28 research outputs found
Late time cosmological approach in mimetic gravity
In this paper, we investigate the late-time cosmic acceleration in mimetic
gravity with Lagrange multiplier and potential in a Universe
containing, besides radiation and dark energy, a self-interacting (collisional)
matter. We obtain through the modified Friedmann equations, the main equation
that can describe the cosmological evolution and with several models from
and the well known particular model , we perform an analysis of
the late-time evolution. We examine the behavior of the Hubble parameter, the
dark energy equation of state and the total effective equation of state and we
compare in each case the resulting picture with the non-collisional matter
(assumed as dust) and also with the collisional matter in mimetic
gravity. The results obtained are in good agreement with the observational data
and show that in presence of the collisional matter the dark energy
oscillations in mimetic f(R, T) gravity can be damped.Comment: 18 pages, 2 figure
Late time cosmological approach in mimetic gravity
In this paper, we investigate the late-time cosmic acceleration in mimetic
gravity with Lagrange multiplier and potential in a Universe
containing, besides radiation and dark energy, a self-interacting (collisional)
matter. We obtain through the modified Friedmann equations, the main equation
that can describe the cosmological evolution and with several models from
and the well known particular model , we perform an analysis of
the late-time evolution. We examine the behavior of the Hubble parameter, the
dark energy equation of state and the total effective equation of state and we
compare in each case the resulting picture with the non-collisional matter
(assumed as dust) and also with the collisional matter in mimetic
gravity. The results obtained are in good agreement with the observational data
and show that in presence of the collisional matter the dark energy
oscillations in mimetic f(R, T) gravity can be damped.Comment: 18 pages, 2 figure
New Static Solutions in f(T) Theory
We consider the equations of motion of an anisotropic space-time in
theory, where is the torsion. New spherically symmetric solutions of black
holes and wormholes are obtained with a constant torsion and the cases for
which the radial pressure is proportional to a real constant, to some algebraic
functions and their derivatives , or vanish identically.Comment: 15 pages, accepted for publication in European Physical Journal
Extended Birkhoff's Theorem in the f(T) Gravity
The f(T) theory, a generally modified teleparallel gravity, has been proposed
as an alternative gravity model to account for the dark energy phenomena.
Following our previous work [Xin-he Meng and Ying-bin Wang, EPJC(2011),
arXiv:1107.0629v1], we prove that the Birkhoff's theorem holds in a more
general context, specifically with the off diagonal tetrad case, in this
communication letter. Then, we discuss respectively the results of the external
vacuum and internal gravitational field in the f(T) gravity framework, as well
as the extended meaning of this theorem. We also investigate the validity of
the Birkhoff's theorem in the frame of f(T) gravity via conformal
transformation by regarding the Brans-Dicke-like scalar as effective matter,
and study the equivalence between both Einstein frame and Jordan frame.Comment: 7 pages, 1 figure, submitted to EPJ-C. arXiv admin note: substantial
text overlap with arXiv:1107.062
Birkhoff's Theorem in f(T) Gravity up to the Perturbative Order
f(T) gravity, a generally modified teleparallel gravity, has become very
popular in recent times as it is able to reproduce the unification of inflation
and late-time acceleration without the need of a dark energy component or an
inflation field. In this present work, we investigate specifically the range of
validity of Birkhoff's theorem with the general tetrad field via perturbative
approach. At zero order, Birkhoff's theorem is valid and the solution is the
well known Schwarzschild-(A)dS metric. Then considering the special case of the
diagonal tetrad field, we present a new spherically symmetric solution in the
frame of f(T) gravity up to the perturbative order. The results with the
diagonal tetrad field satisfy the physical equivalence between the Jordan and
the so-called Einstein frames, which are realized via conformal transformation,
at least up to the first perturbative order.Comment: 8 pages, no figure. Final version, accepted for publication in EPJ
Note on the Evolution of the Gravitational Potential in Rastall Scalar Field Theories
We investigate the evolution of the gravitational potential in Rastall scalar
field theories. In a single component model a consistent perturbation theory,
formulated in the newtonian gauge, is possible only for , which is
the General Relativity limit. On the other hand, the addition of another
canonical fluid component allows also to consider the case .Comment: 16 pages, 3 figures, Sections 2 and 5 enlarged, accepted for
publication in Physics Letters
Scalar models for the generalized Chaplygin gas and the structure formation constraints
The generalized Chaplygin gas model represents an attempt to unify dark
matter and dark energy. It is characterized by a fluid with an equation of
state . It can be obtained from a generalization of the
DBI action for a scalar, tachyonic field. At background level, this model gives
very good results, but it suffers from many drawbacks at perturbative level. We
show that, while for background analysis it is possible to consider any value
for , the perturbative analysis must be restricted to positive values
of . This restriction can be circumvented if the origin of the
generalized Chaplygin gas is traced back to a self-interacting scalar field,
instead of the DBI action. But, in doing so, the predictions coming from
formation of large scale structures reduce the generalized Chaplygin gas model
to a kind of quintessence model, and the unification scenario is lost, if the
scalar field is the canonical one. However, if the unification condition is
imposed from the beginning as a prior, the model may remain competitive. More
interesting results, concerning the unification program, are obtained if a
non-canonical self-interacting scalar field, inspired by Rastall's theory of
gravity, is imposed. In this case, an agreement with the background tests is
possible.Comment: Latex file, 25 pages, 33 figures in eps format. New section on scalar
models. Accepted for publication in Gravitation&Cosmolog