Abstract
The discovery that the cosmic expansion is accelerating has been followed by an intense theoretical and experimental response in physics and astronomy. The discovery implies that our most basic notions about how gravity works are violated on cosmological distance scales. A simple fix is to introduce a cosmological constant into the field equations for general relativity. However, the extremely small value of the cosmological constant, relative to theoretical expectations, has led theorists to explore numerous alternative explanations that involve the introduction of an exotic negative-pressure fluid or a modification of general relativity. Here we review the evidence for cosmic acceleration. We then survey some of the theoretical attempts to account for it, including the cosmological constant, quintessence and its variants, mass-varying neutrinos, and modifications of general relativity. We discuss experimental and observational tests that may allow us to distinguish among some of the theoretical ideas that have been proposed.
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