REVIEW 3 major objections 4 minor 1 cited by
Novel electrically charged wormhole, black hole and black bounce exact solutions in hybrid metric-Palatini gravity
T0 review · 3 major / 4 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read Electrically charged zero-potential HMPG solutions include traversable wormholes, black holes with double horizons, black bounces, and infinite-horizon spacetimes.
desk verdict The advertised 'expanding black universe' is not derived—every solution is static—but the Jordan-frame catalog of charged HMPG wormholes, bounces, and double-horizon black holes is careful, honest, and worth refereeing. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the conformal bridge between the Einstein-frame seed metric (15), the general static spherically symmetric electrovacuum solution with scalar field $\bar\phi=\bar C u+\bar\phi_0$ written in terms of the functions $s^2(k,u)$ and $s^2(h,u+u_1)$, and the Jordan-frame metrics obtained by multiplying by $\cosh^2(\bar\phi/\sqrt6)$ for a canonical scalar ($n=+1$) or $\cos^2(\bar\phi/\sqrt6)$ for a phantom scalar ($n=-1$). The constants are tied by $k^2\,\mathrm{sign}\,k = n\,3C^2 + h^2\,\mathrm{sign}\,h$. The mechanism does the classification work: the zeros of the cosine conformal factor, and their ordering relative to the zeros of the seed trigonometric functions, decide where the Jordan frame develops a singularity, a horizon, a throat, an anti-throat, a second spatial infinity, or a regular centre; the Kretschmann scalar and the embedding construction certify which candidate points are genuine structures.
What would settle it
Directly integrate the Jordan-frame field equations for action (11) with $V(\phi)=0$ and a pure Maxwell source, without assuming the Einstein-frame seed, for a static spherically symmetric metric; if any solution is found that is not a conformal transform of Eqs. (15)-(17), the classification is incomplete. For the 'black universe' reading, a more specific check is to compute the Jordan-frame scale factor and matter fluxes beyond the extremal horizon of a double-horizon solution and see whether the interior is homogeneous, isotropic, and expanding rather than merely regular.
Extended reading notes
Core claim
The central discovery, stated on the paper's own terms, is that the zero-potential HMPG action (11) with a Maxwell field admits Jordan-frame exact solutions whose qualitative global structure is controlled by the sign of the scalar kinetic term and the relative size of the constants $h$ and $k$ in the seed metric (15). For a canonical scalar the conformal factor $\cosh^2(Cu+\psi_0)$ never vanishes, and the resulting spacetimes are naked central singularities, in some cases preceded by a throat or a throat-plus-anti-throat pair because the radius function develops extrema. For a phantom scalar the conformal factor $\cos^2(Cu+\psi_0)$ has zeros; depending on whether these zeros occur before, after, or together with the zeros of the trigonometric functions $s^2(h,u+u_1)$ and $s^2(k,u)$, the geometry becomes a two-way traversable wormhole, a black bounce, a black hole with one degenerate horizon and a singularity, an asymmetric wormhole with a second flat infinity, a regular repulsive surface followed by a singularity, or a solution with infinitely many double horizons. The paper supports each classification with Kretschmann-scalar regularity checks, Penrose diagrams, embedding diagrams, and the summary Table I.
Load-bearing premise
The whole classification rests on the assumption that the imported Einstein-frame seed family is the complete solution set for the zero-potential, electrically charged, spherically symmetric case; if another seed solution exists that is not a conformal transform of Eqs. (15)-(17), the Table I catalogue misses it.
Editorial extensions
If this is right
- HMPG with zero scalar potential and electric charge is not restricted to GR-like black holes: exact, globally regular two-way traversable wormholes with two asymptotically flat ends appear in the phantom branch.
- Black holes in this theory can have a single degenerate double horizon instead of the usual inner/outer pair, with the metric remaining regular at the horizon.
- Black bounce geometries, throats located behind an extremal horizon, exist within the same parameter family, giving regular black holes whose singularity is replaced by a throat.
- In the phantom branch, when the zeros of the conformal factor align with those of the seed metric, the spacetime can contain an infinite sequence of double horizons separating static regions.
- The regular-centre subcase 3C of class [3-] produces an Anti-de Sitter-like core that avoids a central singularity entirely.
Reading between the lines
- The 'black universe' label in the abstract and conclusion implies a genuinely expanding cosmological interior beyond the horizon, but the body of the paper establishes regular interiors, throats, and horizons; it does not explicitly derive a Friedmann-like scale factor or isotropic expansion for those regions, so that cosmological reading should be treated as an open expectation rather than a demo
- Because the paper notes that a magnetic charge would give analogous results, the same catalogue should extend to dyonic and purely magnetic sources without new machinery.
- The parameter regions that produce double horizons and black bounces could be used to compute gravitational-wave ringdown and lensing signatures that distinguish these spacetimes from Schwarzschild and Reissner-Nordström; the paper does not perform those calculations.
