$Λα$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) $P= p_0 +αρ$, resulting in an {\it effective cosmological constant} $ρ_Λ=-p_0/(1+α)$. 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 $P=P(ρ)$, 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, $α$ and $Ω_Λ$, using data from a suite of different cosmological observations, and perform a comparison with the standard $Λ$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 $Λ$CDM, with best fit values $α=0.01 \pm 0.02$ and $Ω_Λ=0.70 \pm 0.04$ (95% confidence intervals). The standard model (best fit $Ω_Λ=0.71\pm 0.04$), 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 $Λ$CDM. For the latter, the best fit values are $Ω_K=-0.02^{+0.01}_{-0.02} $ and $Ω_Λ=0.71 \pm 0.04$, i.e. a closed model is preferred. A phantom UDM with affine EoS is ruled out well beyond $3σ$.