General Relativity and Gravitation最新文献

Recently, it has become increasingly clear that there are constraints on the low-energy effective theories of quantum gravity that cannot be captured by the standard Wilsonian paradigm. For gravitational theories in asymptotically anti-de Sitter spacetimes, we can formulate such constraints and aim to prove or falsify them using the AdS/CFT correspondence. I will review the work I did with Daniel Harlow on constraints on symmetries of quantum gravity. I will also discuss more recent progress in this holographic approach, and present the proof that Yifan Wang and I completed this year, which proves and strengthens a part of the Distance Conjecture that I proposed with Cumrun Vafa in 2006.

I present a review of the Dirac equation in general relativity. Although the generalization of the Dirac equation to a curved spacetime is well known, it is not usually part of the standard toolkit of techniques known to people working on classical general relativity. Recently, there has been some renewed interest in studying solutions of the Einstein–Dirac system of equations, particularly in the context of the so-called “Dirac stars”. Motivated by this, here I present a review of the Dirac equation in general relativity, starting from Minkowski spacetime, and then considering the Lorentz group and the tetrad formalism in order to generalize this equation to the case of a curved spacetime. I also derive the form of the Dirac equation and its associated stress–energy tensor for the case of the 3+1 formalism of general relativity, which can be useful for the study of the evolution of the Dirac field in a dynamical spacetime.

Constraining the Generalized Uncertainty Principle (GUP) parameter is crucial for probing potential quantum gravity effects in regimes that extend beyond the Planck scale. In this study, we place bounds on the (beta ) parameter, associated with the widely studied quadratic GUP model, using existing experimental data from nuclear matter and results from chiral effective field theory ((chi )EFT) calculations. We also assess the compatibility of neutron star (NS) matter prediction based on those extracted from NS observations. The quadratic GUP model shares the same dispersion relation as a specific version of Double Special Relativity (DSR), establishing a connection between one of the rainbow gravity (RG) parameters and the quadratic GUP parameter. We then explore NS properties within the RG framework, defining (X = E/E_p) alongside (beta ). Therefore, we calculate the predictions for slow-rotating NS using the RG effective metric and compare these results with existing observational data. From our analysis, we obtain an upper bound of (beta = 1.5 times 10^{-7}) based on nuclear matter and neutron star matter data. We also find a non-zero lower bound of (beta = -1.5 times 10^{-7}). When using (beta = 1.5 times 10^{-7}) within the RG framework, the maximum mass prediction is lower than the constraints derived from the NICER data. In fact, rather than increasing, the parameter X further decreases the maximum mass prediction. However, when we set (beta = -1.5 times 10^{-7}) and (X = 10^{-38.5}), the maximum neutron star mass remains consistent with NICER and other astrophysical constraints. Our results show that slowly rotating NS favor negative (beta ) within this framework.

Recent observational evidences point out towards a late time acceleration of the universe. In order to study the accelerated expansion, scientists have incorporated the existence of an exotic matter with negative pressure, termed as dark energy. Afterwards a new idea of dark energy has been studied depending on the holographic principle of quantum gravity, called as the Holographic Dark Energy(HDE). Later on modifying Bekestein-Hawking entropy, different generalized entropies have been proposed, one of them being Rényi entropy which leads to Rényi holographic dark energy model (RHDE). We have considered RHDE model with Hubble horizon as the IR cut off and have studied the cosmological behaviour under non interacting, linear and non-linear interacting scenarios with the help of dynamical systems analysis. We have also investigated the stability of the system around hyperbolic critical points along with the type of fluid description, evolution of equation of state parameter as well as matter and energy density parameters.

The latest observations from the LIGO-Virgo indicated the existence of mass-gap region astrophysical objects. This is a rather sensational observation and there are two possibilities for the nature of these mass-gap region astrophysical objects, these are either small black holes that result from the mergers of ordinary mass neutron stars, or these are heavy neutron stars. In the line of research implied by the former possibility, in this work we shall examine the implied neutron star phenomenology from vector f(R) gravity inflationary models. These theories are basically scalar-tensor deformations of the Starobinsky inflationary model. We shall present the essential features of cosmologically viable and non-viable deformations of the Starobinsky model, originating from vector f(R) gravity inflationary theories, and we indicate which models and for which equations of state provide a viable neutron star phenomenology. We solve the Tolman-Oppenheimer-Volkov equations using a robust double shooting LSODA python based code, for the following piecewise polytropic equations of state: the WFF1, the SLy, the APR, the MS1, the AP3, the AP4, the ENG, the MPA1 and the MS1b. We confront the resulting phenomenology with several well-known neutron star constraints, and we indicate which equation of state and model fits the phenomenological constraints. A remarkable feature, also known from other inflationary attractor models, is that the MPA1 is the equation of state which is most nicely fitted to the constraints, for all the theoretical models used, and actually the maximum mass for this equation of state is well inside the mass-gap region. Another mentionable feature that stroked us with surprise is the fact that even cosmologically non-viable inflationary models produced a viable neutron star phenomenology, which most likely has to be a model-dependent feature.

