Chiral primordial gravitational waves in extended theories of Scalar-Tensor gravity

Abstract

We re-examine the problem of parity violation in single field inflation. We look for a systematic way to parametrically approach the scale at which maximal parity violation occurs, which is where we expect to find the presence of the Chern Simons instability. We do so by considering possible realizations of the effective field theory of Scalar-Tensor gravity, which could offer a rich phenomenology. The gravitational action is extended to include derivatively coupled interactions which, by means of a disformal transformation, are scaled by negative powers of a small parameter which is identified with the graviton speed. This results in suppressing the cutoff scale of the effective theory leading to parametrically large chiral tensor fluctuations. We conclude that a change in the physical description of the system is necessary in order to maintain sufficient parity violation as well ensure stability of the modes.