On the Scale of New Physics in Inflation

TL;DR

This paper investigates the validity of effective field theory in inflation, analyzing how the cutoff scale relates to inflaton excursions and the implications for models with super-Planckian field ranges.

Contribution

It provides a detailed estimation of the cutoff scale in inflationary EFTs using unitarity, perturbativity, and entropy calculations, highlighting the role of gravity in setting the cutoff.

Findings

In the decoupling limit, the cutoff depends linearly on inflaton excursion.

With gravity, the cutoff is at the Planck scale.

Super-Planckian inflationary models are not natural from an EFT perspective.

Abstract

Effective field theory is a powerful organizing principle that allows to describe physics below a certain scale model-independently. Above that energy scale, identified with the cutoff, the EFT description breaks down and new physics is expected to appear, as confirmed in many familiar examples in quantum field theory. In this work, we examine the validity of effective field theory methods applied to inflation. We address the issue of whether Planck-suppressed non-renormalizable interactions are suppressed enough to be safely neglected when computing inflationary predictions. We focus on non-derivative non-renormalizable operators and estimate the cutoff that should suppress them using two independent approaches: (i) the usual unitarity and perturbativity argument, (ii) by computing the UV-divergent part of the inflaton entropy, known to scale as the square of the UV-cutoff. We find that in the absence of gravity (decoupling limit) the cutoff appears to depends linearly on the total inflaton excursion. On the other hand, once gravity is restored, the cutoff is brought back to the Planck scale. These results suggest that inflationary scenarios with super-Planckian excursion are not natural from the EFT viewpoint.