Evolution of fNL to the adiabatic limit

TL;DR

This paper investigates how the nonlinear parameter fNL evolves during the adiabatic limit in multi-field inflation models, revealing its dependence on model specifics and providing criteria for large fNL values.

Contribution

It offers a numerical analysis of fNL evolution in multi-field inflation and derives an analytic criterion for large fNL in sum-separable potential models.

Findings

fNL depends on the decay process of isocurvature modes

Analytic criterion for large fNL in certain models

Distinction between evolution types by bispectrum sign

Abstract

We study inflationary perturbations in multiple-field models, for which zeta typically evolves until all isocurvature modes decay--the "adiabatic limit". We use numerical methods to explore the sensitivity of the nonlinear parameter fNL to the process by which this limit is achieved, finding an appreciable dependence on model-specific data such as the time at which slow-roll breaks down or the timescale of reheating. In models with a sum-separable potential where the isocurvature modes decay before the end of the slow-roll phase we give an analytic criterion for the asymptotic value of fNL to be large. Other examples can be constructed using a waterfall field to terminate inflation while fNL is transiently large, caused by descent from a ridge or convergence into a valley. We show that these two types of evolution are distinguished by the sign of the bispectrum, and give approximate expressions for the peak fNL.