UV physics and the speed of sound during inflation

TL;DR

This paper investigates how higher-derivative operators in inflationary effective field theories can significantly alter the speed of sound and tensor-to-scalar ratio without invalidating the theory, impacting observable inflation signatures.

Contribution

It demonstrates that irrelevant higher-derivative operators can notably decrease the sound speed during inflation, affecting observables even with a high UV cutoff.

Findings

Higher-derivative operators reduce the sound speed during inflation.

Decreased sound speed leads to a lower tensor fraction.

Effects occur within the effective field theory's validity regime.

Abstract

We consider inflation as an effective field theory and study the effects of the addition to the Lagrangian of irrelevant operators with higher powers of first derivatives on its dynamics and observables. We find that significant deviations from the two-derivative dynamics are possible within the regime of validity of the effective field theory. Focusing on monomial potentials we show that the main effect of the terms under consideration is to reduce the speed of sound thereby reducing the tensor fraction, while having little impact on the scalar tilt. Crucially, these effects can arise even when the UV cut-off is well above the inflationary Hubble parameter.