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Black hole perturbations in DHOST theories: Master variables, gradient instability, and strong coupling (2106.07128v2)
Published 14 Jun 2021 in gr-qc and hep-th
Abstract: We study linear perturbations about static and spherically symmetric black hole solutions with stealth scalar hair in degenerate higher-order scalar-tensor (DHOST) theories. We clarify master variables and derive the quadratic Lagrangian for both odd- and even-parity perturbations. It is shown that the even modes are in general plagued by gradient instabilities, or otherwise the perturbations would be strongly coupled. Several possible ways out are also discussed.
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