Cosmological Acceleration: Dark Energy or Modified Gravity?

The recently observed accelerating cosmological expansion or ”dark energy” may be either a negative pressure constituent in General Relativity (Dark Energy) or modified gravity (Dark Gravity) without any constituent Dark Energy. Lowor high-curvature modifications of Einstein gravity are distinguished by the spacetime (Ricci) curvature of their vacua. If constituent Dark Energy does not exist, so that our universe is now dominated by pressure-free matter, Einstein gravity must be modified at lowcurvature, becoming asymptotically de Sitter. The dynamics of either kind of ”dark energy” cannot be derived from the homogeneous expansion history alone, but requires also observing the growth of inhomogeneities. Present and projected observations are all consistent with a small fine-tuned cosmological constant, but also allow nearly static Dark Energy or gravity modified at cosmological scales. The growth of cosmological fluctuations will potentially distinguish between static and ”dynamic ”dark energy”. But, cosmologically distinguishing dynamic Dark Energy from Dark Gravity will require a weak lensing shear survey more ambitious than any now projected. Dvali-Gabadadze-Porrati low-curvature modifications of Einstein gravity may also be detected in refined observations in the solar system or in isolated galaxy clusters. We review local and cosmological tests of General Relativity and modified gravity. Dark Energy is epicyclic in character, requires fine-tuning to explain why its energy density is just now comparable to ordinary matter density, and cannot be detected in the laboratory or solar system. This, along with braneworld theories, now motivate searching for Dark Gravity on solar system, galaxy cluster and cosmological scales.