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N. Bahcall (Princeton University)
What is the mass-density of the universe? and how is the mass distributed? We show that several independent methods utilizing clusters of galaxies - including cluster dynamics and the mass-to-light ratio, baryon fraction, and cluster evolution - all indicate the same consistent result: the mass-density of the universe is low, about 0.2-0.3 of the critical density, and mass approximately traces light on large scales. When combined with the SNIa and the microwave background results, the data suggest a universe that is lightweight, accelerating, and flat.
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