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 w(Q) < -1 or in crossing the phantom divide w(Q) = -1. Stability requires that the w(Q) < -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 w(Q) < -1 or crossing the phantom divide. We summarize the theoretical and stability constraints on the quintessential plane (1 + w(Q)) vs. speed of sound squared.