title: No-go theorem for k-essence dark energy.
abstract: We demonstrate that if k-essence can solve the coincidence problem and play the role of dark energy in the Universe, the fluctuations of the field have to propagate superluminally at some stage. We argue that this implies that successful k-essence models violate causality. It is not possible to define a time ordered succession of events in a Lorentz invariant way. Therefore, k-essence cannot arise as a low energy effective field theory of a causal, consistent high energy theory.
authors.affiliation: Département de Physique Théorique, Université de Genève, 24 quai Ernest Ansermet, CH-1211 Genève 4, Switzerland. camille.bonvin@physics.unige.ch
authors.forename: Camille
authors.initials: C
authors.lastname: Bonvin
authors.forename: Chiara
authors.initials: C
authors.lastname: Caprini
authors.forename: Ruth
authors.initials: R
authors.lastname: Durrer