac.uk/physics/about-us/people/david-lyth Juan Maldacena Carl P. Feinberg Professor of Physics, Institute for Advanced Study https://www.sns.ias.edu/malda John C. Mather Senior Astrophysicist and Goddard Fellow, NASA Goddard Space Flight Center and recipient of the Nobel Prize in Physics (2006) Project Scientist, Cosmic Background Explorer (COBE) mission and Senior Project Scientist, James Webb Space Telescope https://science.gsfc.nasa.gov/sed/bio/john.c.mather Hiranya Peiris Professor of Astrophysics, University College London and Director, Oskar Klein Centre for Cosmoparticle Physics, Stockholm Member, Wilkinson Microwave Anisotropy Probe (WMAP) collaboration and Planck collaboration http://zuserver2.star.ucl.ac.uk/~hiranya/ Malcolm Perry Professor of Theoretical Physics, University of Cambridge http://www.damtp.cam.ac.uk/people/m.j.perry/ Lisa Randall Frank B. Baird, Jr., Professor of Science, Department of Physics, Harvard University https://www.physics.harvard.edu/people/facpages/randall Martin Rees Astronomer Royal of Great Britain, former President of the Royal Society of London, and Professor (Emeritus) of Cosmology and Astrophysics, University of Cambridge http://www.ast.cam.ac.uk/~mjr/ Misao Sasaki Professor, Yukawa Institute for Theoretical Physics, Kyoto University http://www2.yukawa.kyoto-u.ac.jp/~misao.sasaki/ Leonardo Senatore Associate Professor of Physics, Stanford University https://physics.stanford.edu/people/faculty/leonardo-senatore Eva Silverstein Professor of Physics, Stanford University https://physics.stanford.edu/people/faculty/eva-silverstein George F. Smoot III Professor of Physics (Emeritus), Founding Director, Berkeley Center for Cosmological Physics, and recipient of the Nobel Prize in Physics (2006) Principal Investigator, Cosmic Background Explorer (COBE) mission http://physics.berkeley.edu/people/faculty/george-smoot-iii Alexei Starobinsky Principal Researcher, Landau Institute for Theoretical Physics, Moscow http://www.itp.ac.ru/en/persons/starobinsky-aleksei-aleksandrovich/ Leonard Susskind Felix Bloch Professor of Physics and Wells Family Director, Stanford Institute for Theoretical Physics, Stanford University https://physics.stanford.edu/people/faculty/leonard-susskind Michael S. Turner Bruce. V. Rauner Distinguished Service Professor, Department of Astronomy and Astrophysics and Department of Physics, University of Chicago https://astro.uchicago.edu/people/michael-s-turner.php Alexander Vilenkin L. and J. Bernstein Professor of Evolutionary Science and Director, Institute of Cosmology, Tufts University http://cosmos2.phy.tufts.edu/vilenkin.html Steven Weinberg Jack S. Josey-Welch Foundation Chair and Regental Professor and Director, Theory Research Group, Department of Physics, University of Texas at Austin, and recipient of the Nobel Prize in Physics (1979) https://web2.ph.utexas.edu/~weintech/weinberg.html Rainer Weiss Professor of Physics (Emeritus), Massachusetts Institute of Technology Chair, Science Working Group, Cosmic Background Explorer (COBE) mission Co-Founder, Laser Interferometric Gravitational-wave Observatory (LIGO) http://web.mit.edu/physics/people/faculty/weiss_rainer.html Frank Wilczek Herman Feshbach Professor of Physics, Massachusetts Institute of Technology, and recipient of the Nobel Prize in Physics (2004) http://web.mit.edu/physics/people/faculty/wilczek_frank.html Edward Witten Charles Simonyi Professor of Physics, Institute for Advanced Study and recipient of the Fields Medal (1990) https://www.sns.ias.edu/witten Matias Zaldarriaga Professor of Astrophysics, Institute for Advanced Study https://www.sns.ias.edu/matiasz THE AUTHORS REPLY: We have great respect for the scientists who signed the rebuttal to our article, but we are disappointed by their response, which misses our key point: the differences between the inflationary theory once thought to be possible and the theory as understood today. The claim that inflation has been confirmed refers to the outdated theory before we understood its fundamental problems. We firmly believe that in a healthy scientific community, respectful disagreement is possible and hence reject the suggestion that by pointing out problems, we are discarding the work of all of those who developed the theory of inflation and enabled precise measurements of the universe. Historically, the thinking about inflation was based on a series of misunderstandings. It was not understood that the outcome of inflation is highly sensitive to initial conditions. And it was not understood that inflation