In a homogeneous and isotropic universe bulk viscosity is the unique viscous effect that is capable of modifying 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 the second law of thermodynamics must be negative-reduces its effective pressure. We discuss the degeneracy in bulk viscous cosmologies and address the possibility that phantom dark energy cosmology could be caused by the existence of nonequilibrium pressure in any one of the cosmic components. We establish the conditions under which either viscous matter or radiation cosmologies can be mapped into the phantom dark energy scenario with constraints from multiple observational data sets.
