11 research outputs found
Stellar configurations in f(R) theories of gravity
We study stellar configurations and the space-time around them in metric
theories of gravity. In particular, we focus on the polytropic model of
the Sun in the model. We show how the stellar configuration in
the theory can, by appropriate initial conditions, be selected to be
equal to that described by the Lane-Emden -equation and how a simple scaling
relation exists between the solutions. We also derive the correct solution
analytically near the center of the star in theory. Previous analytical
and numerical results are confirmed, indicating that the space-time around the
Sun is incompatible with Solar System constraints on the properties of gravity.
Numerical work shows that stellar configurations, with a regular metric at the
center, lead to outside the star ie. the
Schwarzschild-de Sitter -space-time is not the correct vacuum solution for such
configurations. Conversely, by selecting the Schwarzschild-de Sitter -metric as
the outside solution, we find that the stellar configuration is unchanged but
the metric is irregular at the center. The possibility of constructing a
theory compatible with the Solar System experiments and possible new
constraints arising from the radius-mass -relation of stellar objects is
discussed.Comment: 8 pages, 7 figures; typos corrected, reference adde
Consistency of gravity models around solar polytropes
It is stated that a class of gravity models seem to obtain
CDM behaviour for high redshifts and general relativistic behaviour
locally at high curvatures. In the present paper, we numerically study
polytropic configurations that resemble stars like young sun with Hu and
Sawicki gravity field equations and compare the spacetime at the
boundary to the general relativistic counterpart. These polytropes are
stationary spherically symmetric configurations and have regular metrics at the
origin. Since Birkhoff's theorem does not apply for modified gravity, the
solution outside may deviate from Schwarzschild-de Sitter spacetime. At the
boundary, Post-Newtonian parametrization was used to determine how much the
studied model deviates from the general relativistic CDM model.Comment: 10 pages, 3 figures, text revised for publicatio
Geometric and thermodynamic properties in Gauss-Bonnet gravity
In this paper, the generalized second law (GSL) of thermodynamics and entropy
is revisited in the context of cosmological models in Gauss-Bonnet gravity with
the boundary of the universe is assumed to be enclosed by the dynamical
apparent horizon. The model is best fitted with the observational data for
distance modulus. The best fitted geometric and thermodynamic parameters such
as equation of state parameter, deceleration parameter and entropy are derived.
To link between thermodynamic and geometric parameters, the "entropy rate of
change multiplied by the temperature" as a model independent thermodynamic
state parameter is also derived. The results show that the model is in good
agreement with the observational analysis.Comment: 13 pages, 13 figures, to be published in Astrophysics and Space Sc
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom
Entropy and statefinder diagnosis in chameleon cosmology
In this paper, the generalized second law (GSL) of thermodynamics and entropy
is revisited in the context of cosmological models with bouncing behavior such
as chameleon cosmology where the boundary of the universe is assumed to be
enclosed by the dynamical apparent horizon. From a thermodynamic point of view,
to link between thermodynamic and geometric parameters in cosmological models,
we introduce "entropy rate of change multiplied by the temperature" as a model
independent thermodynamic state parameter together with the well known statefinder to differentiate the dark energy models.Comment: 11 pages, 5 figures. will be published in Astrophys. Space Sc