170 research outputs found
Gravitational waves in an anomaly-induced inflation
The behaviour of gravitational waves in the anomaly-induced inflationary
phase is studied. The metric perturbations exhibit a stable behaviour, with a
very moderate growth in the amplitude of the waves. The spectral indice is
computed, revealing an almost flat spectrum.Comment: 4 pages. Talk presented at IRGA 2003 (Renormalization Group and
Anomalies in Gravitation and Cosmology, Ouro Preto, Brazil, 16-23 March,
2003
Ruling out the Modified Chaplygin Gas Cosmologies
The Modified Chaplygin Gas (MCG) model belongs to the class of a unified
models of dark energy (DE) and dark matter (DM). It is characterized by an
equation of state (EoS) , where the case
corresponds to the Generalized Chaplygin Gas (GCG) model. Using a perturbative
analysis and power spectrum observational data we show that the MCG model is
not a sucessful candidate for the cosmic medium unless . In this case, it
reduces to the usual GCG model.Comment: Latex file, 7 pages, 6 figures in eps forma
Thermodynamics of Chaplygin gas
We clarify thermodynamics of the Chaplygin gas by introducing the
integrability condition. All thermal quantities are derived as functions of
either volume or temperature. Importantly, we find a new general equation of
state, describing the Chaplygin gas completely. We confirm that the Chaplygin
gas could show a unified picture of dark matter and energy which cools down
through the universe expansion without any critical point (phase transition).Comment: 5 pages, 4 figures, version "Accepted for publication in Astrophysics
& Space Science
Observational constraints on Rastall's cosmology
Rastall's theory is a modification of General Relativity, based on the
non-conservation of the stress-energy tensor. The latter is encoded in a
parameter such that restores the usual law. We test Rastall's theory in cosmology, on a flat
Robertson-Walker metric, investigating a two-fluid model and using the type Ia
supernovae Constitution dataset. One of the fluids is pressureless and obeys
the usual conservation law, whereas the other is described by an equation of
state , with constant. The Bayesian analysis of the
Constitution set does not strictly constrain the parameter and prefers
values of close to -1. We then address the evolution of small
perturbations and show that they are dramatically unstable if and
, i.e. General Relativity is the favored configuration. The only
alternative is , for which the dynamics becomes independent from
.Comment: Latex file, 14 pages, 6 figures in eps format. Substantial
modifications performed, main conclusions change
Power spectrum in the Chaplygin gas model: tachyonic, fluid and scalar field representations
The Chaplygin gas model, characterized by an equation of state of the type emerges naturally from the Nambu-Goto action of string
theory. This fluid representation can be recast under the form of a tachyonic
field given by a Born-Infeld type Lagrangian. At the same time, the Chaplygin
gas equation of state can be obtained from a self-interacting scalar field. We
show that, from the point of view of the supernova type Ia data, the three
representations (fluid, tachyonic, scalar field) lead to the same results.
However, concerning the matter power spectra, while the fluid and tachyonic
descriptions lead to exactly the same results, the self-interacting scalar
field representation implies different statistical estimations for the
parameters. In particular, the estimation for the dark matter density parameter
in the fluid representation favors a universe dominated almost completely by
dark matter, while in the self-interacting scalar field representation the
prediction is very closed to that obtained in the CDM model.Comment: Latex file, 10 pages, 18 figures in EPS forma
Newtonian Approach to the Matter Power Spectrum of the Generalized Chaplygin Gas
We model the cosmic medium as the mixture of a generalized Chaplygin gas and
a pressureless matter component. Within a neo-Newtonian approach we compute the
matter power spectrum. The 2dFGRS data are used to discriminate between unified
models of the dark sector and different models, for which there is separate
dark matter, in addition to that accounted for by the generalized Chaplygin
gas. Leaving the corresponding density parameters free, we find that the
unified models are strongly disfavored. On the other hand, using unified model
priors, the observational data are also well described, in particular for small
and large values of the generalized Chaplygin gas parameter .Comment: Latex file, 5 pages, 11 figures in eps format. For the proceedings of
the conference Dark Energy and Dark Matter, 7-11 july 2008, Lyon, Franc
Density perturbations in an Universe dominated by the Chaplygin gas
We study the fate of density perturbations in an Universe dominate by the
Chaplygin gas, which exhibit negative pressure. We show that it is possible to
obtain the value for the density contrast observed in large scale structure of
the Universe by fixing a free parameter in the equation of state of this gas.
The negative character of pressure must be significant only very recently.Comment: Latex file, 5 page
Note on the Evolution of the Gravitational Potential in Rastall Scalar Field Theories
We investigate the evolution of the gravitational potential in Rastall scalar
field theories. In a single component model a consistent perturbation theory,
formulated in the newtonian gauge, is possible only for , which is
the General Relativity limit. On the other hand, the addition of another
canonical fluid component allows also to consider the case .Comment: 16 pages, 3 figures, Sections 2 and 5 enlarged, accepted for
publication in Physics Letters
Density Perturbations in the Brans-Dicke Theory
We analyse the fate of density perturbation in the Brans-Dicke Theory, giving
a general classification of the solutions of the perturbed equations when the
scale factor of the background evolves as a power law. We study with details
the cases of vacuum, inflation, radiation and incoherent matter. We find, for
the a negative Brans-Dicke parameter, a significant amplification of
perturbations.Comment: 26 pages, latex fil
Contraints on unified models for dark matter and dark energy using H(z)
The differential age data of astrophysical objects that have evolved
passivelly during the history of the universe (e.g. red galaxies) allows to
test theoretical cosmological models through the predicted Hubble function
expressed in terms of the redshift , . We use the observational data
for to test unified scenarios for dark matter and dark energy.
Specifically, we focus our analysis on the Generalized Chaplygin Gas (GCG) and
the viscous fluid (VF) models. For the GCG model, it is shown that the unified
scenario for dark energy and dark matter requires some priors. For the VF model
we obtain estimations for the free parameters that may be compared with further
analysis mainly at perturbative level.Comment: Latex file, 10 pages, 19 figures in eps format. Accepted for
publication in European Journal of Physics
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