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 sub-critical density. We show that different techniques reveal a consistent picture of a lightweight universe with only ~20-30% of the critical density. Thus, the universe may expand forever. 
Neta A. Bahcall and Xiaohui Fan (Princeton University Observatory)