REVIEW 3 major objections 6 minor 66 references
Spherical photon orbits around Kerr-MOG black hole
T0 review · 3 major / 6 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read Photon orbits around Kerr-MOG black holes are governed by a sextic polynomial whose real root count is set by a critical inclination angle, with four orbits below it, two above, and three at it.
desk verdict A promising extension of Tavlayan-Tekin to Kerr-MOG, but the key factorization behind the critical inclination angle is unverified and the paper has too many internal typos to be reliable as written. 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 central object is the sixth-order polynomial $f(x)=0$ in Eq. (21), obtained by combining the separability of the Hamilton-Jacobi equation (with Carter constant) with the two conditions $R(r)=0$ and $dR/dr=0$ for constant-radius photon orbits. The polynomial encodes the orbit radius $x=r/M$ as a function of the rotation parameter $u=a^2/M^2$, the effective inclination angle $v=\sin^2 i$, and the MOG deformation parameter $\alpha$. The critical-angle analysis rests on the claimed factorization $f(x)=P_4(x)(x+A_5)^2$ at $v_{cr}$, which turns the sextic into solvable pieces whose roots are identified as the photon orbits.
What would settle it
For a concrete parameter pair such as $\alpha=0.3$ and $u=0.5$, compute $v_{cr}$ from Eqs. (30)-(33) and check numerically whether the sextic (21) equals $P_4(x)(x+A_5)^2$ with the coefficients of Eq. (35); any nonzero residual refutes the factorization, and a direct root-count scan in $v$ would confirm or refute the claimed four-to-two transition.
Extended reading notes
Core claim
For Kerr-MOG black holes, the radial photon motion reduces to the sextic polynomial (21), whose real roots are the radii of spherical photon orbits. In the extremal case the polynomial factors as $(x-1)^2P_4(x)$; a slowly rotating extremal hole has two photon orbits outside the horizon, while a rapidly rotating one has only one. In the non-extremal case the paper claims there is a critical inclination angle $v_{cr}=v_{cr}(u,\alpha)$, with four orbits below it, two above it, and three at $v_{cr}$; at the critical angle the sextic is claimed to factor into a quartic times a squared linear term. All of these orbits are radially unstable, and $\alpha$ constrains the spin to $u<1/(1+\alpha)$.
Load-bearing premise
The four-to-two transition at the critical inclination angle rests on the unverified factorization of the sextic into a quartic times a squared linear term at $v_{cr}$; if that factorization is not exact, the formula for $v_{cr}$ and the claimed orbit counts would not follow.
Editorial extensions
If this is right
- The Kerr critical-inclination result of Tavlayan and Tekin extends to MOG, with $v_{cr}$ now a function of the deformation parameter $\alpha$ and the rotation parameter $u$.
- The allowed spin range shrinks to $u<1/(1+\alpha)$, so the extremal hierarchy of orbits is different from Kerr for every nonzero $\alpha$.
- The critical impact parameters decrease as $\alpha$ grows, which would change the size and brightness of the predicted black hole shadow and its photon ring.
- Because all spherical photon orbits are radially unstable, their observable signatures are transient lensing features rather than stable light rings.
Reading between the lines
- The same factorization-plus-root-count method could be reapplied to other rotating modified-gravity metrics, such as Kerr-Newman in scalar-tensor theories, to test whether a critical inclination angle is a generic feature of photon orbits.
- If the factorization at $v_{cr}$ is exact, it implies the discriminant of the sextic vanishes along the $v_{cr}(u,\alpha)$ surface; locating that discriminant surface directly would provide an independent check of the paper's central claim.
- The near-horizon location of the prograde orbit at large $\alpha$ suggests that very-high-spin MOG black holes could be distinguished from Kerr by future near-horizon imaging, if the MOG parameter is not too small.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper studies spherical photon orbits in the Kerr-MOG spacetime of scalar-tensor-vector gravity. Starting from the Hamilton-Jacobi equations, the authors derive a sixth-order polynomial f(x) in the dimensionless radius x, involving the rotation parameter u, the effective inclination angle v, and the MOG deformation parameter α (Eq. 21). They analyze the polar (v=1), equatorial (v=0), and intermediate (0<v<1) cases, for both extremal and non-extremal black holes. The central new claim is that in the non-extremal case there is a critical inclination angle vcr(u,α) such that the sextic has four real roots below vcr, two above vcr, and three at vcr, with two roots outside and one inside the event horizon at the critical point. The paper also reports radial instability of all photon orbits and studies how the critical impact parameter and hence the black hole shadow depend on α.
