Black Hole Scalarization Overview

• Black hole scalarization is a phenomenon where GR black hole solutions acquire nontrivial scalar hair due to tachyonic instabilities from nonminimal curvature couplings.
• The mechanism circumvents classical no-hair theorems by triggering exponential growth of a scalar field via interactions with invariants like the Gauss–Bonnet term.
• Observable implications include altered shadow sizes, quasinormal mode shifts, and thermodynamic favoritism of scalarized branches over standard GR solutions.

Black hole scalarization refers to a class of nonperturbative phenomena in scalar-tensor gravity, whereby a black hole that is a solution of general relativity (GR) becomes unstable and develops nontrivial scalar “hair” when certain criteria involving spacetime curvature or environmental effects are met. This mechanism circumvents classical no-hair theorems and is realized in a range of extended gravity models, including Einstein-scalar-Gauss-Bonnet (EsGB) theories, theories with nonminimal matter couplings, effective field theories with higher-curvature terms, and models motivated by string theory, supergravity, or dark sector physics.

1. Fundamental Mechanism and Linear Onset

The archetypal scalarization scenario involves a real scalar field $\phi$ nonminimally coupled to a curvature invariant (typically the Gauss–Bonnet invariant $\mathcal{G}$) via an interaction term $f(\phi)\mathcal{G}$ in the Lagrangian. For black holes, the defining structure is the linearized equation for small scalar perturbations around a GR background: $$(\Box - \mu_{\rm eff}^2)\,\delta\phi = 0\,,\qquad \mu_{\rm eff}^2 = -\frac{1}{2} f''(\phi_0) \mathcal{G}\,,$$ where $\phi_0$ is the background value, $f''(\phi_0)$ is the second derivative of the coupling at the background, and $\mathcal{G}$ encodes local spacetime curvature (Blázquez-Salcedo et al., 2021, Macedo et al., 2019). A necessary (though not always sufficient) condition for spontaneous scalarization is that $\mu_{\rm eff}^2$ is negative in some region outside the black hole horizon.

This tachyonic instability triggers exponential growth of the scalar in the affected region, leading to bifurcation from the GR solution at a model-dependent threshold set by the mass, charge, spin, or environmental parameters. In the simplest cases, the transition is second-order: the scalar hair develops continuously from zero as the relevant threshold is crossed (Blázquez-Salcedo et al., 2021, Macedo et al., 2019).

2. Theories Realizing Black Hole Scalarization

2.1 Gauss–Bonnet and Higher-Curvature Couplings

EsGB gravity augments the action with a real scalar field and quadratic or shift-symmetry-breaking couplings $F(\phi)\mathcal{G}$ to the Gauss–Bonnet scalar. For $F(0) = 0$ and $F'(0) = 0$, the GR solution persists for $\phi=0$, but the effective mass criterion above is generically satisfied for sufficiently small black hole masses (high curvature). Explicitly, for Schwarzschild $R^2_{\rm GB} = 48M^2/r^6 > 0$, so e.g. with $F(\phi) = \eta\phi^2/2$ and $\eta > 0$, the threshold mass for scalarization is $M_c \sim \lambda$ with dimensionful coupling $\lambda$ (Blázquez-Salcedo et al., 2021, Macedo et al., 2019).

Higher-derivative extensions, such as Starobinsky–Gauss–Bonnet gravity or inclusion of $R^2$ or $R_{\mu\nu} R^{\mu\nu}$ terms, admit massive or self-interacting scalars. Mass and quartic self-interaction terms shift thresholds, regularize solutions, and stabilize against radial perturbations (Macedo et al., 2019, Liu et al., 2020).

2.2 Gauss–Bonnet, Ricci, and Mixed Invariants

Complementary to Gauss–Bonnet, coupling the scalar to the Ricci scalar $R$ via $\phi^2 R$ allows for scalarization sourced by trace anomalies (breakdown of classical scale invariance), or in matter environments where $R\neq 0$ (e.g. nonvacuum, quantum-corrected spacetimes) (Antoniou et al., 2021, Herdeiro et al., 2019). The Ricci coupling tends not to affect linear scalarization thresholds for vacuum black holes but controls the far-zone properties and the non-linear saturation of the hair. Mixed couplings to $R$ and $\mathcal{G}$ can tailor the domain of scalarization and reconcile cosmological and binary-pulsar constraints (Antoniou et al., 2021).

