A2: Mathematical relativity and other progress in classical gravity theory—a session report

The year 2009 signals the birth of a new conference series under the name of Mediterranean Conference on Classical and Quantum Gravity (MCCQG). The main purpose of this new series is to bring together physicists working on General Relativity, Field Theory, and related areas to discuss the present status and latest developments in the classical and quantum treatment of gravitational systems, as well as to support and strengthen the scientific communication between the physicists of the wide Mediterranean region, working in the field of Classical and Quantum Gravity. For the latter reason, we plan to organize all future conferences of this series in the Mediterranean region.

Shape dynamics is a classical theory of gravity which agrees with general relativity in many important cases, but possesses different gauge symmetries and constraints. Rather than spacetime diffeomorphism invariance, shape dynamics takes spatial diffeomorphism invariance and spatial Weyl invariance as the fundamental gauge symmetries associated with the gravitational field. Since the area of the event horizon of a black hole transforms under a generic spatial Weyl transformation, there has been some doubt that one can speak sensibly about the thermodynamics of black holes in shape dynamics. The purpose of this paper is to show that by treating the event horizon of a black hole as an interior boundary, one can recover familiar notions of black hole thermodynamics in shape dynamics and define a gauge invariant entropy that agrees with general relativity.

Zero-point fluctuations are a universal consequence of quantum theory. Vacuum fluctuations of electromagnetic field have provided crucial evidence and guidance for QED as a successful quantum field theory with a defining gauge symmetry through the Lamb shift, Casimir effect, and spontaneous emission. In an accelerated frame, the thermalisation of the zero-point electromagnetic field gives rise to the Unruh effect linked to the Hawking effect of a black hole via the equivalence principle. This principle is the basis of general covariance, the symmetry of general relativity as the classical theory of gravity. If quantum gravity exists, the quantum vacuum fluctuations of the gravitational field should also lead to the quantum decoherence and dissertation of general forms of energy and matter. Here we present a novel theoretical effect involving the spontaneous emission of soft gravitons by photons as they bend around a heavy mass and discuss its observational prospects. Our analytic and numerical investigations suggest that the gravitational bending of starlight predicted by classical general relativity should also be accompanied by the emission of gravitational waves. This in turn redshifts the light causing a loss of its energy somewhat analogous to the bremsstrahlung of electrons by a heavier charged particle. It is suggested that this new effect may be important for a combined astronomical source of intense gravity and high-frequency radiation such as X-ray binaries and that the proposed LISA mission may be potentially sensitive to the resulting sub-Hz stochastic gravitational waves.

The conference brought together people working in mathematical general relativity, a field which lies at the interface of analysis, differential geometry and physics. There was a mix of established workers in the subject with participants at the start of their careers and researchers from neighbouring fields. The Einstein equations are at the heart of general relativity theory. They form a system of evolution equations and their solutions are conveniently parametrized by initial data. These initial data are required to satisfy constraint equations and these were the topic of several lectures at the conference. The talks of Corvino and Pollack were concerned with recent progress in methods for constructing solutions of the constraints. An important concept for solutions of the constraints is that of mass, including quasilocal mass. This was a central theme of the talks of Degeratu, Huisken and Shi. Huisken introduced a striking new approach to the definition of mass based on isoperimetric inequalities. Dain presented a variational characterization of the extreme Reissner–Nordström solution, thus relating the study of the constraint equations to the theory of black holes. The talk of Wohlfarth was rather outside the main area of the conference. He described a new concept of geometry motivated by string theory which may come to enrich the circle of ideas within mathematical relativity. Many of the talks at the conference were on evolution equations related to general relativity. Dafermos and Finster presented results related to the dynamical stability of black holes. Dafermos described new results on the rate of decay of solutions of the wave equation on the Schwarzschild spacetime while Finster explained applications of methods of functional analysis to the wave equation on the Kerr spacetime. Struwe's talk concerned uniqueness for supercritical nonlinear wave equations which from the point of view of the Einstein equations are an important example to compare with. The Maxwell equations are another important comparison system and Bauer showed how they can be used to understand more about radiation formulae. Velázquez gave an introduction to singularity formation in the Keller–Segel model, a parabolic system coming from mathematical biology. This is a possible source of insight for obtaining a rigorous understanding of critical collapse in general relativity. The talks of Andréasson, Choptuik and Lindblom dealt with various aspects of the application of numerical techniques to the study of the Einstein evolution equations. Choptuik showed impressive new simulations of coalescing black holes due to Pretorius which could hardly have been imagined just a year ago. Cosmology is at present a very active area of research in general relativity. This is in part due to the challenge of understanding the observed accelerated expansion of our universe. Mathematics is beginning to make its mark in this subject and this was reflected by talks of Heinzle, Rendall and Tod. Mathematical relativity is a meeting point for many ideas and the abstracts which follow give some idea of the variety of the subject. In fact the spectrum of topics discussed by the participants at the conference was much wider than those for which talks could be scheduled. By limiting the number of presentations it was possible to leave plenty of time for people to exchange insights. The lively interactions observed make us hopeful that this conference has given a boost to the development of the subject in the next few years.

