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"Shadowy" modes in Higher-Order Scalar-Tensor theories

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arxiv 1803.06241 v1 pith:GTG32UHW submitted 2018-03-16 hep-th gr-qc

"Shadowy" modes in Higher-Order Scalar-Tensor theories

Antonio De Felice , David Langlois , Shinji Mukohyama , Karim Noui , Anzhong Wang This is my paper
classification hep-th gr-qc
keywords theoriesmodeu-degenerateapparentdegenerateextrafieldgauge
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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We consider Higher-Order Scalar-Tensor theories which appear degenerate when restricted to the unitary gauge but are not degenerate in an arbitrary gauge. We dub them U-degenerate theories. We provide a full classification of theories that are either DHOST or U-degenerate and that are quadratic in second derivatives of the scalar field, and discuss its extension to cubic and higher order theories. Working with a simple example of U-degenerate theory, we find that, for configurations in which the scalar field gradient is time-like, the apparent extra mode in such a theory can be understood as a generalized instantaneous, or "shadowy" mode, which does not propagate. Appropriate boundary conditions, required by the elliptic nature of part of the equations of motion, lead to the elimination of the apparent instability associated with this extra mode.

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Cited by 4 Pith papers

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