REVIEW 4 major objections 4 minor 37 references
Strong scalar-field collapse can destroy the Hayward black hole's regular core, driving the inner horizon to zero with a universal power-law exponent near 0.5.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-03 19:35 UTC pith:7HOZI57C
load-bearing objection Plausible and useful numerical study, but the central scaling law needs a coordinate-invariant horizon extraction and convergence tests before it can be trusted. the 4 major comments →
Internal structure of Hayward black holes
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that the Hayward inner horizon is not stable against strong scalar-field collapse. With weak scalar data, the inner horizon settles to a finite radius while the Misner–Sharp mass inflates there; with strong data, the inner horizon contracts to r = 0, σ and φ diverge, the Kretschmann scalar diverges at the core, and the near-singularity field equations reduce to the same asymptotic form as collapse to a Schwarzschild black hole, indicating a spacelike singularity. Approaching the critical amplitude, width, or position of the initial Gaussian packet, the final inner-horizon radius vanishes as a power law with exponent 0.5. The same scaling is reported to persist for Reissn
What carries the argument
The argument runs on a double-null (Kruskal-like) coordinate setup, ds² = 4e^{-2σ} du dv + r² dΩ², in which the Hayward metric is nonsingular and the collapse is evolved as coupled nonlinear PDEs for r, σ, and the scalar field φ, with a Gaussian initial profile parameterized by amplitude A, width B, and center x0. The central diagnostic is the final inner-horizon radius r_- extracted from the evolved r(t,x). The near-critical scaling r_- ∝ |p-p*|^γ is established by numerical fitting over three separate parameter directions, and the spacelike nature of the endpoint is inferred from the asymptotic dominance of the same terms that appear in neutral scalar collapse to a Schwarzschild black hole
Load-bearing premise
Everything rests on the numerical evolution being a faithful solution of the field equations: no convergence or resolution study is given, and the reported r_- is read from a fixed spatial slice, so if discretization error or slice dependence changes the near-threshold values, the scaling law and the spacelike-singularity conclusion are unsupported.
What would settle it
Repeat the strong-field runs with twice and four times the grid resolution and with smaller time steps; if the extracted r_- at fixed p shifts by more than the fitting uncertainty, or if choosing a different spatial slice than x=1.2 changes the fitted γ away from 0.5, the central scaling claim is numerical artifact rather than spacetime physics. Also, if a non-zero r_- is found for any p>p* under better resolution, the claimed threshold is not genuine.
If this is right
- If the critical scaling is real, the Hayward spacetime has a threshold: below it the inner horizon is finite, above it the core collapses to a curvature singularity.
- The exponent γ ≈ 0.5 being independent of whether one varies amplitude, width, or position points to a true critical phenomenon in the black-hole interior, not an artifact of one family of data.
- Strong scalar collapse produces a spacelike singularity and Schwarzschild-like geometry, so regular black holes of Hayward type do not by themselves resolve the singularity problem in dynamical collapse.
- Mass inflation at the inner horizon persists even for weak perturbations, meaning the Cauchy horizon remains a locus of instability.
- Because the same scaling appears for Reissner–Nordström interiors, the result likely extends to charged black holes and may unify how Cauchy horizons respond to infalling matter.
Where Pith is reading between the lines
- A testable extension is to derive the 1/2 exponent analytically from the near-singularity equations; if the exponent follows solely from those asymptotic equations, it should hold for any regular black hole whose interior approaches the same form.
- Treating the inner-horizon radius as an order parameter, the collapse of a regular black hole to a singular state resembles a continuous phase transition, and the critical exponent could be measured in independent numerical relativity codes as a consistency check.
- Observationally, if astrophysical black-hole candidates were Hayward-like, strong infall would destroy the regular core, so the absence of such instability in observed systems would constrain the Hayward scale parameter or the existence of regular cores.
