Inflationary NonGaussianity from Thermal Fluctuations

TL;DR

This paper investigates how thermal fluctuations during inflation can produce significant non-Gaussianity, with potential for large $f_{NL}$ values, especially in models with modified equations of state or decoupling effects.

Contribution

It demonstrates that thermal fluctuations can lead to large positive non-Gaussianity in inflation, expanding understanding of thermal effects in early universe models.

Findings

Small radiation component can cause large nonGaussianity.

Thermal nonGaussianity always has positive $f_{NL}$.

Models with modified equations of state can produce very large $f_{NL}$.

Abstract

We calculate the contribution of the fluctuations with the thermal origin to the inflationary nonGaussianity. We find that even a small component of radiation can lead to a large nonGaussianity. We show that this thermal nonGaussianity always has positive $f_{\rm NL}$. We illustrate our result in the chain inflation model and the very weakly dissipative warm inflation model. We show that $f_{NL}\sim {\cal O}(1)$ is general in such models. If we allow modified equation of state, or some decoupling effects, the large thermal nonGaussianity of order $f_{\rm NL}>5$ or even $f_{\rm NL}\sim 100$ can be produced. We also show that the power spectrum of chain inflation should have a thermal origin. In the Appendix A, we made a clarification on the different conventions used in the literature related to the calculation of $f_{\rm NL}$.