7 research outputs found
Bayesian Analysis of the (Generalized) Chaplygin Gas and Cosmological Constant Models using the 157 gold SNe Ia Data
The generalized Chaplygin gas model (GCGM) contains 5 free parameters, here,
they are constrained through the type Ia supernovae data, i.e., the ``gold
sample'' of 157 supernovae data. Negative and large positive values for
are taken into account. The analysis is made by employing the Bayesian
statistics and the prediction for each parameter is obtained by marginalizing
on the remained ones. This procedure leads to the following predictions:
, , , , , . Through the same
analysis the specific case of the ordinary Chaplygin gas model (CGM), for which
, is studied. In this case, there are now four free parameters and
the predictions for them are: , , , , . To complete the
analysis the CDM, with its three free parameters, is considered. For
all these models, particular cases are considered where one or two parameters
are fixed. The age of the Universe, the deceleration parameter and the moment
the Universe begins to accelerate are also evaluated. The quartessence
scenario, is favoured. A closed (and in some cases a flat) and accelerating
Universe is also preferred. The CGM case is far from been ruled
out, and it is even preferred in some particular cases. In most of the cases
the CDM is disfavoured with respect to GCGM and CGM.Comment: 23 pages, LaTeX 2e, 6 tables, 38 EPS figures, uses graphic
On the consistency of a repulsive gravity phase in the early Universe
We exploit the possibility of existence of a repulsive gravity phase in the
evolution of the Universe. A toy model with a free scalar field minimally
coupled to gravity, but with the "wrong sign" for the energy and negative
curvature for the spatial section, is studied in detail. The background
solutions display a bouncing, non-singular Universe. The model is well-behaved
with respect to tensor perturbations. But, it exhibits growing models with
respect to scalar perturbations whose maximum occurs in the bouncing. Hence,
large inhomogeneties are produced. At least for this case, a repulsive phase
may destroy homogeneity, and in this sense it may be unstable. A newtonian
analogous model is worked out; it displays qualitatively the same behaviour.
The generality of this result is discussed. In particular, it is shown that the
addition of an attractive radiative fluid does not change essentially the
results. We discuss also a quantum version of the classical repulsive phase,
through the Wheeler-de Witt equation in mini-superspace, and we show that it
displays essentially the same scenario as the corresponding attractive phase.Comment: Latex file, 15 pages, 7 figures. There is a new figure, a new section
and some other minor correction
Bayesian Statistics and Parameter Constraints on the Generalized Chaplygin Gas Model using SNe Ia Data
The type Ia supernovae (SNe Ia) observational data are used to estimate the
parameters of a cosmological model with cold dark matter and the generalized
Chaplygin gas model (GCGM). The GCGM depends essentially on five parameters:
the Hubble constant, the parameter related to the velocity of the
sound, the equation of state parameter , the curvature of the Universe
and the fraction density of the generalized Chaplygin gas (or the cold dark
matter). The parameter is allowed to take negative values and to be
greater than 1. The Bayesian parameter estimation yields , , , ,
, , and , where is the age
of the Universe and is the value of the deceleration parameter today. Our
results indicate that a Universe completely dominated by the generalized
Chaplygin gas is favoured, what reinforces the idea that the this gas may unify
the description for dark matter and dark energy, at least as the SNe Ia data is
concerned. A closed and accelerating Universe is also favoured. The traditional
Chaplygin gas model (CGM), is not ruled out, even if it does not
give the best-fitting. Particular cases with four or three independent free
parameters are also analysed.Comment: 18 pages, LaTeX 2e, 2 tables, 20 EPS figures, uses graphic
Bayesian Analysis of the Chaplygin Gas and Cosmological Constant Models using the SNe Ia Data
The type Ia supernovae observational data are used to estimate the parameters
of a cosmological model with cold dark matter and the Chaplygin gas. The
Chaplygin gas model depends essentially on four parameters: the Hubble
constant, the velocity of the sound of the Chaplygin gas, the curvature of the
Universe and the fraction density of the Chaplygin gas and the cold dark
matter. The Bayesian parameter estimation yields , , , , , and . These and other results indicate that a Universe
completely dominated by the Chaplygin gas is favoured, at least as the type Ia
supernovae data are concerned. A closed and accelerating Universe is also
favoured. The Bayesian statistics indicates that the Chaplygin gas model is
more likely than the standard cosmological constant () model at
55.3% confidence level when an integration on all free parameters is performed.
Assuming the spatially flat curvature, this percentage mounts to 65.3%. On the
other hand, if the density of dark matter is fixed at zero value, the Chaplygin
gas model becomes more preferred than the model at 91.8%
confidence level. Finally, the hypothesis of flat Universe and baryonic matter
() implies a Chaplygin gas model preferred over the at a confidence level of 99.4%.Comment: 19 pages, LaTeX 2e, 4 EPS figures, uses graphic