JournalTOCs

General Relativity and Gravitation
Journal Prestige (SJR): 0.598

Citation Impact (citeScore): 1
Number of Followers: 2

Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1572-9532 - ISSN (Online) 0001-7701
Published by Springer-Verlag

[2468 journals]

Influence of the cosmological constant on $$\kappa $$ -deformed neutron
star
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  Abstract: Abstract We study a model of the neutron star in $\kappa $ -deformed space-time in the presence of the cosmological constant ( $\Lambda $ ). The Einstein tensor and the energy-momentum tensor are generalized to $\kappa $ -deformed space-time and we construct the field equations with the cosmological constant. Considering the interior of the star to be a perfect fluid as in the commutative case, we find the Tolman–Oppenheimer–Volkoff equations with the inclusion of the cosmological constant in $\kappa $ -deformed space-time. The behavior of the maximum allowed mass of the star and its radius are studied with the variation in the cosmological constant as well as the deformation parameter. We see that the non-commutativity enhances the mass of the star and its maximum mass increases with a decrease in the cosmological constant. The maximum mass varies from 3.44 to $3.68\,\text {M}_{\odot }$ as $\Lambda $ varies from $10^{-10}$ to $10^{-15}\,\text {m}^{-2}$ . We also obtain the compactness factor and surface redshift of the star. We observe that the compactness of the star increases as the cosmological constant decreases, whereas the surface redshift of the star decreases with a decrease in the cosmological constant. The compactness factor and surface redshift corresponding to the maximum mass of the neutron star remains almost constant as $\Lambda $ decreases.
  PubDate: 2024-08-08
Localized chaos due to rotating shock waves in Kerr–AdS black holes and
their ultraspinning version
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  Abstract: Abstract 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 $\kappa =2\pi T_H/(1-\mu \mathcal {L})$ . 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.
  PubDate: 2024-07-29
Elementary considerations on gravitational waves from hyperbolic
encounters
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  Abstract: Abstract 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.
  PubDate: 2024-07-27
Approximating photon trajectories in spherically symmetric spacetimes
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  Abstract: Abstract 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.
  PubDate: 2024-07-24
Exponential correction to Friedmann equations
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  Abstract: Abstract 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 $k=-1$ .
  PubDate: 2024-07-18
Some remarks on Bardeen-AdS black hole surrounded by a fluid of strings
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  Abstract: Abstract 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.
  PubDate: 2024-07-12
The d’Alembert solution in hyperboloidal foliations
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  Abstract: Abstract 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.
  PubDate: 2024-07-08
4D Einstein–Gauss–Bonnet cosmology with Chameleon mechanism
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  Abstract: Abstract 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.
  PubDate: 2024-07-08
A dynamical system analysis of bouncing cosmology with spatial curvature
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  Abstract: Abstract 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.
  PubDate: 2024-07-06
Effeciency of higher dimensional black holes as particle accelerators
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  Abstract: Abstract 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.
  PubDate: 2024-07-05
Cosmological constraints in symmetric teleparallel gravity with bulk
viscosity
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  Abstract: Abstract 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.
  PubDate: 2024-07-05
Nonassociative Einstein–Dirac–Maxwell systems and R-flux modified
Reissner–Nordström black holes and wormholes
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  Abstract: Abstract 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.
  PubDate: 2024-07-04
A study of interacting NTADE and SMHDE models via cosmological parameters
within rastall gravity
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  Abstract: Abstract 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.
  PubDate: 2024-07-02
Spherical orbits around Kerr–Newman and Ghosh black holes
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  Abstract: Abstract 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.
  PubDate: 2024-07-02
New exact solutions, thermodynamics and phase transition in the
Einstein–Maxwell-dilaton theory
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  Abstract: Abstract For the $(n+1)$ -dimensional ( $n\ge 3$ ) 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.
  PubDate: 2024-07-01
Unraveling the mysteries of wormhole formation in Rastall–Rainbow
gravity: a comprehensive study using the embedding approach
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  Abstract: Abstract 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.
  PubDate: 2024-06-25
Cosmological fluids with boundary term couplings
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  Abstract: Abstract 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.
  PubDate: 2024-06-18
Absorption, scattering, quasinormal modes and shadow by canonical acoustic
black holes in Lorentz-violating background
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  Abstract: Abstract 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.
  PubDate: 2024-06-17
  DOI: 10.1007/s10714-024-03263-3
Complete solution of the Einstein field equations for a spherical shell of
truly incompressible liquid
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  Abstract: Abstract 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 $r_{2}$ .
  PubDate: 2024-06-14
  DOI: 10.1007/s10714-024-03262-4
A possible quantum effect of gravitation
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  Abstract: Abstract 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 $1.1\times 10^{21}$ 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.
  PubDate: 2024-06-10
  DOI: 10.1007/s10714-024-03257-1

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