REVIEW 4 major objections 6 minor 59 references
Signatures of cubic gravity in the strong regime
T0 review · 4 major / 6 minor · reviewed 2026-08-09 · deepseek-v4-flash
Pith's one-line read In Einsteinian cubic gravity, a positive coupling shrinks black hole horizons, and Sgr A* keeps its horizon only if the coupling is below roughly 0.1.
desk verdict Solid, code-backed phenomenology of Einsteinian cubic gravity; the main results hold within the single-function branch, but the ansatz needs scrutiny before trusting the quantitative bound. 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 integrated vacuum field equation (7), obtained from the single-function ansatz ds² = -f dt² + $f^{{-1}}$ dr² + r² dΩ², which reduces the six-derivative cubic field equations to one ordinary differential equation containing the ADM mass integration constant C0. At a horizon, f(rh) = 0 and f'(rh) ≥ 0 turn this equation into the cubic relation (24) for f'(rh), which the paper uses to classify horizon existence and sign dependence. For observables, two tools carry the argument: the effective potential VEff = f L²/r², whose maximum locates the photon sphere, and the triangular-array angular difference α from null geodesics, computed numerically using the near-horizon, asymptotic, and weak-coupling series to seed integrations.
What would settle it
Solve the full vacuum field equations without imposing the reciprocal metric relation and compare the resulting horizon curve with Fig. 1; any discrepancy would overturn the derived thresholds. Alternatively, a shadow-radius measurement of Sgr A* accurate to a few percent would expose or rule out the claimed ~50% photon-sphere shift.
Extended reading notes
Core claim
Einsteinian cubic gravity admits exact de Sitter-like solutions with an effective cosmological constant set by λ and Λ, and, away from maximal symmetry, static spherically symmetric solutions whose metric function is governed by one integrated equation with mass integration constant C0. In these solutions the horizon radius is smaller than the Schwarzschild radius for λ > 0 and larger for λ < 0, with λ = 0 recovering Schwarzschild. For C0 fixed to the mass of Sgr A* and ΛEff fixed to the observed cosmological constant, λ ≈ 0.1 is the threshold above which the non-cosmological horizon disappears, leaving a naked singularity with divergent Ricci scalar at r = 0. The photon sphere follows the horizon: positive (negative) λ moves it inward (outward) by up to about 50% for |λ| < 0.1, and the angular difference α between triangular null-geodesic arrays in cubic gravity and in the Kottler or Schwarzschild background grows as the array approaches the source, reaching thousands of arcseconds at impact parameter 10 rs for λ = -5 and about $10^{4}$ arcsec near threshold for positive λ.
Load-bearing premise
Everything downstream assumes the static spherically symmetric solution has the reciprocal relation g_tt = 1/g_rr (the single-function ansatz); if the full cubic equations force a second independent metric function, the horizon radii, photon-sphere positions, and angular differences could all change.
Editorial extensions
If this is right
- If the claim is right, a positive λ reduces the event-horizon radius below the Schwarzschild value for a fixed mass, so mass estimates based on horizon-crossing observables would be systematically shifted unless the cubic term is included.
- For a Sgr A*-mass object, λ ≳ 0.1 removes the black hole horizon entirely; since Sgr A* is observed to have one, the coupling is observationally bounded from above at about 0.1.
- The photon sphere moves by about 50% relative to Schwarzschild for |λ| just under 0.1, implying changes in black hole shadow size of a similar order.
- The angular difference α between cubic gravity and the Kottler or Schwarzschild solution grows sharply when the geodesic triangle approaches the source, so future high-precision deflection measurements near a compact object can isolate the cubic contribution.
- In the Solar System the expected cubic contribution to α is about 10^-9 arcsec, which is below currently planned micro-arcsecond mission sensitivities.
Reading between the lines
- A direct extension not made in the paper would be to compute the predicted shadow image for these non-asymptotically flat metrics and compare it with existing millimetre-wavelength shadow observations of Sgr A*, converting the λ ≲ 0.1 bound into an actual posterior constraint.
- Because the single-function ansatz is assumed, a natural stress test is to solve the full vacuum equations with two independent metric functions; if they differ, the horizon and photon-sphere relations change, especially on the naked-singularity branches.
