2,351 research outputs found
Background cosmological dynamics in gravity and observational constraints
In this paper, we carry out a study of viable cosmological models in
-gravity at the background level. We use observable parameters like
and to form autonomous system of equations and show that the
models under consideration exhibit two different regimes in their time
evolution, namely, a phantom phase followed by a quintessence like behavior. We
employ statefinder parameters to emphasize a characteristic discriminative
signature of these models.Comment: 6 pages, Latex style, 9 eps figures, replaced versions with new
references added, Submitted to Phys.Rev.
Observational signatures of f(R) dark energy models that satisfy cosmological and local gravity constraints
We discuss observational consequences of f(R) dark energy scenarios that
satisfy local gravity constraints (LGC) as well as conditions of the
cosmological viability. The model we study is given by m(r)=C(-r-1)^p (C>0,
p>1) with m=Rf_{,RR}/f_{,R} and r=-Rf_{,R}/f, which cover viable f(R) models
proposed so far in a high-curvature region designed to be compatible with LGC.
The equation of state of dark energy exhibits a divergence at a redshift z_c
that can be as close as a few while satisfying sound horizon constraints of
Cosmic Microwave Background (CMB). We study the evolution of matter density
perturbations in details and place constraints on model parameters from the
difference of spectral indices of power spectra between CMB and galaxy
clustering. The models with p>5 can be consistent with those observational
constraints as well as LGC. We also discuss the evolution of perturbations in
the Ricci scalar R and show that an oscillating mode (scalaron) can easily
dominate over a matter-induced mode as we go back to the past. This violates
the stability of cosmological solutions, thus posing a problem about how the
over-production of scalarons should be avoided in the early universe.Comment: 13 pages, 7 figures, version to appear in Physical Review
Density perturbations in f(R) gravity theories in metric and Palatini formalisms
We make a detailed study of matter density perturbations in both metric and
Palatini formalisms in theories whose Lagrangian density is a general function,
f(R), of the Ricci scalar. We derive these equations in a number of gauges. We
show that for viable models that satisfy cosmological and local gravity
constraints (LGC), matter perturbation equations derived under a sub-horizon
approximation are valid even for super-Hubble scales provided the oscillating
mode (scalaron) does not dominate over the matter-induced mode. Such
approximate equations are especially reliable in the Palatini formalism because
of the absence of scalarons.
Using these equations we make a comparative study of the behaviour of density
perturbations as well as gravitational potentials for a number of classes of
theories. In the metric formalism the parameter m=Rf_{,RR}/f_{,R}
characterising the deviation from the Lambda CDM model is constrained to be
very small during the matter era in order to ensure compatibility with LGC, but
the models in which m grows to the order of 10^{-1} around the present epoch
can be allowed. These models also suffer from an additional fine tuning due to
the presence of scalaron modes which are absent in the Palatini case.
In Palatini formalism LGC and background cosmological constraints provide
only weak bounds on |m| by constraining it to be smaller than ~ 0.1. This is in
contrast to matter density perturbations which, on galactic scales, place far
more stringent constraints on the present deviation parameter m of the order of
|m| < 10^{-5} - 10^{-4}. This is due to the peculiar evolution of matter
perturbations in the Palatini case which exhibits a rapid growth or a damped
oscillation depending on the sign of m.Comment: 36 pages including 8 figures. Accepted for publication in Physical
Review
The phase-space of generalized Gauss-Bonnet dark energy
The generalized Gauss-Bonnet theory, introduced by Lagrangian F(R,G), has
been considered as a general modified gravity for explanation of the dark
energy. G is the Gauss-Bonnet invariant. For this model, we seek the situations
under which the late-time behavior of the theory is the de-Sitter space-time.
This is done by studying the two dimensional phase space of this theory, i.e.
the R-H plane. By obtaining the conditions under which the de-Sitter space-time
is the stable attractor of this theory, several aspects of this problem have
been investigated. It has been shown that there exist at least two classes of
stable attractors : the singularities of the F(R,G), and the cases in which the
model has a critical curve, instead of critical points. This curve is R=12H^2
in R-H plane. Several examples, including their numerical calculations, have
been discussed.Comment: 19 pages, 11 figures, typos corrected, a reference adde
Cosmological coincidence problem in interacting dark energy models
An interacting dark energy model with interaction term is considered. By studying the model near the
transition time, in which the system crosses the w=-1 phantom-divide-line, the
conditions needed to overcome the coincidence problem is investigated. The
phantom model, as a candidate for dark energy, is considered and for two
specific examples, the quadratic and exponential phantom potentials, it is
shown that it is possible the system crosses the w=-1 line, meanwhile the
coincidence problem is alleviated, the two facts that have root in
observations.Comment: 15 pages, LaTeX. Some minor explanations are added. To be published
in Phys. Rev.
Avoidance of future singularities in loop quantum cosmology
We consider the fate of future singularities in the effective dynamics of
loop quantum cosmology. Non-perturbative quantum geometric effects which lead
to modification of the Friedmann equation at high energies result in
generic resolution of singularities whenever energy density diverges at
future singularities of Friedmann dynamics. Such quantum effects lead to the
avoidance of a Big Rip, which is followed by a recollapsing universe stable
against perturbations. Resolution of sudden singularity, the case when pressure
diverges but energy density approaches a finite value depends on the ratio of
the latter to a critical energy density of the order of Planck. If the value of
this ratio is greater than unity, the universe escapes the sudden future
singularity and becomes oscillatory.Comment: 6 pages, 2 figure
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