The Effective Theory of Quintessence: the w<-1 Side Unveiled

TL;DR

This paper develops a comprehensive framework for single-field dark energy models, including those with w<-1, demonstrating their stability and behavior, especially near the phantom divide, and providing constraints on their parameters.

Contribution

It introduces a general parametrization of perturbations in dark energy models, unifying quintessence, k-essence, and ghost condensation, and clarifies stability conditions for w<-1 scenarios.

Findings

No general pathology for w<-1 or crossing the phantom divide.

Stable models with w<-1 behave as k-essence with zero sound speed.

Constraints on the (1+w_Q) vs. sound speed squared plane.

Abstract

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.