Viscous Cardassian universe

TL;DR

This paper explores viscous Cardassian cosmology, analyzing how bulk viscosity affects cosmic evolution, the equation-of-state parameter, and structure formation, with observational data fitting to constrain model parameters.

Contribution

It introduces bulk viscosity into Cardassian models, demonstrating its effects on cosmic dynamics, the equation-of-state crossing -1, and structure collapse, with observational constraints.

Findings

The equation-of-state parameter can cross -1 due to viscosity.

Bulk viscosity slows down the virialization process.

Models fit well with supernovae data, constraining viscosity.

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 $\tau$.