Origin of ultra-light fields during inflation and their suppressed non-Gaussianity

TL;DR

This paper explores how ultra-light isocurvature fields naturally arise during inflation through a scaling symmetry, and demonstrates that their interactions do not produce large non-Gaussianities, leading to specific observational signatures.

Contribution

It introduces a class of inflation models where ultra-light fields emerge from a scaling invariance, explaining their suppressed mass and non-Gaussianity.

Findings

Ultra-light fields can originate from a scaling symmetry in inflation models.

Such fields remain massless after horizon crossing due to invariance.

Interactions with curvature perturbations do not generate large non-Gaussianity.

Abstract

We study the structure of multi-field inflation models where the primordial curvature perturbation is able to vigorously interact with an ultra-light isocurvature field -- a massless fluctuation orthogonal to the background inflationary trajectory in field space. We identify a class of inflationary models where ultra-light fields can emerge as a consequence of an underlying "scaling transformation" that rescales the entire system's action and keeps the classical equations of motion invariant. This scaling invariance ensures the existence of an ultra-light fluctuation that freezes after horizon crossing. If the inflationary trajectory is misaligned with respect to the scaling symmetry direction, then the isocurvature field is proportional to this ultra-light field, and becomes massless. In addition, we find that even if the isocurvature field interacts strongly with the curvature perturbation --transferring its own statistics to the curvature perturbation-- it is unable to induce large non-Gaussianity. The reason is simply that the same mechanism ensuring a suppressed mass for the isocurvature field is also responsible for suppressing its self-interactions. As a result, in models with light isocurvature fields the bispectrum is generally expected to be slow-roll suppressed, but with a squeezed limit that differs from Maldacena's consistency relation.