http://arxiv.org/abs/1310.4531
We studied models of inflation with a preferred clock specifying the end of inflation and giving the curvature perturbations, coupled with another non-equivalent clock that at late times defines the same frame and do not contribute to the density perturbations. This can happen in the framework of dissipative EFT of inflation where the additional degrees of freedom include a fluid developing sound waves propagating with sound speed $c_{sr}$. The fluid defines a preferred frame comoving with it. The paradigmatic example of this is the warm inflation scenario. We studied the dynamics of this systems during inflation and the three-point function. We saw that in the strong dissipation regime the nonlinear parameter induced by the new terms is $|f_{\rm NL}| \sim 1/c_{sr}^2$, not enhanced by the dissipation parameter which enters the two-point function. We checked that the squeezed limit of the three-point function still satisfies the consistency condition with corrections of order $\mathcal{O}(k_L^2/k_S^2)$. We computed the Planck constraints for the case of warm inflation obtaining a bound of $\gamma\lesssim 10^5 H$ for the clock coupled to radiation. For decreasing sound speed the bound decreases. We also checked that the shape of the three-point function corresponding to the model studied here is a mixture between equilateral and orthogonal with a small local component, which is more consistent with Planck’s results.
Date added: Fri, 18 Oct 13