The international conference ‘‘The Modern Physics of Compact Stars and RelativisticGravity’’ took place in Yerevan, Armenia, from 18–21 September 2013. This was the second ina series of conferences which aim to bring together people working in astrophysics of compactstars, physics of dense matter, gravitation and cosmology, observations of pulsars and binaryneutron stars and related fields. The conference was held on the occasion of 100th birthday ofthe founder of the Theoretical Physics Chair at the Department of Physics of Yerevan StateUniversity and prominent Armenian scientist Academician Gurgen S Sahakyan.The field of compact stars has seen extraordinary development since the discovery of pulsarsin 1967. Even before this discovery, pioneering work of a number of theoretical groups had laid thefoundation for this development. A pioneer of this effort was Professor G S Sahakyan who, togetherwith Professor Victor Ambartsumyan and a group of young scientists, started in the early sixtiestheir fundamental work on the properties of superdense matter and on the relativistic structureof compact stellar objects. This conference explored the vast diversity of the manifestationsof compact stars, including the modern aspects of the equation of state of superdense matter,its magnetic and thermal properties, rotational dynamics, superfluidity and superconductivity,phase transition from hadronic to quark matter, etc. The articles on these subjects collected inthis volume are evidence of liveliness of the field and of the continuous feedback betweentheory and the experiment. A part of this volume is devoted to the cosmology and the theories ofgravity — the subfields of astrophysics that are of fundamental importance to our understandingof the universe. The reader will find here articles touching on the most diverse aspects of these fieldssuch as modern problems in Einstein's classical theory of gravity and its alternatives, stringtheory motivated cosmological models, theories of cosmic microwave background, quantum fieldtheory in curved background, Casimir effect, etc. Thus, it is fair to say that the present volumecovers a large number of actively pursued subjects of modern relativistic astrophysics.We would like to thank all those individuals and organizations that helped us in organizinga successful conference. These include the members of the international advisory committeeas well as the local organizing committee (whose names are listed separately above) and thesponsors of the conference — the Volkswagen Stiftung (Hannover, Germany), the HIC for FAIRInstitute (Frankfurt am Main, Germany) and the State Committee for Sciences of Armenia.Edvard Chubaryan, Aram Saharian and Armen Sedrakian Yerevan / Frankfurt-Main, 1 February, 2014 A list of participants is available in the PDF

Modern cosmology successfully describes the main features of the Universe but uses for that some specific initial data for which we do not yet have a reasonable explanation. There is no theory capable of not only describing the observable world but also giving an answer to why and for what reasons the Universe has these or other properties. The most important unsolved issue is the nature of the cosmological singularity whose existence is predicted by the classical (not quantum) relativistic theory of gravity. For a thorough analysis of the singularity one needs a quantum theory of gravity which is yet to be constructed. Beacause of the lack of such a theory one has to start doing cosmology from classical stages by introducing different initial conditions and comparing theiur consequences with real observations.

Einstein’s theory of gravity — the general theory of relativity — is based on the principle that all objects accelerate identically in an external gravitational field. A triple-star system provides a stringent test of this principle. Einstein’s theory of gravity — the general theory of relativity — is based on the principle that all objects accelerate identically in an external gravitational field. A triple-star system provides a stringent test of this principle.