- The same triangular-array observable could be applied to rotating cubic-gravity black holes, where the photon-sphere displacement should be azimuthal and could be separated from the spherically symmetric part.
- If higher-order curvature terms are added, the horizon-shift and photon-sphere relations likely become polynomial in the additional couplings, so the sign-and-magnitude pattern found here may be the first member of a family.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper studies static, spherically symmetric vacuum solutions in Einsteinian cubic gravity with a cosmological constant, using the single-function metric ansatz ds² = -f(r)dt² + dr²/f(r) + r²dΩ². The authors derive approximate weak-coupling, asymptotic, and near-horizon solutions, integrate the field equations numerically, and report that a positive coupling λ shrinks the horizon while a negative λ enlarges it, with a naked-singularity branch appearing for λ ≳ 0.09–0.1 for a SgrA*-mass object. They then compute the angular difference α in triangular null-geodesic arrays and the photon-sphere position, concluding that strong-field observables can constrain the coupling to λ ≲ 0.1 and that the strongest cubic effects occur near the source. The manuscript includes publicly available numerical codes.
Significance. If the results are correct, the paper would provide concrete, falsifiable strong-field predictions for Einsteinian cubic gravity: a horizon-size dependence on the sign of λ, a photon-sphere shift, and angular differences measurable in principle with laser-ranging or interferometric missions. The analytic work extends earlier studies by expressing approximate solutions in terms of a single integration constant C0 and by covering both signs of λ and non-asymptotically flat backgrounds. Credit is due for the explicit asymptotic and near-horizon series, the comparison of these series with numerical integrations, and the public release of the numerical codes. However, the central quantitative claims are currently conditional on an unproved metric ansatz and on numerical outputs without stated uncertainty, and the headline λ ≲ 0.1 bound is derived for a single mass value.
major comments (4)
- [Section II, Eq. (4) and throughout] The single-function ansatz g_tt = 1/g_rr is load-bearing for all results, but the paper does not show that this ansatz is exhaustive for the solutions studied. The statement that the densities C and C' are trivial in static spherically symmetric vacuum, together with Refs. [11,12], establishes the existence of a one-function branch, not uniqueness: the general static spherically symmetric metric has two independent functions, and there is no coordinate freedom to impose g_tt g_rr = -1 while retaining r as the areal radius. Because Eq. (5), the integrated master equation (7), the near-horizon series (20), the horizon equation (24), and all numerical metrics behind Table II and Figs. 5–7 rely on this ansatz, the claimed horizon, photon-sphere, and angular-difference results are conditional on the branch. The paper should either derive the two-function field equations and prove (or verify numerically for a two-function metric with the same boundary conditions) that a solution branch with g_tt g_rr = -1 exists and is the one relevant for black holes, or explicitly restrict every conclusion to the one-function branch and note that the general solution is not covered.
- [Section II, Eq. (7)] The master integrated equation (7), from which nearly all subsequent analytical and numerical results are obtained, is stated with the phrase 'can be integrated' but no derivation is provided. There is no appendix or reference that shows how Eq. (5) is reduced to Eq. (7), and this equation is the starting point for the weak-coupling, asymptotic, near-horizon, and numerical solutions. This gap is load-bearing for the paper's central claims, because any error or missing integration constant in Eq. (7) would propagate through the horizon properties, angular differences, and the λ ≲ 0.1 constraint. The authors should include a derivation of Eq. (7) (or a detailed reference that contains it) and a direct consistency check, such as substituting the final expression back into Eq. (5).
- [Table II and Figs. 5–6] The reported angular differences α are quoted to several significant figures without error bars, convergence tests, or tolerances. In particular, the claim that the results for Kottler and Schwarzschild backgrounds are the same 'under double-precision floating-point numerical resolution' is not accompanied by any estimate of the numerical resolution, and the constant row b1 = 10^4 rs in Table II, where α = 2.04 × 10^-8 arcsec for all λ, could be a numerical floor rather than a physical result. Since α is the central observational signature, the paper should report convergence with respect to the Runge-Kutta tolerances, the grid spacing used for the central-difference derivatives in Eq. (32), the placement of the outer boundary where the asymptotic solution is matched, and the resulting uncertainty on each α value and on the location of the λ ≈ 0.1 threshold.
