Accelerating cosmological expansion from shear and bulk viscosity
  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. 
Stefan Floerchinger, Nikolaos Tetradis, Urs Achim Wiedemann
gr-qc astro-ph.CO hep-ph