Pith. sign in

REVIEW 3 major objections 4 minor 39 references

Evading Cauchy Horizon Excision in Scalarized Regular Black Holes

T0 review · 3 major / 4 minor · reviewed 2026-08-05 · deepseek-v4-flash

Pith's one-line read Scalarization need not erase a black hole's inner horizon

desk verdict Core idea is right—sign-definite L_EM is what kills the Cauchy horizon—but the printed proof has a sign error, the example has a dimensional slip, and the title oversells a necessary condition. read the letter →

arxiv 2608.00557 v1 pith:KUY5FILS submitted 2026-08-01 gr-qc astro-ph.HEhep-th

classification gr-qcastro-ph.HEhep-th MSC 83C5783C75 PACS 04.70.-s04.70.Bw
keywords spontaneousscalarizationCauchyhorizonregularblackholesnonlinearelectrodynamicsP-dualformulationnoinner-horizontheoremmassinflation
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper asks whether spontaneous scalarization, the dynamical growth of a scalar field around a bald black hole, necessarily destroys the inner (Cauchy) horizon of a regular charged black hole. Earlier results said yes for Einstein-Maxwell sources. The paper shows the obstruction is not universal: within the P-dual formulation of nonlinear electrodynamics, a scalarized configuration can keep its Cauchy horizon only if the effective electromagnetic Lagrangian L_EM changes sign between the inner and outer horizons. If L_EM is sign-definite, as in Maxwell theory, the integrated scalar equation plus the usual nodeless scalar-profile conditions force a contradiction at the event horizon. The paper gives a concrete NLED model whose source flips sign at an analytically determined radius, and stresses that the sign change is necessary but not sufficient for a regular scalarized solution.

What carries the argument

The load-bearing identity is Eq. (30), the integral of the scalar equation between adjacent horizons: the integral of r^3 N phi'^3 equals the integral of r^2 f,phi L_EM. Because N < 0 in that interval, the equation becomes a balance condition between the scalar kinetic contribution and the electromagnetic source, so the sign of L_EM decides whether a Cauchy horizon can survive. The other central object is the P-dual (Hamiltonian) formulation of nonlinear electrodynamics, in which the field is described by the conjugate tensor P_munu = L_F F_munu and the metric determines the Hamiltonian through H(P(r)) = -m'(r)/r^2. In this language L_EM = 2P H_P - H, so its sign depends on the relative size

What would settle it

Take the Einstein-Maxwell-scalar action of Ref. [8] and numerically solve the static boundary-value problem (25)-(28) between r- and r+ with N(r-) = N(r+) = 0, a regular center, and a nodeless scalar profile. If a nontrivial scalarized solution with both horizons exists, the claimed sign-definite obstruction is false. Conversely, solve the same boundary-value problem for the model (41) with the sign-flip condition r^-4 = tilde_Upsilon e^(|alpha| phi^2) satisfied; if no regular solution is found, the sign-change criterion is necessary but not sufficient.

Watch

Extended reading notes

Core claim

The central structural claim is that the disappearance of the Cauchy horizon under scalarization is governed by the sign structure of the electromagnetic source, not by scalarization itself. The paper integrates the scalar-field equation between the Cauchy horizon r- and event horizon r+ to obtain the identity integral of r^3 N phi'^3 equals integral of r^2 f,phi L_EM, with N < 0 throughout the interval. For a nodeless scalar branch satisfying the usual Bekenstein sign conditions, a strictly negative L_EM would force phi' to have opposite signs inside and outside r+, so phi would peak at the event horizon; evaluating the equation at r+ then demands L_EM(r+) = 0, contradicting sign-definitene

Load-bearing premise

The whole no-go argument rests on the inference that a sign-definite electromagnetic source forces the scalar field to have opposite slopes inside and outside the event horizon, so it must peak exactly at the horizon; if that monotonicity step fails, the contradiction does not follow as written.