generically leads to eternal inflation and, consequently, a multiverse—an infinite diversity of outcomes. Papers claiming that inflation predicts this or that ignore these problems. Our point is that we should be talking about the contemporary version of inflation, warts and all, not some defunct relic. Logically, if the outcome of inflation is highly sensitive to initial conditions that are not yet understood, as the respondents concede, the outcome cannot be determined. And if inflation produces a multiverse in which, to quote a previous statement from one of the responding authors (Guth), “anything that can happen will happen”—it makes no sense whatsoever to talk about predictions. Unlike the Standard Model, even after fixing all the parameters, any inflationary model gives an infinite diversity of outcomes with none preferred over any other. This makes inflation immune from any observational test. For more details, see our 2014 paper “Inflationary Schism” (preprint available at https://arxiv.org/abs/1402.6980). We are three independent thinkers representing different generations of scientists. Our article was not intended to revisit old debates but to discuss the implications of recent observations and to point out unresolved issues that present opportunities for a new generation of young cosmologists to make a lasting impact. We hope readers will go back and review our article’s concluding paragraphs. We advocated against invoking authority and for open recognition of the shortcomings of current concepts, a reinvigorated effort to resolve these problems and an open-minded exploration of diverse ideas that avoid them altogether. We stand by these principles.
News Article | August 9, 2017
Cosmologists have produced the biggest map yet of the Universe’s structure, and found that matter might be spread more evenly than previously thought. The results, part of the ongoing Dark Energy Survey (DES), chart the distribution of matter in part by measuring how mass bends light, an effect known as weak gravitational lensing. It was the first time the technique had been fed enough data to measure some of the crucial features of cosmic evolution with a precision approaching that for the maps generated from cosmic microwave background (CMB) data, which measure the afterglow of the Big Bang and are the gold standard of cosmology. “We believe that, with these results, we’re no longer the poor cousin” to other efforts, says DES leader Joshua Frieman, a cosmologist at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. “We now have results that have comparable power to constrain cosmology.” There are still some discrepancies with earlier surveys on measurements such as the lumpiness of mass, but they are within the experiments’ margins of error. As the DES maps larger volumes of space, it should become clear whether the disagreements are real, says cosmologist Anthony Tyson, a pioneer of weak gravitational lensing at the University of California, Davis. So far, he says, “I believe they have been very careful and conservative in their interpretations”. The DES, a collaboration of more than 400 researchers, gathers its data using the 4-metre Victor M. Blanco telescope, part of the Cerro Tololo Inter-American Observatory in Chile. Data collection began in 2013; the current map is based on the first year of measurements, which logged 26 million galaxies in the southern sky and their apparent shapes. According to Albert Einstein’s general theory of relativity, mass warps space, so a large amount of matter in the foreground of a galaxy can bend its light in a way that makes the galaxy look slightly squashed. This is true whether the foreground mass is made of ordinary matter or of invisible dark matter. Galaxies can appear squashed for other reasons, including their actual shapes and orientations. But if many galaxies in a certain region of the sky seem on average to be skewed along the same direction, gravitational lensing is the probable culprit. The DES cosmologists were able to tease out the composition of the Universe in a similar way to how the CMB surveys have done in the past — most recently using the European Space Agency’s Planck satellite. Their results confirm that ordinary matter constitutes only 4% the Universe’s contents. But they show a slightly smaller amount of dark matter — about 26% — than the 29% estimated by Planck, with the rest being taken up by ‘dark energy’, the stuff thought to be pushing the cosmos apart at an accelerating speed. More intriguingly, the DES seems to have