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**Session 52 - Clusters & Cosmology.**

*Oral session, Thursday, June 12*

*North Main Hall C/D, *

## [52.01] Constraining Omega with Cluster Evolution

*N. A. Bahcall, X. Fan, R. Cen (Princeton University)*
The evolution of the number density of rich clusters of
galaxies provides a powerful constraint on the cosmological
density parameter, omega. The evolution of high-mass
(Coma-like) clusters is strong in omega=1 models (such as
the standard biased CDM model with sigma 0.5, where
sigma(8) is the normalization of the power-spectrum required
to match the present-day cluster abundance), where the
cluster number-density decreases by a factor of 10^3 from
z=0 to z=0.5; the same clusters show only mild evolution in
low-omega, high-sigma(8) models, where the decrease is a
factor of 10. This diagnostic provides a most powerful
constraint on omega. Using observations of clusters to
z=0.5-1, we find only mild evolution in the observed
cluster abundance. We find omega=0.3+-0.1 and
sigma(8)=0.85+-0.15. The results imply, if confirmed by
future surveys, that we live in a low-density, low-bias
universe.

**Program
listing for Thursday**