A LIGHTWEIGHT UNIVERSE

How much matter is there in the universe? Does the universe have the c ritical density needed to stop its expansion, or is the universe under weight and destined to expand forever? We show that several independen t measures, especially those utilizing the largest bound systems known -clusters of galaxies-all indicate that the mass-density of the univer se is insufficient to halt the expansion. A promising new method, the evolution of the number density of clusters with time, provides the mo st powerful indication so far that the universe has a subcritical dens ity. We show that different techniques reveal a consistent picture of a lightweight universe with only similar to 20-30% of the critical den sity. Thus, the universe may expand forever.