Any unified dark matter cosmology can be decomposed into dark matter interacting with vacuum energy , without introducing any additional degrees of freedom .
We present observational constraints on an interacting vacuum plus dark energy corresponding to a generalised Chaplygin gas cosmology .
We consider two distinct models for the interaction leading to either a barotropic equation of state or dark matter that follows geodesics , corresponding to a rest-frame sound speed equal to the adiabatic sound speed or zero sound speed , respectively .
For the barotropic model , the most stringent constraint on \alpha comes from the combination of CMB+SNIa+LSS ( m ) gives \alpha < 5.66 \times 10 ^ { -6 } at the 95 % confidence level , which indicates that the barotropic model must be extremely close to the \Lambda CDM cosmology .
For the case where the dark matter follows geodesics , perturbations have zero sound speed , and CMB+SNIa+gISW then gives the much weaker constraint -0.15 < \alpha < 0.26 at the 95 % confidence level .