Physics Colloquium Series, Fall 2009 and beyond

How do we weigh the Universe? Where is the Dark Matter? I will discuss these questions and show that several independent methods, including the observed abundance of rich clusters , the baryon-fraction in clusters, the observed Mass-to-Light function from galaxies to superclusters, and other large-scale structure observations, all reveal a universe with a low mass density of ~20% of the critical density. The data suggest that the mass in the Universe, including the dark-matter, follows light on large scales, and most of the mass resides in huge dark halos around galaxies. I will review the combined observational evidence for dark-matter and for dark-energy in the universe and their cosmological implications.