2.3 Nonlinear Electrodynamics, Higher-Derivative Gauge Terms, and Extended Scenarios

Scalarization also occurs in black holes supported by nonlinear electrodynamics (NLED) or higher-derivative gauge field corrections (e.g., Euler–Heisenberg terms or $\mathcal{P}^2$, where $\mathcal{P}=F_{\mu\nu}F^{\mu\nu}$) (Contreras et al., 3 Nov 2025, Zhang et al., 9 Oct 2025, Kiorpelidi et al., 2023). The nonminimal coupling of a scalar to the electromagnetic field, or to its nonlinear corrections, induces a similar tachyonic instability, with a critical charge (or magnetic field) threshold for scalarization. New phenomena include infinite branches of hairy solutions labeled by the number of scalar field nodes, entropy enhancement, and extension of the scalarized domain to overcharged black holes.

Spin also enters as a trigger. When the curvature source is such that only $f''(0)<0$ allows instability, a minimum critical spin parameter is required, giving rise to "spin-induced" scalarization of Kerr black holes (Doneva et al., 2020, Doneva et al., 2022).

2.4 Teleparallel and Torsion-Driven Mechanisms

In teleparallel gravity, "Teleparallel Gauss–Bonnet" (TsGB) scalarization is triggered by coupling the scalar to torsional analogues of $\mathcal{G}$, yielding fundamentally new scalarized branches with non-monotonic metric or scalar profiles (Bahamonde et al., 2022). The domain-of-existence structures in TsGB differ from their Riemannian counterparts and allow new forms of deviation from GR predictions.

2.5 Environmental and Dark Matter-Induced Effects

Scalarization can also be catalyzed by environmental matter, such as perfect-fluid dark matter halos (Tang et al., 21 Apr 2025). Here, scalar hair for nonrotating black holes develops in parameter regimes where it would otherwise be forbidden (e.g., "GB$^-$" regime), provided the dark matter density parameter $b/M$ exceeds a sharp critical threshold.

3. Nonlinear Solution Structure, Stability, and Thermodynamics

The fully nonlinear scalarized black hole is typically constructed by integrating the field equations with a radial ansatz subject to regular horizon and asymptotic boundary conditions. The scalar field profile $\phi(r)$ develops a nontrivial structure—nodeless for the fundamental branch, with arbitrarily many nodes for excited branches (Blázquez-Salcedo et al., 2021, Zhang et al., 9 Oct 2025, Contreras et al., 3 Nov 2025).

Stability analyses show the fundamental, nodeless branch is generically stable against radial perturbations, while excited branches are unstable (Macedo et al., 2019, Zhang et al., 9 Oct 2025). The hairy solutions usually possess larger Wald entropy at fixed global charges than their GR progenitors, marking them as entropically favored; free energy comparisons confirm thermodynamic preference for the scalarized branch within coexistence regions (Blázquez-Salcedo et al., 2021, Contreras et al., 3 Nov 2025, Kiorpelidi et al., 2023).

In extended models, phase structure can exhibit rich behavior: multiple branches, second-order (continuous) and first-order (zeroth-order "cave-of-wind") transitions, and bifurcation or merging lines in $(M,Q)$ or extended phase space $(T,P)$ diagrams (Zhao et al., 22 Nov 2025, Brihaye et al., 2019). For example, in asymptotically AdS spacetimes, the full phase structure parallels that of van der Waals fluids, with critical points and "supercritical" transitions (Zhao et al., 22 Nov 2025).

4. Parameter Dependence, EFT Constraints, and Model No-go Results

The onset and domain of scalarization are controlled by combinations of model parameters: coupling constants, horizon mass, spin, charge, curvature, and sometimes environmental density. In the EFT context, integrating out heavy fields to generate higher-curvature $\mathcal{G}^2$ terms rarely produces the precise sign or hierarchy needed for highly selective scalarization regimes, such as theories in which only supermassive black holes scalarize (Thaalba et al., 26 Jun 2025). Canonical setups with multiple scalars, simple heavy field sectors, or standard Higgs-like mechanisms generally cannot arrange the coupling signs and scale separation to prevent scalarization for lower-mass black holes. More intricate or symmetry-constrained model building is required for such targeted scenarios.

