The Amplitude of Mass Fluctuations

We determine the linear amplitude of mass fluctuations in the universe,sigma_8, from the abundance of massive clusters at redshifts z=0.5 to 0.8. Theevolution of massive clusters depends exponentially on the amplitude of massfluctuations and thus provides a powerful measure of this importantcosmological parameter. The relatively high abundance of massive clustersobserved at z>0.5, and the relatively slow evolution of their abundance withtime, suggest a high amplitude of mass fluctuations: sigma_8=0.9 +-10% forOmega_m=0.4, increasing slightly to sigma_8=0.95 for Omega_m=0.25 andsigma_8=1.0 for Omega_m=0.1 (flat CDM models). We use the cluster abundanceobserved at z=0.5 to 0.8 to derive a normalization relation from thehigh-redshift clusters, which is only weakly dependent on Omega_m:sigma_8*Omega_m^0.14 = 0.78 +-0.08. When combined with recent constraints fromthe present-day cluster mass function (sigma_8*Omega_m^0.6=0.33 +-0.03) we findsigma_8=0.98 +-0.1 and Omega_m=0.17 +-0.05. Low sigma_8 values (<0.7) areunlikely; they produce an order of magnitude fewer massive clusters thanobserved.Comment: 12 pages including 3 figures; updated to match published versio