N A Bahcall | PubMed Health

PubMed1998

A lightweight universe?

How much matter is there in the universe? Does the universe have the critical density needed to stop its expansion, or is the universe underweight and destined to expand forever? We show that several independent measures, especially those utilizing the largest bound systems known-clusters of galaxies-all indicate that the mass-density of the universe is insufficient to halt the expansion. A promising new method, the evolution of the number density of clusters with time, provides the most powerful indication so far that the universe has a subcritical density. We show that different techniques reveal a consistent picture of a lightweight universe with only approximately 20-30% of the critical density. Thus, the universe may expand forever.

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PubMed1993

Clusters, superclusters, and large-scale structure: a consistent picture.

Observations of the large-scale structure in the universe using different tracers and techniques, including the spatial distribution of galaxies, clusters of galaxies, narrow pencil-beam surveys, and quasars, appear to be yielding a consistent picture of the universal structure. A network of large-scale superclusters with scales up to approximately 150h-1 Mpc is suggested (where h approximately 0.5-1 is the Hubble constant in units of 100 km.s-1.Mpc-1; 1 pc = 3.09 x 10(16) m; h = 1 is used throughout this paper). The supercluster network surrounds low-density regions, suggesting a "cellular" structure of the universe. The universal dimensionless cluster correlation function, supported by new data from automated cluster surveys, is consistent with this picture. The "standard" Omega = 1 cold dark matter (CDM) model for the universe appears to be inconsistent with the details of the observed large-scale structure distribution; a low-density, Omega approximately 0.2-0.3, CDM model provides a considerably better fit to the observations.

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