Crossing the phantom divide

We consider fluid perturbations close to the "phantom divide" characterisedby p = -rho and discuss the conditions under which divergencies in theperturbations can be avoided. We find that the behaviour of the perturbationsdepends crucially on the prescription for the pressure perturbation delta-p.The pressure perturbation is usually defined using the dark energy rest-frame,but we show that this frame becomes unphysical at the divide. If the pressureperturbation is kept finite in any other frame, then the phantom divide can becrossed. Our findings are important for generalised fluid dark energy used indata analysis (since current cosmological data sets indicate that the darkenergy is characterised by p ~ -rho so that p < -rho cannot be excluded) aswell as for any models crossing the phantom divide, like some modified gravity,coupled dark energy and braneworld models. We also illustrate the results by anexplicit calculation for the "Quintom" case with two scalar fields.Comment: 14 pages, 10 figures, v2: updated to agree with published version: more readable figures, slightly expanded discussion on modified gravity models and the interpolation across w=-1, results and conclusions unchange