- [Section II F and Section III C] The constraint λ ≲ 0.1 and the associated naked-singularity threshold are derived for a single value of the mass, C0 = 4 × 10^6 M⊙ (later 4.3 × 10^6 M⊙), with no exploration of the SgrA* mass uncertainty or the general dependence on C0. The horizon equation (24) and the near-horizon relation (22) depend on C0, so the threshold is expected to shift with the mass; the text itself quotes λ ≈ 0.09 in one place and λ ≲ 0.1 in the conclusion. The authors should compute the threshold over the observationally allowed mass range for SgrA*, present the dependence of the critical λ on C0, and specify the numerical criterion used to decide that the maximum of the effective potential disappears in Fig. 7.
minor comments (6)
- [Section II F, Eq. (27) and figure captions] The dimensionless rescaling (27) means that values such as λ = 0.05 are only meaningful in the chosen units; the plot axes and table captions should state these units explicitly and distinguish them from dimensionful values used in astrophysical contexts.
- [Section II F, Fig. 3] The left panel of Fig. 3 uses C0 = 4 × 10^21 M⊙ and is described in the text as a non-physical illustrative solution, but the caption does not clearly say this; adding a sentence to the caption would avoid confusion.
- [Section III B, Table II] Table II and the surrounding text use C0 = 4.3 × 10^6 M⊙, while Section II F and Figs. 2, 5, and 6 use C0 = 4 × 10^6 M⊙; these values should be reconciled, since the angular differences depend on C0.
- [Section III A, Eq. (32)] The notation ar g_{\phi\phi} and ar g_{rr} in the tangent formula is not defined; if these are the optical metric components, they should be introduced before Eq. (32).
- [Section III B] The statement that the Kottler and Schwarzschild backgrounds give the same α for b1 < 10^4 rs because the cosmological constant contributes only when b1 > 10^10 rs is asserted without derivation; a quantitative estimate of this threshold would make the statement easier to verify.
- [Throughout] There are several typographical or notation inconsistencies, including 'SgrA∗' versus 'Sgr A*', 'Red. [17]' instead of 'Ref. [17]', and the use of 'and' in footnote 1; these should be cleaned up during revision.
Circularity Check
No significant circularity: the horizon, angular-difference, and photon-sphere claims are computed consequences of the stated action and single-function ansatz, not fitted outputs or self-citation loops.
full rationale
I walked the derivation chain from the action (1) through the field equations (3), the single-function reduction (4)-(7), the approximate solutions (12)-(15) and (20)-(23), the horizon equations (24)-(25), the numerical integrations in Section II F, and the angular-difference/photon-sphere analysis in Section III. Each quantitative claim (smaller/larger horizon for positive/negative lambda, Table II alpha values, photon-sphere shift, lambda <= 0.1 constraint) is obtained by solving the field equations with stated boundary conditions; no parameter is fitted to the output that it is then said to predict. The only self-citation that plays a substantive role is Ref. [22] for the angular-difference method; that method is independently checkable, as the paper itself notes that in the Schwarzschild-versus-Minkowski limit it reproduces the textbook deflection angle, so it is real evidence rather than a circularity. The main caveat is the single-function ansatz (4), which is asserted and supported by Refs. [11,12] but not proven exhaustive for all branches explored; if the full ECG vacuum equations required a second independent metric function, the horizon and photon-sphere numbers could change. That is a limitation or validity risk, not a circular reduction of the paper's outputs to its inputs. I therefore find no significant circularity.
Assumptions & free parameters
free parameters (3)
- Coupling constant lambda =
Scanned by hand: -5.0, -1.5, 0.05, 0.1 (dimensionless units of 16 C0^4 / M_Pl^3)
- Effective cosmological constant Lambda_Eff =
1.1056 x 10^-52 m^-2 (Planck 2018 observed value)
- Near-horizon coefficient a2 = f''(r_h)/2 =
Not given in closed form; selected to match the asymptotic branch for lambda < 0
assumptions (6)
- domain assumption The Einsteinian cubic gravity action (1) with P as the only relevant cubic density for static spherically symmetric vacuum solutions.
- domain assumption The static spherically symmetric metric has a single metric function f(r) with g_tt = 1/g_rr, as in Eq (4).
- domain assumption The theory is treated as an effective low-energy theory with validity regime |lambda P / M_Pl^3| much less than |R| and r_h much greater than (lambda / M_Pl^5)^{1/4}.
