Hill-climbing dark inflation

Within the framework of the scalar-tensor theory we consider a hill-climbing inflation, in which the effective Planck mass increases in time. We obtain the Einstein frame potential with infinitely long and flat plateau as we approach towards the strong coupling regime, together with a run-away vacuum in the GR limit of the theory. The inflation ends with the scalar field rolling down towards infinity, which at the effective level indicates the massless scalar field domination in the Universe. In this scheme we assume that the inflaton is a dark particle, which has no couplings to the Standard Model degrees of freedom (other than the gravitational ones). We discuss the gravitational reheating of the Universe together with its implications on the predictions of the model, including possible amplification of primordial gravitational waves. Our model for the first time realizes explicitly the enhancement of the primordial gravitational waves in the dark inflation scenario.