Latest Papers in General Relativity

General Relativity And Quantum Cosmology

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On the structure and applications of the Bondi-Metzner-Sachs group (1709.05134v3)

Francesco Alessio, Giampiero Esposito

2017-09-15

This work is a pedagogical review dedicated to a modern description of the Bondi-Metzner-Sachs group. The curved space-times that will be taken into account are the ones that suitably approach, at infinity, Minkowski space-time. In particular we will focus on asymptotically flat space-times. In this work the concept of asymptotic symmetry group of those space-times will be studied. In the first two sections we derive the asymptotic group following the classical approach which was basically developed by Bondi, van den Burg, Metzner and Sachs. This is essentially the group of transformations between coordinate systems of a certain type in asymptotically flat space-times. In the third section the conformal method and the notion of asymptotic simplicity are introduced, following mainly the works of Penrose. This section prepares us for another derivation of the Bondi-Metzner-Sachs group which will involve the conformal structure, and is thus more geometrical and fundamental. In the subsequent sections we discuss the properties of the Bondi-Metzner-Sachs group, e.g. its algebra and the possibility to obtain as its subgroup the Poincar'e group, as we may expect. The paper ends with a review of the Bondi-Metzner-Sachs invariance properties of classical gravitational scattering discovered by Strominger, that are finding application to black hole physics and quantum gravity in the literature.

On the asymptotic behavior of static perfect fluids (1801.06614v2)

Lars Andersson, Annegret Y. Burtscher

2018-01-20

Static spherically symmetric solutions to the Einstein-Euler equations with prescribed central densities are known to exist, be unique and smooth for reasonable equations of state. Some criteria are also available to decide whether solutions have finite extent (stars with a vacuum exterior) or infinite extent. In the latter case, the matter extends globally with the density approaching zero at infinity. The asymptotic behavior largely depends on the equation of state of the fluid and is still poorly understood. While a few such unbounded solutions are known to be asymptotically flat with finite ADM mass, the vast majority are not. We provide a full geometric description of the asymptotic behavior of static spherically symmetric perfect fluid solutions with linear and polytropic-type equations of state with index n>5. In order to capture the asymptotic behavior we introduce a notion of scaled quasi-asymptotic flatness, which encodes a form of asymptotic conicality. In particular, these spacetimes are asymptotically simple.

Quantum and classical effects in light-clock falling in Schwarzschild geometry (1901.08000v1)

Maximilian P. E. Lock, Ivette Fuentes

2019-01-23

Quantum theory and relativity offer different conceptions of time. To explore the conflict between them, we study a quantum version of the light-clock commonly used to illustrate relativistic time dilation. This semiclassical model combines elements of both theories. We show for Gaussian states of the light field that the clock times are independent of the initial state. We calculate the discrepancy between two such clocks when one is held in a gravitational field and the other is left to fall a certain distance. Contrasting our results with the case of pointlike observers in general relativity, as well as classical light-clocks, we find both quantitative and qualitative differences. We find that the quantum contribution to the discrepancy between the two clocks increases with the gravitational field strength, and results in a minimum resolution of the dropped clock (distinct from the quantum uncertainty in its measurement).

The future is not always open (1901.07996v1)

James D. E. Grant, Michael Kunzinger, Clemens Sämann, Roland Steinbauer

2019-01-23

We demonstrate the breakdown of several fundamentals of Lorentzian causality theory in low regularity. Most notably, chronological futures (defined naturally using locally Lipschitz curves) may be non-open, and may differ from the corresponding sets defined via piecewise -curves. By refining the notion of a causal bubble from [CG:12],we characterize spacetimes for which such phenomena can occur, and also relate these to the possibility of deforming causal curves of positive length into timelike curves (push-up). The phenomena described here are, in particular, relevant for recent synthetic approaches to low regularity Lorentzian geometry where, in the absence of a differentiable structure, causality has to be based on locally Lipschitz curves.

On Prescription in Cosmology (1810.12324v2)

Ali Kaya

2018-10-29

This is a technical note on the prescription in cosmology where we consider a self-interacting scalar field in the Poincare patch of the de Sitter space whose Hamiltonian has explicit time dependence. We use both path integral and operator formalisms to work out the evolution of states from asymptotic past infinity with prescription, which becomes nontrivial even in the free theory, and explicitly show how arbitrary states are projected onto the vacuum. We establish that in perturbation theory the prescription can be implemented in Weinberg's commutator formula by just inserting dependent convergence factors that make the oscillating time integrals at infinity meaningful.

