If the dark energy equation of state parameter w ( z ) crosses the phantom divide line w = -1 ( or equivalently if the expression \frac { d ( H ^ { 2 } ( z ) ) } { dz } -3 \Omega _ { 0 m } H _ { 0 } ^ { 2 } ( 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-canonical phantom component or general relativity needs to be extended to a more general theory on cosmological scales .
The former possibility requires the existence of a phantom component which has been shown to suffer from serious theoretical problems and instabilities .
Therefore , the latter possibility is the simplest realistic theoretical framework in which such a crossing can be realized .
After providing a pedagogical description of various dark energy observational probes , we use a set of such probes ( including the Gold SnIa sample , the first year SNLS dataset , the 3-year WMAP CMB shift parameter , the SDSS baryon acoustic oscillations peak ( BAO ) , the X-ray gas mass fraction in clusters and the linear growth rate of perturbations at z = 0.15 as obtained from the 2dF galaxy redshift survey ) to investigate the priors required for cosmological observations to favor crossing of the phantom divide .
We find that a low \Omega _ { 0 { m } } prior ( 0.2 < \Omega _ { 0 { m } } < 0.25 ) leads , for most observational probes ( except of the SNLS data ) , to an increased probability ( mild trend ) for phantom divide crossing .
An interesting degeneracy of the ISW effect in the CMB perturbation spectrum is also pointed out .