Resonance spectra of caged black holes

Recent numerical studies of the coupled Einstein-Klein-Gordon system in a cavity have provided compelling evidence that {\it confined} scalar fields generically collapse to form black holes. Motivated by this intriguing discovery, we here use analytical tools in order to study the characteristic resonance spectra of the confined fields. These discrete resonant frequencies are expected to dominate the late-time dynamics of the coupled black-hole-field-cage system. We consider caged Reissner-Nordstr\"om black holes whose confining mirrors are placed in the near-horizon region $x_{\text{m}}\equiv (r_{\text{m}}-r_+)/r_+\ll\tau\equiv (r_+-r_-)/r_+$ (here $r_{\text{m}}$ is the radius of the confining mirror and $r_{\pm}$ are the radii of the black-hole horizons). We obtain a simple analytical expression for the fundamental quasinormal resonances of the coupled black-hole-field-cage system: $\omega_n=-i2\pi T_{\text{BH}}\cdot n[1+O(x^n_{\text{m}}/\tau^n)]$, where $T_{\text{BH}}$ is the temperature of the caged black hole and $n=1,2,3,...$ is the resonance parameter.

Recently there is a renewed interest in the physics of caged black holes. This renewed interest stems from the important work of Bizoń and Rostworowski [16] who revealed that asymptotically anti-de Sitter (AdS) spacetimes are nonlinearly unstable. In particular, it was shown in [16] that the dynamics of massless, sphericallysymmetric scalar fields in asymptotically AdS spacetimes generically leads to the formation of Schwarzschild-AdS black holes.
It is well-known that the AdS spacetime can be regarded as having an infinite potential wall at asymptotic infinity [17]. One therefore expects the dynamics of confined scalar fields [18] to display a qualitatively similar behavior to the one observed in [16]. In an elegant work, Maliborski [19] (see also [20,21]) has recently confirmed this physically motivated expectation. In particular, the recent numerical study by Okawa, Cardoso and Pani [20] provides compelling evidence that spherically-symmetric confined scalar fields generically collapse to form caged black holes.
The late-time dynamics of perturbation fields in a black-hole spacetime [22] is characterized by quasinormal ringing, damped oscillations which reflect the dissipation of energy from the black-hole exterior region (see [23][24][25] for excellent reviews and detailed lists of references). The observation of these characteristic complex resonances may allow one to determine the physical parameters of the newly born black hole.
While there is a vast literature on the quasinormal spectra of black holes in asymptotically AdS spacetimes [23][24][25], much less is known about the corresponding resonances of caged black holes [26]. The recent interest [19][20][21] in the dynamics and formation of caged black holes makes it highly important to study their characteristic resonance spectra. As we shall show below, the resonant frequencies of caged black holes can be determined analytically in the regime of "tightly caged black holes" [27]. Here r m is the radius of the confining cage (mirror) and r ± are the radii of the black-hole horizons [see Eq. (4) below].

II. DESCRIPTION OF THE SYSTEM
The physical system we explore consists of a massless scalar field Ψ linearly coupled to a Reissner-Nordström (RN) black hole of mass M and electric charge Q. In terms of the Schwarzschild coordinates (t, r, θ, φ), the black-hole spacetime is described by the line element [28] where [29] f (r) The radii of the black-hole (event and inner) horizons are determined by the zeros of f (r): The dynamics of the scalar field Ψ in the RN spacetime is governed by the Klein-Gordon wave equation Resolving the field Ψ into spherical harmonics: one obtains a Schrödinger-like wave equation for the radial part of the field [28,30,31]: where the "tortoise" radial coordinate y is defined by The effective scattering potential in (7) is given by

III. BOUNDARY CONDITIONS
We shall be interested in solutions of the radial wave equation (7) with the physical requirement (boundary condition) of purely ingoing waves crossing the black-hole horizon [28]: In addition, following [20] we shall consider two types of boundary conditions at the surface r = r m of the confining cavity: (1) The Dirichlet-type boundary condition implies (2) The Neumann-type boundary condition implies

