Cardassian Expansion: Constraints from Compact Radio Source Angular Size versus Redshift Data

The ``Cardassian Expansion Scenario’’ was recently proposed by Freese and Lewis (2002) as an alternative to a cosmological constant in explaining the current accelerating universe. In this paper we investigate observational constraints on this scenario from recent measurements of the angular size of high-$z$ compact radio sources compiled by Gurvits et al. (1999). We show that the allowed intervals for $n$ and $z_{eq}$, the two parameters of the Cardassian model, are heavily dependent on the value of the mean projected linear size $l$. However, the best fit to the current angular size data prefers the conventional flat $\mathrm{\Lambda }$CDM model to this Cardassian expansion proposal, though the latter is cosmologically credible and compatible with the $\Theta - z$diagramforsomevaluesof$l$.