Biased cold dark matter theory - Trouble from rich clusters?

The expected population of rich clusters in a cold dark matter (CDM) dominated universe is extremely sensitive to the spectrum normalization, the inverse of which defines the bias parameter b. Recent observations of three high-velocity dispersion clusters at redshifts z ~ 0.5 indicate an alarmingly low value of b for the theory. If taken at face value, the observations imply an unbiased model, b ~ 1. However, this value of b predicts roughly 10 times as many low-redshift clusters (z < 0.1) as are observed. The low-redshift data favor values in the range b ~ 1.5-1.9. It is argued that poor statistics and the uncertainty in recovering intrinsic velocity dispersions from redshift measurements preclude a reliable determination of b at present. A survey of cluster X-ray temperatures complete to moderate redshifts would prove invaluable in deciding this issue.