The extreme limit of the generalised Chaplygin gas

Unified Dark Matter models describe Dark Matter and Dark Energy as a single entity which is, in the simplest case, embodied in a perfect barotropic fluid. It is a well-established fact that small adiabatic perturbations of Unified Dark Matter have an evolution characterised by oscillations and decay which provide predictions on the Cosmic Background Radiation anisotropies which are in poor agreement with observation. In this paper we investigate the generalised Chaplygin gas and we find that the Integrated Sachs-Wolfe effect excludes the model for 10(-3) < alpha < 350. We discuss the implications on the background evolution of the Universe if large values of a are considered. In this case, the Universe expansion mimics a matter-dominated phase abruptly followed by a de Sitter one at the transition redshift z(tr). Thanks to an analysis of the type Ia supernovae Constitution set we are able to place z(tr) = 0.22.