We show that the evolution of the number density of rich clusters of galaxies breaks the degeneracy between Ω(the mass density ratio of the universe) and σ_8 (the normalization of the power spectrum), σ_8 Ω~0.5 ≈0.5, that follows from the observed present-day abundance of rich clusters. The evolution of high-mass (Coma-like) clusters is strong in Ω= 1, low-σ_8 models (such as the standard biased cold dark matter model with σ_8 ≈0.5), where the number density of clusters decreases by a factor of about 10~3 from z = 0 to z ≈0.5; the same clusters show only mild evolution in low-Ω, high-σ_8 models, where the decrease is a factor of about 10.
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