Observational CMB predictions from warm inflation
Bastero-Gil, Mar (Universidad de Granada )

We review the calculation of the primordial curvature spectrum generated during warm inflation, including shear viscous effects. The primordial spectrum is dominated by the thermal fluctuations of the radiation bath, sourced by the dissipative term of the inflaton field. The dissipative coefficient $\Upsilon$ computed from first principles in the close-to-equilibrium approximation depends in general on the temperature $T$, and this dependence renders the system of the linear fluctuations coupled, inducing a growing mode in the fluctuations before horizon crossing. However, dissipation intrinsically means departures from equilibrium, and therefore the presence of a shear viscous pressure in the radiation fluid. This in turn acts as an extra friction term for the radiation fluctuations which tends to damp the growth of the perturbations. Independently of the $T$ functional dependence of the dissipation and the shear viscosity, we find the condition under which the shear effect kills the growing mode in the spectrum. We also discuss the implications of the shear for non-gaussianity.