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.
