Dark energy from coexistence of phases

We suggest that the current acceleration of the universe may be explained by the vacuum energy of a hidden sector which is stuck in a state of equilibrium between phases. The phases are associated to a late-time first-order phase transition, where phase coexistence originates at a temperature Tc ∼ 10−3eV and lasts indefinitely. During phase coexistence, the energy density has an effective cosmological constant component with the observed magnitude. This scenario does not require supercooling and arises naturally in realistic models. Evidence for the acceleration of the expansion rate of the universe, coming from Type Ia supernovae observations [1], together with large scale structure and CMB measurements [2], are compatible with a flat universe which is composed of 70% dark energy and 30% dark matter, with an energy density of order (10eV ). These observations raise some naturalness problems, which could be summarized in two questions: why is there a tiny, though non-vanishing, cosmological constant? Why do we happen to live in the epoch in which such constant energy density coincides with that of nonrelativistic matter? Many approaches to solve these problems assume that the cosmological constant actually vanishes in the true vacuum due to some as yet unknown mechanism, the observed vacuum energy being caused by a field which is away from the global minimum of its potential. This is the case, e.g., of quintessence models [2], where a field φ is slowly rolling towards its minimum.