- A sharper test would run the same collapse with non-Gaussian or asymmetric initial data: if γ changes, the scaling is tied to the profile family, whereas if it stays 0.5 it is universal.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies nonlinear collapse of a massless scalar field on a Hayward regular black hole background using a double-null (t,x) numerical evolution scheme. The authors present equations of motion for r, σ, and ϕ derived from an Einstein-nonlinear-electrodynamics action with a scalar field, and report three main results: (i) weak scalar-field perturbations leave the inner horizon at a finite, though contracted, radius; (ii) sufficiently strong scalar fields drive r to zero at a fixed spatial slice, with divergences in σ, ϕ, the Misner–Sharp mass, and the Kretschmann scalar, which are interpreted as formation of a spacelike singularity; and (iii) near a critical threshold p* for the initial Gaussian profile parameters A, B, and x0, the final inner-horizon radius satisfies r− ∝ |p − p*|^γ with γ ≈ 0.5. The central quantitative claim is Eq. (28) and Fig. 7.
Significance. If established, the result would be significant: it would show that a canonical regular black hole model is not dynamically singularity-free under scalar-field collapse, and it would identify a new critical-scaling law for the black-hole interior. The paper also usefully extends the double-null formalism of Ref. [33] to a regular black hole background, and the derived equations of motion (15)–(17) appear plausible and internally consistent. However, the three headline claims rest entirely on a numerical simulation for which no convergence tests, resolution details, or gauge-invariant horizon extraction are presented. The inner-horizon radius is read off from a fixed-x slice rather than from a geometric trapping-horizon condition, and the claimed spacelike character of the singularity is inferred only by analogy with previous work. These omissions leave the main quantitative conclusion unsupported pending additional validation.
major comments (4)
- [§III.B and Fig. 4; Eq. (23)] The inner-horizon radius r− used in the scaling law is never defined geometrically. In §III.B and the Fig. 4 caption the 'inner horizon' is identified with r on the fixed spatial slice x = 1.2, and Fig. 7/Eq. (28) evidently use the same extraction rule. For a spherically symmetric dynamical spacetime the inner horizon is a trapping horizon where the null expansions vanish, equivalently g^{ab} r_{,a} r_{,b} = e^{2σ}(−r_t^2 + r_x^2) = 0; a fixed-x worldline is not such a surface. Since the (t,x) gauge is fixed by the arbitrary initial condition (21), the value r(t, x = const) has no invariant meaning. A coordinate/stopping-time artifact could mimick power-law scaling, especially near criticality where the approach to the final state slows. Please provide an invariant extraction of r−, or at minimum demonstrate slice-independence of the values used in Fig. 7.
- [§II and §III.C; Fig. 7] No numerical convergence study is reported. The leapfrog scheme, the extrapolated boundaries, the domain [−5,5], and the time-step initialization by Taylor expansion are only referenced to [33]; no grid resolution, time-step size, domain-size variation, or convergence-factor test is given. Near a critical threshold the relaxation time diverges, so without a resolution and domain study the values of r− used in Eq. (28) may be contaminated by discretization and finite-domain effects. The critical values A*, B*, x0* are quoted to 11–12 digits, yet no error bars, fit residuals, or even the numerical values of r− entering the fit are reported. A resolution/convergence study and an uncertainty estimate for γ are required to support the central claim.
- [§III.B and Fig. 5–6] The claim that a spacelike singularity forms is not demonstrated. What is shown is that along the x = 1.2 slice r approaches zero and terms in the equations of motion (15)–(17) simplify to (25)–(27), which are said to resemble the collapse studied in Refs. [33–36]. Striking asymptotic similarity on one coordinate slice is not sufficient to establish the causal character of the resulting singularity. Please provide a geometric diagnosis: e.g., behavior of null geodesics, the location of apparent horizons, or an argument based on the full spacetime structure, rather than an analogy to previous solutions.
- [Eq. (28) and §III.C] The universality of the scaling law is not testable from the data presented. The exponents are quoted as 0.49997, 0.500521, and 0.499601 for A, B, and x0 respectively, but no fitting procedure is described, no uncertainties are given, and no tabulated r− versus p data are shown. Moreover, the text states that for p > p* the inner horizon 'vanishes' while for p < p* it is finite, but Fig. 7 plots |p − p*| without indicating whether the two sides behave symmetrically or whether only one side is used. Please provide the fitting details, the data, and a discussion of systematic uncertainties before claiming a clean γ ≈ 1/2 law.
minor comments (4)
- [§III.A, Fig. 2 caption] The caption says the evolution is along the x = 0.5 slice, while §III.B and Fig. 4 use x = 1.2. The choice of slice is not justified; if the purpose is to probe the inner horizon, an invariant justification would help.
