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

TL;DR

This work critically assesses the regime of validity for effective field theory (EFT) during inflation when a heavy field is present. By analyzing adiabaticity and particle production in a two-field setup, it shows that decoupling holds for a heavy field with $M_{eff} > H$ as long as the heavy sector starts in its vacuum and the evolution remains adiabatic; observable EFT violations arise only from localized, non-adiabatic events which are tightly bounded by energy conservation and locality. The paper provides quantitative bounds on the duration and strength of such events, constraining their impact on the power spectrum and non-Gaussianity, and demonstrates that heavy fields near the Hubble scale could produce only small signatures unless the theory is strongly coupled or UV-complete. It also clarifies how these results fit within geometric multi-field inflation and the EFT of Inflation frameworks, linking to recent works on turns in field space and dispersion-relations-based approaches. Overall, the EFT approach remains robust for inflation, with potential observable effects confined to a narrow window around the Hubble scale for heavy degrees of freedom.

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.

Figures (2)

• Figure 1: Non-adiabaticity becomes appreciable for times $t \gtrsim r$ and is peaked at the pole where $\omega^2=0$ and $t=\tau=r+i\mu$. At early and late times adiabaticity holds and time is strictly real.
• Figure 2: Constraints on the level of non-adiabaticity and production of super-Hubble mass particles during a temporary violation of inflation. The dark shaded area above represents the allowed region of production where the constraints overlap. The upper bound (blue) comes from demanding conservation of energy during the violation, whereas the lower bound (red) comes from requiring a large enough violation so that modes are produced. Although our estimates indicate a sharp cutoff in production around $M_{eff}=500 H$ as seen in the graph, this result is in fact overly optimistic given that the regions actually saturate the lower and upper bound. More realistically we must be deep within these regions so that $M_{eff} \simeq H$ and so even with a violation of the EFT only for masses near the Hubble scale are models interesting.