Chaplygin gas in decelerating DGP gravity and the age of the oldest star

Abstract

Accelerating Chaplygin gas combined with the decelerating braneworld Dvali-Gabadadze-Porrati (DGP) model can produce an overall accelerated expansion of the order of magnitude seen. Both models have similar asymptotic properties at early and late cosmic times, and are characterized by a length scale. Taking the length scales to be proportional one obtains a combined model with three free parameters, one more than the LCDM model, which fits supernovae data equally well. We further constrain it by the CMB shift parameter, and by requiring that the model yields a longer age of the Universe than that of the oldest star HE 1523-0901, t * = 13.4\pm 0.8(stat)\pm 1.8(syst). In contrast to generalized DGP and Chaplygin gas models, this is a genuine alternative to the cosmological constant model because it does not reduce to it in any limit of the parameter space.