Racah Institute of Physics

Israel

Racah Institute of Physics

Israel
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Carlesi E.,Racah Institute of Physics | Hoffman Y.,Racah Institute of Physics | Sorce J.G.,Leibniz Institute for Astrophysics Potsdam | Gottlober S.,Leibniz Institute for Astrophysics Potsdam
Monthly Notices of the Royal Astronomical Society | Year: 2017

The mass of the Local Group (LG) is a crucial parameter for galaxy formation theories. However, its observational determination is challenging - its mass budget is dominated by dark matter that cannot be directly observed. To meet this end, the posterior distributions of the LG and its massive constituents have been constructed by means of constrained and random cosmological simulations. Two priors are assumed - the Λ cold dark matter model that is used to set up the simulations, and an LG model that encodes the observational knowledge of the LG and is used to select LG-like objects from the simulations. The constrained simulations are designed to reproduce the local cosmography as it is imprinted on to the Cosmic flows-2 data base of velocities. Several prescriptions are used to define the LG model, focusing in particular on different recent estimates of the tangential velocity of M31. It is found that (a) different vtan choices affect the peak mass values up to a factor of 2, and change mass ratios of MM31 to MMW by up to 20 per cent; (b) constrained simulations yield more sharply peaked posterior distributions compared with the random ones; (c) LG mass estimates are found to be smaller than those found using the timing argument; (d) preferred Milky Way masses lie in the range of (0.6-0.8) × 1012 M⊙; whereas (e) MM31 is found to vary between (1.0-2.0) × 1012 M⊙, with a strong dependence on the vtan values used. © 2016 The Authors.


Carlesi E.,Racah Institute of Physics | Mota D.F.,University of Oslo | Winther H.A.,University of Oxford | Winther H.A.,University of Portsmouth
Monthly Notices of the Royal Astronomical Society | Year: 2017

In this work, we study the dynamics of the Local Group (LG) within the context of cosmological models beyond General Relativity (GR). Using observable kinematic quantities to identify candidate pairs, we build up samples of simulated LG-like objects drawing from f(R), symmetron, Dvali, Gabadadze & Porrati and quintessence N-body simulations together with their Λ cold dark matter (ΛCDM) counterparts featuring the same initial random phase realizations. The variables and intervals used to define LG-like objects are referred to as LG model; different models are used throughout this work and adapted to study their dynamical and kinematic properties. The aim is to determine how well the observed LG dynamics can be reproduced within cosmological theories beyond GR, We compute kinematic properties of samples drawn from alternative theories and ΛCDM and compare them to actual observations of the LG mass, velocity and position. As a consequence of the additional pull, pairwise tangential and radial velocities are enhanced in modified gravity and coupled dark energy with respect to ΛCDM inducing significant changes to the total angular momentum and energy of the LG. For example, in models such as f(R) and the symmetron this increase can be as large as 60 per cent, peaking well outside of the 95 per cent confidence region allowed by the data. This shows how simple considerations about the LG dynamics can lead to clear small-scale observational signatures for alternative scenarios, without the need of expensive high-resolution simulations. © 2017 The Authors.


Lynden-Bell D.,Institute of Astronomy | Bicak J.,Institute of Astronomy | Bicak J.,Charles University | Katz J.,Institute of Astronomy | Katz J.,Racah Institute of Physics
Classical and Quantum Gravity | Year: 2012

A new formula is given for the fast linear gravitational dragging of the inertial frame within a rapidly accelerated spherical shell of deep potential. The shell is charged and is electrically accelerated by an electric field whose sources are included in the solution.


Lynden-Bell D.,Institute of Astronomy | Katz J.,Institute of Astronomy | Katz J.,Racah Institute of Physics
Monthly Notices of the Royal Astronomical Society | Year: 2014

Large contributions to the near closure of the Universe and to the acceleration of its expansion are due to the gravitation of components of the stress-energy tensor other than its mass density. To familiarize astronomers with the gravitation of these components we conduct thought experiments on gravity, analogous to the real experiments that our forebears conducted on electricity. By analogy to the forces due to electric currents we investigate the gravitational forces due to the flows of momentum, angular momentum and energy along a cylinder. Under tension the gravity of the cylinder decreases but the 'closure' of the 3-space around it increases. When the cylinder carries a torque the flow of angular momentum along it leads to a novel helical interpretation of Levi-Civita's external metric and a novel relativistic effect. Energy currents give gravomagnetic effects in which parallel currents repel and antiparallel currents attract, though such effects must be added to those of static gravity. The gravity of beams of light give striking illustrations of these effects and a re-derivation of light bending via the gravity of the light itself. Faraday's experiments lead us to discuss lines of force of both gravomagnetic and gravity fields. A serious conundrum arises if Landau and Lifshitz's definition of gravitational force is adopted. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Lynden-Bell D.,Institute of Astronomy | Lynden-Bell D.,Racah Institute of Physics
Classical and Quantum Gravity | Year: 2014

When a static cylindrical system is subjected to equal and opposite torques top and bottom it transports angular momentum along its axis. The external metric of this static system can be transformed to Levi-Civita's form by using helical coordinates. This gives the external metric of a static cylinder three dimensionless parameters corresponding to the mass per unit length, the total stress along the cylinder, and the total torque. The external vacuum metric of a spherical system is characterised by its mass alone. How many parameters characterise the external metric of a general stationary cylindrical system? Leaving aside the radius of the cylinder which defines the scale we find that there are five parameters, the three above mentioned, to which should be added the momentum along the cylinder per unit length and the angular momentum per unit length. We show how to transform Levi-Civita's one parameter metric to include all five. © 2014 IOP Publishing Ltd.


