The Experts below are selected from a list of 156 Experts worldwide ranked by ideXlab platform
David Garfinkle - One of the best experts on this subject based on the ideXlab platform.
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numerical simulations of general Gravitational Singularities
arXiv: General Relativity and Quantum Cosmology, 2008Co-Authors: David GarfinkleAbstract:This paper covers some of the current techniques and issues involved in performing numerical simulations of the formation of Singularities.
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numerical simulations of stiff fluid Gravitational Singularities
Physical Review D, 2005Co-Authors: Joshua Curtis, David GarfinkleAbstract:Numerical simulations of the approach to the singularity in spacetimes with stiff fluid matter are presented here. The spacetimes examined have no symmetries and can be regarded as representing the general behavior of Singularities in the presence of such matter. It is found that the singularity is spacelike and that as it is approached, the spacetime dynamics becomes local and nonoscillatory.
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the nature of Gravitational Singularities
International Journal of Modern Physics D, 2004Co-Authors: David GarfinkleAbstract:The nature of Gravitational Singularities, long mysterious, has now become clear through a combination of mathematical and numerical analysis. As the singularity is approached, the time derivative terms in the field equations dominate, and the singularity behaves locally like a homogeneous oscillatory spacetime.
Itzhak Bars - One of the best experts on this subject based on the ideXlab platform.
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Wavefunction for the Universe close to its beginning with dynamically and uniquely determined initial conditions
Physical Review D, 2018Co-Authors: Itzhak BarsAbstract:In this paper I will first outline an effective field theory for cosmology (EFTC) that is based on the Standard Model coupled to General Relativity and improved with Weyl symmetry. There are no new physical degrees of freedom in this theory, but what is new is an enlargement of the domain of the existing physical fields and of spacetime via the larger symmetry, thus curing the geodesic incompleteness of the traditional theory. Invoking the softer behavior of an underlying theory of quantum gravity, I further argue that it is reasonable to ban higher curvature terms in the effective action, thus making this EFTC mathematically well behaved at Gravitational Singularities, as well as geodesically complete, thus able to make new physics predictions. Using this EFTC, I show some predictions of surprising behavior of the universe at Singularities including a unique set of big-bang initial conditions that emerge from a dynamical attractor mechanism. I will illustrate this behavior with detailed formulas and plots of the classical solutions and the quantum wavefunction that are continuous across Singularities for a cosmology that includes the past and future of the big bang. The solutions are given in the geodesically complete global mini-superspace that is similar to the extended spacetime of a black hole or extended Rindler spacetime. The analytic continuation of the quantum wavefunction across the horizons describes the passage through the Singularities. This analytic continuation solves a long-standing problem of the singular (-1/r^2) potential in quantum mechanics that dates back to Von Neumann. The analytic properties of the wavefunction also reveal an infinite stack of universes sewn together at the horizons of the geodesically complete space. Finally a critique of recent controversial papers using the path integral approach in cosmology is given.
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Physical Interpretation of Antigravity
Physical Review D, 2016Co-Authors: Itzhak Bars, Albin JamesAbstract:Geodesic incompleteness is a problem in both general relativity and string theory. The Weyl-invariant Standard Model coupled to general relativity ($\mathrm{SM}+\mathrm{GR}$), and a similar treatment of string theory, are improved theories that are geodesically complete. A notable prediction of this approach is that there must be antigravity regions of spacetime connected to gravity regions through Gravitational Singularities such as those that occur in black holes and cosmological bang/crunch. Antigravity regions introduce apparent problems of ghosts that raise several questions of physical interpretation. It was shown that unitarity is not violated, but there may be an instability associated with negative kinetic energies in the antigravity regions. In this paper we show that the apparent problems can be resolved with the interpretation of the theory from the perspective of observers strictly in the gravity region. Such observers cannot experience the negative kinetic energy in antigravity directly, but can only detect in and out signals that interact with the antigravity region. This is no different from a spacetime black box for which the information about its interior is encoded in scattering amplitudes for in/out states at its exterior. Through examples we show that negative kinetic energy in antigravity presents no problems of principles but is an interesting topic for physical investigations of fundamental significance.
