Observations of distant supernovae have provided evidence that the expansion of the universe is accelerating rather than slowing down. The most straightforward explanation for these results is the existence of a nonzero cosmological constant; I will review the observational evidence pointing to this conclusion, and some of its theoretical ramifications. A nonzero cosmological constant seems unnatural in a variety of ways, which leads us to consider more dramatic alternatives to explain this puzzling result. I will speak about some of these alternatives and how we can hope to constrain them observationally.

ANL Physics Division Colloquium Schedule