Crossing the phantom divide: theoretical implications and observational status

Abstract

If the dark energy equation of state parameterw(z) crosses the phantomdivide line w = −1 (or equivalentlyif the expression d(H2(z))/dz−3Ω0mH02(1+z)2 changes sign) at recent redshifts, then there are two possible cosmological implications.Either the dark energy consists of multiple components with at least one non-canonicalphantom component or general relativity needs to be extended to a more general theory oncosmological scales. The former possibility requires the existence of a phantomcomponent which has been shown to suffer from serious theoretical problems andinstabilities. Therefore, the latter possibility is the simplest realistic theoreticalframework in which such a crossing can be realized. After providing a pedagogicaldescription of various dark energy observational probes, we use a set of suchprobes (including the Gold SnIa (supernovae type Ia) sample, the first-year SNLS(Supernova Legacy Survey) data-set, the three-year WMAP CMB (WilkinsonMicrowave Anisotropy Probe cosmic microwave background) shift parameter, theSDSS (Sloan Digital Sky Survey) baryon acoustic oscillations peak (BAO), thex-ray gas mass fraction in clusters and the linear growth rate of perturbations atz = 0.15 as obtained from the 2dF (Two-Degree Field) Galaxy Redshift Survey) to investigate the priorsrequired for cosmological observations to favour crossing of the phantom divide. We find that a lowΩ0m prior(0.2<Ω0m<0.25) leads, for most observational probes (except of the SNLS data), to an increasedprobability (mild trend) for phantom divide crossing. An interesting degeneracy of theintegrated Sachs–Wolfe effect in the CMB perturbation spectrum is also pointed out.