Dynamical properties of Bianchi-I spacetimes in gravity (1805.03237v5)

Saikat Chakraborty, Kazuharu Bamba, Alberto Saa

2018-05-08

We present a dynamical analysis in terms of new expansion-normalized variables for homogeneous and anisotropic Bianchi-I spacetimes in gravity in the presence of anisotropic matter. With a suitable choice of the evolution parameter, the Einstein's equations are reduced to an autonomous 5-dimensional system of ordinary differential equations for the new variables. Further restrictions lead to considerable simplifications. For instance, we show that for a large class of functions , which includes several cases commonly considered in the literature, all the fixed points are polynomial roots, and hence they can be determined with good accuracy and classified for stability. Moreover, typically for these cases, any fixed point corresponding to isotropic solutions in the presence of anisotropic matter will be unstable. The assumption of a perfect fluid as source and or the vacuum cases imply some dimensional reductions and even more simplifications. In particular, we find that the vacuum solutions of , with a constant, are governed by an effective bi-dimensional phase space which can be analytically constructed, leading to an exactly soluble dynamics. Finally, we demonstrate that several results already reported in the literature can be re-obtained in a more direct and easy way by exploring our dynamical formulation.

Observational Constraints in Nonlocal Gravity: the Deser-Woodard Case (1901.07832v1)

Luca Amendola, Yves Dirian, Henrik Nersisyan, Sohyun Park

2019-01-23

We study the cosmology of a specific class of nonlocal model of modified gravity, the so-called Deser-Woodard (DW) model, modifying the Einstein-Hilbert action by a term , where is a free function. Choosing so as to reproduce the cosmological background expansion history within the nonlocal model, we implement the model in a cosmological linear Einstein--Boltzmann solver and study the deviations to GR the model induces in the scalar and tensor perturbations. We observe that the DW nonlocal model describes a modified propagation for the gravitational waves, as well as a lower linear growth rate and a stronger lensing power as compared to , up to several percents. Such prominent growth and lensing features lead to the inference of a significantly smaller value of with respect to the one in , given \textit{Planck} CMB+lensing data. The prediction for the linear growth rate within the DW model is therefore significantly smaller than the one in and the addition of growth rate data from Redshift-space distortion measurements to \textit{Planck} CMB+lensing, opens a (dominant) tension between Redshift-space distortion data and the reconstructed \textit{Planck} CMB lensing potential. However, model selection issues only result in "weak" evidences for against the DW model given the data. Such a fact shows that the datasets we consider are not constraining enough for distinguishing between the models. As we discuss, the addition of galaxy WL data or cosmological constraints from future galaxy clustering, weak lensing surveys, but also third generation gravitational wave interferometers, prove to be useful for discriminating modified gravity models such as the DW one from , within the close future.

Bulk geometry from entanglement entropy of CFT (1807.04646v2)

Ashis Saha, Sourav Karar, Sunandan Gangopadhyay

2018-07-11

In this paper, we compute the exact form of the bulk geometry emerging from a -dimensional conformal field theory using the holographic principle. We first consider the -dimensional asymptotic metric in Poincare coordinates and compute the area functional corresponding to the static minimal surface and obtain the entanglement entropy making use of the holographic entanglement entropy proposal. We then use the results of the entanglement entropy for -dimensional conformal field theory on an infinite line, on an infinite line at a finite temperature and on a circle. Comparing these results with the holographic entanglement entropy, we are able to extract the proper structure of the bulk metric. Finally, we also carry out our analysis in the case of super Yang-Mills theory and obtain the exact form of the dual bulk geometry corresponding to this theory. The analysis reveals the behavior of the bulk metric in both the near boundary region and deep inside the bulk. The results also show the influence of the boundary UV cut-off "" on the bulk metric. It is observed that the reconstructed metrics match exactly with the known results in the literature when one moves deep inside the bulk or towards the turning point.

Configurational entropy of tachyon kinks on unstable Dp-branes (1812.00343v2)

Chong Oh Lee

2018-12-02

We consider tachyon effective theory with Born-Infeld electromagnetic fields and investigate the configurational entropy of the various tachyon kink solutions. We find that the configurational entropy stats at a minimum value and saturates to a maximum value as the negative pressure of pure tachyonic field increases. In particular, when an electric field is turned on and its magnitude is larger than or equal to the critical value, we find the configurational entropy has a global minimum, which is related to the predominant tachyonic states.

Scalar Field Theory Description of the Running Vacuum Model: from the scalaron to the "vacuumon" (1901.06638v2)

Spyros Basilakos, Nick E. Mavromatos, Joan Sola

2019-01-20

We investigate the running vacuum model (RVM) in the framework of scalar field theory.This dynamical vacuum model provides an elegant global explanation of the cosmic history, namely the universe starts from a non-singular initial de Sitter vacuum stage, it passes smoothly from an early inflationary era to a radiation epoch ("graceful exit") and finally it enters the dark matter and dark energy (DE) dominated epochs, where it can explain the large entropy problem and predicts a mild dynamical evolution of the DE. Within this phenomenologically appealing context, we formulate an effective scalar field description of the RVM through a field , called the vacuumon, which turns out to be very helpful for an understanding and practical implementation of the physical mechanisms of the running vacuum during both the early universe and the late time cosmic acceleration. In the early universe, the vacuumon behaves similarly to the scalaron field of Starobinsky-type inflation, whilst in the late universe it provides an effective scalar field description of DE.

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