Instability of scalar perturbations in a phantomic cosmological scenario

Scalar perturbations can grow during a phantomic cosmological phase as the big rip is approached, in spite of the highly accelerated expansion regime, if the equation of state is such that p/rho = w < - 5/3. It is shown that such result is independent of the spatial curvature. The perturbed equations are exactly solved for any value of the curvature parameter k and of the equation of state parameter w. Growing modes are found asymptotically under the condition w < -5/3. Since the Hubble radius decreases in a phantom universe, such result indicates that a phantom scenario may not survive longtime due to gravitational instability. Copyright (C) EPLA, 2008.