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 for the dark matter one that is modeled 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 (LSLT). The model is also compared with cosmological observations at different redshifts: type Ia supernova, the acoustic peak of baryon acoustic oscillation, the Hubble parameter H(z), and the angular scale of the cosmic microwave background encoded in the first peak. Taken together, observations consistently point to a negative value of the bulk viscous coefficient, that is in disagreement with the LSLT. From the different cases that we study, the best model that we find corresponds to the case of a dark matter with a null viscosity, interacting with a phantom dark energy. Also, overall the fitting procedure shows no preference for the model over the standard ΛCDM model.