26,989 research outputs found
Anisotropic fluid for a set of non-diagonal tetrads in f(T) gravity
We consider gravity for a Weitzenbock spherically symmetric and static
spacetime, where the metric is projected in the tangent space to the manifold
for a set of non-diagonal tetrads. The matter content is coupled through the
energy momentum tensor of an anisotropic fluid, generating various classes of
new black hole and wormhole solutions. One of these classes is that of cold
black holes. We also perform the reconstruction scheme of the algebraic
function for two cases where the radial pressure is proportional to
and its first derivative.Comment: 12 pages. Accepted for publication in Physics Letters B (PLB
Locally Rotationally Symmetric Bianchi Type-I cosmological model in gravity: from early to Dark Energy dominated universe
We study the locally rotational symmetry Bianchi type-I dark energy model in
the framework of theory of gravity, where denotes the torsion
scalar. A viable cosmological model is undertaken and the isotropization of
this latter is checked, yielding a result that reflects the real evolution of
our universe. Moreover, still in the anisotropic optic, a more complicated
model is obtained from the cosmological reconstruction scheme and the
analysis shows that the universe is more anisotropic at the beginning if the
terms of higher order in are not considered. This means that the non-linear
model should be favoured by observational data.Comment: 20 pages, 10 figures, Accepted for publication in International
Journal of Modern Physics D (IJMPD
Tolman-Oppenheimer-Volkoff Equations and their implications or the structures of relativistic Stars in f(T) gravity
We investigate in this paper the structures of neutron and quark stars in
f(T) theory of gravity where T denotes the torsion scalar. Attention is
attached to the TOV type equations of this theory and numerical integrations of
these equations are performed with suitable EoS. We search for the deviation of
the mass-radius diagrams for power-law and exponential type correction from the
TT gravity. Our results show that for some values of the input parameters
appearing in the considered models, f(T) theory promotes more the structures of
the relativistic stars, in consistency with the observational data
From the early to the late time universe within gravity
In this paper we perform the reconstruction scheme of the gravitational
action within gravity, where and denote
the torsion scalar and the trace of the energy momentum tensor, respectively.
We particularly focus our attention on the case where the algebraic function
is decomposed as a sum of two functions and
, i.e, . The
description is essentially based on the scale factor and then, we consider two
interesting and realistic expressions of this parameter and reconstruct the
action corresponding to each phase of the universe. Our results show that some
models are able to describe the evolution of the universe
from the inflation phase to the late time dark energy dominated phase.Comment: 23 page
Exploring Cylindrical Solutions in Modified f(G) Gravity
We present cylindrically symmetric solutions for a type of the Gauss-Bonnet
gravity, in details. We derive the full system of the field equations and show
that there exist seven families of exact solutions for three forms of viable
models. By applying the method based on the effective fluid energy momentum
tensor components, we evaluate the mass per unit length for the solutions. From
dynamical point of the view, by evaluating the null energy condition for these
configurations, we show that in some cases the azimuthal pressure breaks the
energy condition. This violation of the null energy condition predicts the
existence of a cylindrical wormhole.Comment: 27 page
Evaporation phenomena in f(T) gravity
We formulate evaporation phenomena in a generic model of generalized
teleparallel gravity in Weitzenbock spacetime with diagonal and non-diagonal
tetrads basis. We also perform the perturbation analysis around the constant
torsion scalar solution named Nariai spacetime which is an exact solution of
field equations as the limiting case of the Schwarzschild-de Sitter and in the
limit where two back hole and their cosmological horizons coincide. By a
carefully analysis of the horizon perturbation equation, we show that
(anti)evaporation can not happen if we use a diagonal tetrad basis. This result
implies that a typical black hole in any generic form of generalized
teleparallel gravity is frozen in its initial state if we use the diagonal
tetrads. But in the case of non-diagonal tetrads the analysis is completely
different. By a suitable non trivial non-diagonal tetrad basis we investigate
the linear stability of the model under perturbations of the metric and torsion
simultaneously. We observe that in spite of the diagonal case, both evaporation
and anti evaporation can happen. The phenomena depend on the initial phase of
the horizon perturbation. In the first mode when we restrict ourselves to the
first lower modes the (anti)evaporation happens. So, in non-diagonal case the
physical phenomena is reasonable. This is an important advantage of using
non-diagonal tetrads instead of the diagonal ones. We also see that this is an
universal feature, completely independent from the form of the model.Comment: 16 pages, 2 figure
Charged Black Holes in Generalized Teleparallel Gravity
In this paper we investigate charged static black holes in 4D for generalized
teleparallel models of gravity, based on torsion as the geometric object for
describing gravity according to the equivalence principle. As a motivated idea,
we introduce a set of non-diagonal tetrads and derive the full system of non
linear differential equations. We prove that the common Schwarzschild gauge is
applicable only when we study linear f(T) case. We reobtain the
Reissner-Nordstrom-de Sitter (or RN-AdS) solution for the linear case of f(T)
and perform a parametric cosmological reconstruction for two nonlinear models.