Significance. If correct, the result would generalize the spherical photon orbit analysis of Tavlayan and Tekin for Kerr black holes to Kerr-MOG black holes and would give a concrete α-dependent shadow prediction that could, in principle, distinguish MOG from GR. The manuscript is self-contained: the polynomial is derived from the Hamilton-Jacobi equation rather than imported, and the paper checks several familiar limits such as Schwarzschild and Kerr. The numerical survey of orbits and stability is extensive. However, at present the central critical-angle result rests on an unverified factorization, and the displayed sextic has apparent algebraic and dimensional inconsistencies that prevent the stated limits from being reproduced. Because the main object of the paper, Eq. (21), and the main new result, the vcr transition, are both affected, the significance cannot be evaluated until the derivation is corrected and the factorization is verified.
major comments (3)
- [III, Eq. (21)] As printed, the sextic polynomial is not consistent with the equations of motion from which it is supposed to follow. In the Kerr limit α=0 and with M=1, the x^4 coefficient is 9+2uva=9+2u^{3/2}v, because u=a^2, whereas the polar condition v=1 (L_z=0) obtained from R(r)=0 and dR/dr=0 is (x^3-3x^2+ux+u)^2=0, whose x^4 coefficient is 9+2u. The x^3 coefficient in Eq. (21) is -4u for α=0, but the correct coefficient at general v is -4uv, so at the equatorial value v=0 the polynomial retains a cubic term instead of reducing to the condition η=0. Thus Eq. (21) does not reduce to Eq. (22) or to Eq. (27), and the root counts shown in the figures are not those of the displayed polynomial. If the intended terms are 2uv and -4uv, the derivation and all subsequent numerical results must be redone with the corrected polynomial.
- [V, Eqs. (30)-(35)] The critical inclination angle is the central new result, but the formulas supporting it cannot be checked. Eqs. (30)-(33) are stated without derivation, and Eq. (31) contains an unbalanced bracket and an unclear division by w. More seriously, at vcr the sextic is asserted to factor as f(x)=P4(x)(x+A5)^2, Eq. (34), but no substitution or verification is supplied; the factorization is not demonstrated. The coefficient list (35) is unusable as printed: in Eq. (35b) the numerator and denominator are identical, forcing A2=1; Eq. (35c) contains an undefined symbol d; and Eq. (35d) contains a bare a instead of α. Since the four-to-two transition and the existence of three orbits at vcr rest on this factorization, this is a load-bearing gap rather than a presentation issue.
- [IV.B, Eq. (27)] The equatorial-plane section contains a contradiction in the definition of v. Eqs. (18)-(19) define v=sin^2 i, so for equatorial orbits (K=0, i=0,π) one has v=0, while the text of Section IV.B states that 'i=0 or π, which leads to v=1'. The abstract and Section IV.A use v=1 for polar orbits, so the two conventions are incompatible. In addition, Eq. (27) does not follow from Eq. (21) for either v=0 or v=1: substituting v=0 into Eq. (21) and factoring out x^2 yields a quartic whose x coefficient differs from that of Eq. (27) by a term involving u and α. The derivation of Eq. (27) must be shown explicitly and the v convention fixed.
minor comments (6)
- [IV.A] In the sentence defining polar orbits, the text writes 'ν=1' where v=1 is meant; the symbol should be made consistent with Eq. (19).
- [IV.B] In the description of the extremal equatorial case, the paper says the black hole has one photon orbit 'outside the event horizon and the other (x1) inside'; the inside orbit should be labeled x3 or x4, not x1.
