We study analytically and numerically the evolution of bound systems in universes with accelerating expansion where the acceleration either increases with time towards a Big Rip singularity (phantom cosmologies) or decreases with time (quintessence). We confirm the finding of Caldwell et. al.[1] that bound structures get dissociated in phantom cosmologies but we demonstrate that this happens earlier than anticipated in Ref. [1]. In particular we find that the 'rip time' when a bound system gets unbounded is not the time when the repulsive phantom energy gravitational potential due to the average (ρ + 3p) balances the attractive gravitational potential of the mass M of the system. Instead, the 'rip time' is the time when the minimum of the time dependent effective potential (including the centrifugal term) disappears. For the Milky Way galaxy this happens approximately 180Myrs before the Big Rip singularity instead of approximately 60Myrs indicated in [1] for a phantom cosmology with w=-1.5. A numerical reconstruction of the dissociating bound orbits is presented.

If the dark energy equation of state parameter w(z) crosses the phantom divide line w = −1 (or equivalently if the expression2 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 Ω0m prior (0.2 < Ω0m < 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.

We have performed a comparative analysis of three recent and reliable SnIa datasets available in the literature: the Full Gold (FG) dataset (157 data points 0 < z < 1.75), a Truncated Gold (TG) dataset (140 data points 0 < z < 1) and the most recent Supernova Legacy Survey (SNLS) dataset (115 data points 0 < z < 1). We have analyzed and compared the likelihood of cosmological constant and dynamical dark energy parametrizations allowing for crossing of the phantom divide line (PDL). We find that even though the constraints obtained using the three datasets are consistent with each other at the 95% confidence level, the latest (SNLS) dataset shows distinct trends which are not shared by the Gold datasets. We find that the best fit dynamical w(z) obtained from the SNLS dataset does not cross the PDL w = −1 and remains above and close to the w = −1 line for the whole redshift range 0 < z < 1 showing no evidence for phantom behavior. The LCDM parameter values (w0 = −1, w1 = 0) almost coincide with the best fit parameters of the dynamical w(z) parametrizations. In contrast, the best fit dynamical w(z) obtained from the Gold datasets (FG and TG) clearly crosses the PDL and departs significantly from the PDL w = −1 line while the LCDM parameter values are about 2σ away from the best fit w(z). In addition, the (Ω0m, ΩΛ) parameters in a LCDM parametrization without a flat prior, fit by the SNLS dataset, favor the minimal flat LCDM concordance model. The corresponding fit with the Gold datasets mildly favors a closed universe and the flat LCDM parameter values are 1σ − 2σ away from the best fit (Ω0m, ΩΛ).

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