Editorial extensions

If this is right

  • In Maxwell-like scalarization, the Cauchy horizon of a regular charged black hole is generically excised; a necessary ingredient for avoiding this is an electromagnetic source that is not sign-definite between the horizons.
  • For NLED theories in the P-dual picture, a sign-flipping L_EM gives a concrete route to scalarized configurations that retain r-, so inter-horizon dynamics and mass inflation, controlled by kappa- = (1/2)|N'(r-)|, become well-posed questions.
  • The derived condition is necessary, not sufficient; a full nonlinear solution with regular boundary conditions at both r- and r+ is still required.
  • The mechanism is tied to the electric sector; purely magnetic configurations lack the H-versus-H_P competition needed to flip the sign, so the evasion does not directly extend there.
  • A practical check for whether a given model can evade the no-go reduces to the geometry and scalar profile via L_EM = 2(G^theta_theta + grad_alpha phi grad^alpha phi)/f(phi), without needing the electromagnetic fields explicitly.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Not stated in the paper: if the sign-change condition becomes the organizing criterion, the space of NLED Lagrangians admitting scalarized regular black holes can be surveyed by locating zeros of 2P H_P - H, a purely algebraic condition per model.
  • A sharper test would be to build an explicit two-horizon solution for the illustrative model; if no such solution exists, the integral constraint would remain necessary but empty.
  • The same integral-constraint technique could be applied to distinguish stable nodeless scalar branches from excited nodal ones, with the expected instability of nodal profiles providing an additional selection rule.
  • The paper's framing suggests treating the coupled scalar-electromagnetic system as fundamental: if the pure-sector L(F) becomes multivalued in P-dual variables, the scalar coupling f(phi)L_EM may smooth the branch structure, but this is left open.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. The paper considers spontaneous scalarization of regular black holes in scalar-tensor theories with nonlinear electrodynamics (NLED), specifically asking whether the Cauchy horizon can survive scalarization. Working in the P-dual Hamiltonian formulation, the authors derive an integrated constraint from the scalar-field equation and argue that, for the fundamental nodeless branch, a sign-definite electromagnetic Lagrangian L_EM (as in Maxwell) is incompatible with a two-horizon regular scalarized solution. They then prove a necessary condition for survival: L_EM must change sign in the inter-horizon region. A particular P-dual model is used to illustrate this sign flip with an analytic zero of the effective source. The paper concludes that Cauchy horizon excision is not universal and identifies candidate theories for which scalarized Cauchy horizons may persist, while explicitly deferring full solution construction to future work.

Significance. If established, the central structural message would be useful: it would show that the no-inner-horizon theorems for scalarized charged black holes depend on the sign properties of the electromagnetic sector, and it would focus future searches on NLED models with sign-changing L_EM. The integral-constraint approach is clean and the P-dual framework is well suited to the problem. However, the paper as written contains a sign error in the main no-go proof and an internally inconsistent illustrative model, and it stops short of demonstrating existence. These issues mean the advertised claim of 'evading' Cauchy horizon excision is not yet supported.

