$Λα$DM: Observational constraints on unified dark matter with constant speed of sound

Balbi, Amedeo; Bruni, Marco; Quercellini, Claudia

$Λα$DM: Observational constraints on unified dark matter with constant speed of sound

Authors

Balbi, Amedeo

Bruni, Marco

Quercellini, Claudia

Description

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 $3σ$.
7 pages, 4 figures. Matching version published on PRD