Bulk Viscous Cosmology: Unified Dark Matter

The bulk viscosity is introduced to model the current observational cosmos and the unified dark matter (dark matter and dark energy). The viscous unified model assumes that the universe is filled with a single nonperfect fluid with a bulk viscosity. We review the general framework for the viscous cosmology model building finding that the Hubble parameter has possessed a direct connection with the bulk viscosity coefficient. For the concrete form of the bulk viscosity, the Hubble parameter which has the scaling relation with the redshift can be obtained generally. We discuss two viscosity models and the cosmological evolution to which they lead in detail. Using SNe Ia data, CMB shift, as well as BAO observations, the viscosity model parameters can be fitted very well. We also briefly review the fitting method in this paper.