as tr o - ph / 9 80 40 82 8 A pr 1 99 8 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 insuucient 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 sub-critical density. We show that diierent techniques reveal a consistent picture of a lightweight universe with only 20-30% of the critical density. Thus, the universe may expand forever. Standard models of innation { how the universe expanded in the beginning { as well as general arguments that demand no \\ne tuning" of cosmological parameters, predict a at universe with the critical density needed to just halt its expansion. The critical density, 1:9 10 ?29 h 2 g cm ?3 (where h refers to Hubble's constant, see below), is equivalent to 10 protons per cubic meter; this density provides the gravitational pull needed to slow down the universal expansion that began with the Big Bang approximately 15 billion years ago, and will eventually bring it to a halt. So far, however, only a small fraction of the critical density has been detected, even when all the unseen \dark matter" in galaxy halos and clusters of galaxies is included. There is no reliable indication so far that most of the matter needed for closing the universe does in fact exist. Here we show that several independent observations of clusters of galaxies, including the mass-to-light ratio of clusters, the high baryon fraction in clusters, and the observed evolution of cluster abundance, all portray a consistent picture of a sub-critical universe.