Lambda alpha DM: Observational constraints on unified dark matter with constant speed of sound

We consider the hypothesis that dark energy and dark matter are the two faces of a single dark component, a unified dark matter (UDM) that we assume can be modeled by the affine equation of state (EoS) P=p(0)+alpha rho, resulting in an effective cosmological constant rho(Lambda)=-p(0)/(1+alpha). The affine EoS arises from the simple assumption that the speed of sound is constant; it may be seen as an approximation to an unknown barotropic EoS P=P(rho), and may as well represent the tracking solution for the dynamics of a scalar field with appropriate potential. Furthermore, in principle the affine EoS allows the UDM to be phantom. We constrain the parameters of the model, alpha and Omega(Lambda), using data from a suite of different cosmological observations, and perform a comparison with the standard Lambda CDM model, containing both cold dark matter and a cosmological constant. First considering a flat cosmology, we find that the UDM model with affine EoS fits the joint observations very well, better than Lambda CDM, with best-fit values alpha=0.01 +/- 0.02 and Omega(Lambda)=0.70 +/- 0.04 (95% confidence intervals). The standard model (best-fit Omega(Lambda)=0.71 +/- 0.04), having one less parameter, is preferred by a Bayesian model comparison. However, the affine EoS is at least as good as the standard model if a flat curvature is not assumed as a prior for Lambda CDM. For the latter, the best-fit values are Omega(K)=-0.02(-0.02)(+0.01) and Omega(Lambda)=0.71 +/- 0.04, i.e. a closed model is preferred. A phantom UDM with affine EoS is ruled out well beyond 3 sigma.