The spectrum of tensor perturbations in warm inflation

Understand

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 $\ell_{\rm mfp}$.