- [IV.A and IV.B, impact parameter discussion] The critical impact parameter is defined by Eq. (25) and evaluated in Eq. (26), but the text and Fig. 4 refer to 'Eq. (29)' when discussing β; the cross-reference should be corrected.
- [Section VI, Conclusion] The conclusion states that as α increases the radii of the prograde and retrograde equatorial orbits expand, which contradicts the description in Section IV.B and the behavior shown in Fig. 6; this discrepancy should be resolved.
- [References] Reference [55] duplicates reference [32], and reference [57] duplicates reference [53]; the bibliography should be consolidated to avoid duplicate entries.
- [Eq. (24)] The notation ddR(2)_i in Eq. (24) is nonstandard and is not defined in the text; it should be written explicitly as the second derivative of R(x) with respect to x evaluated at the orbit x_i.
Circularity Check
No significant circularity: the photon-orbit polynomial and critical-angle analysis are derived from the external Kerr-MOG metric via the Hamilton-Jacobi equation, with no fitted parameter renamed as a prediction and no load-bearing self-citation.
full rationale
The paper's derivation chain is self-contained once the Kerr-MOG metric (Eq. 9-10) is adopted from the cited MOG literature. The sextic polynomial (Eq. 21) is obtained by combining the radial conditions R(r)=0 and dR/dr=0 with the Hamilton-Jacobi equations, and the polar and equatorial reductions follow algebraically from that polynomial. The critical inclination angle in the non-extremal case is obtained from an algebraic factorization ansatz (Eq. 34) with coefficients determined by comparison with the sextic, not by fitting data or by importing a result from the authors' own prior work. The extremal critical angle v_cr is also derived from the quartic P4(x), again algebraically. No parameter is fitted to the quantity it is later said to predict, and no central claim reduces by definition to an input. The paper contains no self-citations by the authors at all; references to prior work by other groups supply the spacetime metric, the stability criterion, and the Kerr-case motivation, which are standard external inputs rather than circular load-bearing assumptions. The concerns raised about Eq. (35) being internally inconsistent or the factorization (34) being unverified are mathematical correctness issues, not circularity: an unproven or erroneous algebraic step is not the same as a derivation that presupposes its conclusion. Therefore, on the circularity axis the paper receives score 0.
Assumptions & free parameters
assumptions (4)
- domain assumption The Kerr-MOG metric (9) is a valid vacuum solution of STVG.
- domain assumption The Hamilton-Jacobi equation is separable in the Kerr-MOG spacetime, allowing a Carter constant.
- domain assumption The radial stability criterion of Ref. [54] applies, namely an orbit is unstable if d²R/dx² > 0.
- ad hoc to paper At the critical inclination angle vcr, the sextic f(x) factorizes as P4(x)P2(x) with P2 = (x + A5)^2.
Cite this review
Pith. "Pith review of Spherical photon orbits around Kerr-MOG black hole." pith.science (2026). https://pith.science/paper/K5MHY3K4
@misc{pith2026241217520,
author = {Pith},
title = {Pith review of: Spherical photon orbits around Kerr-MOG black hole},
year = {2026},
howpublished = {\url{https://pith.science/paper/K5MHY3K4}},
note = {Machine review of arXiv:2412.17520}
}
abstract
This study investigates photon orbits around Kerr-MOG black holes. The equation of photon of motion around the Kerr-MOG black hole is derived by solving the Hamilton-Jacobi equation, expressed as a sixth-order polynomial involving the inclination angle $v$, the rotation parameter $u$, and the deformation parameter $\alpha$ that characterizes modified gravity. We find that $\alpha$ constrains the rotation of the black hole, modifying its gravitational field and leading to distinct photon orbital characteristics. Numerical analysis reveals that the polar plane ($v=1$) has two effective orbits: one outside and one inside the event horizon, while the equatorial plane ($v=0$) has four effective orbits: two outside and two inside the event horizon. Moreover, we derive the exact formula for general photon orbits between the polar and equatorial planes ($0<v<1$). In the extremal case, the rotation speed significantly impacts general photon orbits. A slowly rotating extremal black hole has two general photon orbits outside the event horizon, whereas a rapidly rotating extremal black hole has only one such orbit. In the non-extremal case, a critical inclination angle $v_{cr}$ exists in the parameter space $\left(v, u, \alpha \right)$. Below $v_{cr}$, there are four general photon orbits, while above $v_{cr}$, there are two orbits. At the critical inclination angle, three solutions are found: two photon orbits outside and one inside the event horizon. Additionally, the results indicate that all orbits are radially unstable. Furthermore, by analyzing photon impact parameter, we argue that $\alpha$ influences observational properties of the black hole.
