Dark Energy and Dark Gravity

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.

Friedman-like Robertson–Walker model in generalized metric space-time with weak anisotropy

“…This result is in agreement with the fixing of a similar parameter in [11,12,13] for self-accelerated brane-world cosmology.Since the parameter z t is related to the variation of anisotropy we expect it to have negative sign(self accelerating universe) as it may control a transition of the universe from a state of anisotropy to a smoother isotropic phase [15].…”

Anisotropic Cosmological Models with a Generalized Chaplygin Gas

“…This result is in agreement with the fixing of a similar parameter for a self accelerated brane-world cosmology [27]- [29]. Since the parameter  is related to the variation of anisotropy, we expect it to have a negative sign (self accelerating universe) as it may control a transition of the universe from a state of anisotropy to a smoother isotropic phase.…”

Improving dark energy constraints with high-redshift Type Ia supernovae from CANDELS and CLASH

“…Many possible alternatives have been introduced by our imaginative community, including modifications of the theoretical framework that deviate from general relativity such as Chaplygin gas, Cardassian expansion, DGP brane-world models, f (R) theories, etc. (Tsujikawa 2010;Dvali 2006;Carroll et al 2005;Freese & Lewis 2002;Gorini et al 2003;Bento et al 2002;Sahni et al 2003;Maartens 2007;Sotiriou et al 2010). Among observers, the most popular class of alternative models are the proposals for evolving dark energy.…”