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Healthy theories beyond Horndeski

David Langlois
Federico Piazza


We introduce a new class of scalar-tensor theories 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 the 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.

Dates and versions

cea-01073587 , version 1 (10-10-2014)



Jérôme Gleyzes, David Langlois, Federico Piazza, Filippo Vernizzi. Healthy theories beyond Horndeski. 2014. ⟨cea-01073587⟩
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