Beating the Lyth bound by parametric resonance during inflation

TL;DR

This paper introduces a new mechanism during inflation that amplifies primordial gravitational waves through parametric resonance, potentially allowing detectable tensor modes without large field excursions, challenging the traditional Lyth bound.

Contribution

It presents a novel, scale-dependent mechanism for enhancing gravitational waves during inflation via non-linear scalar field interactions, bypassing the Lyth bound.

Findings

Demonstrates a scale-dependent counter-example to the Lyth bound.

Shows the mechanism can produce detectable tensor modes with low-scale inflation.

Proposes testability with upcoming CMB B-mode observations.

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.