n, 0"8, 4>(L, z) RESULTS, fj PROSPECTS FOR A MEASUREMENT

Rich galaxy clusters are powerful probes of both cosmological and galaxy evolution parameters. The CNOC cluster survey was primarily designed to distinguish between n = 1 and n == 0.2 cosmologies. Projected foreground and background galaxies provide a field sample of compara­ ble size. The results strongly support a low-density universe. The luminous cluster galaxies are about 10-30% fainter, depending on color, than the com­ parable field galaxies, but otherwise they show a slow and nearly parallel evolution. On the average, there is no excess star formation when galaxies fall into clusters. These data provide the basis for a simple A measurement using the survey's clusters and the field data. The errors in nM , ni\, 0"8 and galaxy evolution parameters could be reduced to a few percent with a sample of a few hundred clusters spread over the 0 < z < 1 range.