arXiv subjects

S. Akhshabi

Publications and source records attributed to S. Akhshabi.

6 recordsLinked to original sources

arXiv2019

Orbits of particles and black hole thermodynamics in a spacetime with torsion

We derive the static spherically symmetric vacuum solution for a spacetime with non-vanishing torsion by solving the field equations analytically. The effects of torsion appear as a single parameter in the line element. For the positive values of this parameter, the resulting line element is found to be of the Reissner-Nordstrom type. This parameter is related to the spin of matter and acts as a torsion 'charge' much like the electric charge in conventional Reissner-Nordstrom geometry. We also analyze the existence and stability of the orbits for both massless and massive particles in this setup and compare the results to the corresponding case in general relativity. We also derive the first law of black hole thermodynamics for a black hole with torsion and define the black hole temperature and entropy in terms of its mass and torsion charge.

gr-qc↗

arXiv2016

Scalar perturbation potentials in a homogeneous and isotropic Weitzenböck geometry

We describe the fully gauge invariant cosmological perturbation equations in teleparallel gravity by using the gauge covariant version of the Stewart lemma for obtaining the variations in tetrad perturbations. In teleparallel theory, perturbations are the result of small fluctuations in the tetrad field. The tetrad transforms as a vector in both its holonomic and anholonomic indices. As a result, in the gauge invariant formalism, physical degrees of freedom are those combinations of perturbation parameters which remain invariant under a diffeomorphism in the coordinate frame, followed by an arbitrary rotation of the local inertial (Lorentz) frame. We derive these gauge invariant perturbation potentials for scalar perturbations and present the gauge invariant field equations governing their evolution.

gr-qc↗

arXiv2015

Braneworld Cosmological Perturbations in Teleparallel Gravity

We describe the fully gauge invariant cosmological perturbation equations in braneworld teleparallel gravity. In this theory, perturbations are the result of small fluctuations in the pentad field. We derive the gauge invariant 'potentials' for both geometric and matter variables. Unlike the metric fluctuations in general relativity, pentad perturbations can only contain scalar and vector modes. As a result, in teleparallel gravity, tensor modes including gravitational waves, come from the contortion tensor.

gr-qc↗

arXiv2014

Braneworld setup and embedding in teleparallel gravity

We construct the setup of a five-dimensional braneworld scenario in teleparallel gravity. Both cases of Minkowski and Friedmann-Robertson-Walker branes embedded in Anti de Sitter bulk are studied and the effective 4-D action were studied. 4-dimensional local Lorentz invariance is found to be recovered in both cases. However, due to different junction conditions, the equations governing the 4-D cosmological evolution differ from general relativistic case. Using the results of Ref. [13], we consider a simple inflationary scenario in this setup. The inflation parameters are found to be modified compared to general relativistic case.

gr-qc↗

arXiv2012

Braneworld Teleparallel Gravity

We study the gravity in the context of a braneworld teleparallel scenario. The geometrical setup is assumed to be Randall-Sundrum II model where a single positive tension brane is embedded in an infinite AdS bulk. We derive the equivalent of Gauss-Codacci equations in teleparallel gravity and junction conditions in this setup. Using these results we derive the induced teleparallel field equations on the brane. We show that compared to general relativity, the induced field equations in teleparallel gravity contain two extra terms arising from the extra degrees of freedom in the teleparallel Lagrangian. The term carrying the effects of the bulk to the brane is also calculated and its implications are discussed.

gr-qc↗

arXiv2012

Matter stability in modified teleparallel gravity

We study the matter stability in modified teleparallel gravity or $f(T)$ theories. We show that there is no Dolgov-Kawasaki instability in these types of modified teleparallel gravity theories. This give the $f(T)$ theories a great advantage over their $f(R)$ counterparts because from the stability point of view there isn't any limit on the form of functions that can be chosen.

gr-qc↗