Inflationary NonGaussianity from Thermal Fluctuations

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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 NL. We illustrate our result in the chain inflation model and the very weakly dissipative warm inflation model. We show that fNL 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 NL>5 or even f NL 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 NL.