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Non-adiabatic Chaplygin gas

The split of a generalised Chaplygin gas with an equation of state p = -A/ρ^α into an interacting mixture of pressureless matter and a dark-energy component with equation of state p_Λ = - ρ_Λ 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 α= -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.