Exploring gravitational theories beyond Horndeski

Authors:
  • JGJérôme Gleyzes
    Jérôme Gleyzes
  • DLDavid Langlois
    David Langlois
  • FPFederico Piazza
    Federico Piazza
  • FVFilippo Vernizzi
    Filippo Vernizzi
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Abstract
We have recently proposed a new class of gravitational scalar-tensor theories free from Ostrogradski instabilities, in ref. [1]. As they generalize Horndeski theories, or “generalized” galileons, we call them G3. These theories possess a simple formulation when the time hypersurfaces are chosen to coincide with the uniform scalar field hypersurfaces. We confirm that they contain only three propagating degrees of freedom by presenting the details of the Hamiltonian formulation. We examine the coupling between these theories and matter. Moreover, we investigate how they transform under a disformal redefinition of the metric. Remarkably, these theories are preserved by disformal transformations that depend on the scalar field gradient, which also allow to map subfamilies of G3into Horndeski theories.

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