Cumulative effects in inflation with ultra-light entropy modes

TL;DR

This paper studies how ultralight non-adiabatic modes in multi-field inflation can grow and influence the curvature perturbation, leading to cumulative effects that enhance the adiabatic power spectrum while suppressing isocurvature modes.

Contribution

It introduces the concept of ultralight entropy modes acting as a constant source for curvature perturbations, revealing a shift symmetry and cumulative effects during inflation.

Findings

Ultralight modes cause linear growth in curvature perturbations.

The power spectrum of adiabatic modes is significantly enhanced.

Isocurvature perturbations are effectively suppressed.

Abstract

In multi-field inflation one or more non-adiabatic modes may become light, potentially inducing large levels of isocurvature perturbations in the cosmic microwave background. If in addition these light modes are coupled to the adiabatic mode, they influence its evolution on super horizon scales. Here we consider the case in which a non-adiabatic mode becomes approximately massless ("ultralight") while still coupled to the adiabatic mode, a typical situation that arises with pseudo-Nambu-Goldstone bosons or moduli. This ultralight mode freezes on super-horizon scales and acts as a constant source for the curvature perturbation, making it grow linearly in time and effectively suppressing the isocurvature component. We identify a Stuckelberg-like emergent shift symmetry that underlies this behavior. As inflation lasts for many e-folds, the integrated effect of this source enhances the power spectrum of the adiabatic mode, while keeping the non-adiabatic spectrum approximately untouched. In this case, towards the end of inflation all the fluctuations, adiabatic and non-adiabatic, are dominated by a single degree of freedom.