- [§III.A, §III.B] Figures 1 and 3 would benefit from a clearer statement of which panels correspond to which quantity and what the color gradient means; the repeated list of time labels in the captions is unwieldy. Also, Fig. 6 does not label which curves correspond to which term in Eqs. (15)–(17), making the 'negligible' assertion hard to verify.
- [Eq. (10)–(13)] The notation is occasionally ambiguous: the parameter s in Eq. (10) is not the same as the action in Eq. (1), and ρ0, ρ1, ρ2 are defined with a semicolon-like notation. Please use distinct symbols for the action and the Hayward parameter, and define all symbols at first use.
- [General] The paper cites Refs. [28,29] only in passing as 'the question remains open'; a more substantive discussion of the stability debate and how this work contributes to it would help position the result. Also, the companion paper [37] is mentioned only in the last sentence; if its results are used, they should be stated explicitly in the main text.
Circularity Check
No significant circularity: the scaling law (28) is a numerically measured relation, not a consequence of an input that contains it.
full rationale
The central claim, Eq. (28), is obtained by evolving the coupled Einstein-scalar system (15)-(17) from independent Gaussian initial data (20) and then fitting the late-time inner-horizon radius r_- as a function of the initial-profile parameter p. Neither p nor p_* is defined in terms of r_-, and no fitted parameter is renamed as a prediction; the scaling exponent gamma~0.5 is an output of the fitting, not an input. The self-citations are to a numerical implementation [33], a related spherical-collapse study [34], and a companion paper [37] reporting the analogous Reissner-Nordström result; none of them supplies the Hayward scaling law, and the 'spacelike singularity' inference is supported also by independent references [35,36]. The paper's own caveats (extrapolated boundaries, reference to [33] for numerical details, extraction along a fixed slice x=1.2) are legitimate numerical-validation concerns, but they concern whether the measurement is correct and robust, not whether the conclusion is assumed in the premises. No equation in the paper reduces to another by construction, so there is no significant circularity.
Axiom & Free-Parameter Ledger
free parameters (5)
- q (magnetic charge) =
0.5
- r+ (outer horizon radius) =
3
- A* (critical amplitude) =
0.078606364025
- B* (critical width) =
0.978312460101
- x0* (critical position) =
1.008831954973
axioms (5)
- domain assumption Hayward metric (Eq. 10) is a valid regular black hole solution sourced by the nonlinear electrodynamics Lagrangian L(1) from [7].
- domain assumption Massless scalar field minimally coupled to gravity, obeying the wave equation, is a valid matter model for the perturbation.
- domain assumption Spherical symmetry and the double-null / Kruskal-like coordinate ansatz cover the spacetime including the interior and near-singularity region.
- ad hoc to paper The leapfrog finite-difference scheme with extrapolated boundary conditions converges to the continuum solution on the domain [-5,5].
- ad hoc to paper The final-state inner horizon radius r_- is well-defined and can be extracted from a fixed spatial slice independently of the choice of slice.
read the original abstract
Regular black holes, free of central singularities, provide an ideal laboratory for probing the geometric structure of spacetime. The global structure of some regular black holes, e.g. Hayward black hole, features an event horizon and a Cauchy horizon, raising fundamental questions about the latter's stability. In this work, we investigate collapse of a scalar field in Hayward spacetime. Under weak scalar perturbations, the inner horizon maintains a stable finite radius. In the circumstance of a strong scalar field, the inner horizon shrinks to zero volume, accompanied by the formation of a spacelike singularity. The Hayward geometry is effectively converted into a Schwarzschild-like geometry. Furthermore, the strength of the scalar field governs the contraction dynamics of the inner horizon. As the parameter $p$ of the initial profile for the scalar field approaches the critical threshold ${p_*}$, the radius of the inner horizon ${r_{-}}$ exhibits a universal scaling behavior: ${r_{-}}\propto{|p - {p_*}|^\gamma}$, with a critical exponent $\gamma\approx 0.5$.
Figures
Reference graph
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discussion (0)
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