major comments (3)
  1. [Sec. IV, Eqs. (31)–(39)] The principal no-go argument contains a sign error. With F=e^{-\delta}r^2 N \phi', Eq. (28) and L_EM<0 imply F'<0, not F<0 globally. Since F(r_+)=0 and F'<0, one has F>0 for r<r_+ and F<0 for r>r_+. Because N<0 between the horizons, this gives \phi'<0 in (r_-,r_+), reversing Eq. (33). Consequently \phi' need not vanish at r_+, and Eq. (35) and the contradiction in Eqs. (38)–(39) do not follow as written. The no-go conclusion can be rescued: F(r_-)=F(r_+)=0 with F'<0 everywhere contradicts Rolle's theorem, so a sign-definite L_EM<0 already forbids two-horizon solutions. But that argument is not the one presented, and the printed derivation is invalid.
  2. [Sec. IV, Eqs. (41)–(49)] The illustrative model is internally inconsistent. Eq. (42) defines X=\Upsilon(-P)^{3/4}, while Eq. (45) gives P=-Q^2/(2f^2r^4). Therefore X=\Upsilon(Q^2/(2f^2r^4))^{3/4} \propto r^{-3}, not X=\Upsilon Q^2/(2f^2r^4) as stated in Eq. (46). The subsequent condition (47) and the analytic zero (49), which are built on the 1/r^4 form, do not follow from the stated definitions. In addition, the derivative H_P in Eq. (43) is not compatible with H(P)=P(1+X)^{-4/3}; direct differentiation with X as defined yields (1+2X)/(1+X)^{7/3}, which changes the expression for L_EM and the claimed sign-flip condition X>1. The illustrative example therefore does not demonstrate the mechanism it is intended to illustrate.
  3. [Abstract and Sec. V] The title and abstract claim to 'evade' Cauchy horizon excision and that the obstruction is 'not universal,' but the paper proves only a necessary condition. No scalarized regular black-hole solution with a surviving Cauchy horizon is constructed, and no argument is given that the coupled system (26)–(28) admits such a solution even when L_EM changes sign. The integrated constraint (30) can be satisfied only if signed areas match, but matching is not shown. The concluding limitation statement admits that constructing specific solutions is beyond scope; given that admission, the advertised conclusion overreaches. The paper should either provide a concrete existence argument (or a numerical/perturbative demonstration) or explicitly restrict its claims to a no-go theorem plus a necessary condition.
minor comments (4)
  1. [Sec. IV, text near Eq. (30)] Typo: 'he same derivation' should be 'the same derivation'.
  2. [Fig. 1 caption] The caption refers to 'condition (39)' as necessary for a Cauchy horizon, but Eq. (39) is a derived contradiction for sign-definite L_EM; the intended necessary condition is the sign-change condition discussed later. The reference should be corrected.
  3. [Sec. IV, Eq. (52)] The practical check via Eq. (52) is interesting but the sign of the L_EM obtained from the geometry should be matched with the scalarization condition (23); a one-line consistency check would improve clarity.
  4. [Appendix A] The transcendental equation for the second Ayon-Beato-Garcia model is claimed to be 'straightforwardly extended' but no sign-flip analysis is given for that model; the wording is stronger than the analysis.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: the central necessary condition follows from the paper's own equations and is not an input; the self-citation to Ref. [12] is not load-bearing.

full rationale

The paper's main claim is a necessary condition: for a nodeless scalarized regular black hole to preserve its Cauchy horizon, L_EM must change sign between r_- and r_+. This is derived from the action (19), the scalar equation (28), and the integrated constraint (30), under the stated Bekenstein sign conditions (20). The condition is not assumed or fitted; it is a logical consequence of the sign analysis. The illustrative model (41)-(49) is introduced to exhibit a theory where such a sign flip can occur, and no parameter is fitted to data. The self-citation to Ref. [12] supplies the scalarization framework and Eq. (24), but Eq. (24) is a standard equation of motion rederivable from the action, and the paper even notes and corrects a typo in that reference; it is not load-bearing in a circular sense. External no-go results by Bronnikov and Russo-Townsend are cited as context and explicitly distinguished from the scalar-coupled case. The paper also candidly states that constructing explicit solutions is 'beyond the scope of this paper,' so the claim that the obstruction is 'not universal' is only partially supported: the paper proves a necessary condition but does not construct a surviving-Cauchy-horizon solution. This is a support gap, not circularity. Likewise, the apparent sign inconsistency in Eqs. (31)-(34) and the algebraic mismatch between Eqs. (42)/(45) and Eq. (46) are correctness defects, not cases where a prediction reduces to an input by construction. Overall, no circular step of the enumerated kinds is present; the score 2 reflects only the presence of minor, non-load-bearing self-citation.

