Constraints on Cardassian expansion

High‐redshift supernovae and cosmic microwave background (CMB) data are used to constrain the Cardassian expansion model of Freese & Lewis, a cosmology in which a modification to the Friedmann equation gives rise to a flat, matter‐dominated universe which is 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 the Wilkinson Microwave Anisotropy Probe tightly constrains the cosmology. The available parameter space is further constrained by various high‐redshift supernova data sets taken from Tonry et al., 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 2σ level to 0.19 < Ω 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. There is also a large discrepancy between the 1σ confidence regions defined by the CMB and tightest supernova constraints, with the CMB data favouring a low‐Ω m , high‐ n Cardassian cosmology and z eq >1, as opposed to the supernova data which support a high‐Ω m , low‐ n cosmology.