The GR19 meeting was held in México City from 6–9 July 2010. The decision to have the meeting in México was taken during the GR18 meeting in Sydney, Australia in 2007, and represented a great milestone for the scientific community working in the fields related to gravitation in México. This fact was evidenced by the commitment of the most important institutions in México where the field is developed, by the support the meeting received at various governmental levels, and also by a promotional campaign dedicated to educate the public about our subject, which was undertaken by important segments of the gravitational physics community in Mexico. This campaign was named ‘El Mes de Einstein’ or ‘The Einsteinian Month’, and consisted of a series of presentations, talks and movies about topics related to General Relativity which culminated with the public talk of the GR19 meeting (now a traditional aspect of the GR events). This talk was given by George Smoot, Nobel laureate in physics 2006, on the amazing developments around the detailed studies of the Cosmic Microwave Background, and was held at the Nezahualc’oyotl Hall in the main campus of the National Autonomous University of México, which was filled to capacity by enthusiastic crowds of lay people fascinated with the subject.The meeting itself was a very successful one with participants from dozens of countries spanning the five continents, with a rich, varied and informative plenary program. Highlights, featured in this issue, were perhaps the talk by Veronika Hubeny on the fluid/gravity correspondence, a subject that has grown dramatically during the last few years, the lecture by Tarun Souradeep on the enormous potential for discovery offered by the ever increasing accuracy of cosmological observations, the presentation by Jeffrey McClintock, about accreting black holes and the exciting possibility of measuring their spins, the informative review about Loop Quantum Gravity from one of the pioneers of the subject: Carlo Rovelli, and an interesting update on one the recent developments in the ADF/CFT correspondence ‘the condensed matter–gravity connection’ presented by Gary Horowitz, among other excellent contributions. The material presented by other plenary speakers, which is not represented in this issue, has already been widely reported either by themselves or by others.The parallel sessions covered a wide range of topics currently underinvestigation by the worldwide gravity community, and were enthusiastically attended by the conference participants. In the past, ithas been traditional to ask session chairs to write a brief summary of thetalks presented. This time, however, we asked the parallel session chairsto nominate a small number of individual speakers to contribute to this issue.By focusing on a smaller number of topics, we were able to allocate morespace to each, which we hope will provide a more useful overview of at least some of the most exciting physics at GR19. We hope that this novel approach will become a standard feature of future proceedings of the GR meetings.Anther aspect we should mention is the satellite meeting on Quantum Gravity and its preceding PASI Quantum Gravity Summer School held at the Institute for Mathematics, National Autonomous University of México (UNAM), Campus Morelia, Morelia México, from 23 June to 3 July, 2010 which was attended by many students from Mexican institutions, as well as by several participants who came from abroad to learn about this fascinating subject.The organizers want to thank the generous support of the following institutions: the Mexican National Council for Science and Technology (CONACYT), the National Autonomous University of México (UNAM), the Autonomous Metropolitan University of México UAM, the Center for Research and Advanced Studies CINVESTAV, the Mexican Physical Society (SMF), The Mexican Academy of Sciences AMC, The Institute for Nuclear Sciences of UNAM (ICN), the International Union of Pure and Applied Physics (IUPAP), The International Society on General Relativity and Gravitation, the ‘Benemerita’ Autonomous University of Puebla (BAUP), the University Michoacana San Nicolas de Hidalgo, the University of Veracruz, the University of Guadalajara, the University of Guanajuato, the University Iberoamericana, the University of Sinaloa, the University of Zacatecas, the Advanced Institute of Cosmology (IAC), as well as the Government of the Federal District of Mexico City.Don Marolf and Daniel Sudarsky Guest Editors

Two radically different views about time are possible. According to the first, the universe is three dimensional. It has a past and a future, but that does not mean it is spread out in time as it is spread out in the three dimensions of space. This view requires that there is an unambiguous, absolute, cosmic-wide “now” at each instant. According to the second view about time, the universe is four dimensional. It is spread out in both space and time—in space-time in short. Special and general relativity rule out the first view. There is, according to relativity theory, no such thing as an unambiguous, absolute cosmic-wide “now” at each instant. However, we have every reason to hold that both special and general relativity are false. Not only does the historical record tell us that physics advances from one false theory to another. Furthermore, elsewhere I have shown that we must interpret physics as having established physicalism—in so far as physics can ever establish anything theoretical. Physicalism, here, is to be interpreted as the thesis that the universe is such that some unified “theory of everything” is true. Granted physicalism, it follows immediately that any physical theory that is about a restricted range of phenomena only, cannot be true, whatever its empirical success may be. It follows that both special and general relativity are false. This does not mean of course that the implication of these two theories that there is no unambiguous cosmic-wide “now” at each instant is false. It still may be the case that the first view of time, indicated at the outset, is false. Are there grounds for holding that an unambiguous cosmic-wide “now” does exist, despite special and general relativity, both of which imply that it does not exist? There are such grounds. Elsewhere I have argued that, in order to solve the quantum wave/particle problem and make sense of the quantum domain we need to interpret quantum theory as a fundamentally probabilistic theory, a theory which specifies how quantum entities—electrons, photons, atoms—interact with one another probabilistically. It is conceivable that this is correct, and the ultimate laws of the universe are probabilistic in character. If so, probabilistic transitions could define unambiguous, absolute cosmic-wide “nows” at each instant. It is entirely unsurprising that special and general relativity have nothing to say about the matter. Both theories are pre-quantum mechanical, classical theories, and general relativity in particular is deterministic. The universe may indeed be three dimensional, with a past and a future, but not spread out in four dimensional space-time, despite the fact that relativity theories appear to rule this out. These considerations, finally, have implications for views about the arrow of time and free will.