Assumptions & free parameters 3 free parameters · 5 assumptions · 0 invented entities

No new particles or forces are introduced. The central claim rests on background geometric assumptions and on the illustrative NLED model's constants.

free parameters (3)
  • Upsilon
    Constant in the illustrative H(P) model. It controls the location of the L_EM sign flip but is not fitted to data.
  • alpha
    Scalar coupling exponent in f=e^{|alpha| phi^2}. It sets the exponential factor in the crossing condition and is a theory input.
  • Q
    Electric charge of the background. It sets the scale of P and the crossing radius, and is an input parameter.
assumptions (5)
  • domain assumption Static, spherically symmetric metric ansatz (1)
    All equations and the integral constraint are derived in this ansatz.
  • domain assumption Regular center implies N -> 1 as r -> 0 and therefore a Cauchy horizon exists
    Used to guarantee r- and to set boundary terms to zero in Eq. (30).
  • domain assumption Fundamental nodeless scalar branch has phi > 0 and satisfies the Bekenstein conditions f,phi phi > 0 and phi f,phi > 0
    Used to fix f,phi > 0 and to exclude nodal excited states.
  • standard math Legendre transform L_EM = 2 P H_P - H is valid for electric configurations
    The P-dual Hamiltonian formalism underlies the sign-change analysis.
  • ad hoc to paper Pure NLED no-go theorems do not apply because the electromagnetic sector is nonminimally coupled as f(phi) L_EM
    Needed to allow the scalarized sector to escape Bronnikov and Russo-Townsend results.

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Pith. "Pith review of Evading Cauchy Horizon Excision in Scalarized Regular Black Holes." pith.science (2026). https://pith.science/paper/KUY5FILS

@misc{pith2026260800557,
  author       = {Pith},
  title        = {Pith review of: Evading Cauchy Horizon Excision in Scalarized Regular Black Holes},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/KUY5FILS}},
  note         = {Machine review of arXiv:2608.00557}
}
abstract

Spontaneous scalarization provides a dynamical mechanism to evade the no-hair paradigm, but it has been argued to generically eliminate the Cauchy horizon in charged black holes. We show that this obstruction is not universal, but instead follows from the sign-definite structure of the Einstein-Maxwell source term. Within the $P$-dual formulation of nonlinear electrodynamics, we derive a general condition under which the effective scalar source changes sign between the horizons, allowing the integral constraint to be satisfied without destroying the Cauchy horizon. This establishes that the fate of the Cauchy horizon depends on the electromagnetic coupling and identifies candidate theories where scalarized horizons may persist. The resulting framework opens the possibility of studying scalarization in the inter-horizon region and its interplay with mass inflation.

Figures

Figures reproduced from arXiv: 2608.00557 by the authors.

Figure 1
Figure 1. FIG. 1. Sketched profiles for the integrand [PITH_FULL_IMAGE:figures/full_fig_p006_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. Schematic illustration of the sign change of the [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗

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Works this paper leans on

39 extracted references · 35 canonical work pages

  1. [1]

    Markus Heusler.Black Hole Uniqueness Theorems. 7 1996

  2. [2]

    M. S. Volkov and D. V. Gal’Tsov. Non-Abelian Einstein- Yang-Mills black holes.Soviet Journal of Experimental and Theoretical Physics Letters, 50(7):346–350, October 1989

  3. [3]

    New black hole solutions with hair.Physics Letters B, 268(3-4):371–376, October 1991

    Serge Droz, Markus Heusler, and Norbert Straumann. New black hole solutions with hair.Physics Letters B, 268(3-4):371–376, October 1991

  4. [4]

    Clare Burrage, Pedro G. S. Fernandes, Richard Brito, and Vitor Cardoso. Spinning black holes with axion hair. Classical and Quantum Gravity, 40(20):205021, October 2023

  5. [5]

    Nonper- turbative strong-field effects in tensor-scalar theories of gravitation.Phys

    Thibault Damour and Gilles Esposito-Farese. Nonper- turbative strong-field effects in tensor-scalar theories of gravitation.Phys. Rev. Lett., 70(15):2220–2223, April 1993

  6. [6]

    Doneva and Stoytcho S

    Daniela D. Doneva and Stoytcho S. Yazadjiev. New gauss-bonnet black holes with curvature-induced scalar- ization in extended scalar-tensor theories.Phys. Rev. Lett., 120:131103, 2018

  7. [7]

    Silva, Jeremy Sakstein, Leonardo Gualtieri, Thomas P

    Hector O. Silva, Jeremy Sakstein, Leonardo Gualtieri, Thomas P. Sotiriou, and Emanuele Berti. Spontaneous scalarization of black holes and compact stars from a gauss-bonnet coupling.Phys. Rev. Lett., 120:131104, 2018

  8. [8]

    Evading Cauchy Horizon Excision in Scalarized Regular Black Holes

    invariants; see Ref. [9] for a review. More recently, spontaneous scalarization has been extended to black holes supported by nonlinear electrodynamics (NLED) ∗ ernesto.contreras@ua.es † pedro.bargueno@ua.es ‡ a.suvorov@uni-tuebingen.de [10, 11] and a general framework for scalarization in reg- ular spacetimes was developed in Ref. [12]. NLED has motivati...

