REVIEW 3 major objections 5 minor 1 cited by
Quasinormal mode spectrum of rotating black holes in Einstein-Gauss-Bonnet-dilaton theory
T0 review · 3 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read This paper computes the quasinormal-mode spectrum of rapidly rotating Einstein-Gauss-Bonnet-dilaton black holes non-perturbatively and finds that the dominant ringdown mode can change with the coupling and spin.
desk verdict First non-perturbative rotating EGBd QNM spectrum, with a credible method and honest caveats, but the headline mode-ordering reversal at j=0.8 sits exactly where the authors concede accuracy loss. 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 machinery is the spectral reduction of the coupled perturbation equations. The seven perturbation functions — the metric functions H1, T, N, L, the axial functions h0, h1, and the dilaton perturbation Φ1 — are expanded in Chebyshev polynomials in the compactified radial coordinate x and in associated Legendre functions of y=cosθ, with the outgoing and ingoing boundary behavior factored out. Evaluating the PDEs and boundary conditions on a Gauss-Lobatto and uniform grid converts the system into a quadratic eigenvalue problem (M0 + M1 ω + M2 ω²) C = 0, whose eigenvalues are the complex quasinormal frequencies. The same scheme is validated against Kerr and against second-order-in-spin, sixth-order-in-coupling perturbative results.
What would settle it
Repeat the eigenvalue calculation for the fundamental l=2 scalar-led mode at j=0.2 and ξ near the upper end of the domain (around 0.17) with doubled spectral resolution (larger Nx and Ny) and with an independent time-domain evolution of the linearized EGBd equations; if the sharp drop in MωI that makes the scalar mode longest-lived near the domain boundary shifts or disappears as the resolution changes, the claimed mode-order change is a numerical artifact rather than a property of the theory.
Extended reading notes
Core claim
The central claim is that the quasinormal modes of rapidly rotating EGBd black holes can be extracted without treating either the Gauss-Bonnet coupling or the angular momentum as small parameters. Working with numerically exact stationary axisymmetric backgrounds, the authors linearize the metric and dilaton field equations, impose purely outgoing waves at infinity and purely ingoing waves at the horizon, and expand the seven perturbation functions in Chebyshev and Legendre series, reducing the problem to a quadratic eigenvalue problem. For azimuthal number Mz=2, l=2- and l=3-led fundamental modes, they find that polar-led and axial-led modes split once the coupling is turned on, and that the imaginary parts — the damping times — develop a strong coupling dependence. Toward the boundary of the domain of existence the scalar-led modes cross the metric-led modes and become the longest lived, so the ordering of modes, and hence which mode dominates the ringdown, changes with both the coupling and the angular momentum.
Load-bearing premise
The load-bearing premise is that the spectral expansion of the seven coupled perturbation functions is converged for the EGBd backgrounds, so the complex frequencies it returns are true quasinormal modes; the paper validates the pipeline on Kerr and against weak-coupling perturbative results, but it reports no dedicated convergence or error study for the EGBd eigenvalue problem itself, and it states that for j=0.6 and 0.8 the calculations lose accuracy toward the boundary of the domain of existence.
Editorial extensions
If this is right
- Isospectrality is generically broken in EGBd: once the Gauss-Bonnet coupling is nonzero, polar-led and axial-led modes split, and the splitting grows with the coupling.
- Near the boundary of the domain of existence, scalar-led modes can be the longest lived, so the dominant ringdown mode is not simply the Kerr quadrupole mode; ringdown analyses that assume Kerr isospectrality would misidentify the mode.
- Perturbative expansions in the spin and the coupling deviate increasingly from the exact spectrum at large angular momentum and large coupling, so strong-coupling ringdown predictions require the full numerical treatment.
- The domain of existence of rotating EGBd black holes, including solutions that slightly exceed the Kerr bound j=1, is charted by the background construction, showing where perturbative methods break down.
- Precision ringdown observations could in principle place bounds on the Gauss-Bonnet coupling by comparing the measured frequencies and damping times with these non-perturbative spectra.
Reading between the lines
- (Editorial inference) The crossing pattern suggests that the ringdown of an EGBd black hole could show a sharp transition in the longest-lived mode as a function of spin or coupling; a search that follows the longest-lived mode continuously would be a sharper test than comparing isolated frequencies.
- (Editorial inference) The same spectral method should carry over to other scalar-tensor theories, such as shift-symmetric Einstein-Gauss-Bonnet gravity, and the tendency of scalar-led modes to dominate near the domain boundary is likely a general feature; this is a testable prediction for those theories.
- (Editorial inference) The paper computes the fundamental modes only; overtone spectra could change which mode actually dominates a matched-filter ringdown search at early times, so the dominance claim is a statement about fundamentals, not the full signal.
