  High redshift supernovae and Cosmic Microwave Background data are used to
constrain the Cardassian expansion model (Freese & Lewis 2002), a cosmology in
which a modification to the Friedmann equation gives rise to a flat,
matter-dominated Universe which is currently undergoing a phase of accelerated
expansion. In particular, the precision of the positions of the Doppler peaks
in the CMB angular power spectrum provided by WMAP tightly constrains the
cosmology. The available parameter space is further constrained by various high
redshift supernova datasets taken from Tonry et al. (2003), a sample of 230
supernovae collated from the literature, in which fits to the distance and
extinction have been recomputed where possible and a consistent zero-point has
been applied. In addition, the Cardassian model can also be loosely constrained
by inferred upper limits on the epoch at which the Cardassian term in the
modified Friedmann equation begins to dominate the expansion (z_eq). Using
these methods, a Cardassian cosmology is constrained at the 2 sigma level to
0.19<Omega_m<0.26, 0.01<n<0.24 for the Cardassian expansion parameter, n, and
0.42<z_eq<0.89, in contradiction to the previous constraints of Sen & Sen
(2003). There is also a large discrepancy between the 1 sigma confidence
regions defined by the CMB and tightest supernova constraints, with the CMB
data favouring a low-Omega_m, high-n Cardassian cosmology and z_eq>1, as
opposed to the supernova data which supports a high-Omega_m, low-n cosmology.
