The spectrum of tensor perturbations in warm inflation

TL;DR

This paper calculates the tensor perturbation spectrum in warm inflation, revealing a new thermal contribution that can surpass the standard vacuum fluctuation component under certain conditions.

Contribution

It introduces a novel thermal contribution to tensor perturbations in warm inflation, arising from gravitational wave production by the thermal bath.

Findings

Thermal fluctuations can dominate tensor spectrum at large scales.

New component proportional to mean free path and temperature is significant.

Standard vacuum fluctuation component is complemented by thermal effects.

Abstract

We compute the spectrum of tensor perturbations in warm inflation. We find that the spectrum, besides the standard component $\propto {H^2}/{M_P^2}$ associated to the amplification of the tensor vacuum fluctuations, acquires a component $\propto {\ell_{\rm mfp}\,T^5}/{M_P^4}$, where $\ell_{\rm mfp}$ and $T$ are respectively the mean free path and the temperature of the thermal degrees of freedom. The new contribution is due to the direct production of gravitational waves by the thermal bath, and can exceed the standard one in a viable region of parameter space. This contribution is dominated by thermal fluctuations at scales longer than $\ell_{\rm mfp}$.