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global analysis of new Gravitational Singularities in string and particle theories
Physical Review D, 1992Co-Authors: Itzhak Bars, Konstadinos SfetsosAbstract:We present a global analysis of the geometries that arise in noncompact current algebra (or gauged Wess-Zumino-Witten (WZW)) coset models of strings and particles propagating in curved space-time. The simplest case is the two-dimensional (2D) black hole. In higher dimensions these geometries describe new and much more complex Singularities. For string and particle theories (defined in the text) we introduce general methods for identifying global coordinates and give the general exact solution for the geodesics for any gauged WZW model for any number of dimensions. We then specialize to the 3D geometries associated with SO(2,2)/SO(2,1) (and also SO(3,1)/SO(2,1)) and discuss in detail the global space, geodesics, curvature Singularities, and duality properties of this space. The large-small (or mirror-) type duality property is reformulated as an inversion in group parameter space. The 3D global space has two topologically distinct sectors, with patches of different sectors related by duality. The first sector has a singularity surface with the topology of pinched double trousers.'' It can be pictured as the world sheet of two closed strings that join into a single closed string and then split into two closed strings, but with a pinch in each leg of the trousers. The second sectormore » has a singularity surface with the topology of double saddle,'' pictured as the world sheets of two infinite open strings that come close but do not touch. We discuss the geodesically complete spaces on each side of these surfaces and interpret the motion of particles in physical terms. A cosmological interpretation is suggested and comments are made on possible physical applications.« less
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Superstrings on Curved Spacetimes
arXiv: High Energy Physics - Theory, 1992Co-Authors: Itzhak BarsAbstract:In this lecture I summarize recent developments on strings propagating in curved spacetime. Exact conformal field theories that describe Gravitational backgrounds such as black holes and more intricate Gravitational Singularities have been discovered and investigated at the classical and quantum level. These models are described by gauged Wess-Zumino-Witten models, or equivalently current algebra G/H coset models based on non-compact groups, with a single time coordinate. The classification of such models for all dimensions is complete. Furthermore the heterotic superstrings in curved spacetime based on non-compact groups have also been constructed. For many of the $d\le 4$ models the Gravitational geometry described by a sigma model has been determined. Some general results outlined here include a global analysis of the geometry and the exact classical geodesics for any G/H model. Moreover, in the quantized theory, the conformally exact metric and dilaton are obtained for all orders in an expansion of $k$ (the central extension). All such models have large-small (or mirror) duality properties which we reformulate as an inversion in group space. To illustrate model building techniques a specific 4-dimensional heterotic string in curved spacetime is presented. Finally the methods for investigating the quantum theory are outlined. The construction and analysis of these models at the classical and quantum level involve some aspects of noncompact groups which are not yet sufficiently well understood. Some of the open problems in the physics and mathematics areas are outlined.
George Savvidy - One of the best experts on this subject based on the ideXlab platform.
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gravity with a linear action and Gravitational Singularities
Proceedings of Corfu Summer Institute 2017 "Schools and Workshops on Elementary Particle Physics and Gravity" — PoS(CORFU2017), 2018Co-Authors: George SavvidyAbstract:Motivated by quantum-mechanical considerations we earlier suggested an alternative action for discretised quantum gravity which measures the perimeter of the space-time and has a dimension of length. It is the so called perimeter/linear action, since it is a "square root" of the area action in gravity and has a new constant of dimension one in front. The physical reason to introduce the perimeter/linear action was to suppress singular configurations "spikes" in the quantum-mechanical integral over geometries. Here we shall consider the continuous limit of the discretised perimeter action. We shall demonstrate that in the modified theory during the time evolution of a large massive star, when a star undergoes a collapse and develops an event horizon which confines the light, a smaller space-time region will be created behind the event horizon which is unreachable by test particles. These regions are located in the places where a standard theory of gravity has Singularities. We are confronted here with a drastically new concept that during the time evolution of a massive star a space-time region is created which is excluded from the physical scene, being physically unreachable by test particles or observables. If this concept is accepted, then it seems plausible that the Gravitational Singularities are excluded from the modified theory.
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lecture on quantum gravity with perimeter action and Gravitational Singularities
arXiv: High Energy Physics - Theory, 2018Co-Authors: George SavvidyAbstract:Motivated by quantum-mechanical considerations we earlier suggested an alternative action for discretised quantum gravity which measures the perimeter of the space-time and has a dimension of length. It is the so called perimeter action, since it is a "square root" of the area action in gravity and has a new constant of dimension one in front. The physical reason to introduce the perimeter/linear action was to suppress singular configurations "spikes" in the quantum-mechanical integral over geometries. Here we shall consider the continuous limit of the discretised perimeter/linear action. We shall demonstrate that in the modified theory during the time evolution of a large massive star, when a star undergoes a collapse and develops an event horizon which confines the light, a smaller space-time region will be created behind the event horizon which is unreachable by test particles. These regions are located in the places where a standard theory of gravity has Singularities. We are confronted here with a drastically new concept that during the time evolution of a massive star a space-time region is created which is excluded from the physical scene, being physically unreachable by test particles or observables. If this concept is accepted, then it seems plausible that the Gravitational Singularities are excluded from the modified theory.
