Shear viscous effects on the primordial power spectrum from warm inflation

TL;DR

This paper analyzes how shear viscous effects influence the primordial power spectrum during warm inflation, showing that significant shear viscosity can suppress the growth of fluctuations and alter the spectrum.

Contribution

It introduces the first-principles calculation of shear viscous effects in warm inflation and demonstrates their damping impact on primordial fluctuations.

Findings

Shear viscosity can suppress the growth of fluctuations in warm inflation.

A threshold exists where shear damping dominates over dissipative growth.

Shear viscous effects can prevent the formation of a growing mode in the spectrum.

Abstract

We compute 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. Whenever the dissipative coefficient is larger than the Hubble expansion rate H, there is 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 that tends to damp the growth of the perturbations. Independently of the T functional dependence of the dissipation and the shear viscosity, we find that when the shear viscous coefficient \zeta_s is larger than 3 \rho_r/H at horizon crossing, \rho_r being the radiation energy density, the shear damping effect wins and there is no growing mode in the spectrum.