Physics – High Energy Physics – High Energy Physics - Theory
Scientific paper
2005-02-21
JCAP 0506:007,2005
Physics
High Energy Physics
High Energy Physics - Theory
21 pages, 5 figures; minor clarifications added, typos corrected and references updated; final version to appear in JCAP
Scientific paper
10.1088/1475-7516/2005/06/007
In dark energy models of scalar-field coupled to a barotropic perfect fluid, the existence of cosmological scaling solutions restricts the Lagrangian of the field $\vp$ to $p=X g(Xe^{\lambda \vp})$, where $X=-g^{\mu\nu} \partial_\mu \vp \partial_\nu \vp /2$, $\lambda$ is a constant and $g$ is an arbitrary function. We derive general evolution equations in an autonomous form for this Lagrangian and investigate the stability of fixed points for several different dark energy models--(i) ordinary (phantom) field, (ii) dilatonic ghost condensate, and (iii) (phantom) tachyon. We find the existence of scalar-field dominant fixed points ($\Omega_\vp=1$) with an accelerated expansion in all models irrespective of the presence of the coupling $Q$ between dark energy and dark matter. These fixed points are always classically stable for a phantom field, implying that the universe is eventually dominated by the energy density of a scalar field if phantom is responsible for dark energy. When the equation of state $w_\vp$ for the field $\vp$ is larger than -1, we find that scaling solutions are stable if the scalar-field dominant solution is unstable, and vice versa. Therefore in this case the final attractor is either a scaling solution with constant $\Omega_\vp$ satisfying $0<\Omega_\vp<1$ or a scalar-field dominant solution with $\Omega_\vp=1$.
Gumjudpai Burin
Naskar Tapan
Sami M.
Tsujikawa Shinji
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