General Relativity and Gravitation最新文献

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.

We elaborate on a model of nonassociative and noncommutative Einstein–Dirac–Maxwell, EDM, theory determined by star product R-flux deformations in string theory. Solutions for nonassociative EDM systems and physical properties not studied in modern physics. For modifications of the four-dimensional, 4-d, Einstein gravity, we work on conventional nonassociative 8-d phase spaces modelled as star-deformed co-tangent Lorentz bundles. Generalizing the anholonomic frame and connection deformation method, the nonassociative EDM equations are decoupled and integrated in exact and parametric quasi-stationary forms. Corresponding generic off-diagonal metrics are described by nonlinear symmetries and encode nonassociative effective sources and generating functions depending on space and momentum-like coordinates. For respective nonholonomic parameterizations, such solutions describe nonassociative deformations of the Reissner–Nordström black holes. A variant of nonassociative phase space wormhole solution with fermions possessing anisotropic polarized masses is also analyzed. We conclude that such phase space physical objects can’t be characterized using the concept of Bekenstein–Hawking entropy and show how to compute another type (modified G. Perelman ones) nonassociative geometric and statistical thermodynamic variables.

This article explores the characteristics of interacting new Tsallis agegraphic dark energy (NTADE) and Sharma–Mittal holographic dark energy (SMHDE) models in a flat FLRW universe within Rastall gravity (RG). To check the viability of these models and to distinguish them, we develop important cosmological parameters including equation of state (EoS) parameter, deceleration parameter q, square speed of sound, statefinder pair and Om(z) diagnostic. By constraining the different parameters for both dark energy (DE) models, it is observed that: EoS parameter indicates phantom-like behavior, deceleration parameter is showing a phase transition from decelerating to accelerating phase. The (j, s) plane is indicating a rich behavior as it shows different DE eras like quintessence, phantom and Chaplygin gas for both models depending upon interacting term (d^{2}), parameter (delta ) and Rastall parameter (lambda ). The (omega _{D}-omega ^{'}_{D}) pair is indicating freezing region for NTADE but for SMHDE it initially falls in freezing region then move towards thawing region. We also check stability of NTADE/SMHDE models through the graphical interpretation of square speed of sound.

We conduct a comprehensive study on spherical orbits around two types of black holes: Kerr–Newman black holes, which are charged, and Ghosh black holes, which are nonsingular. In this work, we consider both null and timelike cases of orbits. Utilizing the Mino formalism, all analytical solutions for the geodesics governing these orbits can be obtained. It turns out that all spherical photon orbits outside the black hole horizons are unstable. In the extremal cases of both models, we obtain the photon boomerangs. The existence of charge in the Kerr–Newman allows the orbits to transition between retrograde and prograde motions, and its increase tends to force the orbits to be more equatorial. On the other hand, the Ghosh black hole, characterized by a regular core and a lack of horizons in certain conditions, presents the possibility of observable stable spherical orbits in the so-called no-horizon condition. As the Ghosh parameter k increases, trajectories tend to exhibit larger latitudinal oscillation amplitudes. We observe that as the Ghosh parameter k increases the trajectories tend to have larger latitudinal oscillation amplitudes. Finally, we investigate the existence of innermost stable spherical orbits (ISSOs). Both black holes demonstrate the appearance of two branches of ISSO radii as a function of the Carter constant ({mathcal {C}}). However, there are notable differences in their behavior: in the case of the Kerr–Newman black hole, the branches merge at a critical value, beyond which no ISSO exists, while for the Ghosh black hole, the transcendental nature of the metric function causes the branches to become complex at some finite distance.

For the ((n+1))-dimensional ((nge 3)) dilaton black hole in the Einstein–Maxwell-dilaton theory, we have presented exact analytical solutions of the field equations. These exact solutions include the exact formula of the potential function as well as the exact formula of the metric function. The presence of the dilaton field makes the asymptotic behavior of these black holes no longer flat or anti-de Sitter. We have calculated the electric charge, mass, temperature, entropy and electric potential of these black holes and have shown the correctness of the first law of black hole thermodynamics. As a thermodynamic system, we have analyzed thermal stability of these types of black holes using the canonical ensemble method and, investigated the effect of dilaton field on their stability.

The present work looks for the possible existence of static and spherically symmetric wormhole geometries in Rastall–Rainbow gravity. Since, the Rastall–Rainbow gravity model has been constructed with the combination of Rastall theory and the gravity’s rainbow formalism. Taking advantage of the Karmarkar condition for embedding class one metrics, we solve the modified field equations analytically that describe wormholes for specific choice of redshift function. For specific parameter ranges, the solution represents a traversable wormhole that exhibits the violation of null energy condition and consequently the weak energy condition also. Furthermore, we focus on the wormhole stability via adiabatic sound velocity analysis. This model establishes a strong connection between two model parameters, namely, the Rastall parameters and the Rainbow functions, and how it affects the wormhole solution.

Cosmological models can be studied effectively using dynamical systems techniques. Starting from Brown’s formulation of the variational principle for relativistic fluids, we introduce new types of couplings involving a perfect fluid, a scalar field, and boundary terms. We describe three different coupling models, one of which turns out to be particularly relevant for cosmology. Its behaviour is similar to that of models in which dark matter decays into dark energy. In particular, for a constant coupling, the model mimics well-known dynamical dark energy models while the non-constant couplings offer a rich dynamical structure, unseen before. We are able to achieve this richness whilst working in a two-dimensional phase space. This is a significant advantage which allows us to provide a clear physical interpretation of the key features and draw analogies with previously studied models.

In the present work, we study the scattering for a black hole described by the canonical acoustic metric with Lorentz violation using asymptotic and numerical methods. In this scenario, we also check the effects of quasinormal modes and the acoustic shadow radius. In the eikonal limit the relationship between the shadow radius and the real part of the quasinormal frequency is preserved.

We present the solution of the Einstein field equations, in the static and spherically symmetric case, for an incompressible fluid, that has constant proper energy density at each and every point of the volume where it exists, according to a set of local observers who are stationary with respect to the fluid at each point. In the general case the fluid exists within a spherically symmetric shell with an inner vacuum-matter interface at a radial position (r_{1}) and an outer matter-vacuum interface at a radial position (r_{2}) in the Schwarzschild coordinate system. Therefore, in the general case there is an inner vacuum region with a repulsive singularity at the origin, just like in all other similar shell solutions. We present the parameter plane of the problem, and show that there are limits of solutions that approach the configuration of black holes, with the formation of an event horizon at the radial position (r_{2}).

Beginning from the standard Arnowitt–Deser–Misner (ADM) formulation of general relativity we construct a tentative model of quantum gravity from the point of view of an observer with constant proper acceleration, just outside of a horizon of spacetime. In addition of producing the standard results of black-hole thermodynamics, our model makes an entirely new prediction that there is a certain upper bound for the energies of massive particles. For protons, for instance, this upper bound is around (1.1times 10^{21}) eV. The result is interesting, because this energy is roughly of the same order of magnitude as are the highest energies ever measured for protons in cosmic rays.