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arXiv · 1404.6495

Healthy theories beyond Horndeski

Jérôme Gleyzes·David Langlois·Federico Piazza·Filippo Vernizzi
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Abstract

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.

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Jérôme Gleyzes, David Langlois, Federico Piazza, Filippo Vernizzi. 2015-05-19. Healthy theories beyond Horndeski. https://doi.org/10.1103/physrevlett.114.211101

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