Key points are not available for this paper at this time.
The number of rich galaxy clusters per unit volume is a strong function of Omega, the cosmological density parameter, and sigma₈, the linear extrapolation to z=0 of the density contrast in 8/h Mpc spheres. The CNOC cluster redshift survey provides a sample of clusters whose average mass profiles are accurately known, which enables a secure association between cluster numbers and the filtered density perturbation spectrum. We select from the CNOC cluster survey those EMSS clusters with bolometric Lₓ>=10⁴5 erg/s and a velocity dispersion exceeding 800 km/s in the redshift ranges 0. 18-0. 35 and 0. 35-0. 55. We compare the number density of these subsamples with similar samples at both high and low redshift. Using the Press-Schechter formalism and CDM style structure models, the density data are described with sigma₈~0. 85 and Omega~0. 3, but Omega=0. 2 and sigma₈=1. 05+/-0. 05 also are acceptable, as are other Omega<=0. 6. The predicted cluster density evolution in an Omega=1 CDM model exceeds that observed by more than an order of magnitude.
Carlberg et al. (Thu,) studied this question.