IV. THE RESONANCE CONDITIONS
The boundary conditions (11) and (12) single out two discrete families of complex resonant frequencies {ω(M, Q, r m , l; n)} [32] which characterize the late-time dynamics of the composed black-hole-field-cavity system (these characteristic resonances are also known as "boxed quasinormal frequencies" [11]). The main goal of the present paper is to determine these characteristic resonances analytically.
The Dirichlet-type boundary condition R(x = x m ) = 0 [see Eq. (11)] now reads Using equation 9.1.2 of [35], one can express this boundary condition in the form where Y ν (x) is the Bessel function of the second kind [35].
In the near-horizon region [see Eq. (1)] one may use equations 9.1.7 and 9.1.9 of [35] in order to write the resonance condition (23) in the form [37] The Neumann-type boundary condition dR(x = x m )/dx = 0 [see Eq. (12)] now reads Using equation 9.1.27 of [35], one can express (26) in the form Using equation 9.1.2 of [35], one can express this boundary condition in the form From equations 9.1.7 and 9.1.9 of [35] one finds (24)]. Using these relations, one may write the resonance condition (28) , which in the near-horizon region (24) implies (see equations 9.1.7 and 9.1.9 of [35])

V. THE DISCRETE RESONANCE SPECTRA OF CAGED BLACK HOLES
Taking cognizance of the near-horizon condition (24), one realizes that the r.h.s of the resonance conditions (25) and (29) are small quantities. This observation follows from the fact that, for damped modes with ℑ̟ < 0 [ℜ(i̟) > 0] one has z i̟ m ≪ 1 in the regime (24). We can therefore use an iteration scheme in order to solve the resonance conditions (25) and (29).
for the characteristic resonance spectra of caged black holes in the regime (24).

VI. SUMMARY AND DISCUSSION
Recent numerical studies of the Einstein-Klein-Gordon system in a cavity [20,21] have provided compelling evidence that confined scalar fields [18] generically collapse to form (caged) black holes. Motivated by these intriguing studies, we have explored here the late-time dynamics [22] of these confined fields in the background of caged black holes.
In particular, we have studied the characteristic resonance spectra of confined scalar fields in caged Reissner-Nordström black-hole spacetimes. It was shown that these resonances can be derived analytically for caged black holes whose confining mirrors are placed in the vicinity of the black-hole horizon [that is, in the regime x m ≪ τ , see Eq. (24)]. Remarkably, the resonant frequencies of these caged black holes can be expressed in terms of the Bekenstein-Hawking temperature T BH of the black-hole [39]: Note that, for spherical field configurations (the ones studied in [19,20]), the characteristic resonant frequencies are given by the remarkably simple linear relation [40] Finally, it is worth mentioning other black-hole spacetimes which share this remarkable property (that is, black-hole spacetimes which are characterized by resonant frequencies whose imaginary parts [41] scale linearly with the black-hole temperature): (1) Near-extremal asymptotically flat Kerr black holes [42].
(3) Asymptotically flat charged black holes coupled to charged scalar fields in the highly-charged regime qQ ≫ 1 [44,45]. (4) The "subtracted" black-hole geometries studied in [46]. (5) Asymptotically AdS black holes in the regime r + ≫ R [47,48]. This last example, together with our result (34) for the resonant frequencies of caged black holes, provide an elegant demonstration of the analogy, already discussed in the Introduction, between asymptotically AdS black-hole spacetimes and caged black-hole spacetimes.
The confining cavity (mirror) of our analysis obviously restricts the dynamics of the fields to the near-horizon region x ≤ x m ≪ τ . It is therefore not surprising that the characteristic resonances (33) of these caged black holes are determined by the surface gravity [49] at the black-hole horizon. The fact that the black-hole spacetimes mentioned above [42,43,45,46,48] share this same property (namely, they are characterized by a linear scaling of their resonances with the black-hole temperature) suggests that the dynamics of perturbation fields in these black-hole spacetimes are mainly determined by the nearhorizon properties of these geometries.