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Title: Do black holes carry scalar hair from spontaneous scalarization?
Plain: In some theories a black hole becomes surrounded by a cloud of a new scalar field once it is small enough or spins fast enough. This would change how binaries emit gravitational waves.
Precise: In scalar-Gauss-Bonnet theories with $f(\phi)$ allowing spontaneous scalarization (Doneva and Yazadjiev 2018; Silva et al. 2018), determine from binary inspirals (dipole radiation entering at $-1$ post-Newtonian order) and ringdowns whether scalarized black holes exist, by constraining the coupling length $\sqrt{\alpha}$ relative to observed black hole masses. An answer is a detection of dipole radiation or a bound on $\sqrt{\alpha}$ that excludes scalarization for all observed masses.
Settled by: Gravitational-wave constraints on $\sqrt{\alpha}$ below the horizon scale of the lightest observed black holes, or a detection of scalar dipole emission.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: open
Status note: Current bounds on $\sqrt{\alpha}$ from binary inspirals are approximately of order kilometers, leaving scalarization of the lightest black holes allowed as of 2026.
Posed since: 2018