Does Chaplygin gas have salvation?
  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
M\'esz\'aros equation, is indeed different and the formation of sub-horizon GCG
matter halos may be importantly affected in comparison with the \Lambda CDM
scenario.