Lynden-Bell D.,Institute of Astronomy | Katz J.,Institute of Astronomy | Katz J.,Racah Institute of Physics
Classical and Quantum Gravity | Year: 2012

In electromagnetism a current along a wire tightly wound on a torus makes a solenoid whose magnetic field is confined within the torus. In Einstein's gravity we give a corresponding solution in which a current of matter moves up on the inside of a toroidal shell and down on the outside, rolling around the torus by the short way. The metric is static outside the torus but stationary inside with the gravomagnetic field confined inside the torus, running around it by the long way. This exact solution of Einstein's equations is found by fitting Bonnor's solution for the metric of a light beam, which gives the required toroidal gravomagnetic field inside the torus, to the general Weyl static external metric in toroidal coordinates, which we develop. We deduce the matter tensor on the torus and find when it obeys the energy conditions. We also give the equipotential shells that generate the simple BachWeyl metric externally and find which shells obey the energy conditions. © 2012 IOP Publishing Ltd.


All the Plutonium used on Earth is artificially produced in nuclear reactors. Still, it turns out that it is also produced in nature. "The origin of heavy elements produced in nature through rapid neutron capture ('r-process') by seed nuclei is one of the current nucleosynthesis mysteries," Dr. Kenta Hotokezaka, Prof. Tsvi Piran and Prof. Michael Paul from the Racah Institute of Physics at the Hebrew University of Jerusalem said in their letter. Plutonium is a radioactive element. Its longest-lived isotope is plutonium-244 with a lifetime of 120 million years. Detection of plutonium-244 in nature would imply that the element was synthesized in astrophysical phenomena not so long ago (at least in Galactic time scales) and hence its origin cannot be too far from us. Several years ago it was discovered that the early Solar system contained a significant amount of plutonium-244. Considering its short-lived cycle, plutonium-244 that existed over four billion years ago when Earth formed has long since decayed but its daughter elements have been detected. But recent measurements of the deposition of plutonium-244, including analysis of Galactic debris that fell to Earth and settled in deep sea, suggest that only very small amount of plutonium has reached Earth from outer space over the recent 100 million years. This is in striking contradiction to its presence at the time when the Solar system was formed, and that is why the Galactic radioactive plutonium remained a puzzle. The Hebrew University team of scientists have shown that these contradicting observations can be reconciled if the source of radioactive plutonium (as well as other rare elements, such as gold and uranium) is in mergers of binary neutron stars. These mergers are extremely rare events but are expected to produce large amounts of heavy elements. The model implies that such a merger took place accidentally in the vicinity of our Solar System within less than a hundred million years before it was born. This has led to the relatively large amount of plutonium-244 observed in the early Solar system. On the other hand, the relatively small amount of plutonium-244 reaching Earth from interstellar space today is simply accounted for by the rarity of these events. Such an event hasn't occurred in the last 100 million years in the vicinity of our Solar system. Explore further: Plutonium tricks cells by 'pretending' to be iron More information: Kenta Hotokezaka et al. Short-lived 244Pu points to compact binary mergers as sites for heavy r-process nucleosynthesis, Nature Physics (2015). DOI: 10.1038/nphys3574


