Phantom cosmologies

The dynamics of a minimally coupled scalar field in the expanding universe is discussed with special reference to phantom cosmology. The evolution of the universe with a phantom field vis-à-vis a quintessence field is compared. Phantom cosmologies are found to have two special features: (i) occurrence of a singularity where the scale factor, the energy density, and Ricci curvature scalar diverge to infinity (this singularity occurs at a finite time, depending on the value of w during cosmic evolution), and (ii) degeneracy in the determination of ${w(z}_m)$ for a given transition redshift $z_m$ which seems to impart similar observational properties to corresponding phantom and quintessence models and makes both of them compatible with the cosmological observations. Although due to the uncertainties in the measurement of the Hubble constant $H_0,$ the Hubble dependent observational parameters yield only loose constraints over the range of w, the duality in the determination of w with respect to transition redshift may be used to constrain w. An observational test, based upon the observations of low redshift galactic clusters, is suggested to discriminate between the quintessence and phantom dark energy.