found a deviation from Planck’s prediction of the current lumpiness of matter. Whereas ordinary and dark matter were evenly distributed in the Universe’s infancy 14 billion years ago, that is not the case in present galaxies. Gravity has been pulling the matters together into a web-like structure of clusters and filaments, with enormous voids in-between. The concentration measured by the DES is 7% lower than that predicted by the standard model of cosmology. The gap is not statistically large, at about one standard deviation. But another weak lensing project, the Kilo Degree Survey (KiDS), found the same kind of deviation last year1. If confirmed, the discrepancy could mean that, over cosmic history, mass has been clumping more slowly than expected. And that could potentially reveal new physics, such as unexpected interactions between dark matter and dark energy or new types of neutrino. The DES presented its results on 3 August at a meeting of the American Physical Society at Fermilab, and the authors posted a battery of ten papers online (see go.nature.com/2ubhr8l). Although cosmological observations have been converging towards a consistent, detailed picture in recent decades, the weak-lensing observations are not the only ones still troubling researchers. Astronomers have, for instance, found that the cosmos is expanding faster than predicted on the basis of Planck data. George Efstathiou, director of the Kavli Institute for Cosmology in Cambridge, UK, and a member of both the Planck and DES collaborations, says that the clumping discrepancy is potentially more worrisome than the one relating to cosmic expansion. Overall, researchers are excited to have another tool with which to probe the cosmos in ever-greater detail. “My own view of all of these measurements is that they are stunning tests of the cosmological model, and the precision and accuracy only keep getting better and better,” says astronomer Wendy Freedman of the University of Chicago in Illinois. The final survey, due to conclude in 2018, will cover one-eighth of the sky; the results might be available some time in 2020, Frieman says. Ultimately, the DES aims to map a large-enough region to see how the influence of dark energy has evolved over the Universe’s recent history. “This is exciting,” Tyson says. “The future looks bright for weak gravitational lensing.”
Shaw J.R.,Kavli Institute for Cosmology |
Shaw J.R.,Canadian Institute for Theoretical Astrophysics |
Lewis A.,University of Sussex
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012
Primordial magnetic fields could provide an explanation for the galactic magnetic fields observed today; in which case, they may leave interesting signals in the CMB and the small-scale matter power spectrum. We discuss how to approximately calculate the important nonlinear magnetic effects within the guise of linear perturbation theory and calculate the matter and CMB power spectra including the Sunyaev-Zel'dovich contribution. We then use various cosmological data sets to constrain the form of the magnetic field power spectrum. Using solely large-scale CMB data (WMAP7, QUaD, and ACBAR) we find a 95% C.L. on the variance of the magnetic field at 1 Mpc of B λ<6.4nG. When we include South Pole Telescope data to constrain the Sunyaev-Zel'dovich effect, we find a revised limit of B λ<4.1nG. The addition of Sloan Digital Sky Survey Lyman-α data lowers this limit even further, roughly constraining the magnetic field to B λ<1.3nG. © 2012 American Physical Society.
Shaw J.R.,Kavli Institute for Cosmology |
Lewis A.,Institute of Astronomy
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010
Primordial magnetic fields and massive neutrinos can leave an interesting signal in the CMB temperature and polarization. We perform a systematic analysis of general perturbations in the radiation-dominated universe, accounting for any primordial magnetic field and including leading-order effects of the neutrino mass. We show that massive neutrinos qualitatively change the large-scale perturbations sourced by magnetic fields, but that the effect is much smaller than previously claimed. We calculate the CMB power spectra sourced by inhomogeneous primordial magnetic fields, from before and after neutrino decoupling, including scalar, vector and tensor modes, and consistently modeling the correlation between the density and anisotropic stress sources. In an appendix we present general series solutions for the possible regular primordial perturbations. © 2010 The American Physical Society.
