Dark energy and dark gravity: theory overview |
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Authors: | Ruth Durrer Roy Maartens |
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Institution: | (1) Départment de Physique Théorique, Université de Genève, 24 Quai E. Ansermet, 1211 Genève 4, Switzerland;(2) Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth, PO1 2EG, UK |
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Abstract: | Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological
observations confronts theoretical cosmology with a tremendous challenge, which it has so far failed to meet. Explanations
of cosmic 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 (or dark gravity) provides a new angle on the problem,
but also faces serious difficulties, including in all known cases severe fine-tuning and 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. |
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