Beating the Lyth bound by parametric resonance during inflation

Abstract

We propose a novel mechanism for enhancing the primordial gravitational waves without significantly affecting the curvature perturbations produced during inflation. This is achieved due to non-linear sourcing of resonantly amplified scalar field fluctuations. Our result is an explicit scale-dependent counter-example of the famous Lyth bound, which opens up a promising perspective of producing detectable inflationary tensor modes with low-scale inflation and a sub-Planckian field excursion. We explicitly demonstrate the testability of our mechanism with upcoming Cosmic Microwave Background B-mode observations.

Comments: v1: 6 pages, 2 figures, comments are welcome. v2: 7 pages, 2 figures, updated references, minor modifications, accepted for publication in PRL