title: New class of consistent scalar-tensor theories.
abstract: We introduce a new class of scalar-tensor theories of gravity that extend Horndeski, or "generalized Galileon," models. Despite possessing equations of motion of higher order in derivatives, we show that the true propagating degrees of freedom obey well-behaved second-order equations and are thus free from Ostrogradski instabilities, in contrast to standard lore. Remarkably, the covariant versions of the original Galileon Lagrangians-obtained by direct replacement of derivatives with covariant derivatives-belong to this class of theories. These extensions of Horndeski theories exhibit an uncommon, interesting phenomenology: The scalar degree of freedom affects the speed of sound of matter, even when the latter is minimally coupled to gravity.
authors.affiliation: CEA, Institut de Physique Théorique, F-91191 Gif-sur-Yvette cédex, CNRS, Unité de recherche associée-2306, F-91191 Gif-sur-Yvette cédex, Paris, France.|Université Paris Sud, 15 rue George Clémenceau, 91405 Orsay, France.
authors.forename: Jérôme
authors.initials: J
authors.lastname: Gleyzes
authors.affiliation: APC, (CNRS-Université Paris 7), 10 rue Alice Domon et Léonie Duquet, 75205 Paris, France.
authors.forename: David
authors.initials: D
authors.lastname: Langlois
authors.affiliation: APC, (CNRS-Université Paris 7), 10 rue Alice Domon et Léonie Duquet, 75205 Paris, France.|PCCP, 10 rue Alice Domon et Léonie Duquet, 75205 Paris, France.|CPT, Aix Marseille Université, UMR 7332, 13288 Marseille, France.
authors.forename: Federico
authors.initials: F
authors.lastname: Piazza
authors.affiliation: CEA, Institut de Physique Théorique, F-91191 Gif-sur-Yvette cédex, CNRS, Unité de recherche associée-2306, F-91191 Gif-sur-Yvette cédex, Paris, France.
authors.forename: Filippo
authors.initials: F
authors.lastname: Vernizzi