Conserved vectors are divergencies of superpotentials. In field theory on curved backgrounds, they are useful in calculating global ‘charges’ in arbitrary coordinates and local conserved quantities for small perturbations with specific gauge conditions. Superpotentials are, however, ill–defined. A new criterion of Julia and Silva selects uniquely for Dirichlet boundary conditions the ‘KBL superpotential’ as proposed by Katz, Bičák and Lynden–Bell, which has remarkable properties.Here, we show that a Belinfante–type addition to the KBL superpotential in general relativity gives an expression that is independent of boundary conditions defined by a variational principle. The modified superpotential has the same global properties as the KBL one, except for angular momentum at null infinity, and it does not differ from the KBL superpotential in the linearized theory of gravitation.As an illustration in linearized theory on curved backgrounds, we calculate conserved quantities for small perturbations on a Friedmann–Robertson–Walker spacetime associated with conformal Killing vectors. Our unifying view relates a number of applications in cosmology found in the literature. Globally conserved quantities have simple physical interpretations in the ‘uniform Hubble expansion’ gauge.

This paper is based on the invited talks delivered by the author at GR 19: the 19th International Conference on General Relativity and Gravitation (Ciudad de México, Mexico, July 2010).

Quantum theory is a general theoretical framework to describe states and interactions in Nature. It does so successfully for the strong, weak, and electromagnetic interactions. Gravity is, however, still described by a classical theory – Einstein’s theory of general relativity, also called geometrodynamics. So far, general relativity seems to accommodate all observations which include gravity; there exist some phenomena which could in principle need a more general theory for their explanation (Dark Matter, Dark Energy, Pioneer Anomaly), but this is an open issue. Quantum gravity would ultimately be a physical theory, both mathematically consistent and experimentally tested, that accommodates the gravitational interaction into the quantum framework. Such a theory is not yet available. Therefore, one calls quantum gravity all approaches which are candidates for such a theory or suitable approximations thereof. The following sections will first focus on the general motivation for constructing such a theory, and then introduce the approaches which at the moment look most promising.

General relativity and its Newtonian weak field limit are not sufficient to explain the observed phenomenology in the Universe, from the formation of large-scale structures to the dynamics of galaxies, with the only presence of baryonic matter. The most investigated cosmological model, the ΛCDM, accounts for the majority of observations by introducing two dark components, dark energy and dark matter, which represent ∼95% of the mass-energy budget of the Universe. Nevertheless, the ΛCDM model faces important challenges on the scale of galaxies. For example, some very tight relations between the properties of dark and baryonic matters in disk galaxies, such as the baryonic Tully–Fisher relation (BTFR), the mass discrepancy–acceleration relation (MDAR), and the radial acceleration relation (RAR), which see the emergence of the acceleration scale a0≃1.2×10−10 m s−2, cannot be intuitively explained by the CDM paradigm, where cosmic structures form through a stochastic merging process. An even more outstanding coincidence is due to the fact that the acceleration scale a0, emerging from galaxy dynamics, also seems to be related to the cosmological constant Λ. Another challenge is provided by dwarf galaxies, which are darker than what is expected in their innermost regions. These pieces of evidence can be more naturally explained, or sometimes even predicted, by modified theories of gravity, that do not introduce any dark fluid. I illustrate possible solutions to these problems with the modified theory of gravity MOND, which departs from Newtonian gravity for accelerations smaller than a0, and with Refracted Gravity, a novel classical theory of gravity introduced in 2016, where the modification of the law of gravity is instead regulated by a density scale.

