International Journal of Modern Physics D

We consider the periapsis shifts of bound orbits of stars on static clouds around a black hole. The background spacetime is constructed from a Schwarzschild black hole surrounded by a static and spherically symmetric self-gravitating system of massive particles, which satisfies all the standard energy conditions and physically models the gravitational effect of dark matter distribution around a nonrotating black hole. Using nearly circular bound orbits of stars, we obtain a simple formula for the precession rate. This formula explicitly shows that the precession rate is determined by a positive contribution (i.e. a prograde shift) from the conventional general-relativistic effect and a negative contribution (i.e. a retrograde shift) from the local matter density. The four quantities for such an orbit (i.e. the orbital shift angle, the radial oscillation period, the redshift and the star position mapped onto the celestial sphere) determine the local values of the background model functions. Furthermore, we not only evaluate the precession rate of nearly circular bound orbits in several specific models but also simulate several bound orbits with large eccentricity and their periapsis shifts. The present exact model demonstrates that the retrograde precession does not mean any exotic central objects such as naked singularities or wormholes but simply the existence of significant energy density of matters on the star orbit around the black hole.

Vacuum spherically symmetric loop quantum gravity in the midi-superspace approximation using inhomogeneous horizon-penetrating slices has been studied for a decade, and it has been noted that the singularity is eliminated. It is replaced by a region of high curvature and potentially large quantum fluctuations. It was recently pointed out that the effective semiclassical metric implies the existence of a shell of matter which violates energy conditions in regions where the curvature is largest. Here, we propose an alternative way of treating the problem that is free from the shells. The ambiguity in the treatment is related with the existence of new observables in the quantum theory that characterize the area excitations, and how the counterpart of diffeomorphisms in the discrete quantum theory is mapped to the continuum semiclassical picture. The resulting spacetime in the high curvature region inside the horizon is approximated by a metric of the type of the Simpson–Visser wormhole and it connects the black hole interior to a white hole in a smooth manner.

In this paper, we have studied entanglement between two Dirac modes observed, respectively, by two independently accelerated observers. Due to Unruh effect, the entanglement degrades, but residual entanglement remains in the limit of infinite accelerations. We have also investigated quantum teleportation using entangled Dirac modes, and calculated the fidelities in various cases. In general, the fidelity depends on both the single-qubit state and the Bell state used in the teleportation.

In this paper, we examine whether low-mass Primordial Black Holes (PBHs) can be considered a plausible dark matter candidate in galactic halos. We derive the relativistic dynamics of PBHs around the heavy compact objects and evaluate their collision rate, as well as the likelihood of PBH capture in neutron stars and black holes. Although the rate of these collisions in the Milky Way is lower than our lifetime (i.e. almost one collision per hundred years), it may still be observable on cosmological scales. Additionally, we investigate the gravitational wave emission as an important observable window for PBH-astrophysical black hole merging. For the allowed range of PBH mass, gravitational wave signal is smaller than the sensitivity of present gravitational wave detectors. We provide observational prospect for detection of these events in future.

We study numerically a combined gravitational and nonlinear magnetic lensing effect on electromagnetic flux. A magnetar with a dipole magnetic field and background gravitational field is considered to deflect the light rays which passed through its magnetosphere. We assume a square wave front with sides $(150\times 150)\times 10^3$m as a grid with the $10^3$m dynamic step. At the nodes of this grid, the rays enter perpendicularly into the cubic area, which covers the main magnetic lensing region with a magnetar at the center. On the basis of general relativity (GR) and nonlinear vacuum electrodynamics, the distribution of rays by the deflection angle in the combined field of the magnetar was obtained. On the basis of the analysis of the obtained data, it is possible to assert that the magnetic field distorts the result of gravitational lensing. Therefore, the magnetar is regarded as a gravitational-magnetic lensing object, wherein the magnetic field induces axial distortion within $10^{-6\circ }$. These results are expected to be detectable by modern instruments.

We have conducted research on the presence of wormholes characterized by a logarithmic shape function within the framework of exponential $f(R,T)$ gravity. This gravity model is defined as $f(R,T)=R+2e^{\beta t}$, with $\beta$ representing an arbitrary constant. We find that the logarithmic shape function serves as expected, providing all the necessary conditions for traversable and asymptotically flat wormholes. We have examined the solutions with three distinct sets of physical constraints placed on $p_r$, $p_t$ and $\rho$. viz. Model-I: $p_r=k{p}_t$ with k being variable dependent on r and $p_t=\omega \rho$ with $\omega$ being EoS parameter, Model-II: $p_r={\omega }_1\rho$ and $p_t={\omega }_2\rho$ with ${\omega }_1$ and ${\omega }_2$ being EoS parameters, Model-III: Trace is zero i.e. $T=0$ or $\rho =p_r+2p_t$ i.e. $f(R)$ gravity model. From Model-I, we have obtained a new parametrization for EoS parameter $\omega (r)$ and from Model-III, we have obtained a new form of shape function $b(r)$ in $f(R)$ gravity theory.

In this paper, we discuss the corrections to the photon orbits of a nonrotating black hole due to semiclassical fluctuations of the metric. It is found that the photon orbit impact parameter differences with the critical impact parameter become of the order of the semiclassical fluctuations. We calculate the effect of the semiclassical fluctuations on the photon orbits and show that instead of circulating the black hole infinite number of times at the critical orbit, the photons bounce off the semiclassical geometry.

In this paper, we consider a charged BTZ black hole in asymptotically AdS spacetime of massive gravity to study the effect of the thermal fluctuations on the black hole thermodynamics. We consider the Einstein–Born–Infeld solution and investigate critical points and stability. We also compare the results with the case of Einstein–Maxwell solutions. Besides, we find that thermal fluctuations, which appear as a logarithmic term in the entropy, affect the stability of the black hole and change the phase transition point. Moreover, we study the geometrical thermodynamics and find that the behavior of the linear Maxwell solution is the same as the nonlinear one.