We describe an attempt of string theoretic derivation of the Gibbons-Hawking entropy. Despite not admitting a de Sitter vacuum, the string theory, by the power of open-close correspondence, captures the Gibbons-Hawking entropy as the entropy of Chan-Paton species on a de Sitter-like state obtained via D-branes. Moreover, this derivation sheds a new light at the origin of the area-form, since the equality takes place for a critical ’t Hooft coupling for which the species entropy of open strings saturates the area-law unitarity bound.

The gravitational field equations in general relativity (GR) consist of a sophisticated system of nonlinear partial differential equations. Solving such equations in some generic off-diagonal forms is usually a hard analytic or numeric task. Physically important solutions in GR were constructed using diagonal ansatz for metrics with maximum 4 independent coefficients. The Einstein equations can be solved in exact or parametric forms determined by some integration constants for corresponding assumptions on spherical or cylindric spacetime symmetries. The anholonomic frame and connection deformation method allows us to construct generic off-diagonal solutions described by 6 independent coefficients of metrics depending, in general, on all spacetime coordinates. New types of exact and parametric solutions are determined by generating and integration functions and (effective) generating sources. They may describe vacuum gravitational and matter fields solitonic hierarchies; locally anisotropic polarizations of physical constants for black holes, wormholes, black toruses, or cosmological solutions; various types of off-diagonal deformations of horizons etc. The additional degrees of freedom (related to off-diagonal coefficients) can be used to describe dark energy and dark matter configurations and elaborate locally anisotropic cosmological scenarios. In general, the generic off-diagonal solutions do not involve certain hypersurface or holographic configurations and can’t be described in the framework of the Bekenstein-Hawking thermodynamic paradigm. We argue that generalizing the concept of G. Perelman’s entropy for relativistic Ricci flows allows us to define and compute geometric thermodynamic variables for all possible classes of solutions in GR.

In this manuscript we confirm the presence of a Rindler horizon at CERN-NA63 by exploring its thermodynamics induced by the Unruh effect in their high energy channeling radiation experiments. By linking the entropy of the emitted radiation to the photon number, we find the measured spectrum to be a simple manifestation of the second law of Rindler horizon thermodynamics and thus a direct measurement of the recoil Fulling-Davies-Unruh (FDU) temperature. Moreover, since the experiment is born out of an ultra-relativistic positron, and the FDU temperature is defined in the proper frame, we find that temperature boosts as a length and thus fast objects appear colder. The spectrum also provides us with a simple setting to measure fundamental constants, and we employ it to measure the positron mass.

In this short paper, we investigate the consequences of observer dependence of the quantum effective potential for an interacting field theory. Specializing to (d+2) dimensional Euclidean Rindler space, we develop the formalism to calculate the effective potential. While the free energy diverges due to the presence of the Rindler horizon, the effective potential, which is a local function of space, is finite after the necessary renormalization procedure. We apply the results of our formalism to understand the restoration of spontaneously broken (mathbb {Z}_2) symmetry in three and four dimensions.

We shall investigate the inflation for the D-brane model, motivated by the modified gravity (F(phi ,T)). This gravity has been recently introduced in the literature. The feasibility of the D-brane inflation theory in the (F(phi ,T))-gravity has been studied in conjunction with the most recent Planck data. We shall analyze the slow-roll inflation in the context of the (F(phi )T)-gravity, via the D-brane model. Then, we shall calculate the inflation dynamics to obtain the scalar spectral index “(n_s)” and the tensor-to-scalar ratio “r”. Besides, we investigate the dynamics of the reheating for this model. Our model accurately covers the left-hand side of the Planck data and the D-brane inflation.