5. Astrophysical and Observational Signatures

Scalarized black holes imprint distinctive deviations from GR in both electromagnetic and gravitational observables (Blázquez-Salcedo et al., 2021, Contreras et al., 3 Nov 2025, Kiorpelidi et al., 2023). Shadow size modifications, percent-level shifts in photon sphere and ISCO radii, and changes in the spectrum and damping rates of quasinormal modes are generic, and typically remain within the present observational bounds for natural coupling strengths and charge or spin parameters. In gravitational-wave astronomy, scalar charges of $\mathcal{O}(0.1)$ (in $\sqrt{\eta}$ units) induce shifts in ringdown frequencies and possibly prompt new polarizations or dipole emission channels, especially in binaries where only one component scalarizes. Scalar radiation can also dominate the energy output in stellar core collapse leading to black holes (Kuan et al., 2021).

Nontrivial environmental or topological extensions (e.g., AdS, higher dimensions, NLED) broaden the scope of possible phase transitions and observable predictions. In AdS contexts, scalarization is dual to symmetry-breaking phase transitions in the boundary CFT, closely related to holographic superconductors (Brihaye et al., 2019, Zhao et al., 22 Nov 2025). Teleparallel and dark matter-induced scalarization scenarios provide further possible channels to test modified gravity and matter-sector physics in strong-field environments (Bahamonde et al., 2022, Tang et al., 21 Apr 2025).

6. Generalizations and Theoretical Landscape

Black hole scalarization is a robust, universal strong-gravity phase transition that manifests in a wide variety of gravitational settings: static and rotating black holes, higher-dimensional geometries, asymptotically flat and AdS backgrounds, and with matter or gauge sector involvement (Blázquez-Salcedo et al., 2021, Astefanesei et al., 2020, Contreras et al., 3 Nov 2025, Bahamonde et al., 2022). Its model dependence is reflected in both the phenomenology (e.g., charge gap, overcharging, thermodynamic behavior) and the requirements for the threshold (e.g., curvature sign, presence of matter, boundary terms).

A major open direction is the precise mapping of scalarized solution families, stability regions, and observable signatures across the extended landscape of scalar-tensor and higher-derivative gravities. On the computational front, stable and efficient numerical frameworks (e.g., fixing-the-equations approaches for nonlinear evolution in sGB theory) have been developed to enable dynamical and fully coupled simulations (Lara et al., 13 Mar 2024).

For targeted scenarios, such as exclusive scalarization for supermassive black holes, simple EFT or multi-scalar models have been shown to be inadequate, requiring additional dynamics, specific sign structures, or symmetry protection beyond minimal setups (Thaalba et al., 26 Jun 2025).

7. Table: Representative Scalarization Models and Key Features

Model/Class	Trigger	Theory Structure	Scalarization Threshold	Thermodynamics
EsGB (quadratic, static)	Curvature	$f(\phi)\mathcal{G}$ with $f''(0)>0$	$M/M_\mathrm{pl} < \mathrm{const}\cdot\lambda$	Hair branch entropically favored
EsGB (spin-induced)	Spin	$f''(0)<0$, Kerr background	$a/M > j_c(\lambda)$	Hair branch for fast rotators
NLED-coupled scalar	Charge	$f(\phi)L(F)$, $L(F)$ nonlinear	$Q/M > q_c(\alpha)$	Multiple branches, stable $n=0$
Ricci-coupling, $R\neq 0$	Anomaly/environment	$-\xi\phi^2R$	Discrete $\xi_n$ for $R\neq 0$	Hair branch entropically favored
Teleparallel Gauss–Bonnet	Torsion	$f_i(\psi)T_G$, $f_i(\psi)B_G$	Model-dependent on $\alpha_i$	Non-monotonic solutions
Higher derivatives, gauge	$F^4$, $\mathcal{P}^2$	$f(\phi)\mathcal{P}^2$	$Q/M > q_c(\alpha, \beta)$	Hair branch overcharged, favored
AdS topological	Topology, T,P	Charged scalar + Maxwell in AdS	$T<T_c(k,P)$	1st, 2nd, $0$th order, COW
Dark matter-induced	Matter	$S_\mathrm{gravity} + \mathcal{L}_\mathrm{PFDM}$	$b/M > 1.86287$	New regime: DM-dependent

All key results above are drawn from the referenced literature, including (Blázquez-Salcedo et al., 2021, Macedo et al., 2019, Thaalba et al., 26 Jun 2025, Contreras et al., 3 Nov 2025, Zhang et al., 9 Oct 2025, Doneva et al., 2022, Zhao et al., 22 Nov 2025, Brihaye et al., 2019, Kiorpelidi et al., 2023, Lara et al., 13 Mar 2024, Astefanesei et al., 2020, Antoniou et al., 2021, Bahamonde et al., 2022, Tang et al., 21 Apr 2025, Herdeiro et al., 2019).