Institute for Theoretical Physics

Innsbruck, Austria

Institute for Theoretical Physics

Innsbruck, Austria

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News Article | May 10, 2017
Site: www.scientificamerican.com

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 | May 17, 2017
Site: www.sciencedaily.com

What happened right after the beginning of the universe? How can we understand the structure of quantum materials? How does the Higgs-Mechanism work? Such fundamental questions can only be answered using quantum field theories. These theories do not describe particles independently from each other; all particles are seen as a collective field, permeating the whole universe. But these theories are often hard to test in an experiment. At the Vienna Center for Quantum Science and Technology (VCQ) at TU Wien, researchers have now demonstrated how quantum field theories can be put to the test in new kinds of experiments. They have created a quantum system consisting of thousands of ultra cold atoms. By keeping them in a magnetic trap on an atom chip, this atom cloud can be used as a "quantum simulator," which yields information about a variety of different physical systems and new insights into some of the most fundamental questions of physics. Complex Quantum Systems -- More than the Sum of their Parts "Ultra cold atoms open up a door to recreate and study fundamental quantum processes in the lab," says Professor Jörg Schmiedmayer (VCQ, TU Wien). A characteristic feature of such a system is that its parts cannot be studied independently. The classical systems we know from daily experience are quite different: The trajectories of the balls on a billiard table can be studied separately -- the balls only interact when they collide. "In a highly correlated quantum system such as ours, made of thousands of particles, the complexity is so high that a description in terms of its fundamental constituents is mathematically impossible," says Thomas Schweigler, the first author of the paper. "Instead, we describe the system in terms of collective processes in which many particles take part -- similar to waves in a liquid, which are also made up of countless molecules." These collective processes can now be studied in unprecedented detail using the new methods. In high-precision measurements, it turns out that the probability of finding an individual atom is not the same at each point in space -- and there are intriguing relationships between the different probabilities. "When we have a classical gas and we measure two particles at two separate locations, this result does not influence the probability of finding a third particle at a third point in space," says Jörg Schmiedmayer. "But in quantum physics, there are subtle connections between measurements at different points in space. These correlations tell us about the fundamental laws of nature which determine the behaviour of the atom cloud on a quantum level." "The so-called correlation functions, which are used to mathematically describe these relationships, are an extremely important tool in theoretical physics to characterize quantum systems," says Professor Jürgen Berges (Institute for Theoretical Physics, Heidelberg University). But even though they have played an important part in theoretical physics for a long time, these correlations could hardly be measured in experiments. With the help of the new methods developed at TU Wien, this is now changing: "We can study correlations of different orders -- up to the tenth order. This means that we can investigate the relation between simultaneous measurements at ten different points in space," Schmiedmayer explains. "For describing the quantum system, it is very important whether these higher correlations can be represented by correlations of lower order -- in this case, they can be neglected at some point -- or whether they contain new information." Using such highly correlated systems like the atom cloud in the magnetic trap, various theories can now be tested in a well-controlled environment. This allows us to gain a deep understanding of the nature of quantum correlations. This is especially important because quantum correlations play a crucial role in many, seemingly unrelated physics questions: Examples are the peculiar behaviour of the young universe right after the big bang, but also for special new materials, such as the so-called topological insulators. Important information on such physical systems can be gained by recreating similar conditions in a model system, like the atom clouds. This is the basic idea of quantum simulators: Much like computer simulations, which yield data from which we can learn something about the physical world, a quantum simulation can yield results about a different quantum system that cannot be directly accessed in the lab.


