Formation times for rich clusters of galaxies

The author proposes that the present-day dimensions of rich clusters make it difficult to find a process that enables them to be formed within the Hubble time. Shorter formation times are obtained when the gravitational constant is a decreasing function of time. To obtain times that are compatible with the existence of high redshift quasars within a unified theory, requires G to vary about in proportion to tsup(-1/2). (J.R.)