Cardassian Universe Constrained by Latest Observations

Several Cardassian universe models including the original, modified polytropic and exponential Cardassian models are constrained by the latest Constitution Type Ia supernova data, the position of the first acoustic peak of CMB from the five years WMAP data and the size of baryonic acoustic oscillation peak from the SDSS data. Both the spatial flat and curved universe are studied, and we also take account of the possible bulk viscosity of the matter fluid in the flat universe case.