Superluminality in DHOST theory with extra scalar
S. Mironov, V. Rubakov, V.Volkova
TL;DR
This paper investigates how an extra scalar field affects the propagation speeds of modes in DHOST Ia theories, revealing conditions for superluminal propagation and identifying subclasses without this issue.
Contribution
It demonstrates that a slowly rolling scalar field generally causes superluminal modes in DHOST Ia theories and identifies subclasses where all modes are subluminal.
Findings
Superluminal propagation occurs with a slowly rolling scalar field.
A subclass of DHOST Ia theories avoids superluminality.
Implications for stability and causality in modified gravity theories.
Abstract
We consider DHOST Ia theory interacting gravitationally with an additional conventional scalar field minimally coupled to gravity. At the linearized level of perturbations about cosmological background, we find that in the presence of a slowly rolling extra scalar field, one of the modes generically propagates at superluminal speed. This result is valid for any stable cosmological background. We identify a subclass of DHOST Ia theories in which this superluminality property is absent, and all modes may propagate (sub)luminally. We discuss possible implications for the interacting DHOST Ia theories.
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