Phantom Dark Energy Models with Negative Kinetic Term

We examine phantom dark energy models derived from a scalar field with a negative kinetic term for which V (φ) → ∞ asymptotically. All such models can be divided into three classes, corresponding to an equation of state parameter wφ with asymptotic behavior wφ → −1, wφ → w0 < −1, and wφ → −∞. We derive the conditions on the potential V (φ) which lead to each of these three types of behavior. For models with wφ → −1, we derive the conditions on V (φ) which determine whether or not such models produce a future big rip. Observational constraints are derived on two classes of these models: power-law potentials with V (φ) = λφ (with α positive or negative) and exponential potentials of the form V (φ) = βe α . It is shown that these models spend more time in a state with Ωm ∼ Ωφ than do corresponding models with a constant value of wφ, thus providing a more satisfactory solution to the coincidence problem.