- (Editorial inference) A practical next step would be to compute excitation coefficients from binary merger initial data, since spectral dominance alone does not determine how strongly each mode is rung up in a real event.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents a numerical spectral method for computing the quasinormal mode (QNM) spectrum of rapidly rotating Einstein-Gauss-Bonnet-dilaton (EGBd) black holes without perturbative expansions in either the Gauss-Bonnet coupling or the angular momentum. The authors first obtain stationary, axially symmetric background solutions with the FIDISOL/CADSOL package, then linearize the metric and dilaton equations using a seven-function perturbation ansatz (polar, axial, and scalar sectors), apply ingoing/outgoing boundary conditions at the horizon and infinity, and solve the resulting quadratic eigenvalue problem with a Chebyshev-Legendre spectral decomposition. They report fundamental l=2- and l=3-led modes with azimuthal number Mz=2 for angular momenta j=0.2, 0.4, 0.6, 0.8 and both signs of the real frequency. The results are validated against the Kerr limit and against second-order perturbative results for small coupling and rotation, and they exhibit isospectrality breaking and a reordering of modes with increasing coupling, including a claimed dominance of scalar-led modes near the boundary of the domain of existence.
Significance. If the results are correct, this is a significant advance: it provides the first fully non-perturbative QNM spectrum for rapidly rotating black holes in a well-motivated modified gravity theory, with direct implications for ringdown-based tests of gravity. The paper's strengths include a detailed description of the background construction and perturbation method, extensive numerical tables, and genuine external checks against Kerr and second-order perturbative results in the weak-coupling regime. However, the central phenomenological claim that the dominant ringdown mode changes to the scalar-led sector—and in particular that the l=3 scalar mode can dominate at high angular momentum—rests on data in a region where the authors themselves state that accuracy is degraded, and no EGBd-specific convergence or error study is provided. As presented, the qualitative scalar-dominance trend is plausible, but the stronger quantitative claims are not yet fully established.
major comments (3)
- [Section 4, Tables 13 and 16] The claimed octupole dominance at j=0.8 is decided by the last tabulated point xi=0.11221: the scalar l=3 mode's M*omega_I changes from -0.06379 at xi=0.10926 to -0.05660, overtaking the polar l=2 mode (M*omega_I=-0.06271) by only about 0.006. This is precisely the regime covered by the sentence in Section 4: 'for j=0.6 and 0.8 our calculations lose accuracy towards the boundary of the domain of existence.' Since a spectral method can produce spurious eigenvalues near a boundary, the crossing at the final tabulated point cannot be distinguished from a numerical artifact without a convergence or resolution study. The conclusion in Section 5 that 'the octupole modes may also dominate over the quadrupole modes for large coupling' is therefore not secure at j=0.8.
- [Section 3.4, Eqs. (93)-(102)] No convergence tests or error estimates are reported for the EGBd eigenvalue problem itself. The Kerr limit (xi=0) checks the method only in the decoupled case, and the perturbative comparison in Fig. 5 explicitly covers polar-led modes only (the text notes 'the axial modes were not given in there'). The scalar-led and axial-led strong-coupling modes are therefore validated neither against an independent solver nor against the static EGBd limit. To support the central mode-ordering claim, the paper should provide, at minimum for the j=0.6 and j=0.8 cases, a convergence study in Nx and Ny for the fundamental polar, axial, and scalar modes, and report the resulting errors on M*omega_R and M*omega_I, or state explicitly which tabulated points meet a chosen accuracy threshold.
- [Section 4 and Fig. 4] The scalar-led modes in the strong-coupling regime are not checked against the static (j=0) EGBd QNMs of Refs. [38,41] or any other independent non-Kerr limit. Such a comparison would directly test the scalar sector and the axial/polar splitting at finite coupling, where no perturbative anchor exists. If this check cannot be performed, the claim that 'scalar modes tend to dominate' toward the boundary should be restricted to the j=0.2 and 0.4 data, where the trend is visible before the accuracy caveat becomes severe, and the stronger j=0.8 octupole-ordering statement should be deferred until the numerical accuracy is demonstrated.
minor comments (5)
- [Section 3.1 and Fig. 3] The classification of modes as polar-led, axial-led, or scalar-led is based on visual inspection of the parity of Re(T~), Re(h0~), and Re(Phi~) at a single point (j=0.6, xi=0.11). Because rotation and coupling mix multipoles, and the mixing increases with xi, a quantitative criterion (for instance, the relative magnitude of the dominant spectral coefficients) would make the mode labels reproducible and less subjective.
- [Section 4 and Appendix] The text states that for j=0.6 and 0.8 the calculations lose accuracy towards the boundary of the domain of existence and that the authors 'refrained from showing their values there,' yet Tables 9-16 do list values at the largest couplings. Please clarify which entries are considered unreliable and mark them in the tables (for example, with parentheses or italics).
