International Journal of Modern Physics D

Nonadditive entropies have been proposed as alternatives to understanding the thermodynamics of Black Holes. Moreover, it has been suggested that the difficulties of quantization of gravity are an indication that gravity is an emergent phenomenon, and therefore an entropic formulation is warranted. In this work, we explore cosmologies that come from nonadditive entropies. We focus our study on the cosmologies derived from entropies in superstatistics.

In this paper, we compute the shadow cast by a charged Morris–Thorne wormhole when the light source is a star located beyond the mouth which is opposite to the observer. First, we provide an extensive analysis of the geodesic properties of the spacetime, both for null and massive particles. The geometrical properties of this solution are such that independently of the viewing angle, some light rays always reach the observer. Additionally, the structure of the images is preserved among the different values of the charge and scales proportionally to the charge value.

Gravitational waves (GW) from eccentric binaries have intricate signals encoding important features about the location, creation and evolution of the sources. Eccentricity shortens the merger time, making the emitted GW statistically predominant in the observed data once detectors will reach the required sensitivity. We present a novel implementation of fully analytical GW templates from eccentric binary black hole (BBH) mergers within the Wolfram Mathematica software. We increase the accuracy by identifying and minimizing the possible source of errors. We start with an overview of the physics involved in eccentric mergers, then assemble the strain for the inspiral by employing up to six post-Newtonian (PN) corrections. We complete the eccentric inspiral with the quasi-circular Backwards one Body (BOB) merger model in frequency, amplitude and phase, then we build the hybrid GW strain for the whole evolution of the binary. For low eccentricity, we reach coincidence in the overlap, with no ambiguity in the time interval, a remarkable improvement from the usual matching techniques. For high-eccentricity, we compensate for the implicit quasi-circular assumption of the BOB approach, by introducing a small rescaling in amplitude. Our streamlined implementation is relevant for the new field of GW astronomy and is straightforward to understand, use and extend, offering researchers in the field a valuable open resource tool.

We study the impact of nonlocal modifications of General Relativity on stellar structure. In particular, assuming an analytic distortion function and specific equations of state, we made use of remnant stars to put qualitative constraints on a parameter not directly restricted by solar system tests. Using current data sets available for white dwarfs and strange quark stars candidates, we find that the most stringent bounds come from the objects displaying the highest core densities, namely strange quark stars. Specifically, the constraints obtained from this class of stars are three to four orders of magnitude tighter than those obtained using white dwarfs.

We investigate the destroying of charged and rotating black holes in Einstein–Maxwell-(dilaton) theory. We show that a test particle with some appropriate properties in the black hole background can turn the black hole into a naked singularity. In this work, we neglect the self-force, self-energy and radiative effects as considered by some others in literature. As the result, we are able to show that the Kaluza–Klein black holes can be destroyed by the test particle. Furthermore, we consider the overspinning of the black hole by a neutral test scalar field.

In this paper, we study the evolution of random hypermagnetic fields (HMFs) in the symmetric phase of the early universe before the electroweak phase transition. The behavior of HMFs is driven by the analog of the chiral magnetic effect accounting for the asymmetries of leptons and Higgs bosons. These asymmetries are also dynamical variables of the model and evolve together with HMFs. Moreover, we account for the contribution of the hyper-MHD turbulence in the effective diffusion coefficient and the $\alpha$-dynamo parameter. The realistic spectrum of seed HMFs consists of two branches: Batchelor and Kolmogorov ones. The impact of HMFs on the production of relic gravitational waves (GWs) and the baryon asymmetry of the universe (BAU), as well as flavor oscillations of supernova neutrinos in the stochastic GWs generated is considered. We establish the constraint on the strength of the seed HMF comparing the spectral density of produced GWs with the observations of the LIGO-Virgo-KAGRA collaborations. The stronger upper bound on the seed HMF is obtained from the condition of not exceeding the observed value of BAU.

The use of correlations between various observable quantities for Gamma-ray bursts (GRBs) has been the subject of much interest and discussion. In particular, many studies have explored the E_iso-E_i,peak relation, its origin and its potential use as a means to discriminate between GRBs from various progenitors. Observations of GRBs through the Fermi Gamma-Ray Space Telescope have also opened up the possibility of studying this correlation over a wide range of energies from keVs to GeVs. While the correlation has been studied through Fermi Large Area Telescope (LAT) GRBs with measured redshifts, its compatibility with LAT GRBs without measured redshifts is yet to be tested. In this paper, we study this correlation with a sample of 24 Fermi LAT GRBs with unknown redshift. We analyze their data between 8 keV and 100 GeV through single and multiple-component spectral models. We use the results of this analysis to study the compatibility of these GRBs with the E_iso-E_i,peak relation by varying the redshift continuously between 0.3 and 10.5. We find that all 24 GRBs are compatible with the E_iso-E_i,peak relation within a 1-sigma uncertainty for a certain redshift range. The range of compatible redshifts found for each GRB remains consistent with observed redshifts for long GRBs.

We relate the known Oberth effect and the nonrelativistic analogue of the Penrose process. When a particle decays to two fragments, we derive the conditions on the angles under which debris can come out for such a process to occur. We also consider the decay and the Oberth effect in the relativistic case, when a particle moves in the background of the Schwarzschild black hole. This models the process when a rocket ejects fuel. Different scenarios are analyzed depending on what data are fixed. The efficiency of the process is found, in particular, near the horizon and for a photon rocket (when the ejected particle is massless). We prove directly that the most efficient process occurs when fuel is ejected along the rocket trajectory. When this occurs on the horizon, the efficiency reaches 100% for a photon rocket. We compare in two ways how a rocket can reverse its direction of motion to a black hole near the event horizon by restoring the initial energy-to-mass ratio: (i) by a single ejection or (ii) in the two-step process when it stops and moves back afterwards. For a nonphotonic rocket, in case (ii) a larger mass can be taken out from the vicinity of a horizon. For a photonic one, there is no difference between (i) and (ii) in this respect. We also consider briefly the scenario when a rocket hangs over a black hole due to continuous ejection of fuel. Then, the fuel mass decays exponentially with the proper time.

This work deals with a multi-field cosmological model in a spatially flat FLRW spacetime geometry. The usual Brans–Dicke (BD) field and another scalar field are minimally coupled to gravity while they interact with each other through the Kinetic terms. The main aim of this work is to examine whether the model is compatible with cosmic observations. So cosmological solutions are obtained using symmetry analysis only. By imposing Noether Symmetry to the Lagrangian of the system, the potential of the scalar field as well as the coupling function had been determined. The classical solutions are determined after simplifying the Lagrangian using cyclic variables. Finally, Wheeler–DeWitt (WD) equation in quantum cosmology has been formulated and conserved momenta corresponding to Noether symmetry will show the periodic part of the wave function and it helps to have the complete integral for the wave function.

By using the Kaniadakis statistics, we discuss the modifications of Newtonian gravity and radial velocity profile in the light of Verlinde’s formalism for gravitational entropy. After considering the implications of $\kappa$-statistics on the gravitational potential, it is shown that an accelerated universe may be obtained by considering the Friedmann first equation in this nonextensive statistics.