ar X iv : 0 81 1 . 08 27 v 2 [ as tr o - ph ] 1 3 N ov 2 00 8 The Effective Theory of Quintessence : the w < − 1 Side Unveiled

We study generic single-field dark energy models, by a parametrization of the most general theory of their perturbations around a given background, including higher derivative terms. In appropriate limits this approach reproduces standard quintessence, k-essence and ghost condensation. We find no general pathology associated to an equation of state wQ < −1 or in crossing the phantom divide wQ = −1. Stability requires that the wQ < −1 side of dark energy behaves, on cosmological scales, as a k-essence fluid with a virtually zero speed of sound. This implies that one should set the speed of sound to zero when comparing with data models with wQ < −1 or crossing the phantom divide. We summarize the theoretical and stability constraints on the quintessential plane (1+wQ) vs. speed of sound squared.