- ad hoc to paper Lambda_Eff is fixed to the observed cosmological constant and Eq (16) is inverted to determine the bare Lambda.
- standard math Null geodesics obey the weak equivalence principle, giving the effective potential V_Eff = f(r) L^2 / r^2.
- domain assumption Numerical solutions are seeded from the asymptotic solution for lambda > 0 and from the near-horizon solution for lambda < 0.
Cite this review
Pith. "Pith review of Signatures of cubic gravity in the strong regime." pith.science (2026). https://pith.science/paper/GY2DZYZZ
@misc{pith2026250201747,
author = {Pith},
title = {Pith review of: Signatures of cubic gravity in the strong regime},
year = {2026},
howpublished = {\url{https://pith.science/paper/GY2DZYZZ}},
note = {Machine review of arXiv:2502.01747}
}
read the original abstract
We investigate the effects of Einsteinian cubic gravity in the strong gravitational regime. In the first part, we explore analytical solutions for a static, spherically symmetric metric, establishing the existence of maximally symmetric de Sitter solutions, as well as asymptotically de Sitter solutions, with an effective cosmological constant. We also study, analytically and numerically, how the horizon properties are affected by cubic gravity. Our results reveal that a positive coupling constant reduces the horizon size, while a negative one increases it. In the second part, we analyze potential observational signatures of cubic terms, focusing on their effects on the bending of light. Specifically, we investigate the angular difference, related to the deflection angle but valid near the source, along with the behavior of the photon sphere. Our findings show that the strongest effects of the cubic terms occur in the strong gravity regime, and there exists a direct relationship between the value of the coupling constant and the photon sphere position, opening up the possibility to constrain cubic gravity with black hole shadows.
Figures
Figures from the paper (4 more)
Reference graph
Works this paper leans on
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Furthermore, we assume f ′(rh) ≥ 0 at the (non- cosmological) horizon. This is justified, since we are interested in the exterior black hole horizon, where the metric function f (r) is expected to change sign from pos- itive (outside the horizon) to negative (inside the hori- zon). Notice, however, that we are allowing for f (r) to have a minimum at rh, s...
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[2]
V acuum covariant field equations In Section II, the vacuum covariant field equations (3), were presented, where the modifications introduced by the cubic terms are represented by the tensor Pµν, de- fined as follows: 15 Pµν =gµνA − 24Rµν□R + 48Rµ ρAνρ + 48∇νRσρ(∇ρRµ σ − ∇µRσρ) + Rµσνρ (96□Rσρ − 72∇ρ∇σR) − 24RσρAσµρν − 144∇ρRµγνσ ∇γRσρ + 48(Rνρσγ − 3Rνσργ...
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Maximally symmetry solutions In Section II A, we demonstrated the existence of max- imally symmetric de Sitter-like solutions in Einsteinian cubic gravity. For real and positive (negative) Λ Eff roots of the equation: Λ3 Eff + M 5 Pl 16λ ΛEff − M 5 PlΛ 48λ = 0, (A3) the solutions exhibit behavior corresponding to de Sitter (anti-de Sitter) solutions. Next...
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[4]
W eak coupling solution In Section II B, the weak coupling solution for Eq. (7) is obtained iteratively. The functions h1, h2, and h3 are presented in Eq. (13). The full expression for the H3 term in h3 is: H3 = − 10935C0 14Λ3M 2 Plr7 1 + 3699C0 35M 2 Plr − 1411589C 2 0 5670M 4 Plr2 − 10797C0 28ΛM 2 Plr3 + 12158C 2 0 7ΛM 4 Plr4 + 3078C0 7Λ2M 2 Plr5 − 1022...
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(7), showing the main terms in Eq
Asymptotic solution In Section II C, we obtain the iterative asymptotic so- lution for Eq. (7), showing the main terms in Eq. (15). Next, we present the corresponding terms for the first ten terms: 16 fasy(r) =1 − ΛEff 3 r2 − 6M 3 Pl 3M 5 Pl + 16λΛ2 Eff C0 r + 432λ M 9 Pl 1 + 16λΛ2 Eff 3M 5 Pl 3 1 − 14ΛEff 27 r2 C 2 0 r6 + 1202688λ2ΛEff M 16 Pl 1 + 16λΛ2 ...
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