Simple single-field inflation models with arbitrarily small tensor/scalar ratio

TL;DR

This paper presents simple single-field inflation models that can have arbitrarily small tensor-to-scalar ratios, challenging previous assumptions about lower bounds in such models.

Contribution

The authors construct a family of models with small tensor amplitude and no spectral index running, showing no lower limit on tensor/scalar ratio in simple single-field inflation.

Findings

Models can have arbitrarily small tensor/scalar ratio.

Constructed models are consistent with Planck and BICEP Keck bounds.

No lower bound exists on tensor/scalar ratio in simple single-field inflation.

Abstract

We construct a family of simple single-field inflation models consistent with Planck / BICEP Keck bounds which have a parametrically small tensor amplitude and no running of the scalar spectral index. The construction consists of a constant-roll hilltop inflaton potential with the end of inflation left as a free parameter induced by higher-order operators which become dominant late in inflation. This construction directly demonstrates that there is no lower bound on the tensor/scalar ratio for simple single-field inflation models.