Latest Observational Constraints on Cardassian Models

Constraints on the original Cardassian model and the modified polytropic Cardassian model are examined from the latest derived 397 Type Ia supernova (SNe Ia) data, the size of baryonic acoustic oscillation peak from the Sloan Digital Sky Survey (SDSS), the position of first acoustic peak of the Cosmic Microwave Background radiation (CMB) from the five years Wilkinson Microwave Anisotropy Probe (WMAP), the x-ray gas mass fractions in clusters of galaxies, and the observational H(z) data. In the original Cardassian model with these combined data set, we find $\Omega_{m0}=0.271^{+0.014}_{-0.014}, n=0.035^{+0.049}_{-0.049}$ at $1 \sigma$ confidence level. And in the modified polytropic Cardassian model, we find that $\Omega_{m0}=0.271^{+0.014}_{-0.015}$, $n=-0.091^{+0.331}_{-1.908}$ and $\beta=0.824^{+0.750}_{-0.622}$ within $1\sigma$ confidence level. According to these observations, the acceleration of the universe begins at $z_T=0.55^{+0.05}_{-0.05} (1\sigma)$ for the original Cardassian model, and at $z_T=0.58^{+0.12}_{-0.12} (1\sigma)$ for the modified polytropic Cardassian model. Evolution of the effective equation of state $w_{eff}$ for the modified polytropic Cardassian model is also examined here and results show that an evolutionary quintessence dark energy model is favored.