Non-adiabatic Chaplygin gas
  The split of a generalised Chaplygin gas with an equation of state p =
-A/\rho^{\alpha} into an interacting mixture of pressureless matter and a
dark-energy component with equation of state p_{\Lambda} = - \rho_{\Lambda}
implies the existence of non-adiabatic pressure perturbations. We demonstrate
that the square of the effective (non-adiabatic) sound speed c_s of the medium
is proportional to the ratio of the perturbations of the dark energy to those
of the dark matter. Since, as demonstrated explicitly for the particular case
\alpha = -1/2, dark-energy perturbations are negligible compared with
dark-matter perturbations on scales that are relevant for structure formation,
we find |c_s^2| << 1. Consequently, there are no oscillations or instabilities
which have plagued previous adiabatic Chaplygin-gas models.
