Charge Gravastars in f(T)f(T) Modified Gravity

In the present work, we have discussed the four dimensional spherically symmetric steller system in the framework of modified $f(T)$ gravity theory with electro-magnetic field. The field equations have been written for two cases, either $T'=0$ or $f_{TT}=0$. Next we have discussed the charged gravastar model which has three regions: interior region, shell region and exterior region. In the interior region, we have found the solutions of all physical quantities like density, pressure, electro-magnetic field and also the metric coefficients for both the cases. For $T'=0$, gravastar cannot form but it forms only for the case $f_{TT}=0$. In the exterior region, we have obtained the exterior solution for vacuum model. In the shell region, we have assumed that the interior and exterior regions join together at a place, so the intermediate region must be thin shell with the approximation $h\ll 1$. Under this approximation, we have found the analytical solutions. The proper length of the thin shell, entropy and energy content inside the thin shell have been found and they are directly proportional to the proper thickness of the shell $\epsilon$ under the approximation ($\epsilon\ll 1$). According to the Darmois-Israel formalism, we have studied the matching between the surfaces of interior and exterior regions of the gravastar. The energy density, pressure, equation of state parameter on the surface and mass of the thin shell have been obtained.Comment: 8 pages, Comments are welcom

Accretion and Evaporation of Modified Hayward Black Hole

First we have assumed the most general static spherically symmetric black hole metric. The accretion of any general kind of fluid flow around the black hole have been investigated. The accretion of fluid flow around the modified Hayward black hole have been analyzed and we then calculated the critical point, fluid 4 velocity and velocity of sound during accretion process. Also the nature of the dynamical mass of black hole during accretion of fluid flow and taking into consideration of Hawking radiation from black hole i.e., evaporation of black hole have been analyzed.Comment: 5 pages, two columns, no figures, Accepted for publication in Eur. Phys. J.

Reconstructing Einstein-Aether Gravity from Ordinary and Entropy-Corrected Holographic and New Agegraphic Dark Energy Models

Here we briefly discuss the Einstein-Aether gravity theory by modification of Einstein-Hilbert action. We find the modified Friedmann equations and then from the equations we find the effective density and pressure for Einstein-Aether gravity sector. These can be treated as dark energy provided some restrictions on the free function $F(K)$, where $K$ is proportional to $H^{2}$. Assuming two types of the power law solutions of the scale factor, we have reconstructed the unknown function $F(K)$ from HDE and NADE and their entropy-corrected versions (ECHDE and ECNADE). We also obtain the EoS parameter for Einstein-Aether gravity dark energy. For HDE and NADE, we have shown that the type I scale factor generates the quintessence scenario only and type II scale factor generates phantom scenario. But for ECHDE and ECNADE, the both types of scale factors can accommodate the transition from quintessence to phantom stages i.e., phantom crossing is possible for entropy corrected terms of HDE and NADE models. Finally, we show that the models are classically unstable.Comment: 19 pages, 8 figures. arXiv admin note: text overlap with arXiv:1004.1805 by other author

Accretion of Dark Energy onto Higher Dimensional Charged BTZ Black Hole

In this work, we have studied the accretion of $(n+2)$-dimensional charged BTZ black hole (BH). The critical point and square speed of sound have been obtained. The mass of the BTZ BH has been calculated and we have observed that the mass of the BTZ BH is related with square root of the energy density of dark energy which accretes onto BH in our accelerating FRW universe. We have assumed modified Chaplygin gas (MCG) as a candidate of dark energy which accretes onto BH and we have found the expression of BTZ BH mass. Since in our solution of MCG, this model generates only quintessence dark energy (not phantom) and so BTZ BH mass increases during the whole evolution of the accelerating universe. Next we have assumed 5 kinds of parametrizations of well known dark energy models. These models generate both quintessence and phantom scenarios i.e., phantom crossing models. So if these dark energies accrete onto the BTZ BH, then in quintessence stage, BH mass increases upto a certain value (finite value) and then decreases to a certain finite value for phantom stage during whole evolution of the universe. We have shown these results graphically.Comment: 15 pages, 6 figures. arXiv admin note: substantial text overlap with arXiv:1312.7462; and text overlap with arXiv:1007.2704 by other author