
Scalartensor 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 this type have selfaccelerating solutions. In an example inspired by string dilaton couplings, scalartensor gravity coupled to ordinary matter exhibits a de Sitter type expansion, even in the presence of a {\it negative} cosmological constant whose magnitude exceeds that of the matter density. This unusual behavior does not require phantoms, ghosts or other exotic sources. More generally, we show that any expansion history can be interpreted as arising partly or entirely from scalartensor gravity. To distinguish any quintessence or inflation model from its scalartensor variants, we use the fact that scalartensor models imply deviations of the postNewtonian parameters of general relativity, and time variation of the Newton's gravitational coupling G. We emphasize that nextgeneration probes of modified GR and the time variation of G are an essential complement to dark energy probes based on luminositydistance measurements.
