2016/01/31 by Marco Crisostomi, Matthew Hull, K. Koyama +2 · 8 citations
Physics and Astronomy · #Black Holes and Theoretical Physics #Classical mechanics #Constraint (computer-aided design) #Cosmology #Cosmology and Gravitation Theories #Curvature #Dark energy #Effective field theory #Geometry #Instability #Invariant (physics) #Mathematical physics #Physics #Quantum mechanics #Scalar (mathematics) #Scalar field #Solar and Space Plasma Dynamics #Theoretical physics #astro-ph.CO #gr-qc #hep-th
paper · pdf · doi:10.1088/1475-7516/2016/03/038
published as JCAP03 (2016) 038 · 19 pages, published version
openalex publication_date 2016/03/21 · arxiv created 2016/03/22 · arxiv updated 2016/03/23 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
Determining the most general, consistent scalar tensor theory of gravity is important for building models of inflation and dark energy. In this work we investigate the number of degrees of freedom present in the theory of beyond Horndeski. We discuss how to construct the theory from the extrinsic curvature of the constant scalar field hypersurface, and find a simple expression for the action which guarantees the existence of the primary constraint necessary to avoid the Ostrogradsky instability. Our analysis is completely gauge-invariant. However we confirm that, mixing together beyond Horndeski with a different order of Horndeski, obstructs the construction of this primary constraint. Instead, when the mixing is between actions of the same order, the theory can be mapped to Horndeski through a generalised disformal transformation. This mapping however is impossible with beyond Horndeski alone, since we find that the theory is invariant under such a transformation. The picture that emerges is that beyond Horndeski is a healthy but isolated theory: combined with Horndeski, it either becomes Horndeski, or likely propagates a ghost.