General Relativity and Gravitation最新文献

We present a review of concepts of thermodynamic of spacetime that allows for an understanding of the gravitational dynamics encoding in it, discussing also the recovery of Weyl transverse gravity instead of General Relativity. We also discuss how these tools can provide some hints in the search of quantum gravity phenomenology, by introducing a formalism to analyze low-energy quantum gravity modifications in a completely general framework based on the thermodynamics of spacetime. For that purpose, we consider quantum gravity effects via a parametrized modification of entropy by an extra logarithmic term in the area, predicted in most of the different approaches to quantum gravity. These results provide a general expression for quantum phenomenological equations of gravitational dynamics.

Using Darmois-Israel-Sen junction conditions, and with help of Visser’s cut-and-paste method, we study the dynamics of thin-shell wormholes that are made of two conformally Killing gravity (a.k.a Harada gravity) black holes. We check the energy conditions for different values of the new parameter that Harada introduced, as alternative for dark energy. We examine the radial acceleration to reveal the attractive and repulsive characteristics of the thin-shell wormhole throat. We consider the dynamics and stability of the wormhole around the static solutions of the linearized radial perturbations at the wormhole throat. Finally, we determine the regions of stability by applying the concavity test on the “speed of sound” as a function in the throat radius and other spacetime parameters, particularly the new Harada parameter.

We explore black hole solutions and some of its physical properties in Einstein’s theory in 4D, modified by a cubic gravity term and in the presence of non-linear electrodynamics. In the context of Effective Field Theories (EFT) and under certain assumptions, these curvature and non-linear electromagnetic terms represent the first corrections to the Einstein-Maxwell theory. We obtain static and spherically symmetric generalizations to the asymptotically flat Reissner-Nordström metric using perturbative methods, showing how an asymptotic expansion solution connects with a near horizon solution for a small coupling of the curvature correction term. We perform a thermodynamic stability analysis of the solutions. Finally, we discuss how these EFT corrections change the event horizon properties and also lead to measurable effects on black hole shadows and gravitational lensing around these solutions.

The status of the equivalence principle in modified symmetric teleparallel gravity is examined. In this theory, minimum length geodesics are distinct from autoparallel geodesics, that is, the “shortest” paths are not the “straightest” paths. We show that a standard argument that singles out metric geodesics in general relativity does not apply in modified symmetric teleparallel gravity. This is because the latter theory does not obey the equivalence principle in the sense of Weinberg. We argue, however, that the structure of the theory makes it inevitable that a freely falling test particle follows a shortest path, a geodesic of the metric. The geodesic equation that governs the motion of a freely falling test particle involves the Levi-Civita connection, not some other connection obtained by solving the connection field equations of the theory. This also has bearing on whether, under appropriate conditions, modified symmetric teleparallel gravity is fully equivalent to general relativity.

This study examines the compatibility of the generalized holographic equipartition proposed in Sheykhi (Phys Rev D 87(6):061501, 2013) with the maximization of horizon entropy in an (n + 1)-dimensional non-flat Friedmann–Robertson–Walker (FRW) universe. Here, the entropy associated with the apparent horizon is described by Kaniadakis entropy, as well as truncated Kaniadakis entropy, which is expanded and truncated to third order when the Kaniadakis parameter ((K)) is small, indicating minor deviations from the standard Bekenstein–Hawking entropy. Initially, we derive the conditions required for maximizing both Kaniadakis horizon entropy and truncated Kaniadakis horizon entropy. We then examine whether the generalized holographic equipartition aligns with the constraints of horizon entropy maximization. Our findings reveal that the generalized holographic equipartition is consistent with the maximization of Kaniadakis horizon entropy and truncated Kaniadakis horizon entropy in a universe with non-zero spatial curvature.