Figures
Figures from the paper (11 more)
Reference graph
Works this paper leans on
-
[1]
K. Akiyama et al. (Event Horizon Telescope), Astrophys. J. Lett. 930, L12 (2022) , arXiv:2311.08680
arXiv 2022
-
[2]
K. Akiyama et al. (Event Horizon Telescope), Astrophys. J. Lett. 875, L5 (2019) , arXiv:1906.11242
arXiv 2019
-
[3]
The orange and blue surfaces illustrate the stability of photon orbits x1 and x2, respectively. It is observed that ddR (2) 1 and ddR (2) 2 remain positive, indicating that the polar photon orbits x1 and x2 are unstable under radial perturbations. Since photon orbit x2 resides inside the event horizon, it cannot be observed by detectors at infinity. In c o...
-
[4]
B. P. Abbott et al. (LIGO Scientific, Virgo), Phys. Rev. Lett. 116, 221101 (2016) , arXiv:1602.03841
arXiv 2016
-
[5]
R. Abbott et al. (KAGRA, VIRGO, LIGO Scientific), Astrophys. J. Suppl. 267, 29 (2023) , 23 arXiv:2302.03676
arXiv 2023
-
[6]
J. M. Bardeen and W. H. Press, J. Math. Phys. 14, 7 (1973)
1973
-
[7]
0 0. 2 0. 4 0. 6 0. 8 1. 0 0 1 2 3 4 u 0.45 0.50 0.55 0.60 0.65 0.70 0.75
-
[8]
5 u 0.50 0.52 0.54 0.56 0.58 0.60 0.62
Show all 66 references
-
[9]
5 u 0.48 0.49 0.50 0.51 0.52
-
[10]
5 u (a) α=0 (b) α=0.3 (c) α=0.6 (d) α=0.9 FIG. 12. The behavior of general photon orbits xi and event horizon xh versus the rotation parameter u for different deviation parameter α . reduces the critical impact parameters for both retrograde and prograde orbits. Therefore, the ...
- [11]
-
[12]
J. M. Bardeen, W. H. Press, and S. A. Teukolsky, Astrophys. J. 178, 347 (1972)
1972
-
[13]
Akiyama et al
K. Akiyama et al. (Event Horizon Telescope), Astrophys. J. Lett. 875, L6 (2019) , arXiv:1906.11243
2019 arXiv
- [14]
-
[15]
Nealon, D
R. Nealon, D. Price, and C. Nixon, Mon. Not. Roy. Astron. Soc. 448, 1526 (2015) , arXiv:1501.01687
2015 arXiv
-
[16]
Kumar and S
R. Kumar and S. G. Ghosh, Class. Quant. Grav. 38, 8 (2021) , arXiv:2004.07501
2021 arXiv
-
[17]
Chandrasekhar and K
S. Chandrasekhar and K. S. Thorne, The Mathematical Theory of Black Holes (1985) pp. 1013–1015
1985
-
[18]
According to the Descartes’ rule of signs, the quartic P4 (x) = 0 can have at most two positive real roots, denoted as x1 and x2
corresponds to the event horizon radius, given by x = 1. According to the Descartes’ rule of signs, the quartic P4 (x) = 0 can have at most two positive real roots, denoted as x1 and x2. The analytical expressions for these roots are complex, so their relationships with the pa...
- [19]
- [20]
-
[21]
with u, one obtains (x − 1)2P4(x) = 0 , (28) 15 β1 β2 FIG. 8. The critical impact parameters βi for photons in the retrograde and prograde orbits on the equatorial plane of the Kerr-MOG black hole. where P4 (x) = u2v + (x − 2)2x2 + 2ux [(2 + x) v − 2x] . (29) It is easily foun...
