ar X iv : a st ro - p h / 06 03 17 1 v 1 7 M ar 2 00 6 Scalar perturbations and the possible self - destruction of the phantom menace

Some analysis of the supernovae type Ia observational data seems to indicate that the Universe today is dominated by a phantom field, for which all energy conditions are violated. Such phantom field may imply a singularity in a future finite time, called big rip. Studying the evolution of scalar perturbations for such a field, we show that if the pressure is negative enough, the Universe can become highly inhomogeneous and this phantom menace may be avoided. Using the supernova type Ia as standard candle to evaluate the distance versus redshift relation for high values of the redshift, two observational projects arrived at the conclusion that the Universe today is in a state of accelerated expansion, in opposition to what was believed until almost one decade ago [1]. This accelerated expansion can be achieved if the Universe today is dominated by a fluid with negative pressure, that remains a smooth component, not suffering local agglomeration, as it happens with ordinary matter. This fluid can be represented by an equation of state p = wρ, with w < −1/3, in order the strong energy condition to be violated, leading to a negative deceleration parameter q = −aä/ ˙ a 2 , where a is the scale factor describing the evolution of a homogeneous and isotropic Universe. Initially, the SNe type Ia data leads to the estimation that w ∼ −0.8, but the cosmological constant case (w = −1) was not excluded. Using a sample of 230 SN type Ia, and the hypothesis of a flat Universe, the authors of reference [2] found −1.48 < w < −0.72 at 95% confidence level. With a more restricted, and selected, sample of 176 SN type Ia, it was found in reference [3] that w = −1.02