Simple Bounds from the Perturbative Regime of Inflation

TL;DR

This paper derives bounds on the sound speed during inflation to ensure the validity of perturbative expansion, impacting model viability and eternal inflation conditions.

Contribution

It provides new bounds on sound speed and e-folds in inflation models, extending understanding of perturbative regime constraints with scale dependence.

Findings

Lower limit on sound speed related to power spectrum amplitude

Constraints on e-folds from scale-dependent sound speed

Eternal inflation cannot occur perturbatively at low sound speeds

Abstract

We examine the conditions under which a perturbative expansion around an inflating background is valid. When inflation is driven by a single field with a general sound speed, we find a lower limit on the sound speed related to the amplitude of the inflationary power spectrum. Generalizing the sound speed constraints to include scale dependence can limit the number of e-folds obtained in the perturbative regime and restrict otherwise apparently viable models. We also show that for models with a low sound speed, eternal inflation cannot occur in the perturbative regime.