52 research outputs found
Birkhoff's theorem in the f(T) gravity
Generalized from the so-called teleparallel gravity which is exactly
equivalent to general relativity, the gravity has been proposed as an
alternative gravity model to account for the dark energy phenomena. In this
letter we prove that the external vacuum gravitational field for a spherically
symmetric distribution of source matter in the gravity framework must be
static and the conclusion is independent of the radial distribution and
spherically symmetric motion of the source matter that is, whether it is in
motion or static. As a consequence, the Birkhoff's theorem is valid in the
general theory. We also discuss its application in the de Sitter
space-time evolution phase as preferred to by the nowadays dark energy
observations.Comment: 5p
Accelerating universe from F(T) gravity
It is shown that the acceleration of the universe can be understood by
considering a F(T) gravity models. For these F(T) gravity models, a variant of
the accelerating cosmology reconstruction program is developed. Some explicit
examples of F(T) are reconstructed from the background FRW expansion history.Comment: 13 pages, references adde
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
Probing the course of cosmic expansion with a combination of observational data
We study the cosmic expansion history by reconstructing the deceleration
parameter from the SDSS-II type Ia supernova sample (SNIa) with two
different light curve fits (MLCS2k2 and SALT-II), the baryon acoustic
oscillation (BAO) distance ratio, the cosmic microwave background (CMB) shift
parameter, and the lookback time-redshift (LT) from the age of old passive
galaxies. Three parametrization forms for the equation of state of dark energy
(CPL, JBP, and UIS) are considered. Our results show that, for the CPL and the
UIS forms, MLCS2k2 SDSS-II SNIa+BAO+CMB and MLCS2k2 SDSS-II SNIa+BAO+CMB+LT
favor a currently slowing-down cosmic acceleration, but this does not occur for
all other cases, where an increasing cosmic acceleration is still favored.
Thus, the reconstructed evolutionary behaviors of dark energy and the course of
the cosmic acceleration are highly dependent both on the light curve fitting
method for the SNIa and the parametrization form for the equation of state of
dark energy.Comment: 19 pages, 6 figures, accepted for publication in JCA
Parametrization for the Scale Dependent Growth in Modified Gravity
We propose a scale dependent analytic approximation to the exact linear
growth of density perturbations in Scalar-Tensor (ST) cosmologies. In
particular, we show that on large subhorizon scales, in the Newtonian gauge,
the usual scale independent subhorizon growth equation does not describe the
growth of perturbations accurately, as a result of scale-dependent relativistic
corrections to the Poisson equation. A comparison with exact linear numerical
analysis indicates that our approximation is a significant improvement over the
standard subhorizon scale independent result on large subhorizon scales. A
comparison with the corresponding results in the Synchronous gauge demonstrates
the validity and consistency of our analysis.Comment: 10 pages, 5 figures. Minor modifications and references added to
match published versio
QCD ghost f(T)-gravity model
Within the framework of modified teleparallel gravity, we reconstruct a f(T)
model corresponding to the QCD ghost dark energy scenario. For a spatially flat
FRW universe containing only the pressureless matter, we obtain the time
evolution of the torsion scalar T (or the Hubble parameter). Then, we calculate
the effective torsion equation of state parameter of the QCD ghost f(T)-gravity
model as well as the deceleration parameter of the universe. Furthermore, we
fit the model parameters by using the latest observational data including
SNeIa, CMB and BAO data. We also check the viability of our model using a
cosmographic analysis approach. Moreover, we investigate the validity of the
generalized second law (GSL) of gravitational thermodynamics for our model.
Finally, we point out the growth rate of matter density perturbation. We
conclude that in QCD ghost f(T)-gravity model, the universe begins a matter
dominated phase and approaches a de Sitter regime at late times, as expected.
Also this model is consistent with current data, passes the cosmographic test,
satisfies the GSL and fits the data of the growth factor well as the LCDM
model.Comment: 19 pages, 9 figures, 2 tables. arXiv admin note: substantial text
overlap with arXiv:1111.726
A thermodynamic motivation for dark energy
It is argued that the discovery of cosmic acceleration could have been
anticipated on thermodynamic grounds, namely, the generalized second law and
the approach to equilibrium at large scale factor. Therefore, the existence of
dark energy -or equivalently, some modified gravity theory- should have been
expected. In general, cosmological models that satisfy the above criteria show
compatibility with observational data.Comment: 22 pages, 7 eps figures; Key words: dark energy, thermodynamics,
modified gravity. Comments added and arguments sharpene
Cosmic acceleration and phantom crossing in -gravity
In this paper, we propose two new models in gravity to realize
universe acceleration and phantom crossing due to dark torsion in the
formalism. The model parameters are constrained and the observational test are
discussed. The best fit results favors an accelerating universe with possible
phantom crossing in the near past or future followed respectively by matter and
radiation dominated era.Comment: 20 pages, 18 figures, Will appear in Astrophys Space Sc
Dilaton Dark Energy Model in f(R), f(T) and Horava-Lifshitz Gravities
In this work, we have considered dilaton dark energy model in Weyl-scaled
induced gravitational theory in presence of barotropic fluid. It is to be noted
that the dilaton field behaves as a quintessence. Here we have discussed the
role of dilaton dark energy in modified gravity theories namely, f(R); f(T) and
Horava-Lifshitz gravities and analyzed the behaviour of the dilaton field and
the corresponding potential in respect to these modified gravity theories
instead of Einstein's gravity. In f(R) and f(T) gravities, we have considered
some particular forms of f(R) and f(T) and we have shown that the potentials
always increase with the dilaton fields. But in Horava-Lifshitz gravity, it has
been seen that the potential always decreases as dilation field increases
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