Self-accelerating solutions of scalar–tensor gravity

Abstract

Scalar–tensor gravity is the simplest and best understood modification of general relativity,consisting of a real scalar field coupled directly to the Ricci scalar curvature. Models of thistype have self-accelerating solutions. In an example inspired by string dilaton couplings,scalar–tensor gravity coupled to ordinary matter exhibits a de Sitter type expansion, evenin the presence of a negative cosmological constant whose magnitude exceedsthat of the matter density. This unusual behavior does not require phantoms,ghosts or other exotic sources. More generally, we show that any expansion historycan be interpreted as arising partly or entirely from scalar–tensor gravity. Todistinguish any quintessence or inflation model from its scalar–tensor variants, weuse the fact that scalar–tensor models imply deviations of the post-Newtonianparameters of general relativity and time variation of Newton’s gravitational couplingG. We emphasize that next-generation probes of modified GR and the time variation ofG are an essential complement to dark energy probes based on luminosity–distancemeasurements.