- [Fig. 4 caption] The dotted vertical lines are said to represent the maximal value of the scaled coupling constant for which background solutions exist, but the caption does not explain how those values are obtained; a reference to Fig. 1b or to the boundary curves would help the reader.
- [Section 3.4, Eq. (102)] The quadratic eigenvalue problem is solved 'with the same numerical methods described previously [49],' but the present paper does not specify the linearization procedure, matrix sizes, or tolerance settings used for the EGBd runs; adding one sentence with these details would improve reproducibility.
- [Appendix tables] The tables report four or five significant digits without error bars; if convergence information is obtained, including error estimates in the tables would make the numerical content more useful to future comparisons.
Circularity Check
No significant circularity: the quasinormal-mode frequencies are outputs of an independently solved linearized eigenvalue problem, checked against Kerr and perturbative limits rather than fitted to them.
full rationale
The paper's derivation chain is self-contained as a numerical calculation: the EGBd background solutions (Section 2) are obtained by solving the coupled field equations with stated boundary conditions, and the perturbation equations (Section 3) are linearized about those backgrounds. The quasinormal-mode frequencies are the eigenvalues of the quadratic matrix problem (M0 + M1 omega + M2 omega^2) C = 0, Eq. (102), with purely ingoing and outgoing boundary conditions at the horizon and infinity. Nothing in this construction fits omega_R or omega_I to the values used for validation. The checks performed—reduction to Kerr modes at xi = 0 and comparison with second-order perturbative results of Ref. [43]—are genuine external comparisons: the perturbative data are obtained by an independent method, and the agreement is reported as confirmation rather than used as input. The mode classification into polar-led, axial-led, and scalar-led sectors is made by inspecting the parity of the perturbation eigenfunctions (Fig. 3), not by assigning labels to match known results. The paper's own caveat that for j = 0.6 and 0.8 'our calculations lose accuracy towards the boundary of the domain of existence' is a numerical-convergence concern about strong-coupling points near the boundary; it is a correctness risk, not a circularity, because the affected values are still outputs of the same eigenvalue problem rather than quantities forced by definition or by self-citation. The only self-citations are to the authors' earlier construction of the background solutions and to their earlier Kerr implementation of the same spectral method, both of which are independent support (previously published numerical results and a tested code) and are not used to define the present eigenvalues. No step reduces by construction to its own input, and no fitted parameter is later renamed as a prediction. The central claim—that mode ordering can change with coupling and angular momentum—is an emergent property of the computed spectrum, not an assumption. Therefore the paper exhibits no significant circularity and merits a score of 0.
Assumptions & free parameters
assumptions (5)
- domain assumption The action (1) with f(phi)=exp(-gamma phi), gamma=1, is an appropriate low-energy EGBd theory.
- ad hoc to paper The axial/polar perturbation ansatz with functions H1,T,N,L,h0,h1,Phi captures all linear gravitational and scalar perturbations relevant for the QNM spectrum.
- domain assumption Quasinormal modes are selected by purely outgoing waves at infinity and purely ingoing waves at the horizon.
- standard math The double Chebyshev-Legendre spectral series converges to the true perturbation solutions on EGBd backgrounds.
- ad hoc to paper The numerical background solutions computed with FIDISOL/CADSOL with estimated error 10^-5 are accurate enough for the perturbation calculation.
Cite this review
Pith. "Pith review of Quasinormal mode spectrum of rotating black holes in Einstein-Gauss-Bonnet-dilaton theory." pith.science (2026). https://pith.science/paper/T3B3KR7U
@misc{pith2026241217073,
author = {Pith},
title = {Pith review of: Quasinormal mode spectrum of rotating black holes in Einstein-Gauss-Bonnet-dilaton theory},
year = {2026},
howpublished = {\url{https://pith.science/paper/T3B3KR7U}},
note = {Machine review of arXiv:2412.17073}
}
read the original abstract
Quasinormal modes are excited during the ringdown phase of black holes after merger. Determination of quasinormal modes of rapidly rotating black holes in alternative theories of gravity has remained a challenge for a long time. Here we discuss in detail our recently developed method to extract the quasinormal modes for rapidly rotating black holes in Einstein-Gauss-Bonnet-dilaton theory. We first obtain numerically the exact rapidly rotating background solutions, which also clarify their domain of existence. Then we solve the equations for the linear perturbations of the metric and the dilaton field by employing an appropriate set of boundary conditions and a spectral decomposition of the perturbation functions. The resulting spectrum agrees well with the known limits obtained for slow rotation and weak coupling, while it exhibits larger deviations for stronger coupling.
