Clusters and cosmology

Rich clusters of galaxies, the largest virialized systems known, place powerful constraints on cosmology. Some of the fundamental questions that can be addressed with clusters of galaxies include: What is the mass-density of the universe? and how is the mass distributed? We show that several independent methods utilizing clusters of galaxies - cluster mass-to-light ratio, baryon fraction in clusters, and cluster evolution - all indicate the same robust result: the mass-density of the universe is low, Ω_m≃0.25, and the mass approximately traces light on large scales. Combining the cluster results with recent observations of high-redshift supernovae and the cosmic microwave background anisotropy spectrum indicates a remarkable consistency among these independent, nearly orthogonal methods; the combined results, presented in a cosmic triangle plot, suggest a universe that is lightweight, is accelerating, and is flat.