Show all 39 references
  1. [9]

    Doneva, Leonardo G

    Daniela D. Doneva, Leonardo G. Collodel, Constantinos Kr¨ uger, Stoytcho S. Yazadjiev, and Sven Zschocke. Spon- taneous scalarization.Rev. Mod. Phys., 96:015004, 2024

  2. [10]

    Carlos A. R. Herdeiro, Eugen Radu, Nicolas Sanchis- Gual, and Jose A. Font. Spontaneous scalarisation of charged black holes.Phys. Rev. Lett., 121:101102, 2018

  3. [11]

    Conun- 9 drum of regular black holes with nonlinear electromag- netic fields.Phys

    Ana Bokuli´ c, Tajron Juri´ c, and Ivica Smoli´ c. Conun- 9 drum of regular black holes with nonlinear electromag- netic fields.Phys. Rev. D, 113(2):024044, 2026

  4. [12]

    Leonardo Balart and Elias C. Vagenas. Regular black holes with a nonlinear electrodynamics source.Phys. Rev. D, 90:124045, 2014

  5. [13]

    Born and L

    M. Born and L. Infeld. Foundations of the New Field Theory.Proceedings of the Royal Society of London Se- ries A, 144(852):425–451, March 1934

  6. [14]

    Ernesto Contreras, Mikaela Carrasco-Hidalgo, Pedro Bargue˜ no, and Arthur G. Suvorov. General framework for the spontaneous scalarization of regular black holes. Phys. Rev. D, 112(12):124053, 2025

  7. [15]

    Pellicer and R

    R. Pellicer and R. J. Torrence. Nonlinear Electrody- namics and General Relativity.Journal of Mathematical Physics, 10(9):1718–1723, September 1969

  8. [16]

    E. S. Fradkin and A. A. Tseytlin. Quantum string theory effective action.Nuclear Physics B, 261:1–27, January 1985

  9. [17]

    No cauchy horizon theorem for nonlinear electrodynamics black holes with charged scalar hairs.Phys

    Yu-Sen An, Li Li, and Fu-Guo Yang. No cauchy horizon theorem for nonlinear electrodynamics black holes with charged scalar hairs.Phys. Rev. D, 104:024040, 2021

  10. [18]

    Instability of Nonsingular Black Holes in Nonlinear Electrodynamics

    Antonio De Felice and Shinji Tsujikawa. Instability of Nonsingular Black Holes in Nonlinear Electrodynamics. Phys. Rev. Lett., 134(8):081401, February 2025

  11. [19]

    On the viability of regular black holes.Journal of High Energy Physics, 2018(7):23, July 2018

    Ra´ ul Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Costantino Pacilio, and Matt Visser. On the viability of regular black holes.Journal of High Energy Physics, 2018(7):23, July 2018

  12. [20]

    Hennigar, and David Kubiznak

    Tom´ aˇ s Hale, Robie A. Hennigar, and David Kubiznak. Excising Cauchy horizons with nonlinear electrodynam- ics.Phys. Rev. D, 113(6):L061502, 2026

  13. [21]

    Cauchy Horizon (In)Stability of Regular Black Holes

    Alfio Bonanno, Antonio Panassiti, and Frank Saueres- sig. Cauchy Horizon (In)Stability of Regular Black Holes. arXiv e-prints, page arXiv:2507.03581, July 2025

  14. [22]

    J. Ovalle. Schwarzschild black hole revisited: Before the complete collapse.Physical Review D, 109(10):104032, May 2024

  15. [23]

