Viscous Cardassian universe

Abstract

The viscous Cardassian cosmology is discussed, assuming that there is a bulk viscosity in the cosmic fluid. The dynamical analysis indicates that there exists a singular curve in the phase diagram of viscous Cardassian model. In the viscous PL model, the equation-of-state parameter $w_{k}$ is no longer a constant and it can cross the cosmological constant divide $w_{\Lambda}=-1$, in contrast with same problem of the ordinary PL model. Other models possess with similar characteristics. For MP and exp models, $w_{k}$ evolves more near -1 than the case without viscosity. The bulk viscosity also effect the virialization process of a collapse system in the universe: $\frac{R_{vir}}{R_{ta}}$ is increasingly large when the bulk viscosity is increasing. In other words, the bulk viscosity retards the progress of collapse system. In addition, we fit the viscous Cardassian models to current type Ia supernovae data and give the best fit value of the model parameters including the bulk viscosity coefficient $\tau$.