Bulk viscous cosmology: Statefinder and entropy

The statefinder diagnostic pair is adopted to differentiate viscous cosmology models and it is found that the trajectories of these viscous cosmology models on the statefinder pair s-r plane are quite different from those of the corresponding non-viscous cases. Particularly for the quiessence model, the singular properties of state parameter w=−1 are obviously demonstrated on the statefinder diagnostic pair planes. We then discuss the entropy of the viscous/dissipative cosmology system which may be more practical to describe the present cosmic observations as we understand that the perfect fluid model is just a global approximation to the complicated cosmic media in universe evolution. When the bulk viscosity takes the form of ζ=ζ1a˙/a (ζ1 is constant), the relationship between the entropy S and the redshift z is explicitly given out. We find that the entropy of the viscous cosmology is always increasing and consistent with the thermodynamics arrow of time for the universe evolution. With the parameter constraints by fitting to the 157 gold data of type Ia supernova observations, it is shown that this viscous cosmology model is rather well consistent to the observational data at the lower redshifts, and together with the diagnostic statefinder pair analysis it is concluded that the viscous cosmic models tend to the favored ΛCDM model in the later cosmic evolution, agreeable to lots of cosmological simulation results, especially to the fact of confidently observed current accelerating cosmic expansion.