Splitting the Gravitational Atom: Instabilities of Black Holes with Synchronized/Resonant Hair

Abstract

Black holes (BHs) with synchronized bosonic hair challenge the Kerr paradigm, linking superradiance from ultralight fields -- creating gravitational atoms -- to bosonic stars across parameter space. In the ''very hairy'' regime, where a small horizon lies inside a bosonic star containing most of the energy, they deviate sharply from Kerr, but their dynamics remain unexplored. We show that for such solutions the horizon gets naturally ejected from the center of its scalar environment, and observe a similar dynamics in a cousin model of BHs with resonant scalar hair, albeit with a different fate. This dynamical splitting is likely to be generic for sufficiently hairy BHs in the broader class of models with synchronized or resonant hair, but possible exceptions may exist.

Figures (9)

• Figure 1: (Left) BHsSH in the simplest scalar model: a horizon inside a toroidal boson star. (Right) A thin ring of constant mass density (bottom) and its Newtonian potential (top).
• Figure 2: The BHSH configuration C spirals out from the origin, disrupts the scalar field and absorbs most of it when reaching high density regions. (Top) Snapshots of the density of the scalar field in the $xy$ plane. The horizon trajectory is depicted as solid black arrows. They are ordered from left to right, top to bottom. (Bottom) Time evolution of the absolute value of the Cartesian coordinates and equatorial radius $\rho\equiv\sqrt{x^2+y^2}$ of the puncture, in log scale.
• Figure 3: Mass transfer between the BH and scalar field. The most dynamical phase occurs between $t \approx 700$ and $1500$.
• Figure 4: Fraction of scalar energy (main panel) and total spacetime energy $E$ (inset) vs.$\omega$ for sequences of solutions with different $r_H$. Two connected branches arise, i.e. two solutions for the same $\omega$. The dots mark the solutions numerically evolved.
• Figure 5: Snapshots of the density of the scalar field in the $xy$ plane. The BH is expelled by the hair, leaving behind a stable boson star with non-zero linear momentum.