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gravity with linear action and Gravitational Singularities
arXiv: High Energy Physics - Theory, 2017Co-Authors: George SavvidyAbstract:Motivated by quantum mechanical considerations we earlier suggested an alternative action for discretised quantum gravity which has a dimension of length. It is the so called "linear" action. The proposed action is a "square root" of the classical area action in gravity and has in front of the action a new constant of dimension one. Here we shall consider the continuous limit of the discretised linear action. We shall demonstrate that in the modified theory of gravity there appear space-time regions of the Schwarzschild radius scale which are unreachable by test particles. These regions are located in the places where standard theory of gravity has Singularities. We are confronted here with a drastically new concept that there may exist space-time regions which are excluded from the physical scene, being physically unreachable by test particles or observables. If this concept is accepted, then it seems plausible that the Gravitational Singularities are excluded from the modified theory.
Joshua Curtis - One of the best experts on this subject based on the ideXlab platform.
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numerical simulations of stiff fluid Gravitational Singularities
Physical Review D, 2005Co-Authors: Joshua Curtis, David GarfinkleAbstract:Numerical simulations of the approach to the singularity in spacetimes with stiff fluid matter are presented here. The spacetimes examined have no symmetries and can be regarded as representing the general behavior of Singularities in the presence of such matter. It is found that the singularity is spacelike and that as it is approached, the spacetime dynamics becomes local and nonoscillatory.
J. Haro - One of the best experts on this subject based on the ideXlab platform.
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can quantum effects due to a massless conformally coupled field avoid Gravitational Singularities
Theoretical and Mathematical Physics, 2012Co-Authors: J. HaroAbstract:Using quantum corrections from massless fields conformally coupled to gravity, we study the possibility of avoiding Singularities that appear in the flat Friedmann-Robertson-Walker model. We assume that the universe contains a barotropic perfect fluid with the state equation p = ωρ, where p is the pressure and ρ is the energy density. We study the dynamics of the model for all values of the parameter ω and also for all values of the conformal anomaly coefficients α and β. We show that Singularities can be avoided only in the case where α > 0 and β −1 (only a one-parameter family of solutions, but no a general solution, has this behavior at late times), the initial conditions of the nonsingular solutions at early times must be chosen very exactly. These nonsingular solutions consist of a general solution (a two-parameter family) exiting the contracting de Sitter phase and a one-parameter family exiting the contracting Friedmann phase. On the other hand, for ω < −1 (a phantom field), the problem of avoiding Singularities is more involved because if we consider an expanding Friedmann phase at early times, then in addition to fine-tuning the initial conditions, we must also fine-tune the parameters α and β to obtain a behavior without future Singularities: only a oneparameter family of solutions follows a contracting Friedmann phase at late times, and only a particular solution behaves like a contracting de Sitter universe. The other solutions have future Singularities.
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Can quantum effects due to a massless conformally coupled field avoid Gravitational Singularities?
Theoretical and Mathematical Physics, 2012Co-Authors: J. HaroAbstract:Using quantum corrections from massless fields conformally coupled to gravity, we study the possibility of avoiding Singularities that appear in the flat Friedmann-Robertson-Walker model. We assume that the universe contains a barotropic perfect fluid with the state equation p = ωρ, where p is the pressure and ρ is the energy density. We study the dynamics of the model for all values of the parameter ω and also for all values of the conformal anomaly coefficients α and β. We show that Singularities can be avoided only in the case where α > 0 and β < 0 . To obtain an expanding Friedmann universe at late times with ω > − 1 (only a one-parameter family of solutions, but no a general solution, has this behavior at late times), the initial conditions of the nonsingular solutions at early times must be chosen very exactly. These nonsingular solutions consist of a general solution (a two-parameter family) exiting the contracting de Sitter phase and a one-parameter family exiting the contracting Friedmann phase. On the other hand, for ω < − 1 (a phantom field), the problem of avoiding Singularities is more involved because if we consider an expanding Friedmann phase at early times, then in addition to fine-tuning the initial conditions, we must also fine-tune the parameters α and β to obtain a behavior without future Singularities: only a oneparameter family of solutions follows a contracting Friedmann phase at late times, and only a particular solution behaves like a contracting de Sitter universe. The other solutions have future Singularities.