- The zero-potential assumption removes the scalar potential from the field equations, but the conformal zeros that create the horizons depend on the scalar charge $\bar C$ and $\psi_0$; adding a potential will generically shift those zeros, so the stability and survival of the catalogue under small potentials is an immediate open question.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper constructs exact static, spherically symmetric, electrically charged solutions in hybrid metric-Palatini gravity with zero scalar potential by conformally transforming the known Einstein-frame electrovacuum family of Refs. [85-87]. It presents the Jordan-frame line elements for the canonical (n=+1) and phantom (n=-1) scalar branches, analyzes their asymptotic behavior, Kretschmann regularity, horizon and throat structures, and provides Penrose and embedding diagrams. The claimed outcomes are traversable wormholes, black holes with double horizons, black bounces, and 'black universe' models.
Significance. If the overstatement about expanding interiors is removed, the paper is a useful exact-solution catalog for HMPG. The conformal-frame method is standard and not circular: the solutions are obtained by transforming a previously known seed family, not by fitting a target metric. The explicit line elements, the systematic class-by-class treatment of throats and horizons, and the comparison with the Simpson-Visser black bounce are concrete strengths. The main limitation is that the most prominently advertised novelty, the 'expanding cosmological' black universe, is not actually derived; the paper's own analysis shows every solution remains in the static sector with unchanged signature across double horizons. Additionally, the completeness of the classification is inherited from an unverified seed-family claim.
major comments (3)
- [Abstract and Sec. VI] The abstract and conclusion claim 'black universe' models in which spacetime beyond the horizon leads to an expanding cosmological solution, but no such time-dependent or interior-region solution is derived. All metrics in the paper are static and spherically symmetric (line element (13)); the horizons identified are double zeros of g00, and the text itself states in Sec. IV that the metric signature remains (+---) beyond such horizons. Hence t remains timelike and r remains spacelike across the horizon, so there is no region with a timelike radius or a scale factor. The only regular-centre solution, class [3-] subcase 3C in Sec. V F, has g00 ~ c (u - u_max)^2 and is Anti-de Sitter-like, not an expanding cosmology. I recommend removing the phrase 'expanding cosmological solution' from the abstract and conclusion, or replacing it with a precise statement about regular static interiors and black bounces.
- [Sec. III, Eq. (15)] The paper adopts Eqs. (15)-(17) as 'a general solution' from Refs. [85-87] without derivation or field-equation verification. Since the classification in Table I is presented as exhaustive within the listed classes, the central completeness claim rests on the seed family being the full static spherically symmetric electrovacuum solution of action (12). The authors should either state this inheritance explicitly and qualify the completeness claim, or include a verification that Eqs. (15)-(20) satisfy the field equations derived from action (12).
- [Sec. V, Kretschmann analysis] The regularity and singularity classification is made on the basis of K1 expressions (e.g., Eqs. (38), (42), (49)-(51), (56)-(57), (62)-(63), and (68)), with the repeated assertion that 'the remaining terms of K' follow the same behaviour. Because K is a sum of four independent components (Eq. (34)), the unsupplied K2-K4 terms are needed to rule out additional divergences at the same points. I request that the authors include these terms, or provide a supplementary computational file, so that the singularity statements can be checked directly.
minor comments (4)
- [Sec. IV] The paper defines a black bounce as 'a particular case of a black universe' but never gives a precise definition of 'black universe'; align this terminology with the abstract and conclusion to avoid the impression that an expanding interior is being asserted.
- [Eq. (33)] The mass formula is written as 'm = lim eβγ′/β′', which is ambiguous; it should be typeset as e^{β} γ'/β' with the exponential made explicit.
- [Eq. (21)] The condition s2(h,u1) = 1/q^2 imposes both asymptotic flatness and real-valuedness restrictions (for h < 0 it requires |q| ≥ |h|); state this restriction explicitly where u1 is introduced.
- [Figs. 2-4, 9-10] The z(x) versus r(x) panels would benefit from explicit axis labels and units; several figures currently rely on the caption alone to identify the plotted functions.
Circularity Check
No circularity: Jordan-frame solutions follow by conformal transformation of an imported Einstein-frame solution; the 'black universe' wording is an overclaim, not a circular reduction.
full rationale
The paper's derivation chain is: HMPG action (7) is conformally transformed to the Einstein-frame action (12); the static spherically symmetric electrovacuum solution (15)-(17) is imported from refs. [85-87]; the inverse conformal transformation (24)-(27) yields the Jordan-frame line elements (36)-(67); and each class is then analyzed for horizons, throats, and singularities. Every step is an explicit mathematical operation, and no parameter is fitted to a target outcome. The imported Einstein-frame solution is not a self-citation of the present authors: refs. [85-87] are by Bronnikov and collaborators, with no author overlap with the present paper, and the paper does not invoke a uniqueness theorem to exclude alternatives. It adopts a known solution family and systematically classifies the sign combinations (22)-(23). The 'black universe' phrasing in the abstract and conclusion, which mentions 'an expanding cosmological structure' beyond the horizon, is not supported by the static solutions derived in the body; the body itself uses 'black universe' only as a generic term for regular black-bounce geometries, citing Simpson-Visser [90]. That is an overclaim or inconsistency, but it is not a circular reduction of a derived result to an input. No fitted quantity is renamed as a prediction, and no load-bearing self-citation chain forces the stated conclusions. Therefore no circular step is exhibited, and the appropriate finding is no significant circularity.
