Backreaction effects on the matter side of Einsteins field equations

Abstract: I will discuss how the dissipation of energy from local velocity perturbations in the cosmological fluid modifies the expansion history of the universe. This backreaction effect on the matter side of Einsteins equations depends on shear and bulk viscosity and other material properties of the dark sector, as well as the spectrum of perturbations. If sufficiently large, it could account for the acceleration of the cosmological expansion. But even if dissipative effects are smaller they are of some interest in the era of precision cosmology.