Crossing the phantom divide: Dark energy internal degrees of freedom

Abstract

Dark energy constraints have forced viable alternatives that differ substantially from a cosmological constant $\ensuremath{\Lambda}$ to have an equation of state $w$ that evolves across the phantom divide set by $\ensuremath{\Lambda}$. Naively, crossing this divide makes the dark energy gravitationally unstable, a problem that is typically finessed by unphysically ignoring the perturbations. While this procedure does not affect constraints near the favored cosmological constant model it can artificially enhance the confidence with which alternative models are rejected. Similar to the general problem of stability for $w<0$, the solution lies in the internal degrees of freedom in the dark energy sector. We explicitly show how to construct a two scalar field model that crosses the phantom divide and mimics the single field behavior on either side to substantially better than $1%$ in all observables. It is representative of models where the internal degrees of freedom keep the dark energy smooth out to the horizon scale independently of the equation of state.