Heavy fields, reduced speeds of sound and decoupling during inflation

TL;DR

This paper clarifies how effective single-field inflation models with suppressed sound speed can be consistent with decoupling, slow-roll, and observational constraints, even with strong turn dynamics and large non-Gaussianity.

Contribution

It demonstrates the construction of inflationary models with suppressed sound speed that maintain decoupling and weak coupling despite strong turn effects.

Findings

Heavy fields influence adiabatic modes without breaking decoupling.

The mass gap increases with turn strength, maintaining adiabaticity.

Models can produce large equilateral non-Gaussianity compatible with observations.

Abstract

We discuss and clarify the validity of effective single field theories of inflation obtained by integrating out heavy degrees of freedom in the regime where adiabatic perturbations propagate with a suppressed speed of sound. We show by construction that it is indeed possible to have inflationary backgrounds where the speed of sound remains suppressed and slow-roll persists for long enough. In this class of models, heavy fields influence the evolution of adiabatic modes in a manner that is consistent with decoupling of physical low and high energy degrees of freedom. We emphasize the distinction between the effective masses of the isocurvature modes and the eigenfrequencies of the propagating high energy modes. Crucially, we find that the mass gap that defines the high frequency modes increases with the strength of the turn, even as the naive heavy (isocurvature) and light (curvature) modes become more strongly coupled. Adiabaticity is preserved throughout, and the derived effective field theory remains in the weakly coupled regime, satisfying all current observational constraints on the resulting primordial power spectrum. In addition, these models allow for an observably large equilateral non-Gaussianity.