  After reviewing the cosmological constant problem - why is Lambda not huge? -
I outline the two basic approaches that had emerged by the late 1980s, and note
that each made a clear prediction. Precision cosmological experiments now
indicate that the cosmological constant is nonzero. This result strongly favors
the environmental approach, in which vacuum energy can vary discretely among
widely separated regions in the universe. The need to explain this variation
from first principles constitutes an observational constraint on fundamental
theory. I review arguments that string theory satisfies this constraint, as it
contains a dense discretuum of metastable vacua. The enormous landscape of
vacua calls for novel, statistical methods of deriving predictions, and it
prompts us to reexamine our description of spacetime on the largest scales. I
discuss the effects of cosmological dynamics, and I speculate that weighting
vacua by their entropy production may allow for prior-free predictions that do
not resort to explicitly anthropic arguments.
