Scalar phantom energy as a cosmological dynamical system

Phantom energy can be visualized as a scalar field with a (non-canonical) negative kinetic energy term. We use the dynamical system formalism to study the attractor behavior of a cosmological model containing a phantom scalar field $\phi$ endowed with an exponential potential of the form $V(\phi)=V_0 \exp(-\lambda \kappa \phi)$, and a perfect fluid with constant equation of state $\gamma$; the latter can be of the phantom type too. As in the canonical case, three characteristic solutions can be identified. The scaling solution exists but is either unstable or of no physical interest. Thus, there are only two stable critical points which are of physical interest, corresponding to the perfect fluid and scalar field dominated solutions, respectively. The most interesting case arises for $0> \gamma > -\lambda^2/3$, which allows the coexistence of the three solutions. The main features of each solution are discussed in turn.Comment: 6 pages, 3 eps figures; uses RevTex4. New references added, and changes made according to referee's suggestions. Matches published version in JCA

Two Loop Scalar Self-Mass during Inflation

We work in the locally de Sitter background of an inflating universe and consider a massless, minimally coupled scalar with a quartic self-interaction. We use dimensional regularization to compute the fully renormalized scalar self-mass-squared at one and two loop order for a state which is released in Bunch-Davies vacuum at t=0. Although the field strength and coupling constant renormalizations are identical to those of lfat space, the geometry induces a non-zero mass renormalization. The finite part also shows a sort of growing mass that competes with the classical force in eventually turning off this system's super-acceleration.Comment: 31 pages, 5 figures, revtex4, revised for publication with extended list of reference

Coupled dark energy: Towards a general description of the dynamics

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.Comment: 21 pages, 5 figures; minor clarifications added, typos corrected and references updated; final version to appear in JCA