20 research outputs found
Constraining gravity models with disappearing cosmological constant
The gravity models proposed by Hu-Sawicki and Starobinsky are generic
for local gravity constraints to be evaded. The large deviations from these
models either result into violation of local gravity constraints or the
modifications are not distinguishable from cosmological constant. The curvature
singularity in these models is generic but can be avoided provided that proper
fine tuning is imposed on the evolution of scalaron in the high curvature
regime. In principle, the problem can be circumvented by incorporating
quadratic curvature correction in the Lagrangian though it might be quite
challenging to probe the relevant region numerically.Comment: 9 pages and 4 figures, minor clarifications and corrections added,
final version to appear in PR
How delicate are the f(R) gravity models with a disappearing cosmological constant?
We consider stability of spherically symmetric solutions in f(R) gravity model proposed by Starobinsky. We find that the model suffers from a severe fine-tuning problem when applied to compact objects like neutron stars. The problem can be remedied by introducing a cutoff on the mass of the scalar degree of freedom present in the model. A new mass scale associated with neutron stars density is then required for the stabilities of f(R) gravity solutions inside relativistic stars
Reheating the D-brane universe via instant preheating
We investigate a possibility of reheating in a scenario of D-brane inflation
in a warped deformed conifold background which includes perturbative
corrections to throat geometry sourced by chiral operator of dimension 3/2 in
the CFT. The effective D-brane potential, in this case, belongs to the class of
non-oscillatory models of inflation for which the conventional reheating
mechanism does not work. We find that gravitational particle production is
inefficient and leads to reheating temperature of the order of . We
show that instant preheating is quite suitable to the present scenario and can
easily reheat universe to a temperature which is higher by about three orders
of magnitudes than its counter part associated with gravitational particle
production. The reheating temperature is shown to be insensitive to a
particular choice of inflationary parameters suitable to observations.Comment: 6 pages and 4 figures, replaced with revised version, to appear in
PR
Generic f(R) theories and classicality of their scalarons
We study quantum stability bound on the mass of scalaron in generic theories
of gravity. We show that in these scenarios, the scalaron mass increases
faster with local density of the environment than one loop quantum correction
to it thereby leading to violation of quantum bound on the chameleon mass. The
introduction of quadratic curvature corrections in the action are shown to
stabilize the model.Comment: 7 pages, no figures, typos corrected, to match with the PLB published
versio
Delicate f(R) gravity models with disappearing cosmological constant and observational constraints on the model parameters
We study the theory of gravity using metric approach. In particular we
investigate the recently proposed model by Hu-Sawicki, Appleby Battye and
Starobinsky. In this model, the cosmological constant is zero in flat space
time. The model passes both the Solar system and the laboratory tests. But the
model parameters need to be fine tuned to avoid the finite time singularity
recently pointed in the literature. We check the concordance of this model with
the and baryon acoustic oscillation data. We find that the model
resembles the CDM at high redshift. However, for some parameter values
there are variations in the expansion history of the universe at low redshift.Comment: 16 pages and 9 figures, typos corrected, few references and minor
clarifications added, revised version to appera in PR
Dark energy generated from a (super)string effective action with higher order curvature corrections and a dynamical dilaton
We investigate the possibility of a dark energy universe emerging from an
action with higher-order string loop corrections to Einstein gravity in the
presence of a massless dilaton. These curvature corrections (up to order)
are different depending upon the type of (super)string model which is
considered. We find in fact that Type II, heterotic, and bosonic strings
respond differently to dark energy. A dark energy solution is shown to exist in
the case of the bosonic string, while the other two theories do not lead to
realistic dark energy universes. Detailed analysis of the dynamical stability
of the de-Sitter solution is presented for the case of a bosonic string. A
general prescription for the construction of a de-Sitter solution for the
low-energy (super)string effective action is also indicated. Beyond the
low-energy (super)string effective action, when the higher-curvature correction
coefficients depend on the dilaton, the reconstruction of the theory from the
universe expansion history is done with a corresponding prescription for the
scalar potentials.Comment: 15 pages, 7 eps figures, minor corrections, published versio
Non-minimally coupled f(R) Cosmology
We investigate the consequences of non-minimal gravitational coupling to
matter and study how it differs from the case of minimal coupling by choosing
certain simple forms for the nature of coupling, The values of the parameters
are specified at (present epoch) and the equations are evolved backwards
to calculate the evolution of cosmological parameters. We find that the Hubble
parameter evolves more slowly in non-minimal coupling case as compared to the
minimal coupling case. In both the cases, the universe accelerates around
present time, and enters the decelerating regime in the past. Using the latest
Union2 dataset for supernova Type Ia observations as well as the data for
baryon acoustic oscillation (BAO) from SDSS observations, we constraint the
parameters of Linder exponential model in the two different approaches. We find
that there is a upper bound on model parameter in minimal coupling. But for
non-minimal coupling case, there is range of allowed values for the model
parameter.Comment: 7 pages, Latex style, 8 eps figure
Equilibrium thermodynamics in modified gravitational theories
We show that it is possible to obtain a picture of equilibrium thermodynamics
on the apparent horizon in the expanding cosmological background for a wide
class of modified gravity theories with the Lagrangian density ,
where is the Ricci scalar and is the kinetic energy of a scalar field
. This comes from a suitable definition of an energy momentum tensor of
the "dark" component that respects to a local energy conservation in the Jordan
frame. In this framework the horizon entropy corresponding to equilibrium
thermodynamics is equal to a quarter of the horizon area in units of
gravitational constant , as in Einstein gravity. For a flat cosmological
background with a decreasing Hubble parameter, globally increases with
time, as it happens for viable inflation and dark energy models. We also
show that the equilibrium description in terms of the horizon entropy is
convenient because it takes into account the contribution of both the horizon
entropy in non-equilibrium thermodynamics and an entropy production
term.Comment: 11 pages, 2 figures, version to appear in Physics Letters B, typos
correcte
Is the future universe singular: Dark Matter versus modified gravity?
The fundamental problem of the occurrence/removal of finite-time future
singularity in the universe evolution for coupled dark energy (DE) is
addressed. It is demonstrated the existence of the (instable or local minimum)
de Sitter space solution which may cure the Type II or Type IV future
singularity for DE coupled with DM as the result of tuning the initial
conditions. In case of phantom DE, the corresponding coupling may help to
resolve the coincidence problem but not the Big Rip (Type I) singularity issue.
We show that modified gravity of special form or inhomogeneous DE fluid may
offer the universal scenario to cure the Type I,II,III or IV future singularity
of coupled (fluid or scalar) DE evolution.Comment: LaTeX 9 page
Phantom and non-phantom dark energy: The cosmological relevance of non-locally corrected gravity
In this paper we have investigated the cosmological dynamics of non-locally
corrected gravity involving a function of the inverse d'Alembertian of the
Ricci scalar, . Casting the dynamical equations into local
form, we derive the fixed points of the dynamics and demonstrate the existence
and stability of a one parameter family of dark energy solutions for a simple
choice, . The effective EoS
parameter is given by, and the
stability of the solutions is guaranteed provided that . For
and , the underlying system exhibits phantom
and non-phantom behavior respectively; the de Sitter solution corresponds to
. For a wide range of initial conditions, the system mimics dust
like behavior before reaching the stable fixed point. The late time phantom
phase is achieved without involving negative kinetic energy fields. A brief
discussion on the entropy of de Sitter space in non-local model is included.Comment: 5 pages and 2 figures, typos corrected, final version to appear in
Phys. Lett.