We also study in detail a type of the no-go theorem in the framework of this
modified teleparallel gravity.Comment: 16 pages, 3 figure, version accepted for publication in Journal of
Cosmology and Astroparticle Physics (JCAP
Perfect fluid and F(T) gravity descriptions of inflationary universe and comparison with obervational data
We describe in this paper the observables of inflationary models, in
particular the spectrum index of torsion scalar perturbations, the
tensor-to-scalar ratio, and the running of the spectral index, in the framework
of perfect fluid models and F(T) gravity theories through the reconstruction
methods. Then, our results on the perfect fluid and F(T) gravity theories of
inflation are compared with recent cosmological observations such as the Planck
satellite and BICEP2 experiment. Ours studies prove that the perfect fluid and
F(T) gravity models can reproduce the inflationary universe consistent above
all with the Planck data. We have reconstructed several models and considered
others which give the best fit values compatible with the spectral index of
curvature perturbations, the tensor-to-scalar ratio, and the running of the
spectral index within the allowed ranges suggested by the Planck and BICEP2
results.Comment: 18 pages. Accepted for publication in Astrophys Space Science. arXiv
admin note: text overlap with arXiv:1410.3993 by other author
Thermodynamics of phantom Reissner-Nordstrom-AdS black hole
We obtain a new solution of the Einstein-anti-Maxwell theory with
cosmological constant, called anti-Reissner-Nordstrom-(A)de Sitter
(anti-RN-(A)dS) solution. The basic properties of this solution is reviewed.
Its thermodynamics is consistently established, with the extreme cases and
phase transitions, where the analysis is performed through two methods, the
usual one and that of Geometrothermodynamics . The Geometrothermodynamics
analysis does not provide a result in agreement with the usual method or by the
specific heat. We establish local and global thermodynamic stabilities of
anti-RN-AdS solution through the specific heat and the canonical and
grand-canonical ensembles.Comment: 13 pages, 11 figure
Anisotropic Universe Models in f(T) Gravity
We investigate the cosmological reconstruction in anisotropic universe for
both homogeneous and inhomogeneous content of the universe. Special attention
is attached to three interesting cases: Bianchi type-I, Bianchi type-III and
Kantowski-Sachs models. The de Sitter, power-law and general exponential
solutions are assumed for the scale factor in each spatial direction and the
corresponding cosmological models are reconstructed. Moreover, for the general
exponential solutions, from which the de Sitter and power-law solutions may be
obtained, we obtain models which reproduce the early universe, assumed as the
inflation, and the late time accelerated expanding universe. The models
obtained for the late time universe are consistent with a known result in
literature where a power-law type correction in T is added to a power-law type
of f(T) for guaranteeing the avoidance of the Big Rip and the Big Freeze.Comment: 16 pages, revtex4 format, version accepted for publication in
Physical Review
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