Does Chaplygin gas have salvation?

We investigate the unification scenario provided by the generalized Chaplygin gas model (a perfect fluid characterized by an equation of state p=−A/ρ α ). Our concerns lie with a possible tension existing between background kinematic tests and those related to the evolution of small perturbations. We analyze data from the observation of the differential age of the universe, type Ia supernovae, baryon acoustic oscillations, and the position of the first peak of the angular spectrum of the cosmic background radiation. We show that these tests favor negative values of the parameter α: we find α=−0.089+0.161−0.128 at the 2σ level and that α<0 with 85 % confidence. These would correspond to negative values of the square speed of sound which are unacceptable from the point of view of structure formation. We discuss a possible solution to this problem, when the generalized Chaplygin gas is framed in the modified theory of gravity proposed by Rastall. We show that a fluid description within this theory does not serve the purpose, but it is necessary to frame the generalized Chaplygin gas in a scalar field theory. Finally, we address the standard general relativistic unification picture provided by the generalized Chaplygin gas in the case α=0: this is usually considered to be undistinguishable from the standard ΛCDM model, but we show that the evolution of small perturbations, governed by the Mészáros equation, is indeed different and the formation of sub-horizon GCG matter halos may be importantly affected in comparison with the ΛCDM scenario