# The effective theory of quintessence: the w&lt;-1 side unveiled

### APA

Creminelli, P. (2009). The effective theory of quintessence: the w&lt;-1 side unveiled. Perimeter Institute. https://pirsa.org/09070027

### MLA

Creminelli, Paolo. The effective theory of quintessence: the w&lt;-1 side unveiled. Perimeter Institute, Jul. 17, 2009, https://pirsa.org/09070027

### BibTex

@misc{ pirsa_PIRSA:09070027, doi = {10.48660/09070027}, url = {https://pirsa.org/09070027}, author = {Creminelli, Paolo}, keywords = {Quantum Fields and Strings, Particle Physics, Cosmology}, language = {en}, title = {The effective theory of quintessence: the w\&lt;-1 side unveiled}, publisher = {Perimeter Institute}, year = {2009}, month = {jul}, note = {PIRSA:09070027 see, \url{https://pirsa.org}} }

The Abdus Salam International Centre for Theoretical Physics (ICTP)

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Talk Type

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