Rescaled Einstein–Gauss–Bonnet gravity inflation

Abstract

We study the inflationary phenomenology of a rescaled Einstein–Gauss–Bonnet (EGB) gravity. In this framework, the gravitational constant of the Einstein–Hilbert term is rescaled due to effective terms active in the high curvature era. Basically, the total theory is an theory with the Gauss–Bonnet part contributing only a non-minimal coupling to the scalar field, so it is a theory with string theory origins and with a non-trivial F(R) gravity part. The F(R) gravity part in the high curvature regime contributes only a rescaled Einstein–Hilbert term and thus the resulting theory is effectively a rescaled version of a standard EGB theory. We develop the formalism of rescaled EGB gravity, taking in account the GW170817 constraints on the gravitational wave speed. We show explicitly how the rescaled theory affects directly the primordial scalar and tensor perturbations, and how the slow-roll and observational indices of inflation are affected by the rescaling of the theory. We perform a thorough phenomenological analysis of several models of interest and we show that is it possible to obtain viable inflationary theories compatible with the latest Planck data. Also among the studied models there are cases that yield a relatively large blue tilted tensor spectral index and we demonstrate that these models can lead to detectable primordial gravitational waves in the future gravitational wave experiments. Some of the scenarios examined, for specific values of the reheating temperature may be detectable by SKA, LISA, BBO, DECIGO and the Einstein Telescope.