Abstract
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) , resulting in an effective cosmological constant . 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 , 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, and , using data from a suite of different cosmological observations, and perform a comparison with the standard 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 , with best-fit values and (95% confidence intervals). The standard model (best-fit ), 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 . For the latter, the best-fit values are and , i.e. a closed model is preferred. A phantom UDM with affine EoS is ruled out well beyond .
- Received 15 February 2007
DOI:https://doi.org/10.1103/PhysRevD.76.103519
©2007 American Physical Society