    Salazar, A

    H. Salazar, A. Garcia, and J. Plebanski. Duality rota- tions and type d solutions to einstein equations with non- linear electromagnetic sources.Journal of Mathematical Physics, 28:2171, 1987

  16. [24]

    Pellicer and R

    R. Pellicer and R. J. Torrence. Nonlinear electrodynamics and general relativity.Journal of Mathematical Physics, 10:1718, 1969

  17. [25]

    Ayon-Beato and A

    E. Ayon-Beato and A. Garcia. New regular black hole solution from nonlinear electrodynamics.Phys. Lett. B, 464:25, 1999

  18. [26]

    Ayon-Beato and A

    E. Ayon-Beato and A. Garcia. Regular black hole in general relativity coupled to nonlinear electrodynamics. Phys. Rev. Lett., 80:5056, 1998

  19. [27]

    A Primer on Energy Conditions.Einstein Stud., 13:43–104, 2017

    Erik Curiel. A Primer on Energy Conditions.Einstein Stud., 13:43–104, 2017

  20. [28]

    Dymnikova and E

    I. Dymnikova and E. Galaktionov. Regular rotating elec- trically charged black holes and solitons in non-linear electrodynamics minimally coupled to gravity.Class. Quant. Grav., 32:165015, 2015

  21. [29]

    J. Ovalle. Interior Dynamics of Regular Schwarzschild Black Holes.arXiv: 2509.00816

  22. [30]

    Regular Black Holes: A Short Topic Review.Interna- tional Journal of Theoretical Physics, 62(9):202, Septem- ber 2023

    Chen Lan, Hao Yang, Yang Guo, and Yan-Gang Miao. Regular Black Holes: A Short Topic Review.Interna- tional Journal of Theoretical Physics, 62(9):202, Septem- ber 2023

  23. [31]

    Schwarzschild black hole singularity formation

    Jorge Ovalle, Roberto Casadio, and Alexander Kamen- shchik. Schwarzschild black hole singularity formation. Phys. Rev. D, 113(6):064042, 2026

  24. [32]

    Suvorov and Pedro Bargue˜ no

    Arthur G. Suvorov and Pedro Bargue˜ no. Doubly regular black holes.Physical Review D, 112(4):044027, August 2025

  25. [33]

    No Inner- Horizon Theorem for Black Holes with Charged Scalar Hairs.JHEP, 03:263, 2021

    Rong-Gen Cai, Li Li, and Run-Qiu Yang. No Inner- Horizon Theorem for Black Holes with Charged Scalar Hairs.JHEP, 03:263, 2021

  26. [34]

    Quasinor- mal modes of black holes in Einstein-power-Maxwell theory.International Journal of Modern Physics D, 27(3):1850034, January 2018

    Grigoris Panotopoulos and ´Angel Rinc´ on. Quasinor- mal modes of black holes in Einstein-power-Maxwell theory.International Journal of Modern Physics D, 27(3):1850034, January 2018

  27. [35]

    Devecioglu and Mu-In Park

    Deniz O. Devecioglu and Mu-In Park. No scalar-haired Cauchy horizon theorem in charged Gauss–Bonnet black holes.Eur. Phys. J. C, 84(2):168, 2024

  28. [36]

    Devecioglu and Mu-In Park

    Deniz O. Devecioglu and Mu-In Park. No scalar-haired Cauchy horizon theorem in Einstein-Maxwell-Horndeski theories.Phys. Lett. B, 829:137107, 2022

  29. [37]

    standard

    showed that regular charged black holes are excluded within broad classes of causal NLED. It should be em- phasized, however, that these no-go results are estab- lished for treated as an independent matter sector, with- out the nonminimal scalar coupling considered here. In th...

  30. [38]

    Bronnikov

    Kirill A. Bronnikov. Regular magnetic black holes and monopoles from nonlinear electrodynamics.Phys. Rev. D, 63:044005, 2001

  31. [39]

    Russo and Paul K

    Jorge G. Russo and Paul K. Townsend. Black holes and causal nonlinear electrodynamics.arXiv: 2601.07789

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