## [astro-ph/0410680] Crossing the Phantom Divide: Dark Energy Internal Degrees of Freedom

 Authors: Wayne Hu (KICP; U Chicago) Abstract: Dark energy constraints have forced viable alternatives that differ substantially from a cosmological constant Lambda to have an equation of state w that evolves across the phantom divide set by 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 alternate 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. [PDF]  [PS]  [BibTex]  [Bookmark]

Discussion related to specific recent arXiv papers
Antony Lewis
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### [astro-ph/0410680] Crossing the Phantom Divide: Dark Energy

This paper provides a nice continuous scheme for constructing dark energy parameterizations that evolve from equation of state w<-1 to w>-1 (or vice versa). It works by taking two separate fluids, one with constant w<-1, and one with constant w>-1.

Question: is there actually anything unphysical about a single fluid going through w=-1? Various cases involving scalar fields seem to be problematic (e.g. astro-ph/0407107), but what about general fluids?

It seems that in general the fluid velocity $v \sim \log(|t-t_c|)$ where $w(t_c)=-1$, and so it is singular. However the heat flux $q=(\rho+p)v$ is perfectly regular. The density constrast $\delta$ seems to be continuous, even though its derivative $\delta'$ diverges. Are these properties unphysical?

In other words, is there anything inherently wrong with evolving a single fluid and just allowing v to become singular? (as in e.g. astro-ph/0409574, with some suitable numerical massaging)