2,927 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
Anisotropic Cosmological Constant and the CMB Quadrupole Anomaly
There are evidences that the cosmic microwave background (CMB) large-angle
anomalies imply a departure from statistical isotropy and hence from the
standard cosmological model. We propose a LCDM model extension whose dark
energy component preserves its nondynamical character but wield anisotropic
vacuum pressure. Exact solutions for the cosmological scale factors are
presented, upper bounds for the deformation parameter are evaluated and its
value is estimated considering the elliptical universe proposal to solve the
quadrupole anomaly. This model can be constructed from a Bianchi I cosmology
with cosmological constant from two different ways: i) a straightforward
anisotropic modification of the vacuum pressure consistently with
energy-momentum conservation; ii) a Poisson structure deformation between
canonical momenta such that the dynamics remain invariant under scale factors
rescalings.Comment: 8 pages, 2 columns, 1 figure. v2: figure improved, added comments on
higher eccentricity powers and references. v3: typos corrected, version to
appear in PR
Type I singularities and the Phantom Menace
We consider the future dynamics of a transient phantom dominated phase of the
universe in LQC and in the RS braneworld, which both have a non-standard
Friedmann equation. We find that for a certain class of potentials, the Hubble
parameter oscillates with simple harmonic motion in the LQC case and therefore
avoids any future singularity. For more general potentials we find that damping
effects eventually lead to the Hubble parameter becoming constant. On the other
hand in the braneworld case we find that although the type I singularity can be
avoided, the scale factor still diverges at late times.Comment: More references added. Final PRD versio
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
Probing the Coupling between Dark Components of the Universe
We place observational constraints on a coupling between dark energy and dark
matter by using 71 Type Ia supernovae (SNe Ia) from the first year of the
five-year Supernova Legacy Survey (SNLS), the cosmic microwave background (CMB)
shift parameter from the three-year Wilkinson Microwave Anisotropy Probe
(WMAP), and the baryon acoustic oscillation (BAO) peak found in the Sloan
Digital Sky Survey (SDSS). The interactions we study are (i) constant coupling
delta and (ii) varying coupling delta(z) that depends on a redshift z, both of
which have simple parametrizations of the Hubble parameter to confront with
observational data. We find that the combination of the three databases
marginalized over a present dark energy density gives stringent constraints on
the coupling, -0.08 < delta < 0.03 (95% CL) in the constant coupling model and
-0.4 < delta_0 < 0.1 (95% CL) in the varying coupling model, where delta_0 is a
present value. The uncoupled LambdaCDM model (w_X = -1 and delta = 0) still
remains a good fit to the data, but the negative coupling (delta < 0) with the
equation of state of dark energy w_X < -1 is slightly favoured over the
LambdaCDM model.Comment: 9 pages, 7 figures, RevTeX, minor corrections, references added,
accepted for publication in Phys. Rev.
APSIS - an Artificial Planetary System in Space to probe extra-dimensional gravity and MOND
A proposal is made to test Newton's inverse-square law using the perihelion
shift of test masses (planets) in free fall within a spacecraft located at the
Earth-Sun L2 point. Such an Artificial Planetary System In Space (APSIS) will
operate in a drag-free environment with controlled experimental conditions and
minimal interference from terrestrial sources of contamination. We demonstrate
that such a space experiment can probe the presence of a "hidden" fifth
dimension on the scale of a micron, if the perihelion shift of a "planet" can
be measured to sub-arc-second accuracy. Some suggestions for spacecraft design
are made.Comment: 17 pages, revtex, references added. To appear in Special issue of
IJMP
- …