News Article | May 17, 2017
Site: www.eurekalert.org

A new way to characterize many-particle quantum systems has been presented in the journal "Nature" by TU Wien (Vienna) and Heidelberg University. Quantum simulators can now be used to take a deeper look at previously unanswered questions What happened right after the beginning of the universe? How can we understand the structure of quantum materials? How does the Higgs-Mechanism work? Such fundamental questions can only be answered using quantum field theories. These theories do not describe particles independently from each other; all particles are seen as a collective field, permeating the whole universe. But these theories are often hard to test in an experiment. At the Vienna Center for Quantum Science and Technology (VCQ) at TU Wien, researchers have now demonstrated how quantum field theories can be put to the test in new kinds of experiments. They have created a quantum system consisting of thousands of ultra cold atoms. By keeping them in a magnetic trap on an atom chip, this atom cloud can be used as a "quantum simulator", which yields information about a variety of different physical systems and new insights into some of the most fundamental questions of physics. Complex Quantum Systems -- More than the Sum of their Parts "Ultra cold atoms open up a door to recreate and study fundamental quantum processes in the lab", says Professor Jörg Schmiedmayer (VCQ, TU Wien). A characteristic feature of such a system is that its parts cannot be studied independently. The classical systems we know from daily experience are quite different: The trajectories of the balls on a billiard table can be studied separately -- the balls only interact when they collide. "In a highly correlated quantum system such as ours, made of thousands of particles, the complexity is so high that a description in terms of its fundamental constituents is mathematically impossible", says Thomas Schweigler, the first author of the paper. "Instead, we describe the system in terms of collective processes in which many particles take part -- similar to waves in a liquid, which are also made up of countless molecules." These collective processes can now be studied in unprecedented detail using the new methods. In high-precision measurements, it turns out that the probability of finding an individual atom is not the same at each point in space -- and there are intriguing relationships between the different probabilities. "When we have a classical gas and we measure two particles at two separate locations, this result does not influence the probability of finding a third particle at a third point in space", says Jörg Schmiedmayer. "But in quantum physics, there are subtle connections between measurements at different points in space. These correlations tell us about the fundamental laws of nature which determine the behaviour of the atom cloud on a quantum level." "The so-called correlation functions, which are used to mathematically describe these relationships, are an extremely important tool in theoretical physics to characterize quantum systems", says Professor Jürgen Berges (Institute for Theoretical Physics, Heidelberg University). But even though they have played an important part in theoretical physics for a long time, these correlations could hardly be measured in experiments. With the help of the new methods developed at TU Wien, this is now changing: "We can study correlations of different orders - up to the tenth order. This means that we can investigate the relation between simultaneous measurements at ten different points in space", Schmiedmayer explains. "For describing the quantum system, it is very important whether these higher correlations can be represented by correlations of lower order -- in this case, they can be neglected at some point -- or whether they contain new information." Using such highly correlated systems like the atom cloud in the magnetic trap, various theories can now be tested in a well-controlled environment. This allows us to gain a deep understanding of the nature of quantum correlations. This is especially important because quantum correlations play a crucial role in many, seemingly unrelated physics questions: Examples are the peculiar behaviour of the young universe right after the big bang, but also for special new materials, such as the so-called topological insulators. Important information on such physical systems can be gained by recreating similar conditions in a model system, like the atom clouds. This is the basic idea of quantum simulators: Much like computer simulations, which yield data from which we can learn something about the physical world, a quantum simulation can yield results about a different quantum system that cannot be directly accessed in the lab. Prof. Jörg Schmiedmayer Institute of Atomic and Subatomic Physics Vienna University of Technology Stadionallee 2, 1020 Wien T: +43-1-58801-141801 M: +43-664-605883888 hannes-joerg.schmiedmayer@tuwien.ac.at


