Consequences of a cosmic scalar with kinetic coupling to curvature

The classical gravitational theory of a scalar field with a gradient coupling to the Ricci tensor is examined. This is a scalar–vector–tensor gravitational theory, but in the case that the coupling is weak and the scalar evolves like a quintessence field on cosmological time scales, the field equations within the solar system are similar to a vector–tensor theory predicting tightly constrained preferred-frame effects. In the early universe, it is shown that strong-coupling effects can damp the evolution of the scalar field rolling down a potential to help drive an inflationary epoch. In the absence of a potential, the strong-coupling effects drive a coasting expansion epoch which ultimately terminates in a sudden singularity.