News Article | February 10, 2017
Site: www.cemag.us

Antibiotic resistance is a major and growing problem worldwide. According to the World Health Organization, antibiotic resistance is rising to dangerously high levels in all parts of the world, and new resistance mechanisms are emerging and spreading globally, threatening our ability to treat common infectious diseases. But how these bacterial resistance mechanisms occur, and whether we can predict their evolution, is far from understood. Researchers have previously shown that one way bacteria can survive antibiotics is to evolve a "timer" that keeps them dormant for the duration of antibiotic treatment. But the antibiotic kills them when they wake up, so the easy solution is to continue the antibiotic treatment for a longer duration. Now, in new research published in the prestigious journal Science, researchers at the Hebrew University of Jerusalem report a startling alternative path to the evolution of resistance in bacteria. After evolving a dormancy mechanism, the bacterial population can then evolve resistance 20 times faster than normal. At this point, continuing to administer antibiotics won't kill the bacteria. To investigate this evolutionary process, a group of biophysicists, led by Professor Nathalie Balaban and PhD student Irit Levin-Reisman at the Hebrew University's Racah Institute of Physics, exposed bacterial populations to a daily dose of antibiotics in controlled laboratory conditions, until resistance was established. By tracking the bacteria along the evolutionary process, they found that the lethal antibiotic dosage gave rise to bacteria that were transiently dormant, and were therefore protected from several types of antibiotics that target actively growing bacteria. Once bacteria acquired the ability to go dormant, which is termed "tolerance," they rapidly acquired mutations to resistance and were able to overcome the antibiotic treatment. Thus, first the bacteria evolved to "sleep" for most of the antibiotic treatment, and then this "sleeping mode" not only transiently protected them from the lethal action of the drug, but also actually worked as a stepping stone for the later acquisition of resistance factors. The results indicate that tolerance may play a crucial role in the evolution of resistance in bacterial populations under cyclic exposures to high antibiotic concentrations. The key factors are that tolerance arises rapidly, as a result of the large number of possible mutations that lead to it, and that the combined effect of resistance and tolerance promotes the establishment of a partial resistance mutation on a tolerant background. These findings may have important implications for the development of new antibiotics, as they suggest that the way to delay the evolution of resistance is by using drugs that can also target the tolerant bacteria. Unveiling the evolutionary dynamics of antibiotic resistance was made possible by the biophysical approach of the research team. The experiments were performed by a team of physicists, who developed a theoretical model and computer simulations that enabled a deep understanding of the reason behind the fast evolution of resistance that were observed.


News Article | February 15, 2017
Site: www.eurekalert.org

Antibiotic resistance is a major and growing problem worldwide. According to the World Health Organization, antibiotic resistance is rising to dangerously high levels in all parts of the world, and new resistance mechanisms are emerging and spreading globally, threatening our ability to treat common infectious diseases. But how these bacterial resistance mechanisms occur, and whether we can predict their evolution, is far from understood. Researchers have previously shown that one way bacteria can survive antibiotics is to evolve a "timer" that keeps them dormant for the duration of antibiotic treatment. But the antibiotic kills them when they wake up, so the easy solution is to continue the antibiotic treatment for a longer duration. Now, in new research published in the prestigious journal Science, researchers at the Hebrew University of Jerusalem report a startling alternative path to the evolution of resistance in bacteria. After evolving a dormancy mechanism, the bacterial population can then evolve resistance 20 times faster than normal. At this point, continuing to administer antibiotics won't kill the bacteria. To investigate this evolutionary process, a group of biophysicists, led by Prof. Nathalie Balaban and PhD student Irit Levin-Reisman at the Hebrew University's Racah Institute of Physics, exposed bacterial populations to a daily dose of antibiotics in controlled laboratory conditions, until resistance was established. By tracking the bacteria along the evolutionary process, they found that the lethal antibiotic dosage gave rise to bacteria that were transiently dormant, and were therefore protected from several types of antibiotics that target actively growing bacteria. Once bacteria acquired the ability to go dormant, which is termed "tolerance," they rapidly acquired mutations to resistance and were able to overcome the antibiotic treatment. Thus, first the bacteria evolved to "sleep" for most of the antibiotic treatment, and then this "sleeping mode" not only transiently protected them from the lethal action of the drug, but also actually worked as a stepping stone for the later acquisition of resistance factors. The results indicate that tolerance may play a crucial role in the evolution of resistance in bacterial populations under cyclic exposures to high antibiotic concentrations. The key factors are that tolerance arises rapidly, as a result of the large number of possible mutations that lead to it, and that the combined effect of resistance and tolerance promotes the establishment of a partial resistance mutation on a tolerant background. These findings may have important implications for the development of new antibiotics, as they suggest that the way to delay the evolution of resistance is by using drugs that can also target the tolerant bacteria. Unveiling the evolutionary dynamics of antibiotic resistance was made possible by the biophysical approach of the research team. The experiments were performed by a team of physicists, who developed a theoretical model and computer simulations that enabled a deep understanding of the reason behind the fast evolution of resistance that were observed. Researchers involved in the research are affiliated with the Racah Institute of Physics and the Harvey M. Kruger Family Center for Nanoscience and Nanotechnology at the Hebrew University of Jerusalem, and the Broad institute of Harvard and MIT.


Katz J.,Racah Institute of Physics
Classical and Quantum Gravity | Year: 2014

Since the geometry of our universe seems to depend very little on baryonic matter, we consider a variational principle involving only dark matter and dark energy which in addition make them depend on each other. There are no adjustable parameters or scalar fields with appropriate equations of state. No quintessence. For a pressure-less, 3-flat FRW model, the cosmological 'constant' is now a function of time, positive by definition and always small. Its time derivative or rather its associated parameter w is always negative and close to -1. The most interesting point is that the age of the universe and w itself are correlated. Moreover, this rather unsophisticated model provides a very limited range for both these quantities and results are in surprising agreement with observed values. The problem of vacuum energy remains what it was; the problem of coincidence is significantly less annoying. © 2014 IOP Publishing Ltd.

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