2007/02/28 by A. Balbi, Amedeo Balbi, Marco Bruni +1 · 2 citations
Physics and Astronomy · #Cosmology and Gravitation Theories #Dark Matter and Cosmic Phenomena #Galaxies: Formation, Evolution, Phenomena #astro-ph #hep-ph #hep-th
paper · pdf · doi:10.1103/physrevd.76.103519
published as Phys.Rev.D76:103519,2007 · 7 pages, 4 figures. Matching version published on PRD
arxiv created 2007/11/12 · openalex publication_date 2007/11/12 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/28
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=p0+\ensuremathα\ensuremathρ, resulting in an effective cosmological constant \ensuremathρ_\ensuremathΛ=\ensuremath-p0/(1+\ensuremathα). 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(\ensuremathρ), 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, \ensuremathα and \ensuremathΩ_\ensuremathΛ, using data from a suite of different cosmological observations, and perform a comparison with the standard \ensuremathΛCDM 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 \ensuremathΛCDM, with best-fit values \ensuremathα=0.01\ifmmode±\else\textpm\fi0.02 and \ensuremathΩ_\ensuremathΛ=0.70\ifmmode±\else\textpm\fi0.04 (95% confidence intervals). The standard model (best-fit \ensuremathΩ_\ensuremathΛ=0.71\ifmmode±\else\textpm\fi0.04), 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 \ensuremathΛCDM. For the latter, the best-fit values are \ensuremathΩK=\ensuremath-0.02_\ensuremath-0.02+0.01 and \ensuremathΩ_\ensuremathΛ=0.71\ifmmode±\else\textpm\fi0.04, i.e. a closed model is preferred. A phantom UDM with affine EoS is ruled out well beyond 3\ensuremathσ.