Cantalupo S.,Kavli Institute for Cosmology |
Porciani C.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2011
We present a new three-dimensional radiative transfer (RT) code, radamesh (Radiative-transfer on ADAptive MESH), based on a ray-tracing, photon-conserving and adaptive (in space and time) scheme. radamesh uses a novel Monte Carlo approach to sample the radiation field within the computational domain on a 'cell-by-cell' basis. Thanks to this algorithm, the computational efforts are now focused where actually needed, i.e. within the Ionization-fronts (I-fronts). This results in an increased accuracy level and, at the same time, a huge gain in computational speed with respect to a 'classical' Monte Carlo RT, especially when combined with an Adaptive Mesh Refinement (AMR) scheme. Among several new features, radamesh is able to adaptively refine the computational mesh in correspondence of the I-fronts, allowing to fully resolve them within large, cosmological boxes. We follow the propagation of ionizing radiation from an arbitrary number of sources and from the recombination radiation produced by H and He. The chemical state of six species (Hi, Hii, Hei, Heii, Heiii, e) and gas temperatures are computed with a time-dependent, non-equilibrium chemistry solver. We present several validating tests of the code, including the standard tests from the RT code comparison project and a new set of tests aimed at substantiating the new characteristics of radamesh. Using our AMR scheme, we show that properly resolving the I-front of a bright quasar during reionization produces a large increase of the predicted gas temperature within the whole Hii region. Also, we discuss how H and He recombination radiation is able to substantially change the ionization state of both species (for the classical Strömgren sphere test) with respect to the widely used 'on-the-spot' approximation. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.
News Article | January 19, 2016
KISSIMMEE, Fla. — Many galaxies are LIERS, says Francesco Belfiore, a graduate student at the Kavli Institute for Cosmology at the University of Cambridge. He's not throwing shade at these objects, but rather trying to explain why new stars are no longer born inside them. Earth lies in a galaxy that is flush with new star birth. The Milky Way's spiral shape and blue color are both signs that baby stars are being made inside it. But in elliptical-shaped galaxies with more reddish hues, star birth has stopped, and scientists don't understand why. In trying to study the chemistry of these "dead" galaxies, researchers have found a different chemical fingerprint than the one that dominates star-forming galaxies. To describe what they were seeing, researchers came up with the acronym LINER, which stands for "low-ionization nuclear emission-line region." Belfiore's explanation of the name is more direct: "The reason why we use this acronym is because we don't know what they are," he said. [Gallery: 65 All-Time Great Galaxy Hits] More specifically, scientists don't know what's creating the "LINER" chemical signature in the dead galaxies, and whether it might help explain why they stopped forming stars. Now, new observations by Belfiore and colleagues have added another twist to this LINER mystery: rather than coming from the black hole at the center of the dead galaxy, as researchers previously thought, the signature can be found throughout the galaxies, all the way out to their fringes. According to Belfiore, this new finding that means the "N" in LINER (which stood for "nuclear," referring to the center of the galaxy) should be removed, and these galaxies should be called "LIERs." Belfiore spoke about the new findings on Jan. 8 during a press briefing here at the 227th meeting of the American Astronomical Society. By itself, the new information doesn't answer the big question of why star formation has stopped in those galaxies, but it may resolve a seemingly contradictory observation found in many previous studies: that select patches of living galaxies also exhibit this LINER/LIER chemical fingerprint. In other words, it may help scientists understand what turns living galaxies into LIERs. What does a galactic LIER look like? These "dead" galaxies are different from star-forming galaxies in many ways. They tend to be redder in color, because blue stars have shorter lives than red stars, so for the most part, the blue stars have all died out in the LIER galaxies. The stellar nurseries in star-forming galaxies like the Milky Way emit large amounts of light, and appear as bright, glowing beacons. These are also missing from images of LIER galaxies, which have a more diffuse glow. Dead galaxies are also shaped differently. They're more often elliptical, like an American football, instead of a flat, circular spiral. Without star birth, these galaxies can't develop the massive arms that wrap around the centers of star-forming galaxies like the Milky Way. Once again, scientists don't fully understand why the stop of star birth also means a change in shape for the galaxy, but there is a theory