General Relativity and Gravitation最新文献

The butterfly velocity of four-dimensional rotating charged asymptotically AdS black hole is calculated to probe chaos using localized rotating shock waves. In this work, we obtain the angular momentum dependence of the butterfly velocity due to rotation in the shock wave probes. In general, the angular momentum (mathcal {L}) of the shock waves increases the butterfly velocity. The localized shocks also generate butterfly velocities which vanish when we approach extremality, indicating no entanglement spread near extremality. One of the butterfly velocity modes is well bounded by both the speed of light and the Schwarzschild-AdS result, while the other may become superluminal. Aside from the logarithmic behavior of the scrambling time which indicates chaos, the Lyapunov exponent is also positive and bounded by (kappa =2pi T_H/(1-mu mathcal {L})). The Kerr–NUT–AdS and Kerr–Sen–AdS solutions and their ultraspinning versions are used as examples to attain a better understanding of the chaotic phenomena in rotating black holes, especially those with extra conserved charges.

We examine the main properties of gravitational waves (GWs) emitted by transient hyperbolic encounters of black holes. We begin by building the set of basic variables most relevant to setting our problem. After exposing the ranges of masses and eccentricities accessible at a given GW frequency, we analyze the dependence of the gravitational strain on those parameters and determine the trajectories resulting in the most sizeable strains. Some non-trivial behaviors are unveiled, showing that highly eccentric events can be more easily detectable than parabolic ones. In particular, we underline the correct way to extend formulas from hyperbolic to parabolic orbits. Our reasonings are as general as possible, and we make a point of explaining our considerations pedagogically. The majority of the work is based on Newtonian dynamics and aims at being a benchmark to which more accurate calculations can be compared.

In this paper we use the Homotopy analysis method to obtain an analytic approximation for the entire photon trajectory in the Schwarzschild spacetime. This is usually expressed exactly in terms of an elliptic integral. We compare our approximation with other formulae found in the literature, which were specifically obtained for the Schwarzschild solution. Unlike some of these formulae, our approximation can be applied and maintains a good accuracy for emission point close to the event horizon and also for emission angles close to and greater than (pi /2). We show that our method can easily be applied to other spherically symmetric solutions such as the Reissner-Nordström solution. Such an approximation would be useful when accurate determination of the light trajectories around compact objects is required without the need to revert to time consuming numerical integration of elliptic integrals.

In this paper, employing the exponential corrected entropy (Chatterjee and Ghosh in Phys Rev Lett 125:041302, 2020), we derive the modified Friedmann equations from the first law of thermodynamics at apparent horizon and Verlinde’s entropic gravity scenario. First, we derive the modified Friedmann equations from the first law of thermodynamics. We investigate the validity of generalised second law (GSL) of thermodynamics and find that it is always satisfied for the all eras of universe. Moreover, we investigate the deceleration parameter for the case (k=0) in two frameworks. Finally, we numerically study the bouncing behaviour for the modified Friedmann equations obtained from entropic gravity. The results indicate that the bouncing behaviour is possible for the cases (k=1) and (k=-1).

We obtain a class of solutions that correspond to a generalization of the Bardeen black hole solution by solving the Einstein equations coupled to a particular nonlinear electromagnetic field. The generalization is realized by considering, additionally, the presence of the cosmological constant and a source corresponding to an anisotropic fluid, namely, a fluid of strings, which surrounds the black hole. We show that the obtained class of solutions preserve or not the regularity of the original Bardeen black hole solution, depending on the values of the parameter (beta ) which labels the different solutions. We discuss the characteristics of the solutions, from the point of view of the singularities of the spacetime, by examining the behavior of the Kretschmann scalar as well as of the geodesics with respect to their completeness. We analyze some aspects of the thermodynamics, particularizing to one of the solutions obtained, namely, for (beta = nicefrac {-1}{2}), in which case the regularity of Bardeen black hole is preserved. We also show that there is an incompatibility between the temperature arising from the first law of the black hole thermodynamics and the one using the surface gravity. Some thermodynamic quantities are obtained and analyzed, as for example, pressure, heat capacity, and the critical points, and we show how these quantities change for different values of the parameter q associated with the original Bardeen solution, as well as with the parameter b associated with the presence of the fluid of strings. The phase transitions are also analyzed by using the equation of state and the Helmholtz free energy.

We explore the impact of the chameleon mechanism in scalar field Einstein–Gauss–Bonnet gravity on the dynamics of cosmological parameters. Conducting a thorough analysis of the phase space, we identify conditions under which the future attractor does not depict a singular universe. Our conclusion is that although Einstein–Gauss–Bonnet scalar field gravity offers an inflationary solution as a future attractor, it is unable to account for the late-time acceleration of the universe and it can not describe a universe similar to that provide by the (Lambda )CDM model. On the other hand, when the matter source is described by a massless scalar field, then the de Sitter universe is a future attractor. However, the hyperbolic inflationary solution provided by the two scalar fields does not exist.

We explicitly construct the analogue of the d’Alembert solution to the 1+1 wave equation in an hyperboloidal setting. This hyperboloidal d’Alembert solution is used, in turn, to gain intuition into the behaviour of solutions to the wave equation in a hyperboloidal foliation and to explain an apparently anomalous permanent displacement of the solution in numerical simulations discussed in the literature.

The present work deals with a FLRW cosmological model with spatial curvature and minimally coupled scalar field as the matter content. The curvature term behaves as a perfect fluid with the equation of state parameter (omega _{mathcal {K}}=-frac{1}{3}). Using suitable transformation of variables, the evolution equations are reduced to an autonomous system for both power law and exponential form of the scalar potential. The critical points are analyzed with center manifold theory and stability has been discussed. Also, critical points at infinity have been studied using the notion of Poincaré sphere. Finally, the cosmological implications of the critical points and cosmological bouncing scenarios are discussed. It is found that the cosmological bounce takes place near the points at infinity when the non-isolated critical points on the equator of the Poincaré sphere are saddle or saddle-node in nature.

The center-of-mass energy of two colliding particles could be arbitrarily high in the vicinity of event horizons of the extremal Myers-Perry black holes if the angular momentum of colliding particles is fine-tuned to the critical values. We investigate the maximum efficiency of two colliding particles in four and six dimensions. The efficiency of collision for two particles near the four-dimensional Kerr black holes is 130%. We show that the efficiency increases to 145% for collision in six dimensions. We also show that the region for the polar angle in which the particle can reach the high energy is larger when the dimension of space-time increases.

In this study, we explore the accelerated expansion of the universe within the framework of modified f(Q) gravity. The investigation focus on the role of bulk viscosity in understanding the universe’s accelerated expansion. Specifically, a bulk viscous matter-dominated cosmological model is considered, with the bulk viscosity coefficient expressed as (zeta = zeta _0 rho H^{-1} + zeta _1 H ). We consider the power law f(Q) function (f(Q)=alpha Q^n ), where (alpha ) and n are arbitrary constants and derive the analytical solutions for the field equations corresponding to a flat FLRW metric. Subsequently, we used the combined Cosmic Chronometers (CC)+Pantheon+SH0ES sample to estimate the free parameters of the obtained analytic solution. We conduct Bayesian statistical analysis to estimate the posterior probability by employing the likelihood function and the MCMC random sampling technique, along with the AIC and BIC statistical assessment criteria. In addition, we explore the evolutionary behavior of significant cosmological parameters. The effective equation of state (EOS) parameter predicts the accelerating behavior of the cosmic expansion phase. Further, by the statefinder and Om(z) diagnostic test, we found that our viscous model favors quintessence-type behavior and can successfully describe the late-time scenario.