Interacting viscous dark fluids
  We revise the conditions for the physical viability of a cosmological model
in which dark matter has bulk viscosity and also interacts with dark energy. We
have also included radiation and baryonic matter components; all matter
components are represented by perfect fluids, except the dark matter, that is
treated as an imperfect fluid. We impose upon the model the condition of a
complete cosmological dynamics that results in an either null or negative bulk
viscosity, but the latter also disagrees with the Local Second Law of
Thermodynamics. The model is also compared with cosmological observations at
different redshifts: type Ia supernova, the shift parameter of CMB, the
acoustic peak of BAO, and the Hubble parameter H(z). In general, observations
consistently point out to a negative value of the bulk viscous coefficient, and
in overall the fitting procedure shows no preference for the model over the
standard LCDM model.