that many elliptical galaxies are created when two or more galaxies collide and merge together. Perhaps that process somehow shuts off star birth, scientists have suggested. These red, dead, football-shaped galaxies contain a cocktail of chemicals that's different from that of their living counterparts. Previous observations of elliptical galaxies have been limited in their resolution, and suggested that the LINER gas signature was coming from the center of these galaxies. This is important because, at the center of most (if not all) large galaxies is a supermassive black hole. According to Belfiore, the leading theory for why dead galaxies have this LIER signature is because of activity near the black hole at the center of the galaxy. This idea is bolstered by a third category of galaxies called active galactic nuclei, or AGNs. The black holes at the center of AGNs are extremely active, meaning they have lots of material falling into them, producing jets of material that spew out into space, and radiating an incredible amount of light. AGNs also have a chemical signature that's different from that of LINER galaxies and star-forming galaxies, Belfiore noted in his talk. Some scientists suspect that LINER galaxies are simply weaker examples of AGNs, where a lower amount of activity around the black hole produces this unique chemical fingerprint, he said. But the new observations presented by Belfiore have allowed scientists to look at the source of the LINER emission at a higher resolution, and revealed that they are not limited to the galaxy's center. Using the Sloan Digital Sky Survey (SDSS), a 2.5-meter (8.2 feet) telescope in New Mexico, as part of a project called MaNGA (Mapping Nearby Galaxies at Apache Point Observatory), Belfiore and colleagues found that, in some cases, LIER signatures can be found emanating from throughout a LINER galaxy, or from separate locations near the outskirts. The new results indicate that the process creating the LINER signature is, most likely, something that can occur throughout the galaxy, Belfiore said. [Gallery: Black Holes of the Universe] Living galaxies can be LIERS, too Belfiore and his collaborators theorize that the LIER chemical fingerprint might be coming from older stars as they reach the twilight hours of their lives. In that stage of life, many stars shed their outer layers, and it may be the chemical signature of this dispelled stellar material that is being detected, Belfiore explained. This would explain why the LIER chemical signature is seen in some galaxies where star formation is still happening, Belfiore told Space.com in an email. In many spiral galaxies, star formation does not shut off suddenly, but gradually. Some spiral galaxies develop regions (often near their centers) where star formation stops, and in those cases, it is possible for scientists to see a LIER emission from the "dead" region of the galaxy. Of course, dying stars are present even where new ones are forming, but the LIER emission is "always fainter than the emission due to star formation," Belfiore told Space.com in an email. Hence, in those living galaxies, the weak LIER emission would be swamped by photons from the star-forming regions and would go undetected by telescopes. In other words, Belfiore said, it's possible that most galaxies are LIERs. The idea that LINER galaxies may actually be LIERs — that this chemical signature is not coming from the galactic center but from another source, such as dying stars throughout the galaxy — has been building for some years, Belfiore told Space.com. "Although most astronomers not directly working on this topic would assume that LINERs are weak active galactic nuclei, a paradigm shift has been happening slowly for several years now," he said. He mentioned that some astronomers remain highly skeptical of the idea, but the new SDSS observations may change that. "Compared to previous studies, the new MaNGA data allows, for the first time, a direct test of the stellar hypothesis for LIER emission in a well-defined large sample of galaxies, covering (crucially) both spirals and ellipticals," Belfiore said. "It is the weight of this very direct evidence and its statistical significance that I feel is most compelling about the SDSS MaNGA result." Belfiore said he and his colleagues are preparing to submit their results for publication. Copyright 2016 SPACE.com, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
Becker G.D.,Kavli Institute for Cosmology |
Hewett P.C.,Kavli Institute for Cosmology |
Worseck G.,University of California at Santa Cruz |
Xavier Prochaska J.,University of California at Santa Cruz
Monthly Notices of the Royal Astronomical Society | Year: 2013
