We analyze spherical and odd-parity linear perturbations of hairy black holes with a minimally coupled scalar field.
Spherical ModesIn order to have an asymptotically flat hairy black hole with a minimally coupled scalar field a necessary condition is a scalar field potential with a negative region. For a recent review see, for example.
1As is well-known the stability problem can be mapped to the analysis of the spectrum of a Schrödinger operator, which appears in the master equation for perturbations. An everywhere positive spectrum implies there are no modes which exponentially grow in time. Using the method of Ref. 2 we study the equations of motion for the radial perturbations of the formwhere V eff (ρ) (explicitly given below) is an effective potential in which the modes u(ρ) propagate and ρ is a "tortoise" radial coordinate sending the horizon at minus infinity; it always exhibits a negative region. A sufficient condition for the existence of bound states with negative E 2 (for bounded V eff that fall-off faster than |ρ| −2 ) is the Simon criteria, which states that if S ≡ +∞ −∞ V eff dρ is negative there will always be at least one bound state with negative E 2 ; hence, we only study positive Simon integrals. Using "shooting" techniques to solve (1), we do indeed find unstable modes in Ref. 1. However, there is only a finite number of unstable modes and, moreover, their characteristic time of growth can be made arbitrarily large for certain values of the black holes parameters, as is the case if the size of the black hole is small enough.We are interested in studying the linearized dynamics around a background solution. Hence, starting from the metric in the formwhere r is a general radial coordinate, not necessarily ρ, and the scalar field is assumed in the form φ = φ 0 (r) + φ 1 (r, t). We expand the scalar field potential. As a consequence of spherical symmetry all the dynamics is driven by the scalar field. Indeed, it is possible to write the metric perturbations in terms of the φ 1 (r, t) by using the Einstein field equations. We introduce the master variable ψ(ρ, t) = φ 1 (r, t)C(r) 1/2 ,The Fourteenth Marcel Grossmann Meeting Downloaded from www.worldscientific.com by 54.202.27.70 on 05/11/18. For personal use only.