Inflationary NonGaussianity from Thermal Fluctuations

TL;DR

Inflationary physics can generate large non-Gaussianity from thermal fluctuations rather than solely from inflaton dynamics. The authors develop a framework to compute thermal 2-point and 3-point correlation functions, map them to curvature perturbations, and derive the resulting power spectrum and $f_{NL}$, showing a strong dependence on a thermal horizon length $L$. They apply the formalism to chain inflation and warm inflation, finding that $f_{NL}$ is generically order unity or larger, with potential values up to ~100 for small $L$ or certain $m$ and $w_r$. This work suggests that even a small radiation component during inflation can leave a large, positive thermal non-Gaussian signature, offering a new probe of thermal effects in the early universe.

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 $f_{\rm NL}$.

Figures (1)

• Figure 1: This figure illustrates how the initial condition of perturbation is prepared by the thermal fluctuations. The black cycle represents the thermal horizon. A fluctuation $\delta \rho$ of the system can exit the thermal horizon $L$(shown as a shell in the figure) during inflation. This shell outside the thermal horizon can not return to thermal equilibrium with respect to the original volume. It provides the initial condition of inflationary fluctuations. Fluctuations are created shell by shell.