We investigate the unification scenario provided by the generalised Chaplygin gas model ( a perfect fluid characterized by an equation of state p = - A / \rho ^ { \alpha } ) .
Our concerns lie with a possible tension existing between background kinematic tests and those related to the evolution of small perturbations .
We analyse 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 favour negative values of the parameter \alpha : we find \alpha = -0.089 ^ { +0.161 } _ { -0.128 } at the 2 \sigma level and that \alpha < 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 generalised 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 generalised Chaplygin gas in a scalar field theory .
Finally , we address the standard general relativistic unification picture provided by the generalised Chaplygin gas in the case \alpha = 0 : this is usually considered to be undistinguishable from the standard \Lambda CDM 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 \Lambda CDM scenario .