-
[22]
( 24), we depict the behavior of ddR (2) i as functions of u and α in Fig
into Eq. ( 24), we depict the behavior of ddR (2) i as functions of u and α in Fig
-
[23]
and Eq. ( 29). Fig. 13 shows that ddR (2) i > 0, indicating that all general orbits are radially unstable. Moreover, Fig. 14 illustrates that the impact parameter of general photon orbits are very similar to those on th e equatorial plane. Thus, it can be concluded that the ro...
-
[24]
Carter, Phys
B. Carter, Phys. Rev. 174, 1559 (1968)
1968
-
[25]
J. M. Bardeen and J. A. Petterson, Astrophysical Journa l Letters v. 195, p. L65 195, L65 (1975)
1975
-
[26]
A. S. Alam, L. C. Andaru, B. N. Jayawiguna, and H. S. Ramad han, Gen. Rel. Grav. 56, 79 (2024) , arXiv:2404.17277. 24
2024 arXiv
-
[27]
S. U. Islam, J. Kumar, and S. G. Ghosh, JCAP 10, 013 (2021) , arXiv:2104.00696
2021 arXiv
-
[28]
Galison, M
P. Galison, M. D. Johnson, A. Lupsasca, T. Gravely, and R . Berens, Proc. SPIE Int. Soc. Opt. Eng. 13092, 130926R (2024) , arXiv:2406.11671
2024 arXiv
-
[29]
and illustrate the results in Fig. 8. The orange and blue surfaces correspond to the critical impact p arameters βi for 14 ddR1 (2) ddR2 (2) ddR3 (2) ddR4 (2) FIG. 7. The ddR (2) i as functions of α and u for equatorial photon orbits xi. photons in retrograde and prograde orbi...
-
[30]
Chatterjee, Z
K. Chatterjee, Z. Younsi, M. Liska, A. Tchekhovskoy, S. B. Markoff, D. Yoon, D. van Eijnatten, C. Hesp, A. Ingram, and M. van der Klis, Mon. Not. Roy. Astron. Soc. 499, 362 (2020) , arXiv:2002.08386
2020 arXiv
-
[31]
H. Yang, D. A. Nichols, F. Zhang, A. Zimmerman, Z. Zhang, and Y. Chen, Phys. Rev. D 86, 104006 (2012) , arXiv:1207.4253
2012 arXiv
-
[32]
Tavlayan and B
A. Tavlayan and B. Tekin, Phys. Rev. D 102, 104036 (2020) , arXiv:2009.07012
2020 arXiv
-
[33]
W. Cao, W. Liu, and X. Wu, Phys. Rev. D 105, 124039 (2022) , arXiv:2206.09518
2022 arXiv
-
[34]
Fathi, M
M. Fathi, M. Olivares, and J. R. Villanueva, Eur. Phys. J. Plus 138, 7 (2023) , arXiv:2207.04076
2023 arXiv
-
[35]
Tavlayan and B
A. Tavlayan and B. Tekin, Phys. Rev. D 107, 024016 (2023) , arXiv:2209.14873
2023 arXiv
-
[36]
Chen, J.-H
Y.-X. Chen, J.-H. Huang, and H. Jiang, Phys. Rev. D 107, 044066 (2023) , arXiv:2210.08509
2023 arXiv
-
[37]
D. C. Rodrigues, P. S. Letelier, and I. L. Shapiro, JCAP 04, 020 (2010) , arXiv:0911.4967
2010 arXiv
- [38]
-
[39]
M. J. Geller, A. Diaferio, and K. J. R. A. L. Serra, Astrophys. J. 764, 58 (2013) , arXiv:1209.5675
2013 arXiv
-
[40]
M. T. Hogan, B. R. McNamara, F. Pulido, P. E. J. Nulsen, H. R. Russell, A. N. Vantyghem, A. C. Edge, and R. A. Main, Astrophys. J. 837, 51 (2017) , arXiv:1610.04617
