Confronting Dark Energy Anisotropic Stress
  We use the currently available cosmic observations to probe and constrain an imperfect dark energy fluid which is characterized by a constant equation of state $w$ and a constant speed of viscosity $c^2_{vis}$. The model parameter space was scanned by using Markov chain Monte Carlo method. The results show that the speed of viscosity $c^2_{vis}$ was not well constrained when currently available date sets, which include the cosmic microwave background radiation from {\it Planck}2013, type Ia supernovae and baryon acoustic oscillations, are used. But the cosmic data sets favor phantom dark energy with a negative speed of viscosity $c^2_{vis}$ slightly. 
Baorong Chang, Lixin Xu
astro-ph.CO