99 research outputs found

### Brans-Dicke Theory and Thermodynamical Laws on Apparent and Event Horizons

In this work, we have described the Brans-Dicke theory of gravity and given a
particular solution by choosing a power law form of scalar field $\phi$ and
constant $\omega$. If we assume first law and entropy formula on apparent
horizon then we recover Friedmann equations. Next, assuming first law of
thermodynamics, the validity conditions of GSL on event horizon are presented.
Also without use first law, if we impose the entropy relation on the horizon,
then we also obtain the condition of validity of GSL on event horizon. The
validity of GSL completely depends on the model of BD scalar field solutions.
We have justified that on the apparent horizon the two process are equivalent,
but on the event horizon they are not equivalent. If first law is valid on the
event horizon then GSL may be satisfied in BD solution, but if first law is not
satisfied then GSL is not satisfied in BD solution. So first law always favours
GSL on event horizon. In our effective approach, the first law and GSL is
always satisfied in apparent horizon, which do not depend on BD theory of
gravity.Comment: 6 pages, 2 figure

### Cosmological Constant, Gauge Hierarchy and Warped Geometry

It is suggested that the mechanism responsible for the resolution of the
gauge hierarchy problem within the warped geometry framework can be generalized
to provide a new explanation of the extremely tiny vacuum energy density rho_V
suggested by recent observations. We illustrate the mechanism with some 5D
examples in which the true vacuum energy is assumed to vanish, and rho_V is
associated with a false vacuum energy such that rho_V^{1/4} ~ TeV^2/M_{Pl} ~
10^{-3} eV, where M_{Pl} denotes the reduced Planck mass. We also consider a
quintessence-like solution to the dark energy problem.Comment: 10 pages, LaTeX, 2 figures, section on quantum corrections added,
version to appear in Phys. Rev.

### A New Cosmological Model of Quintessence and Dark Matter

We propose a new class of quintessence models in which late times
oscillations of a scalar field give rise to an effective equation of state
which can be negative and hence drive the observed acceleration of the
universe. Our ansatz provides a unified picture of quintessence and a new form
of dark matter we call "Frustrated Cold Dark Matter" (FCDM). FCDM inhibits
gravitational clustering on small scales and could provide a natural resolution
to the core density problem for disc galaxy halos. Since the quintessence field
rolls towards a small value, constraints on slow-roll quintessence models are
safely circumvented in our model.Comment: Revised. Important new results added in response to referees comment

### Variable Modified Chaplygin Gas and Accelerating Universe

In this letter, I have proposed a model of variable modified Chaplygin gas
and shown its role in accelerating phase of the universe. I have shown that the
equation of state of this model is valid from the radiation era to quiessence
model. The graphical representations of statefinder parameters characterize
different phase of evolution of the universe. All results presented in the
letter concerns the case $k=0$.Comment: 7 Latex pages, 5 figures, revtex styl

### A Two-Field Quintessence Model

We study the dynamics of a quintessence model based on two interacting scalar
fields. The model can account for the (recent) accelerated expansion of the
Universe suggested by astronomical observations. Acceleration can be permanent
or temporary and, for both scenarios, it is possible to obtain suitable values
for the cosmological parameters while satisfying the nucleosynthesis constraint
on the quintessence energy density. We argue that the model dynamics can be
made consistent with a stable zero-energy relaxing supersymmetric vacuum.Comment: 4 pages, 3 eps figures, to be published in Phys. Rev.

### A single model of traversable wormholes supported by generalized phantom energy or Chaplygin gas

This paper discusses a new variable equation of state parameter leading to
exact solutions of the Einstein field equations describing traversable
wormholes. In addition to generalizing the notion of phantom energy, the
equation of state generates a mathematical model that combines the generalized
phantom energy and the generalized Chaplygin gas models.Comment: 6 pages, no figure

### Observational Constraints of Modified Chaplygin Gas in Loop Quantum Cosmology

We have considered the FRW universe in loop quantum cosmology (LQC) model
filled with the dark matter (perfect fluid with negligible pressure) and the
modified Chaplygin gas (MCG) type dark energy. We present the Hubble parameter
in terms of the observable parameters $\Omega_{m0}$, $\Omega_{x0}$ and $H_{0}$
with the redshift $z$ and the other parameters like $A$, $B$, $C$ and $\alpha$.
From Stern data set (12 points), we have obtained the bounds of the arbitrary
parameters by minimizing the $\chi^{2}$ test. The best-fit values of the
parameters are obtained by 66%, 90% and 99% confidence levels. Next due to
joint analysis with BAO and CMB observations, we have also obtained the bounds
of the parameters ($B,C$) by fixing some other parameters $\alpha$ and $A$.
From the best fit of distance modulus $\mu(z)$ for our theoretical MCG model in
LQC, we concluded that our model is in agreement with the union2 sample data.Comment: 14 pages, 10 figures, Accepted in EPJC. arXiv admin note: text
overlap with arXiv:astro-ph/0311622 by other author

### Role of Brans-Dicke Theory with or without self-interacting potential in cosmic acceleration

In this work we have studied the possibility of obtaining cosmic acceleration
in Brans-Dicke theory with varying or constant $\omega$ (Brans- Dicke
parameter) and with or without self-interacting potential, the background fluid
being barotropic fluid or Generalized Chaplygin Gas. Here we take the power law
form of the scale factor and the scalar field. We show that accelerated
expansion can also be achieved for high values of $\omega$ for closed Universe.Comment: 12 Latex pages, 20 figures, RevTex styl

### Generalized Chaplygin Gas, Accelerated Expansion and Dark Energy-Matter Unification

We consider the scenario emerging from the dynamics of a generalized
$d$-brane in a $(d+1, 1)$ spacetime. The equation of state describing this
system is given in terms of the energy density, $\rho$, and pressure, $p$, by
the relationship $p = - A/\rho^{\alpha}$, where $A$ is a positive constant and
$0 < \alpha \le 1$. We discuss the conditions under which homogeneity arises
and show that this equation of state describes the evolution of a universe
evolving from a phase dominated by non-relativistic matter to a phase dominated
by a cosmological constant via an intermediate period where the effective
equation of state is given by $p = \alpha \rho$.Comment: 5 pages, 4 figures, revte

### Can the Chaplygin gas be a plausible model for dark energy?

In this note two cosmological models representing the flat Friedmann Universe
filled with a Chaplygin fluid, with or without dust, are analyzed in terms of
the recently proposed "statefinder" parameters. Trajectories of both models in
the parameter plane are shown to be significantly different w.r.t. "quiessence"
and "tracker" models. The generalized Chaplygin gas model with an equation of
state of the form $p = -A/\rho^{\alpha}$ is also analyzed in terms of the
statefinder parameters.Comment: 6 pages, 2 figure

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