Sudden gravitational transition

Abstract

We investigate the properties of a cosmological scenario which undergoes a gravitational phase transition at late times. In this scenario, the Universe evolves according to general relativity in the standard, hot big bang picture until a redshift $z\ensuremath{\lesssim}1$. Nonperturbative phenomena associated with a minimally-coupled scalar field catalyzes a transition, whereby an order parameter consisting of curvature quantities such as ${R}^{2}$, ${R}_{ab}{R}^{ab}$, ${R}_{abcd}{R}^{abcd}$ acquires a constant expectation value. The ensuing cosmic acceleration appears driven by a dark-energy component with an equation-of-state $w<\ensuremath{-}1$. We evaluate the constraints from type 1a supernovae, the cosmic microwave background, and other cosmological observations. We find that a range of models making a sharp transition to cosmic acceleration are consistent with observations.