Coupled quintessence and the coincidence problem

We consider a model of interacting cosmological constant/quintessence, where dark matter and dark energy behave as, respectively, two coexisting phases of a fluid, a thermally excited Bose component and a condensate, respectively. In a simple phenomenological model for the dark components interaction we find that their energy density evolution is strongly coupled during the universe evolution. This feature provides a possible way out for the coincidence problem affecting many quintessence models. PACS 98.80.-k, 98.80.Cq The extensive astrophysical observations of Supernovae (SN) Ia provide a powerful tool to single out the main cosmological energy density contributions at redshift z ∼ 1. The evidence for a presently accelerated cosmic expansion [1, 2] suggests the presence of a new dominant energy source whose equation of state p = ωρ is compatible with a cosmological constant. The analysis of the recent experimental data favors in fact the range −1 ≤ ω ≤ −0.4 [1, 3, 4]. In particular for a pure cosmological constant the favored energy density, in unit of the critical density, would be of the order of ΩΛ ' 0.7. Unfortunately, this value would be several orders of magnitude smaller than what can