Phantom dark energy as an effect of bulk viscosity
  In a homogeneous and isotropic universe bulk viscosity is the unique viscous
effect capable to modify the background dynamics. Effects like shear viscosity
or heat conduction can only change the evolution of the perturbations. The
existence of a bulk viscous pressure in a fluid, which in order to obey to the
second law of thermodynamics is negative, reduces its effective pressure. We
discuss in this study the degeneracy in bulk viscous cosmologies and address
the possibility that phantom dark energy cosmology could be caused by the
existence of non-equilibrium pressure in any cosmic component. We establish the
conditions under which either matter or radiation viscous cosmologies can be
mapped into the phantom dark energy scenario with constraints from multiple
observational data-sets
