Imperfect dark energy from kinetic gravity braiding
Abstract
We introduce a large class of scalartensor models with interactions containing the second derivatives of the scalar field but not leading to additional degrees of freedom. These models exhibit peculiar features, such as an essential mixing of scalar and tensor kinetic terms, which we have named kinetic braiding. This braiding causes the scalar stress tensor to deviate from the perfectfluid form. Cosmology in these models possesses a rich phenomenology, even in the limit where the scalar is an exact Goldstone boson. Generically, there are attractor solutions where the scalar monitors the behaviour of external matter. Because of the kinetic braiding, the position of the attractor depends both on the form of the Lagrangian and on the external energy density. The latetime asymptotic of these cosmologies is a de Sitter state. The scalar can exhibit phantom behaviour and is able to cross the phantom divide with neither ghosts nor gradient instabilities. These features provide a new class of models for Dark Energy. As an example, we study in detail a simple oneparameter model. The possible observational signatures of this model include a sizeable Early Dark Energy and a specific equation of state evolving into the final deSitter state from a healthy phantom regime.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 October 2010
 DOI:
 10.1088/14757516/2010/10/026
 arXiv:
 arXiv:1008.0048
 Bibcode:
 2010JCAP...10..026D
 Keywords:

 High Energy Physics  Theory;
 Astrophysics  Cosmology and Extragalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology
 EPrint:
 41 pages, 7 figures. References and some clarifying language added. This version was accepted for publication in JCAP