Stability of disformally coupled accretion disks

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Koivisto , T S & Nyrhinen , H J 2017 , ' Stability of disformally coupled accretion disks ' , Physica Scripta , vol. 92 , no. 10 , 105301 .

Title: Stability of disformally coupled accretion disks
Author: Koivisto, Tomi S.; Nyrhinen, Hannu J.
Contributor organization: Helsinki Institute of Physics
Department of Physics
Date: 2017-10
Language: eng
Number of pages: 11
Belongs to series: Physica Scripta
ISSN: 0031-8949
Abstract: The no-hair theorem postulates that the only externally observable properties of a black hole are its mass, its electric charge, and its angular momentum. In scalar-tensor theories of gravity, a matter distribution around a black hole can lead to the so called 'spontaneous scalarisation' instability that triggers the development of scalar hair. In the Brans-Dicke type theories, this effect can be understood as a result of tachyonic effective mass of the scalar field. Here we consider the instability in the generalised class of scalar-theories that feature non-conformal, i.e. 'disformal', couplings to matter. Such theories have gained considerable interest in the recent years and have been studied in a wide variety of systems, both cosmological and astrophysical. In view of the prospects of gravitational wave astronomy, it is relevant to explore the implications of the theories in the strong-gravity regime. In this article, we concentrate on the spontaneous scalarisation of matter configurations around Schwarzschild and Kerr black holes. We find that in the more generic scalar-tensor theories, the instability of the Brans-Dicke theory can be enhanced, suggesting violations of the no-hair theorem. On the other hand, we find that, especially if the coupling is very strong, or if the gradients in the matter distribution are negligible, the disformal coupling tends to stabilise the system.
Subject: modified gravity
scalar-tensor gravity
black holes
strong field gravity
accretion disks
disformal coupling
115 Astronomy, Space science
114 Physical sciences
Peer reviewed: Yes
Rights: unspecified
Usage restriction: openAccess
Self-archived version: acceptedVersion

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