On the importance of heavy fields during inflation

TL;DR

This paper investigates when a low energy effective field theory accurately describes two-field inflation models with sharp turns, emphasizing the role of heavy fields and the adiabaticity condition in preserving the power spectrum.

Contribution

It establishes the conditions under which the single-field EFT remains valid during sharp turns in multi-field inflation, highlighting the importance of the adiabaticity condition.

Findings

EFT accurately reproduces power spectra when the turn's angular velocity change is slow compared to heavy mode masses.

Sharp turns can generate large features in the power spectrum, well captured by the EFT.

The adiabaticity condition is key to the EFT's reliability in multi-field inflation scenarios.

Abstract

We study the dynamics of two-field models of inflation characterized by a hierarchy of masses between curvature and isocurvature modes. When the hierarchy is large, a low energy effective field theory (EFT) exists in which only curvature modes participate in the dynamics of perturbations. In this EFT heavy fields continue to have a significant role in the low energy dynamics, as their interaction with curvature modes reduces their speed of sound whenever the multi-field trajectory is subject to a sharp turn in target space. Here we analyze under which general conditions this EFT remains a reliable description for the linear evolution of curvature modes. We find that the main condition consists on demanding that the rate of change of the turn's angular velocity stays suppressed with respect to the masses of heavy modes. This adiabaticity condition allows the EFT to accurately describe a large variety of situations in which the multi-field trajectory is subject to sharp turns. To test this, we analyze several models with turns and show that, indeed, the power spectra obtained for both the original two-field theory and its single-field EFT are identical when the adiabaticity condition is satisfied. In particular, when turns are sharp and sudden, they are found to generate large features in the power spectrum, accurately reproduced by the EFT.