Effective Field Theory and Decoupling in Multi-field Inflation: An Illustrative Case Study

TL;DR

This paper investigates how heavy fields influence light mode perturbations in multifield inflation, revealing that sharp turns in field space cause strong coupling, non-standard perturbations, and observable features in the power spectrum and bispectrum.

Contribution

It demonstrates that sharp turns in field space invalidate the effective single-field description and induce distinctive oscillatory and resonant features in inflationary perturbations.

Findings

Sharp turns cause non Bunch-Davis components in perturbations.

Strong coupling prevents effective single-field modeling.

Oscillatory features and resonant bispectrum are produced.

Abstract

We explore the effects of heavy degrees of freedom on the evolution and perturbations of light modes in multifield inflation. We use a simple two-field model as an example to illustrate the subtleties of integrating out massive fields in a time-dependent background. We show that when adiabaticity is violated due to a sharp turn in field space, the roles of massive and massless field are interchanged, and furthermore the fields are strongly coupled; thus the system cannot be described by an effective single field action. Further analysis shows that the sharp turn imparts a non Bunch-Davis component in each perturbation mode, leading to oscillatory features in the power spectrum, and a large resonantly enhanced bispectrum.