Observational tests of non-adiabatic Chaplygin gas
  In a previous paper it was shown that any dark sector model can be mapped
into a non-adiabatic fluid formed by two interacting components, one with zero
pressure and the other with equation-of-state parameter $\omega = -1$. It was
also shown that the latter does not cluster and, hence, the former is
identified as the observed clustering matter. This guarantees that the dark
matter power spectrum does not suffer from oscillations or instabilities. It
applies in particular to the generalised Chaplygin gas, which was shown to be
equivalent to interacting models at both background and perturbation levels. In
the present paper we test the non-adiabatic Chaplygin gas against the Hubble
diagram of type Ia supernovae, the position of the first acoustic peak in the
anisotropy spectrum of the cosmic microwave background and the linear power
spectrum of large scale structures. We consider two different samples of SNe
Ia, namely the Constitution and SDSS compilations, both calibrated with the
MLCS2k2 fitter, and for the power spectrum we use the 2dFGRS catalogue. The
model parameters to be adjusted are the present Hubble parameter, the present
matter density and the Chaplygin gas parameter $\alpha$. The joint analysis
best fit gives $\alpha \approx - 0.5$, which corresponds to a constant-rate
energy flux from dark energy to dark matter, with the dark energy density
decaying linearly with the Hubble parameter. The $\Lambda$CDM model, equivalent
to $\alpha = 0$, stands outside the $3\sigma$ confidence interval. This result
is still valid if we use, as supernovae samples, the SDSS and Union2.1
compilations calibrated with the SALT2 fitter.
