The number of rich galaxy clusters per unit volume is a strong function of \Omega , the cosmological density parameter , and \sigma _ { 8 } , the linear extrapolation to z = 0 of the density contrast in 8 h ^ { -1 } 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 L _ { x } \geq 10 ^ { 45 } { erg s ^ { -1 } } and a velocity dispersion exceeding 800 km s ^ { -1 } 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 best described with \sigma _ { 8 } \simeq 0.75 \pm 0.1 and \Omega \simeq 0.4 \pm 0.2 ( 90 % confidence ) .
The cluster dynamical analysis gives \Omega = 0.2 \pm 0.1 for which \sigma _ { 8 } = 0.95 \pm 0.1 ( 90 % confidence ) .
The predicted cluster density evolution in an \Omega = 1 CDM model exceeds that observed by more than an order of magnitude .