Spontaneous scalarization of Gauss-Bonnet black holes: Analytic treatment in the linearized regime

It has recently been proved that nontrivial couplings between scalar fields and the Gauss-Bonnet invariant of a curved spacetime may allow a central black hole to support spatially regular scalar hairy configurations. Interestingly, former numerical studies of the intriguing black-hole spontaneous scalarization phenomenon have demonstrated that the composed hairy black-hole-scalar-field configurations exist if and only if the dimensionless coupling parameter $\overline{\ensuremath{\eta}}$ of the theory belongs to a discrete set ${[{\overline{\ensuremath{\eta}}}_{n}^{\ensuremath{-}},{\overline{\ensuremath{\eta}}}_{n}^{+}]{}}_{n=0}^{n=\ensuremath{\infty}}$ of scalarization bands. We have examined the numerical data that are available in the physics literature and found that the newly discovered hairy black-hole-linearized-massless-scalar-field configurations are characterized by the asymptotic universal behavior ${\mathrm{\ensuremath{\Delta}}}_{n}\ensuremath{\equiv}\sqrt{{\overline{\ensuremath{\eta}}}_{n+1}^{+}}\ensuremath{-}\sqrt{{\overline{\ensuremath{\eta}}}_{n}^{+}}\ensuremath{\simeq}2.72$. Motivated by this intriguing observation, in the present paper we study analytically the physical and mathematical properties of the spontaneously scalarized Schwarzschild black holes in the linearized (weak-field) regime. In particular, we provide a remarkably compact analytical explanation for the numerically observed universal behavior ${\mathrm{\ensuremath{\Delta}}}_{n}\ensuremath{\simeq}2.72$ which characterizes the discrete resonant spectrum ${{\overline{\ensuremath{\eta}}}_{n}^{+}{}}_{n=0}^{n=\ensuremath{\infty}}$ of the composed hairy black-hole-linearized-scalar-field configurations.

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