ar X iv : a st ro - p h / 05 03 16 9 v 1 7 M ar 2 00 5 The Phantom of the OPRA

The Observed Perlmutter-Riess Acceleration (OPRA) implies that the expansion of the Universe is currently increasing and is motivation for the so-called phantom energy models. We consider the dynamics of phantom scalar field models. An important physical time constraint, which can be used to rule out many cosmological models, is obtained from the condition that all forms of energy density, including the field causing OPRA (e.g., the phantom field), must be non-negligible for an extended period, which is conservatively estimated to be of the order of a few Gyr. We find that this physical time constraint cannot be satisfied in conventional phantom cosmological models. Evidence from supernovae observations [1] strongly suggest the possibility of late-time accelerated expansion of the Universe; the Observed Perlmutter-Riess Acceleration (OPRA). The energy density is usually described by an effective " equation-of-state " parameter γ ef f −1 ≡ p/ρ, the ratio of the energy pressure p to its energy density ρ. A value γ ef f < 2/3 is required for cosmic acceleration. The most recent observations, including supernovae, cosmic microwave background experiments, quasar-lensing statistics and galaxy clustering observations (see [2] and references therein), taken together suggest a Universe with γ ef f satisfying γ ef f ≤ 0; this is consistent with γ ef f = 0,