Large Tensor-to-Scalar Ratio in Small-Field Inflation

TL;DR

This paper proposes a mechanism where additional light fields suppress inflaton-induced density perturbations, enabling small-field inflation models to produce large tensor-to-scalar ratios, challenging the traditional Lyth bound.

Contribution

It introduces a novel approach where light degrees of freedom modulate inflationary perturbations, allowing large gravitational waves from small-field inflation models.

Findings

Density perturbations can be suppressed via light fields.

Large tensor-to-scalar ratios are achievable in small-field inflation.

A generalized Lyth bound applies to the total field space.

Abstract

We show that density perturbations seeded by the inflaton can be suppressed when having additional light degrees of freedom contributing to the production of perturbations. The inflaton fluctuations affect the light field dynamics by modulating the length of the inflationary period, hence produce additional density perturbations in the post-inflationary era. Such perturbations can cancel those generated during inflation as both originate from the same inflaton fluctuations. This allows production of large gravitational waves from small-field inflation, which is normally forbidden by the Lyth bound on the inflaton field excursion. We also find that the field bound is taken over by the light scalar when the inflaton-induced perturbations are suppressed, thus present a generalized form of the Lyth bound that applies to the total field space.