International Journal of Modern Physics D

In this work, we review thoroughly the origin, development and present status of gravastar which has been thought to be an alternative to black hole along with its future feasibility in the sense of astrophysical observation. The Mazur–Mottola model of the gravastar is introduced first followed by a review of its generalizations within the context of general relativity including physical applications like primordial and cylindrical gravastars. The study of gravastar in higher- and lower-dimensional general relativity (GR) including the higher-dimensional theories with physical insight like the Randall–Sundrum model is presented. The gravastar models in a number of modified gravity models starting from $f(R,T)$ to Rastall–Rainbow gravity have been reviewed.

We complete the stability study of general-relativistic spherically symmetric polytropic perfect fluid spheres, concentrating our attention on the newly discovered polytropes containing region of trapped null geodesics. We compare the methods of treating the dynamical stability based on the equation governing infinitesimal radial pulsations of the polytropes and the related Sturm–Liouville eigenvalue equation for the eigenmodes governing the pulsations, to the methods of stability analysis based on the energetic considerations. Both methods are applied to determine the stability of the polytropes governed by the polytropic index $n$ in the whole range $0<n<5$, and the relativistic parameter $\sigma$ given by the ratio of the central pressure and energy density, restricted by the causality limit. The critical values of the adiabatic index for stability are determined, together with the critical values of the relativistic parameter $\sigma$. For the dynamical approach, we implemented a numerical method which is independent on the choice of the trial function, and compare its results with the standard trial function approach. We found that the energetic and dynamic method give nearly the same critical values of $\sigma$. We found that all the configurations having trapped null geodesics are unstable according to both methods.

Neutrinos produced in extragalactic sources may experience flavor-oscillations and decoherence on their way to Earth due to their interaction with dark matter (DM). As a result, they may be detected in pointer-states other than the flavor states produced at the source. The oscillation pattern and the structure of the pointer-states can give us information on the characteristics of the DM and the kind of interaction that has taken place. From this perspective, neutrinos can be viewed as DM-tracers. We study the local evolution of neutrino flavor-eigenstates due to local effects produced by the presence of DM. To explore the sensitivity of the model, we consider different DM density profiles, masses and interactions. Starting from the eigenstates of the neutrino mass Hamiltonian, we construct flavor-states with the neutrino mixing-matrix in vacuum. We then include local interactions with DM, acting along the neutrino path towards the Earth, and analyze the resulting probabilities. In doing so, we adopt different DM density profiles, e.g. a constant, a local isotropic, and a Navarro–Frenk–White density distribution. Finally, by following the time-evolution of the flavor-states, we identify pointer-states and interpret the results in terms of the adopted DM model. In addition, neutrinos may experience changes in the oscillation parameters and resonances (MSW effect), the extent of which depends on the DM density profile. Thus, the presence of DM may enhance or suppress the oscillation pattern. The suppression of components of the neutrino wave-packet (decoherence effects) may also take place. The features of the calculated response seem to support the notion that these neutrinos can be taken as DM tracers. From a theoretical point of view, the coexistence and/or competition of decoherence and matter effects is sustained by the results.

We construct exact black hole solutions to Einstein gravity with nonlinear electrodynamic field. In these solutions, there are, in general, four parameters. They are physical mass, electric charge, cosmological constant and the coupling constant. These solutions differ significantly from the Reissner–Nordström–de Sitter solution in Einstein–Maxwell gravity with a cosmological constant, due to the presence of coupling constant. For example, some of them are endowed with a topological defect on angle $𝜃$ and the electric charge of some can be much larger or smaller than their mass by varying the coupling constant. On the other hand, these spacetimes are all asymptotically de Sitter (or anti-de Sitter). As a result, their causal structure is similar to the Reissner–Nordström–de Sitter spacetime. Finally, the investigations on the thermodynamics reveal that the coupling constant except for solution-4 has the opposite effect as temperature on the phase, structure of black holes. Concretely, the phase-space changes from single phase to three phases with the decrease of temperature. On the contrary, it changes from three phases to a single phase with the decrease of coupling constant.

The operation of a wide field-of-view (WFOV) Cherenkov telescope is described. The detection of extensive air showers (EASs) of cosmic rays (CRs) is based upon the coincidence with signals from the Yakutsk array. The data acquisition system of the telescope yields signals connected with EAS development parameters: presumably, shower age and position of shower maximum in the atmosphere. Here, we describe the method of signal processing used to reconstruct Cherenkov radiation signals induced by CR showers. An analysis of signal parameters results in the confirmation of the known correlation of the duration of the Cherenkov radiation signal with the distance to the shower core. The measured core distance dependence is used to set an upper limit to the dimensions of the area along the EAS axis where the Cherenkov radiation intensity is above half-peak amplitude.

In this paper, we have examined the validity of a proposed definition of gravitational entropy in the context of accelerating black hole solutions of the Einstein field equations, which represent the realistic black hole solutions. We have adopted a phenomenological approach proposed in Rudjord et al. [Phys. Scr. 77, 055901 (2008)] and expanded by Romero et al. [Int. J. Theor. Phys. 51, 925 (2012)], in which the Weyl curvature hypothesis is tested against the expressions for the gravitational entropy. Considering the $C$-metric for the accelerating black holes, we have evaluated the gravitational entropy and the corresponding entropy density for four different types of black holes, namely, nonrotating black hole, nonrotating charged black hole, rotating black hole and rotating charged black hole. We end up by discussing the merits of such an analysis and the possible reason of failure in the particular case of rotating charged black hole and comment on the possible resolution of the problem.

In this paper, we study the semiclassical evolution of a “coherently oscillating massive scalar field”. This field is nominally linked to the spatially levelled Friedmann–Robertson–Walker (FRW) metric. By use of two-mode quantum optical states formalism, we evaluated the power-law cosmology with scale factor as a power function of cosmic time. In the oscillatory phase of scalar field, it was found that, in all the four different combinations of quantum states, quantum scalar field obeying the time-dependent Schrödinger equation leads to the power-law expansion, in good agreement with that of the matter-dominated era. However, in the semiclassical theory of gravity in contrary to the classical description of gravity, we obtained two behavioral solutions corresponding to two different pattern evolution of the scale factor. This behavioral pattern evolution of scale factor of the universe has an important quantum implication associated with it.

This paper studies the collapse of stellar filaments in the presence of dark matter (DM). We use $f(R)$ gravity to involve DM in the collapse. We apply Darmois junction conditions (DJCs) on the surface of collapsing boundary $\mathrm{\Sigma }$ and obtain the collapse equation. The radial pressure associated with the seen matter is found to be nonzero at $\mathrm{\Sigma }$. We then use Starobinsky model, $f(R)=R+\alpha R^2$, as a candidate of DM to obtain stability criteria (SC) of the collapsing body. It is found that the stability of filamentary structure relates radial pressure of baryonic directly with the gravitational effects of DM. Stability of polytropic family of filaments are studied by applying polytropic equation of state to baryonic contribution. For all polytropic stable filaments, it turns out that the visible matter density is exponentially linked to effects of DM. Finally, we discuss connection between exotic terms and gravitational waves (GW). It is theoretically indicated that the presence of DM can affect the GW propagation.