Stability of disformally coupled accretion disks

Understand

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.