Confronting dark energy anisotropic stress

Abstract

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 cvis2. The model parameter space was scanned using the Markov chain Monte Carlo method. The results show that the speed of viscosity cv_i^s2 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 fσ₈(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 cv_i^s2.