Compact stars have long served as a test bed of gravitational models and their coupling with stellar matter. In this work, we explore the behavior of an exponential model in f(T) gravity through the Tolman-Oppenheimer-Volkoff equation. This is performed for different envelope thicknesses. Finally, constraints on the models parameters are obtained, which are comparable to the results obtained using cosmological survey data. This consistency across the strong astrophysical and weak cosmological scales shows reasonable viability of the underlying model.

Starting from a Wheeler–DeWitt type equation for an uncharged black hole (Q=0), and by choosing the order parameter ((s=2)) and running the gravitational degrees of freedom it is possible to reduce the Wheeler–DeWitt equation to the canonical form of a quantum harmonic oscillator. In this direction, a natural frequency of oscillation is identified for the black hole. The entanglement entropy of a pair of interacting quantum black holes of mass M is obtained and analyzed. Here we consider as a starting model a pair of identical oscillators of frequency (omega ) coupled by a quadratic potential and with interaction constant given by (omega _{c}). Given the relation between the oscillation frequency (omega ) of an isolated quantum black hole and its mass, the entanglement entropy of this system is obtained by analogy with a pair of quantum oscillators coupled by a quadratic interaction potential of frequency (omega _{c}). The analysis of the entanglement entropy is performed by introducing the reduced variables ({widetilde{omega }}) and ({widetilde{A}}). An interesting result arises when we consider ({widetilde{A}}gg 1) and the interaction parameter is set ({widetilde{omega }}=1). In this case, the entanglement entropy can be replaced by its asymptotic expansion, where the dominant term is of logarithmic character in the reduced area. Another case of analysis emerges when the reduced area ({widetilde{A}}=1) and ({widetilde{omega }}) varies. In this case, the entanglement entropy depends uniquely on the interaction parameter between the two black holes.

String theory has provided a resolution of the puzzles that arise in the quantum theory of black holes. The emerging picture of the hole, encoded in the ‘fuzzball paradigm’, offers deep lessons about the role of quantum gravity on macroscopic length scales. In this article we list these puzzles and explain how they get resolved. We extract the lessons of this resolution in a form that does not involve the technical details of string theory; it is hoped that this form will allow the lessons to be absorbed into other approaches to quantum gravity.

This collection of perspective pieces captures recent advancements and reflections from a dynamic research community dedicated to bridging quantum gravity, hydrodynamics, and emergent cosmology. It explores four key research areas: (a) the interplay between hydrodynamics and cosmology, including analog gravity systems; (b) phase transitions, continuum limits and emergent geometry in quantum gravity; (c) relational perspectives in gravity and quantum gravity; and (d) the emergence of cosmological models rooted in quantum gravity frameworks. Each contribution presents the distinct perspectives of its respective authors. Additionally, the introduction by the editors proposes an integrative view, suggesting how these thematic units could serve as foundational pillars for a novel theoretical cosmology framework termed “hydrodynamics on superspace”.

The existence of a set of 10 Intrinsic Conformal Symmetries, which acts on three-dimensional hypersurfaces (spacelike or timelike), leads to the existence of two distinct families of 5D geometries. These models represent the general solutions of the bulk field equations where their energy-momentum tensor, includes only two components: a negative cosmological constant and a parallel pressure (p_{parallel }) aligned with the extra spatial dimension. Significantly, these models offer a novel perspective for investigating the impacts of spatial inhomogeneity and anisotropy on the cosmological evolution of the Universe, particularly within the context of the braneworld scenarios. It is shown that one of these families reduces to a fully inhomogeneous, anisotropic and conformally flat brane model with a perfect fluid equation of state and corresponds to the Stephani Universe (i.e. a Stephani brane) which implies that our model can be matched smoothly with the standard FRW model. We provide the generalized Friedmann and Raychaudhuri equations and we present how the additional quantities could affect the cosmological evolution. In particular we show that the new constituents are the terms (p_{parallel }), (sigma ^{2}) and the four acceleration of the brane observers that could affect the observational measurement of the Hubble parameter depending on which term dominates therefore provide us a potential answer to the Hubble tension and cosmic acceleration problems due to local inhomogeneities and anisotropies.