2017 arXiv
-
[41]
Tenjes, T
P. Tenjes, T. Tuvikene, A. Tamm, R. Kipper, and E. Tempel , Astronomy & Astrophysics 600, A34 (2017)
2017
-
[43]
J. R. Brownstein and J. W. Moffat, Mon. Not. Roy. Astron. Soc. 382, 29 (2007) , arXiv:astro-ph/0702146
2007 arXiv
-
[44]
J. W. Moffat, Phys. Lett. B 763, 427 (2016) , arXiv:1603.05225
2016 arXiv
-
[45]
J. W. Moffat, Eur. Phys. J. C 75, 175 (2015) , arXiv:1412.5424
2015 arXiv
-
[46]
J. W. Moffat and V. T. Toth, Phys. Rev. D 91, 043004 (2015) , arXiv:1411.6701
2015 arXiv
- [47]
-
[48]
J. W. Moffat, Eur. Phys. J. C 81, 119 (2021) , arXiv:1806.01903
2021 arXiv
-
[49]
J. W. Moffat and V. T. Toth, Eur. Phys. J. C 81, 836 (2021) , arXiv:2109.11133
2021 arXiv
-
[50]
W. Liu, X. Fang, J. Jing, and J. Wang, JCAP 11, 057 (2023) , arXiv:2306.03599
2023 arXiv
-
[51]
J. W. Moffat and V. T. Toth, Mon. Not. Roy. Astron. Soc. 397, 1885 (2009) , arXiv:0805.4774
2009 arXiv
-
[52]
J. W. Moffat, Eur. Phys. J. C 75, 130 (2015) , arXiv:1502.01677
2015 arXiv
-
[53]
J. R. Mureika, J. W. Moffat, and M. Faizal, Phys. Lett. B 757, 528 (2016) , arXiv:1504.08226
2016 arXiv
-
[54]
Manfredi, J
L. Manfredi, J. Mureika, and J. Moffat, Phys. Lett. B 779, 492 (2018) , arXiv:1711.03199
2018 arXiv
- [55]
-
[56]
M. Guo, N. A. Obers, and H. Yan, Phys. Rev. D 98, 084063 (2018) , arXiv:1806.05249
2018 arXiv
-
[57]
Sheoran, A
P. Sheoran, A. Herrera-Aguilar, and U. Nucamendi, Phys. Rev. D 97, 124049 (2018) , arXiv:1712.03344
2018 arXiv
-
[58]
¨Ovg¨ un, I
A. ¨Ovg¨ un, I. Sakallı, and J. Saavedra, Annals Phys. 411, 167978 (2019) , arXiv:1806.06453
2019 arXiv
- [59]
-
[60]
Rahvar and J
S. Rahvar and J. W. Moffat, Mon. Not. Roy. Astron. Soc. 482, 4514 (2019) , arXiv:1807.07424
2019 arXiv
-
[61]
R. N. Izmailov, R. K. Karimov, E. R. Zhdanov, and K. K. Nan di, 25 Mon. Not. Roy. Astron. Soc. 483, 3754 (2019) , arXiv:1905.01900
2019 arXiv
-
[62]
X. Qiao, M. Wang, Q. Pan, and J. Jing, Eur. Phys. J. C 80, 509 (2020)
2020
-
[64]
P. V. P. Cunha, C. A. R. Herdeiro, and E. Radu, Phys. Rev. D 96, 024039 (2017) , arXiv:1705.05461
2017 arXiv
-
[65]
J. W. Moffat, JCAP 03, 004 (2006) , arXiv:gr-qc/0506021
2006 arXiv
-
[66]
D. C. Wilkins, Phys. Rev. D 5, 814 (1972)
1972
-
[67]
W. Liu, D. Wu, X. Fang, J. Jing, and J. Wang, JCAP 08, 035 (2024) , arXiv:2406.00579
2024 arXiv
-
[68]
Kuang, Z.-Y
X.-M. Kuang, Z.-Y. Tang, B. Wang, and A. Wang, Phys. Rev. D 106, 064012 (2022) , arXiv:2206.05878
2022 arXiv
Reviewed August 11, 2026 · model on record in the stance chip above.
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