Generalized Chaplygin Gas Models tested with SNIa

The Generalized Chaplygin Gas (GCG) with the equation of state p = − A ρ was recently proposed as a candidate for dark energy in the Universe. In this paper we confront the GCG with SNIa data using avaliable samples. Specifically we have tested the GCG cosmology in three different classes of models with (1) Ωm = 0.3, ΩCh = 0.7; (2) Ωm = 0.05, ΩCh = 0.95 and (3) Ωm = 0, ΩCh = 1, as well as a model without prior assumptions on Ωm. The best fitted models are obtained by minimalizing the χ function. We supplement our analysis with confidence intervals in the (A0, α) plane by marginalizing the probability density functions over remaining parameters assuming uniform priors. We have also derived one-dimensional probability distribution functions for ΩCh obtained from joint marginalization over α and A0. The maximum value of such PDF provides the most probable value of ΩCh within the full class of GCG models. The general conclusion is that SNIa data give support to the Chaplygin gas (with α = 1). However noticeable preference of A0 values close to 1 means that the α dependence becomes insignificant. It is reflected on one dimensional PDFs for α which turned out to be flat meaning that the power of present supernovae data to discriminate between various GCG models (differing by α) is weak. Extending our analysis by relaxing the prior assumption of the flatness of the Universe leads to