International Journal of Modern Physics D

In this paper, we consider head-on collisions of two particles near the event horizon. Particle 1 is outgoing, particle 2 is ingoing. We elucidate the conditions when the energy $E_{c.m.}$ in the center of mass frame can grow unbounded. In doing so, we dispel some misconceptions popular in the literature on high energy collisions near black holes and carry out clear distinction when such a collision happens near a black hole and when this occurs near a white one. If the proper time between the horizon and an arbitrary point outside it for particle 1 is finite, we deal with a white hole. If it is infinite, we deal with a black hole. Particles can be either free or experience the action of a finite force. Our results are complementary to those for the standard BSW effect when particles move in the same direction. The results rely on classification of particles developed in our previous work Ovcharenko and Zaslavskii [Phys. Rev. D 108 (2023) 064029]. The aforementioned article performed the first step towards a building classification of all possible high energy collisions near black/white hole for different types of horizons and nonzero force. In this paper, we perform the second step.

We address the question of the end point of the quantum evaporation of regular black holes with the de Sitter centers specified by the algebraic structure of their stress–energy tensors $T_t^t=T_r^r$. Most of the presented in the literature regular solutions belong to this class. In the case of a nonzero background cosmological constant stress–energy tensors with $T_t^t=T_r^r$ connect smoothly two de Sitter vacua, in the center and at infinity, and generate regular solutions to the Einstein equations, which describe the spacetime structure of regular black holes and their remnants. We show that for this class black holes their spacetime symmetry prevents a complete evaporation and provides the existence of thermodynamically stable remnants.

We introduce a theoretical framework to alleviate the Hubble tension. This framework is based on dynamics of a minimally coupled scalar field which either belongs to the Brans–Dicke theory with a self-interacting potential or is the scalar partner of $f(R)$ gravity. These two theories are dynamically equivalent when the Brans–Dicke parameter is zero. We will use the dynamical equivalence to interpret the Hubble tension in the same theoretical framework. For both theories, we write one set of field equations in which the value of a parameter distinguishes between the two theories. We will show that $H_0$ actually evolves with redshift so that its value is consistent with that measured from the local distance ladder and then drops to the value measured from CMB at high redshift. We argue that even though both theories exhibit this behavior, Brans–Dicke theory with an exponential potential is more successful than power-law $f(R)$ gravity to relieve the Hubble tension.

In this paper, we consider stationary rotating cylindrically symmetric dust spacetimes. We first show that the Maitra spacetime is the unique nonrigidly (nonnull shear scalar) rotating solution with a regular axis and that is the most general one of the field equations. We are also able to demonstrate what Maitra’s paper does not show, where the solution is merely stated without any demonstration. Then we find that the nonrigidly rotating spacetimes, not necessarily regular, can be matched, across timelike cylindrical hypersurfaces, to a one-parameter family of stationary vacuum exteriors given by the Weyl class but not to the Lewis class, generalizing a result by Bonnor and Steidman. Among other properties, it is shown that the amount of rotating dust packed inside a cylinder is bigger if it is rigidly rotating than nonrigidly rotating. Finally, we show that in the nonrigidly rotating cases the fluid necessarily has vorticity and the spacetimes are not silent, where the magnetic part of the Weyl tensor is nonzero.

In this paper, we investigate the relativistic harmonic oscillator in the Ellis–Bronnikov-type wormhole spacetime with a global monopole and cosmic string. We obtain the angular and radial wave functions exactly as well as the eigenenergies. Additionally, the radial wave function is expressed in terms of Heun polynomial, and as a consequence, the oscillation frequency is quantized. Finally, we graphically analyze the influence of the wormhole, as well as the influence of the global monopole and cosmic string on the radial probability density and the eigenenergies.

In this paper, we derive the exact form of effective potential in Kerr geometry from the general relativistic radial momentum equation. The effective potential accurately mimics the general relativistic features, over the entire range of the spin parameter $-1<a<1$. We obtain the exact expression of the rate of dragging of inertial frames that can be used to study the relativistic precession of twisted accretion disks that are formed when the disk outskirts are tilted relative to the equatorial plane of the black hole. We then present an effective potential that provides a simplistic approach to study particle dynamics using physical concepts analogous to the Newtonian physics. We compare the equatorial as well as off-equatorial particle trajectories obtained using our potential with the general relativistic solutions. We find that our approach can capture the salient features of Kerr geometry and is applicable to studies of accretion processes around Kerr black holes.

There is ongoing interest in the nonmetricity formulation of gravity. The nonlinear extension of the theory, called $f(Q)$ gravity, has recently been proposed and offers a promising avenue for addressing some of the long-standing challenges in cosmology and fundamental physics. A number of solutions have already been found that have confirmed the usefulness of the theory for astrophysics and cosmology. Motivated by earlier work on Gödel-type spacetimes, we investigate whether $f(Q)$ gravity admits such cosmological solutions, which entail causality violation. In the coincident gauge, which is shown to be a legitimate gauge because the connection field equations are satisfied, we find that Gödel-type metrics are allowed by this modified gravity theory for any function $f(Q)$. Noncausal and causal solutions exist depending on the matter content. For a perfect fluid, out of all Gödel-type metrics only Gödel’s original model is allowed in $f(Q)$ gravity, implying violation of causality. The requirement that energy conditions be obeyed by the fluid leads to mild restrictions on $f(Q)$. For a massless scalar field in the presence of a cosmological constant, there are causal solutions for any reasonable function $f(Q)$.

The Hubble tension has become the most significant crisis in modern cosmology, suggesting that the $\mathrm{\Lambda }$CDM model may not accurately describe cosmic evolution. In this paper, we test the spatially flat $\mathrm{\Lambda }$CDM model by comparing constraints on cosmological parameters from observational data across different redshift regions. Utilizing Pantheon$+$ SN Ia sample, cosmic chronometer data, and baryon acoustic oscillation data, we derive six distinct sets of constraints within the redshift range $z\in [0.01,0.8]$. At the first three and the last redshift points, the observed values of the present matter density parameter, ${\mathrm{\Omega }}_{\mathrm{\text{m}}0}$, are consistent with those from the full dataset within $1\sigma$ confidence level (CL), whereas values at $z_i=0.45$ and $0.55$ are not, and the maximum deviation occurring at $z_i=0.45$ reaches $2.3\sigma$. The data indicate a decreasing trend in the Hubble constant, $H_0$, and the SN Ia absolute magnitude, M, with increasing redshift. Despite the variation of both ${\mathrm{\Omega }}_{\mathrm{\text{m}}0}$ and $H_0$ across different redshifts, the values of ${\mathrm{\Omega }}_{\mathrm{\text{m}}0}{h}^2$ remain consistent within $1\sigma$ CL across all regions.