Confronting dark energy anisotropic stress

We use currently available cosmic observations to probe and constrain an imperfect dark-energy fluid that is characterized by a constant equation of state w and a constant speed of viscosity c(vis)(2). The model parameter space was scanned using the Markov chain Monte Carlo method. The results show that the speed of viscosity c(vis)(2) was not well constrained when currently available data sets are used (including the cosmic microwave background radiation from Planck-2013 and WMAP9, type Ia supernovae, baryon acoustic oscillations, redshift space distortion f sigma(8)(z), and gravitational weak lensing from Canada-France Hawaii Telescope Lensing Survey (CFHTLenS). However, the cosmic data sets slightly favor phantom dark energy with a negative speed of viscosity c(vis)(2).