A Universal Bound on Excitations of Heavy Fields during Inflation

TL;DR

This paper establishes a universal limit on the excitation of heavy fields during inflation, linking their energy density to observable oscillations in the power spectrum, with implications for inflationary models.

Contribution

It introduces a universal bound on heavy field excitations during inflation based on the change in the equation of state, validated through a sudden transition approximation.

Findings

Heavy field energy density must be less than 1% of the inflationary energy density.

The bound is derived from the change in the equation of state parameter.

The sudden transition approximation agrees with full multi-field results.

Abstract

We discuss a universal bound on any excitation of heavy fields during inflation: the ratio of the heavy field's energy density to the one driving inflation must be less than the maximally allowed relative amplitude of oscillations in the power-spectrum (less than one percent according to PLANCK). This bound can be traced back to the sudden change of the equation of state parameter across the excitation event. We employ a sudden transition approximation at the perturbed level, which has been used before in different settings; we check its validity by comparison to the full multi-field result in a concrete case study involving a sudden mass change of an inflaton.