News Article | April 28, 2017
Site: www.eurekalert.org

Australian and German researchers have collaborated to develop a genetic algorithm to confirm the rejection of classical notions of causality. Dr Alberto Peruzzo from RMIT University in Melbourne said: "Bell's theorem excludes classical concepts of causality and is now a cornerstone of modern physics. "But despite the fundamental importance of this theorem, only recently was the first 'loophole-free' experiment reported which convincingly verified that we must reject classical notions of causality. "Given the importance of this data, an international collaboration between Australian and German institutions has developed a new method of analysis to robustly quantify such conclusions." The team's approach was to use genetic programming, a powerful machine learning technique, to automatically find the closest classical models for the data. Together, the team applied machine learning to find the closest classical explanations of experimental data, allowing them to map out many dimensions of the departure from classical that quantum correlations exhibit. Dr Chris Ferrie, from the University of Technology Sydney, said: "We've light-heartedly called the region mapped out by the algorithm the 'edge of reality,' referring to the common terminology 'local realism' for a model of physics satisfying Einstein's relativity. "The algorithm works by building causal models through simulated evolution imitating natural selection - genetic programming. "The algorithm generates a population of 'fit' individual causal models which trade off closeness to quantum theory with the minimisation of causal influences between relativistically disconnected variables." The team used photons, single particles of light, to generate the quantum correlations that cannot be explained using classical mechanics. Quantum photonics has enabled a wide range of new technologies from quantum computation to quantum key distribution. The photons were prepared in various states possessing quantum entanglement, the phenomenon which fuels many of the advantages in quantum technology. The data collected was then used by the genetic algorithm to find a model that best matches the observed correlations. These models then quantify the region of models which are ruled out by nature itself. The team includes theoretical physicists and computer scientists from the ARC Centre for Engineered Quantum Systems (EQuS) at the University of Sydney, the Centre for Quantum Software and Information at the University of Technology Sydney and the Institute for Theoretical Physics at the University of Cologne as well as the experimental group at RMIT University's Quantum Photonics Laboratory. The research, "Explaining quantum correlations through evolution of causal models", has been published in Physical Review A and can be accessed online. For interviews: Dr Chris Ferrie, csferrie@gmail.com, or Dr Alberto Peruzzo, alberto.peruzzo@rmit.edu.au or +61 410 790 860. For general media enquiries: David Glanz, +61 3 9925 2807 or +61 438 547 723 or david.glanz@rmit.edu.au.


News Article | April 17, 2017
Site: www.eurekalert.org

The international Event Horizon Telescope (EHT) Collaboration, which is imaging for the first time the black-hole candidate at the center of our Milky Way, has a major research focus in Germany. A significant contribution to this experiment is part of "BlackHoleCam", a German-Dutch experiment founded in 2014. The research group of Prof. Rezzolla at the Institute for Theoretical Physics at the Goethe University Frankfurt is part of the collaboration. BlackHoleCam is supported by the European Research Council via an ERC Synergy Grant of 14 Million Euros. Due to the strong pull of gravity, not even light can escape from black holes, whose surface, i.e., the event horizon, cannot be observed directly. However, the boundary which separates photons that are trapped from those that can escape from the incredible gravitational pull is called the black-hole "shadow", because it would appear as a shadow against a bright lit background. It is such a shadow that is the target of series of observations presently ongoing of Sgr A*, the name of the black-hole candidate in our Milky Way. During the observations, the researchers will analyze the radio emission emitted by Sgr A*, whose mass is 4.5 million times that of our Sun and whose shadow is about half of the size of the distance between the Sun and the Earth. Despite being so massive, Sgr A* is also very far from us, at a 26,000 light years, making the angular size of the shadow extremely small. Measuring the emission from this surface is therefore equivalent to imaging an apple on the surface of the Moon. To accomplish this ambitious project several radio telescopes across the globe are connected and thus form a virtual telescope with a diameter comparable to the Earth. This technique is called Long Baseline Interferometry (VLBI). The work of BlackHoleCam is lead by Prof. Luciano Rezzolla (ITP, Frankfurt), Prof. Michael Kramer (Max Planck Institute for Radio Astronomy, Bonn), and by Prof. Heino Falcke (Radboud-University Nijmegen, Netherlands); all of them are important contributors of the EHT collaboration. In the current observations of Sgr A*, network of radiotelescopes from Europe, the United States of America, Middle- and South America, and the South Pole telescope are participating at the same time. During the observations, each telescope records the data on hard disks which are shipped after the end of the campaign to one of the high-performance computer centers in the US or to Bonn. In these centers the individual data of the telescopes are combined by supercomputers and an image can be reconstructed. This shadow image can be regarded as the starting point for the theoretical research of Prof. Rezzolla's group. Besides predicting theoretically what type of image scientists is expected to observe, the group in Frankfurt is also working on determining whether it will be possible to establish if Einstein's theory of general relativity is the correct theory of gravity. There are several other theories of gravity besides the well-known one by Einstein and the observations of the black-hole shadow may help to identify the true one. Because of this, scientists in Frankfurt analyze the size and the geometry of the shadow and compare them to synthetic images generated on supercomputers which model accretion flows onto black holes.. These images are computed by solving the equations of relativistic magneto-hydrodynamics and tracing the orbit of photons around black holes in different theories of gravity using state-of-the art numerical tools developed in the group of Prof. Rezzolla. Comparing the synthetic shadow to the observed one may shed light on the existence of one of the most extreme predictions of Einstein's theory of gravity: the existence of black holes. However, as Prof. Rezzolla remarks, "These observations represent a major step forward in the international attempt of understanding the nature of the dark and compact object at the centre of our Galaxy. However, they are just the first step and it is likely that many more observations of increasing precision will be necessary for finally settling this fundamental issue".


