  The dissipation of energy from local velocity perturbations in the
cosmological fluid affects the time evolution of spatially averaged fluid
dynamic fields and the cosmological solution of Einstein's field equations. We
show how this backreaction effect depends on shear and bulk viscosity and other
material properties of the dark sector, as well as the spectrum of
perturbations. If sufficiently large, this effect could account for the
acceleration of the cosmological expansion.