We present new measurements of the mean transmitted flux in the Lya forest over 2 < z < 5 made using 6065 quasar spectra from the Sloan Digital Sky Survey data release 7 (SDSS DR7). We exploit the general lack of evolution in the mean quasar continuum to avoid the bias introduced by continuum fitting over the Lyα forest at high redshifts, which has been the primary systematic uncertainty in previous measurements of the mean Lyα transmission. The individual spectra are first combined into 26 composites with mean redshifts spanning 2.25 ≤ zcomp ≤ 5.08. The flux ratios of separate composites at the same rest wavelength are then used, without continuum fitting, to infer the mean transmitted flux, F(z), as a fraction of its value at z ~ 2. Absolute values for F(z) are found by scaling our relative values to measurements made from high-resolution data by Faucher-Giguére et al. at z < 2.5, where continuum uncertainties are minimal. We find that F(z) evolves smoothly with redshift, with no evidence of a previously reported feature at z ≲ 3.2. This trend is consistent with a gradual evolution of the ionization and thermal state of the intergalactic medium over 2 < z < 5. Our results generally agree with the most careful measurements to date made from high-resolution data, but offer much greater precision and extend to higher redshifts. This work also improves upon previous efforts using SDSS spectra by significantly reducing the level of systematic errors. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
Amin M.A.,Kavli Institute for Cosmology
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013
Oscillons are long-lived, localized, oscillatory scalar field configurations. In this work we derive a condition for the existence of small-amplitude oscillons (and provide solutions) in scalar field theories with noncanonical kinetic terms. While oscillons have been studied extensively in the canonical case, this is the first example of oscillons in scalar field theories with noncanonical kinetic terms. In particular, we demonstrate the existence of oscillons supported solely by the noncanonical kinetic terms, without any need for nonlinear terms in the potential. In the small-amplitude limit, we provide an explicit condition for their stability in d+1 dimensions against long-wavelength perturbations. We show that for d≥3, there exists a long-wavelength instability which can lead to radial collapse of small-amplitude oscillons. © 2013 American Physical Society.
Efstathiou G.,Kavli Institute for Cosmology
Monthly Notices of the Royal Astronomical Society | Year: 2014
I reanalyse the Riess et al. (hereafter R11) Cepheid data using the revised geometric maser distance to NGC 4258 of Humphreys et al. (hereafter H13). I explore different outlier rejection criteria designed to give a reduced X2 of unity and compare the results with the R11 rejection algorithm, which produces a reduced X2 that is substantially less than unity and, in some cases, leads to underestimates of the errors on parameters. I show that there are sub-luminous low-metallicity Cepheids in the R11 sample that skew the global fits of the period-luminosity relation. This has a small but non-negligible impact on the global fits using NGC 4258 as a distance scale anchor, but adds a poorly constrained source of systematic error when using the Large Magellanic Cloud as an anchor. I also show that the small Milky Way Cepheid sample with accurate parallax measurements leads to a distance to NGC 4258 that is in tension with the maser distance. I conclude that H0 based on the NGC 4258 maser distance is H0 = 70.6 ± 3.3 kms-1 Mpc-1, compatible within 1s with the recent determination from Planck for the base six-parameter cold dark matter cosmology. If the H-band period-luminosity relation is assumed to be independent of metallicity and the three distance anchors are combined, I find H0 = 72.5 ± 2.5 kms-1 Mpc-1, which differs by 1.9σ from the Planck value. The differences between the Planck results and these estimates of H0 are not large enough to provide compelling evidence for new physics at this stage. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
Cantalupo S.,Kavli Institute for Cosmology
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010
Current models of galaxy formation lack an efficient and physically constrained mechanism to regulate star formation (SF) in low and intermediate mass galaxies. We argue that the missing ingredient could be the effect of photoionization by local sources on the gas cooling. We show that the soft X-ray and EUV flux generated by SF is able to efficiently remove the main coolants (e.g. He+, O4+ and Fe8+) from the halo gas via direct photoionization. As a consequence, the cooling and accretion time of the gas surrounding star-forming galaxies may increase by one or two orders of magnitude. For a given halo mass and redshift, the effect is directly related to the value of the star formation rate (SFR). Our results suggest that the existence of a critical SFR above which 'cold' mode accretion is stopped, even for haloes with Mvir well below the critical shock-heating mass suggested by previous studies. The evolution of the critical SFR with redshift, for a given halo mass, resembles the respective steep evolution of the observed SFR for z < 1. This suggests that photoionization by local sources would be able to regulate gas accretion and SF, without the need for additional, strong feedback processes. © 2010 The Author. Journal compilation © 2010 RAS.