Horndeski: beyond, or not beyond?

Journal Article · Tue Mar 01 04:00:00 EST 2016 · Journal of Cosmology and Astroparticle Physics

DOI:https://doi.org/10.1088/1475-7516/2016/03/038· OSTI ID:22524932

Crisostomi, Marco; Hull, Matthew; Koyama, Kazuya ^[1]; Tasinato, Gianmassimo ^[2]

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1. Institute of Cosmology and Gravitation, University of Portsmouth, Burnaby Road, PO1 3FX, Portsmouth (United Kingdom)
2. Department of Physics, Swansea University, Singleton Park, SA2 8PP, Swansea (United Kingdom)

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.

• OSTI ID: 22524932
• Journal Information: Journal of Cosmology and Astroparticle Physics, Journal Name: Journal of Cosmology and Astroparticle Physics Journal Issue: 03 Vol. 2016; ISSN 1475-7516
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACTION INTEGRAL
COSMOLOGICAL INFLATION
DEGREES OF FREEDOM
GAUGE INVARIANCE
GRAVITATION
INFLATIONARY UNIVERSE
LIMITING VALUES
MAPPING
NONLUMINOUS MATTER
SCALAR FIELDS
SCALARS
TENSORS
TRANSFORMATIONS