Dark energy and dark gravity

Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological observations confronts cosmological theory with a tremendous challenge, which it has so far failed to meet. Explanations of the acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are nearly all based on a dark energy field with fine-tuned, unnatural properties. There is a great variety of models, but all share one feature in common-an inability to account for the gravitational properties of the vacuum energy. Speculative ideas from string theory may hold some promise, but it is fair to say that no convincing model has yet been proposed. An alternative to dark energy is that gravity itself may behave differently from general relativity on the largest scales, in such a way as to produce acceleration. The alternative approach of modified gravity provides a new angle on the problem, but also faces severe difficulties, including the problem of explaining why the vacuum energy does not gravitate. The lack of an adequate theoretical framework for the late-time acceleration of the universe represents a deep crisis for theory-but also an exciting challenge for theorists. It seems likely that an entirely new paradigm is required to resolve this crisis