Non-minimal derivative coupling theories compatible with GW170817

Abstract

In this work we aim to revive the interest for non-minimal derivative coupling theories of gravity, in light of the GW170817 event. These theories include a string motivated non-minimal kinetic term for the scalar field of the form ∼ξ(ϕ)Gμν∂μϕ∂νϕ and predict that the primordial tensor perturbations have a speed that it is distinct from the speed of light. Due to the fact that the Universe is classical during and in the post-inflationary epoch, there is no fundamental reason for the graviton to change its mass, the GW170817 event severely constrained these theories. We analyze and formalize the inflationary phenomenology of these theories, using both the latest Planck data and the GW170817 event data to constrain these theories. Due to the fact that there is no constraint in the choice of the scalar potential and the non-minimal coupling function, we provide several classes of viable models, using convenient forms of the minimal coupling function in terms of the scalar potential, aiming for analyticity, and we discuss the advantages and disadvantages of each viable model.