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The acceleration of the universe presents us with mysteries and opportunities. The fact that this behavior is so puzzling is a sign that there is something fundamental we don't understand. We don't even know whether our misunderstanding originates with gravity as described by general relativity, with some source of dynamical or constant dark energy, or with the structure of the universe on ultra-large scales. Regardless of what the answer is, we seem poised to discover something profound about how the universe works.


It is a pleasure to thank Wendy Freedman for organizing a stimulating meeting, and participants at the Seven Pines Symposium on "The Concept of the Vacuum in Physics" and the Kavli Institute for Theoretical Physics program on "String Theory and Cosmology" for numerous helpful conversations. This work was supported in part by U.S. Dept. of Energy contract DE-FG02-90ER-40560, National Science Foundation Grants PHY01-14422 (CfCP) and PHY99-07949 (KITP), and the David and Lucile Packard Foundation.