Decoupling Survives Inflation: A Critical Look at Effective Field Theory Violations During Inflation

TL;DR

This paper examines the validity of effective field theory during inflation, showing decoupling generally holds unless slow-roll conditions break down, with implications for observable signatures like non-Gaussianity.

Contribution

It provides a detailed analysis of when decoupling fails during inflation, especially during non-adiabatic periods, clarifying the limits of effective field theory methods in cosmology.

Findings

Decoupling holds unless slow-roll is violated.

Non-adiabatic periods can lead to EFT breakdown.

Heavy fields' effects are tightly constrained by locality and energy conservation.

Abstract

We investigate the validity of effective field theory methods and the decoupling of heavy fields during inflation. Considering models of inflation in which the inflaton is coupled to a heavy (super-Hubble) degree of freedom initially in its vacuum state, we find that violations of decoupling are absent unless there is a breakdown of the slow-roll conditions. Next we allow for a temporary departure from inflation resulting in a period of non-adiabaticity during which effective field theory methods are known to fail. We find that the locality of the event and energy conservation lead to a tight bound on the size of the effects of the heavy field. We discuss the implications for the power spectrum and non-gaussianity, and comment on the connection with recent studies of the dynamics of multi-field inflation models. Our results further motivate the use of effective field theory methods to characterize cosmic inflation, and focus the question of observability of additional degrees of freedom during inflation to near the Hubble scale or below - as anticipated from the Wilsonian notions of decoupling and naturalness.