Assumptions & free parameters
free parameters (4)
- q (electric charge)
- C (scalar field slope)
- h
- psi0 (scalar field offset)
assumptions (4)
- domain assumption Eqs. (15)-(17) with relation (20) is the general static spherically symmetric electrovacuum solution of the Einstein-frame action (12).
- domain assumption The conformal transformations (26)-(27), with the Maxwell term unchanged as in Eq. (12), correctly map the HMPG Jordan frame to the Einstein frame.
- ad hoc to paper Physical solutions are selected by imposing m>0 and by excluding some psi0 values (e.g., psi0 = pi/2 + c pi) as non-physical.
- domain assumption The coordinate transformations (30)-(32) and analytic continuation through zeros of the Einstein-frame metric are valid.
Cite this review
Pith. "Pith review of Novel electrically charged wormhole, black hole and black bounce exact solutions in hybrid metric-Palatini gravity." pith.science (2026). https://pith.science/paper/2XCLBKYX
@misc{pith2026241210324,
author = {Pith},
title = {Pith review of: Novel electrically charged wormhole, black hole and black bounce exact solutions in hybrid metric-Palatini gravity},
year = {2026},
howpublished = {\url{https://pith.science/paper/2XCLBKYX}},
note = {Machine review of arXiv:2412.10324}
}
read the original abstract
This paper presents a systematic exploration of exact solutions for electrically charged wormholes, black holes, and black bounces within the hybrid metric-Palatini gravity (HMPG) framework. HMPG combines features of the metric and Palatini formulations of modified gravity, offering a powerful approach to address challenges in General Relativity, particularly those related to cosmic acceleration and dark matter. We examine configurations characterized by a zero scalar potential under spherical symmetry, and present solutions in both the Jordan and Einstein conformal frames. A diverse set of solutions emerges, including traversable wormholes, black holes with double horizons, and ``black universe'' models in which spacetime beyond the horizon leads to an expanding cosmological solution rather than a singularity. Each configuration is categorized according to the properties of the scalar field, with an in-depth analysis of the horizon and throat structures, asymptotic behaviour, and singularities. These findings underscore the versatility of HMPG in capturing complex gravitational phenomena and broadens the scope of the theory, offering a robust framework for modelling gravitational phenomena across a range of astrophysical contexts. Future work will benefit from extending these solutions to include scalar potentials, addressing both early-universe inflation and late-time acceleration, and applying observational data, such as gravitational lensing and gravitational wave measurements.
Figures
Figures from the paper (12 more)
Forward citations
Cited by 1 Pith paper
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Black bounce sourced by non-minimally coupled linear electrodynamics and a canonical scalar field through a thin shell at the throat
A new family of black bounce and wormhole solutions in general relativity is constructed from a canonical scalar field non-minimally coupled to linear electrodynamics, with the required energy-condition violation conf...
Reference graph
Works this paper leans on
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Copy of Our Universe
ψ0 ̸= π 2 + c2π The first zero, relative to spatial infinity, at u = 0, of the conformal factor is at u = π/2−ψ0+f π C (where f is a given integer that guarantees this expression corresponds to the first positive zero, as explained in class [1 −]), of sin(h[u + u1]) is at u = π |h| − u1 if u1 > 0, and at u = −u1 if u1 < 0, and of sin( ku) is at u = π |k| ...
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end of the horn
ψ0 = π 2 + c2π Now, as aforementioned, we must find a point of spa- tial infinity, since u = 0 is not a possibility ( r is finite, in particular equal to |C|). As was already shown in the previous discussion, any isolated zero of the sin(ku) func- tion may be assumed as a spatial infinity. We will, in the following analysis, consider h >0 and h = 0 separa...
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Each of these integrations generates an embedding di- agram
Innermost region: from xs to xC. Each of these integrations generates an embedding di- agram. We emphasize that in order to avoid divergences, the condition q2/m2 > 8/9 must be guaranteed so that the singularity xs remains accessible for integration. The different regions can then be connected with the help of suitable initial conditions. The result is sh...
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ψ0 = π 2 + c2π Now, considering these particular values of ψ0 we may perform an analysis similar to that at the end of the previous class. Accordingly, we have to choose another point of first spatial infinity, as u = 0 is not one in this case. For that, we may choose any of the isolated zeros of the sin( ku) function, which correspond to flat spatial inf...
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7, which is similar to the diagram of the well known Morris- Thorne wormhole [95]
Nevertheless, we only present the possibility in which the throat is located in the middle, represented in Fig. 7, which is similar to the diagram of the well known Morris- Thorne wormhole [95]. Finally, in case 3 we find, as aforementioned, that at u = π/2−ψ0+f π C = π |k| there is an double event horizon and there we define u = uH . Apart from this, ana...
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