General Relativity and Gravitation最新文献

The question of black hole evaporation is reviewed in the framework of quantum field theory in curved spacetimes and semiclassical gravity. We highlight the importance of taking backreaction effects into account to have a consistent picture of the fate of gravitational collapse in this framework. We describe the difficulties of solving the backreaction semiclassical equations due to practical complications of renormalizing the stress-energy tensor of quantum fields in general 3+1 spacetimes. We end with some personal views and plans on the subject.

The gravitational path-integral of Gauss–Bonnet gravity is investigated and the transition from one spacelike boundary configuration to another is analyzed. Of particular interest is the case of Neumann and Robin boundary conditions which is known to lead to a stable Universe in Einstein–Hilbert gravity in four spacetime dimensions. After setting up the variational problem and computing the necessary boundary terms, the transition amplitude is computed exactly in the mini-superspace approximation. The (hbar rightarrow 0) limit brings out the dominant pieces in the path-integral which is traced to an initial configuration corresponding to Hartle–Hawking no-boundary Universe. A deeper study involving Picard–Lefschetz methods not only allow us to find the integration contour along which the path-integral becomes convergent but also aids in understanding the crossover from Euclidean to Lorentzian signature. Saddle analysis further highlights the boundary configurations giving dominant contribution to the path-integral which is seen to be those corresponding to Hartle–Hawking no-boundary proposal and agrees with the exact computation. To ensure completeness, a comparison with the results from Wheeler–DeWitt equation is done.

We consider a free-falling observer who crosses the event horizon in the Schwarzschild background. In the course of this fall, he/she can receive signals from an object (like a star surface) that emits radiation. We study how the frequency received by an observer changes depending on the proper time on his/her trajectory. The scenarios are classified depending on whether the frequency is infinite, finite or zero near the singularity and the horizon. This depends crucially on the angular momenta of an observer and a photon. In this work we consider also emission process, and, as we show, conditions of emission strongly influence parameters of a photon, and thus received frequency. As one of our main results, we present numerical calculations showing evolution of the received frequency during the process of diving into a black hole, depending on parameters of an observer and emitter. We also analyze how a falling observer will see a night sky as he/she approaches the singularity. We show that there appear several blind zones, which were not analyzed previously.

We present a novel perspective on gravity-induced wave function reduction using Bohmian trajectories. This study examines the quantum motion of both point particles and objects, identifying critical parameters for the transition from quantum to classical regimes. By analyzing the system’s dynamics, we define the reduction time of the wave function through Bohmian trajectories, introducing a fresh viewpoint in this field. Our findings align with results obtained in standard quantum mechanics, confirming the validity of this approach.

We investigate the quantum fate of the classical singularities that occur by gravitational collapse of a dust cloud. For this purpose, we address the quantization of a model first proposed by Georges Lemaître in 1933. We find that the singularities can generically be avoided. This is a consequence of unitary evolution in the quantum theory, whereby the quantum dust cloud collapses, bounces at a minimal radius and re-expands.

Quantum black holes, a broad class of objects that refine the solutions of general relativity by incorporating semiclassical and/or quantum gravitational effects, have recently attracted renewed attention within the scientific community. This resurgence of interest is largely driven by advances in gravitational wave astronomy, which have opened the possibility of testing some of these models in the near future. In this essay, we provide a concise overview of the key discussions that emerged during the “Black Hole Inside/Out" meeting, held in August 2024 in Copenhagen. We report these ideas, their connections to the information paradox, and the potential use of analogue gravity as a test bed for these concepts.

In this work, we explore non-commutative effects in fractional classical and quantum schemes using the anisotropic Bianchi type I cosmological model coupled to a scalar field in the K-essence formalism. We introduce non-commutative variables considering that all minisuperspace variables (q^{i}_{nc}) do not commute, so the symplectic structure was modified, resulting in some changes concerning the traditional formalism. In the quantum regime, the probability density presents a new structure in the scalar field corresponding to the value of the non-commutative parameter.

I briefly describe motivation for, and the current state of research into understanding the structure and dynamics of black hole “imposters”: objects that could be misidentified as Kerr black holes given the current precision of LIGO/Virgo gravitational wave observations, or EHT accretion disk measurements. I use the term “weak imposter” to describe an object which is a black hole, i.e. it has an event horizon, but whose structure and dynamics is governed by a modified gravity theory. At the other end of the spectrum are “strong imposters”: hypothetical horizonless, compact objects conjectured to form instead of black holes during gravitational collapse. To discover or rule-out imposters will require a quantitative understanding of their merger dynamics. This is hampered at present by a dearth of well-posed theoretical frameworks to describe imposters beyond perturbations of Kerr black holes and their general relativistic binary dynamics. That so little is known about non-perturbative modifications to dynamical, strongfield gravity is, I argue, due to a lamppost effect.

In this paper, we consider a nonminimal coupling model between gravity and nonlinear electrodynamics with cosmological constant. This cosmological model is designed to account for both the inflationary epoch of the early universe and the current phase of accelerated cosmic expansion. The nonlinear electrodynamic fields provide a mechanism for a graceful exit from the inflationary period, preventing the universe from entering an eternal inflation state. The addition of nonminimal coupling plays a crucial role in shaping the early evolution of the universe. We compare the theoretical predictions of our model with recent observational data and other leading cosmological models, showing that our approach provides a viable and competitive explanation for key aspects of the universe’s evolution. Our results suggest that this model offers a consistent and compelling framework to explain both early-time inflation and the late-time accelerated expansion of the universe, in line with current observations.

This study explores the feasibility of an effective Friedmann equation in removing the classical Big Bang initial singularity and replacing it with a non-singular bounce occurring at a critical energy density value. In a spatially flat, homogeneous, and isotropic universe, the effective theory is obtained by introducing a function parametrically dependent on the critical energy density. This function measures the deviation from the benchmark theory, which is recovered as the critical energy density approaches infinity. Focusing on the covariant single-field formulation in viable Horndeski gravity, our analysis shows that both the effective and the benchmark theories belong to the same scalar–tensor theory, without any additional propagating degrees of freedom: the cuscuton and extended cuscuton models.