Power-counting and the Validity of the Classical Approximation During Inflation

TL;DR

This work develops a standard EFT power-counting approach to quantify quantum corrections in slow-roll inflation and to delineate the regime where the classical inflationary dynamics remains valid. By applying the framework to Higgs inflation with a large nonminimal coupling and to curvature-squared inflation, it shows that the semiclassical description is often reliable but can become precariously near the EFT cutoff, requiring a narrow window for the cutoff scale $M$ relative to the Planck scale $M_p$ and the inflationary parameters $ξ$ or $ζ$. Heavy fields tend to decouple and only leave suppressed corrections, while light fields (e.g., the inflaton itself) can generate observable quantum effects, such as $\langle φ^2 \rangle \sim (H/4π)^2$, but the overall impact on observables is governed by the slow-roll parameters and the UV completion. The findings quantify unitarity bounds and show that achieving successful inflation in these models often necessitates UV physics to lie well above the naive EFT cutoff, thereby highlighting the importance of UV completions for robust inflationary predictions.

Abstract

We use the power-counting formalism of effective field theory to study the size of loop corrections in theories of slow-roll inflation, with the aim of more precisely identifying the limits of validity of the usual classical inflationary treatments. We keep our analysis as general as possible in order to systematically identify the most important corrections to the classical inflaton dynamics. Although most slow-roll models lie within the semiclassical domain, we find the consistency of the Higgs-Inflaton scenario to be more delicate due to the proximity between the Hubble scale during inflation and the upper bound allowed by unitarity on the new-physics scale associated with the breakdown of the semiclassical approximation within the effective theory. Similar remarks apply to curvature-squared inflationary models.