Event Abstract Back to Event Sensory-motor rhythms are modulated in response to perception of pain in others Steven Whitmarsh1*, Ole Jensen1, Ingrid Niewenhuis1 and Henk Barendregt2 1 Donders Institute for Brain, Cognition and Behaviour, Netherlands 2 Radboud University, Netherlands Recent years have seen the emergence of a neuroscientific understanding of empathic responses as embodied sensory and motor processing. For instance remarkable overlap in first- and third-person perspectives has been demonstrated when processing tactile, nociceptive, emotional or behavioral information. In the current study we used MEG (VSM-CTF; 275 sensors) to measure neuromagnetic responses to the observation of static pictures depicting hand and feet in painful and non-painful situations. Our main objective was to investigate oscillatory responses reflecting the perception of pain in others. We found that observation of the hand and feet in non-painful situations resulted in significant suppression of mu and beta oscillations over sensorimotor and visual regions. Observing the limbs in painful situations significantly suppressed these sensorimotor rhythms further. The effect was most pronounced above left sensorimotor regions. In addition, the painful stimuli also induced significantly higher power in the gamma band (65-100Hz) over occipital areas. In future analysis we will localize the source using beamformer techniques. In addition, we intend to characterize the functional cross-frequency coupling between central alpha/beta oscillations and the induced posterior gamma oscillations. Our findings so far suggest that viewing painful compared to neutral stimuli engages both sensorimotor areas and visual areas. This supports the notion that embodied representations are involved in processing the valence of perceived third-person events. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Poster Presentation Topic: Sensory Processing and Functional Connectivity Citation: Whitmarsh S, Jensen O, Niewenhuis I and Barendregt H (2010). Sensory-motor rhythms are modulated in response to perception of pain in others. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00154 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 26 Mar 2010; Published Online: 26 Mar 2010. * Correspondence: Steven Whitmarsh, Donders Institute for Brain, Cognition and Behaviour, Nijmegen, Netherlands, s.whitmarsh@cs.ru.nl Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Steven Whitmarsh Ole Jensen Ingrid Niewenhuis Henk Barendregt Google Steven Whitmarsh Ole Jensen Ingrid Niewenhuis Henk Barendregt Google Scholar Steven Whitmarsh Ole Jensen Ingrid Niewenhuis Henk Barendregt PubMed Steven Whitmarsh Ole Jensen Ingrid Niewenhuis Henk Barendregt Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Event Abstract Back to Event Long-range coupling of prefrontal cortex and visual (MT) or polysensory (STP) cortical areas in motion perception Lucia M. Vaina1, 2*, Finnegan Calabro1, Fa-Hsuan Lin2 and Matti Hamalainen3 1 Boston University, United States 2 Harvard University, United States 3 Massachusetts General Hospital, United States To investigate how, where and when moving auditory cues interact with the perception of object-motion during self-motion, we conducted psychophysical, and MEG experiments in which the subjects viewed nine textured objects during simulated forward self-motion. On each trial, one object was randomly assigned its own looming motion within the scene. Subjects reported which of four labeled objects had independent motion within the scene in two conditions: (1) visual information only and (2) with additional auditory cue. In psychophysics, observers performed slightly better in the presence of an auditory cue moving in the same direction with the object target (p=0.003, t=4.18, df=8, min 1.2%, and max 8.1). When the auditory cue was static, uninformative, it had not impact on performance. In cortically-constrained MEG source estimates, comparison of the two conditions showed: (i) MT (middle temporal area) activity similar across conditions; (ii) additional activity in the auditory cue condition ventral to MT late after the stimulus presentation; (iii) with the auditory cue, early activity at the right auditory cortex (AC) together with STP (superior temporal polysensory area), (iv) different time courses of these two activities with STP signals later in the epoch together with frontal activity in the right hemisphere, (v) stronger activity in PPC (posterior parietal cortex) in the visual-only condition than in the auditory-cue condition. In addition, Dynamic Granger Causality analysis among cortical regions showed for the auditory cues condition a strong connection of the AC with STP but not with MT suggesting binding of visual and auditory information at STP. Also, while in the visual-only condition PFC is connected with MT, in the auditory-cue condition PFC (prefrontal cortex) is connected to STP. These results indicate that PFC allocates attention to the “object” as a whole, in STP to a moving visual-auditory object, and in MT to a moving visual object. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Poster Presentation Topic: Sensory Processing and Functional Connectivity Citation: Vaina LM, Calabro F, Lin F and Hamalainen M (2010). Long-range coupling of prefrontal cortex and visual (MT) or polysensory (STP) cortical areas in motion perception. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00212 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 30 Mar 2010; Published Online: 30 Mar 2010. * Correspondence: Lucia M Vaina, Boston University, Boston, United States, vaina@bu.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Lucia M Vaina Finnegan Calabro Fa-Hsuan Lin Matti Hamalainen Google Lucia M Vaina Finnegan Calabro Fa-Hsuan Lin Matti Hamalainen Google Scholar Lucia M Vaina Finnegan Calabro Fa-Hsuan Lin Matti Hamalainen PubMed Lucia M Vaina Finnegan Calabro Fa-Hsuan Lin Matti Hamalainen Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.