Quintom cosmology: Theoretical implications and observations
Abstract
We review the paradigm of quintom cosmology. This scenario is motivated by the observational indications that the equationofstate of dark energy across the cosmological constant boundary is mildly favored, although the data are still far from being conclusive. As a theoretical setup we introduce a nogo theorem existing in quintom cosmology, and based on it we discuss the conditions for the equationofstate of dark energy realizing the quintom scenario. The simplest quintom model can be achieved by introducing two scalar fields with one being quintessence and the other phantom. Based on the doublefield quintom model we perform a detailed analysis of dark energy perturbations and we discuss their effects on current observations. This type of scenario usually suffers from a manifest problem due to the existence of a ghost degreeoffreedom, and thus we review various alternative realizations of the quintom paradigm. The developments in particle physics and string theory provide potential clues indicating that a quintom scenario may be obtained from scalar systems with higher derivative terms, as well as from nonscalar systems. Additionally, we construct a quintom realization in the framework of braneworld cosmology, where the cosmic acceleration and the phantom divide crossing result from the combined effects of the field evolution on the brane and the competition between four and fivedimensional gravity. Finally, we study the outsets and fates of a universe in quintom cosmology. In a scenario with null energy condition violation one may obtain a bouncing solution at early times and therefore avoid the Big Bang singularity. Furthermore, if this occurs periodically, we obtain a realization of an oscillating universe. Lastly, we comment on several open issues in quintom cosmology and their connection to future investigations.
 Publication:

Physics Reports
 Pub Date:
 August 2010
 DOI:
 10.1016/j.physrep.2010.04.001
 arXiv:
 arXiv:0909.2776
 Bibcode:
 2010PhR...493....1C
 Keywords:

 High Energy Physics  Theory;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology
 EPrint:
 105 pages, 36 figures, version published at Physics Reports