Rezzolla L.,Institute for Theoretical Physics | Kumar P.,University of Texas at Austin
Astrophysical Journal | Year: 2015

The merger of a binary of neutron stars provides natural explanations for many of the features of short gamma-ray bursts (SGRBs), such as the generation of a hot torus orbiting a rapidly rotating black hole, which can then build a magnetic jet and provide the energy reservoir to launch a relativistic outflow. However, this scenario has problems explaining the recently discovered long-term and sustained X-ray emission associated with the afterglows of a subclass of SGRBs. We propose a new model that explains how an X-ray afterglow can be sustained by the product of the merger and how the X-ray emission is produced before the corresponding emission in the gamma-band, though it is observed to follow it. Overall, our paradigm combines in a novel manner a number of well-established features of the emission in SGRBs and results from simulations. Because it involves the propagation of an ultra-relativistic outflow and its interaction with a confining medium, the paradigm also highlights a unifying phenomenology between short and long GRBs. © 2015. The American Astronomical Society. All rights reserved.


Nieuwenhuizen T.M.,Institute for Theoretical Physics
Foundations of Physics | Year: 2011

It is explained on a physical basis how absence of contextuality allows Bell inequalities to be violated, without bringing an implication on locality or realism. Hereto we connect first to the local realistic theory Stochastic Electrodynamics, and then put the argument more broadly. Thus even if Bell Inequality Violation is demonstrated beyond reasonable doubt, it will have no say on local realism, because absence of contextuality prevents the Bell inequalities to be derived from local realistic models. © 2010 The Author(s).


Koivisto T.,Institute for Theoretical Physics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

A variational principle was recently suggested by Goenner, where an independent metric generates the spacetime connection. It is pointed out here that the resulting theory is equivalent to the usual Palatini theory. However, a bimetric reformulation of the variational principle leads to theories which are physically distinct from both the metric and the metric-affine ones, even for the Einstein-Hilbert action. They are obtained at a decoupling limit of C-theories, which contain also other viable generalizations of the Palatini theories. © 2011 American Physical Society.


Koivisto T.S.,Institute for Theoretical Physics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

C-theory provides a unified framework to study metric, metric-affine and more general theories of gravity. In the vacuum weak-field limit of these theories, the parameterized post-Newtonian parameters β and γ can differ from their general relativistic values. However, there are several classes of models featuring long-distance modifications of gravity but nevertheless passing the Solar System tests. Here it is shown how compute the parameterized post-Newtonian parameters in C-theories and also in nonminimally coupled curvature theories, correcting previous results in the literature for the latter. © 2011 American Physical Society.


Koivisto T.S.,Institute for Theoretical Physics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

Nonsingular cosmologies are investigated in the framework of f(R) gravity within the first order formalism. General conditions for bounces in isotropic and homogeneous cosmology are presented. It is shown that only a quadratic curvature correction is needed to predict a bounce in a flat or to describe cyclic evolution in a curved dust-filled universe. Formalism for perturbations in these models is set up. In the simplest cases, the perturbations diverge at the turnover. Conditions to obtain smooth evolution are derived. © 2010 The American Physical Society.

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