Bayesian analysis of the Chaplygin gas and cosmological constant models using the SNe Ia data

The type la supernovae observational data axe used to estimate the parameters of a cosmological model with cold dark matter and the Chaplygin gas. This exotic gas, which is characterized by a negative pressure varying with the inverse of density, represents in this model the dark energy responsible for the acceleration of the Universe. 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 H-0 = 62.1(-3.4)(+3.3) km/Mpcs, Omega(k0) = -0.84(-1.23)(+1.51), Omega(m0) = 0.0(-0.0)(+0.82), Omega(c0) = 1.40(-1.16)(1.15), (A) over bar = c(s)(2) = 0.93(-0.21)(+0.07) c, t(0) = 14.2-1.3+2.8 Gy and q(0) = -0.98(-0.62)(+1.02). These and other results indicate that a Universe completely dominated by the Chaplygin gas is favoured, what reinforces the idea that the Chaplygin gas may unify the description for dark matter and dark energy, at least as the type la supernovae data axe 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 (ACDM) 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 ACDM model at 91.8% confidence level. Finally, the hypothesis of flat Universe and baryonic matter (Omega(b0) = 0.04) implies a Chaplygin gas model preferred over the ACDM at a confidence level of 99.4%.