Figures
Figures from the paper (3 more)
Forward citations
Cited by 1 Pith paper
-
Schr\"odinger perturbation theory for black hole quasinormal modes
A bilinear-form framework computes black-hole quasinormal-mode frequency shifts to any order, but the mode-sum expansion of the first-order mode shift diverges and needs a continuum piece.
Reference graph
Works this paper leans on
-
[1]
Beyond Einstein Gravity: A Surve y of Gravitational Theories for Cosmology and Astrophysics,
V. Faraoni and S. Capozziello, “Beyond Einstein Gravity: A Surve y of Gravitational Theories for Cosmology and Astrophysics,” Springer (2011)
work page 2011
- [2]
-
[3]
Modified Gravity and Cosmology: An Update by the CANTATA Network,
E. N. Saridakis et al. [CANTATA], “Modified Gravity and Cosmology: An Update by the CANTATA Network,” Springer (2021)
work page 2021
-
[4]
Theory and Experiment in Gravitational Physics,
C. M. Will, “Theory and Experiment in Gravitational Physics,” Cambr idge Univer- sity Press (2018)
work page 2018
-
[5]
B. P. Abbott et al. [LIGO Scientific and Virgo], Phys. Rev. Lett. 116, 061102 (2016) 19 Axial Polar Axial Polar α/M2 ωR ωI ωR ωI ωR ωI ωR ωI 0 0.40215 -0.08831 0.40215 -0.08831 -0.35105 -0.08918 -0.35105 -0.08918 0.00196 0.40217 -0.08830 0.40215 -0.08831 -0.35105 -0.08917 -0.35105 -0.08918 0.00784 0.40215 -0.08830 0.40213 -0.08831 -0.35105 -0.08918 -0.3510...
work page 2016
-
[6]
B. P. Abbott et al. [KAGRA, LIGO Scientific and Virgo], Living Rev. Rel. 19, 1 (2016)
work page 2016
-
[7]
Cahillane and G
C. Cahillane and G. Mansell, Galaxies 10, 36 (2022)
2022
-
[8]
M. Punturo, M. Abernathy, F. Acernese, B. Allen, N. Andersso n, K. Arun, F. Barone, B. Barr, M. Barsuglia and M. Beker, et al. Class. Quant. Grav. 27, 194002 (2010)
work page 2010
Show all 59 references
-
[9]
Dwyer, D
S. Dwyer, D. Sigg, S.W. Ballmer, L. Barsotti, N. Mavalvala, and M. E vans, Phys. Rev. D 91, 082001 (2015)
2015
-
[10]
Colpi, K
M. Colpi, K. Danzmann, M. Hewitson, P. Jetzer, G. Nelemans, A. Petiteau, D. Shoe- maker, C. Sopuerta, R. Stebbins and N. Tanvir, et al. [arXiv:2402.07571 [astro- ph.CO]]
-
[11]
K. D. Kokkotas and B. G. Schmidt, Living Rev. Rel. 2, 2 (1999)
1999
-
[12]
Berti, V
E. Berti, V. Cardoso and A. O. Starinets, Class. Quant. Grav. 26, 163001 (2009)
2009
-
[13]
R. A. Konoplya and A. Zhidenko, Rev. Mod. Phys. 83, 793 (2011)
2011
-
[14]
S. A. Teukolsky, Astrophys. J. 185, 635 (1973)
1973
-
[15]
´O. J. C. Dias, M. Godazgar and J. E. Santos, Phys. Rev. Lett. 114, 151101 (2015)
2015
-
[16]
O. J. C. Dias, M. Godazgar, J. E. Santos, G. Carullo, W. Del Poz zo and D. Laghi, Phys. Rev. D 105, 084044 (2022)
2022
-
[17]
O. J. C. Dias, M. Godazgar and J. E. Santos, JHEP 07, 076 (2022)
2022
-
[18]
P. A. Cano, K. Fransen, T. Hertog and S. Maenaut, Phys. Rev . D 108, 024040 (2023)
2023
-
[19]
P. A. Cano, K. Fransen, T. Hertog and S. Maenaut, Phys. Rev . D 108, 124032 (2023)
2023
-
[20]
P. A. Cano, L. Capuano, N. Franchini, S. Maenaut and S. H. V¨ o lkel, Phys. Rev. D 110, no.12, 124057 (2024)
2024
-
[21]
F. S. Miguel, Phys. Rev. D 109, 104016 (2024)
2024
-
[22]
S. S. Bohra, S. Sarkar and A. A. Sen, Phys. Rev. D 109, 104021 (2024)
2024
-
[23]
C. P. Burgess, Living Rev. Rel. 7 (2004) 5
2004
-
[24]
G. W. Horndeski, Int. J. Theor. Phys. 10, 363 (1974)
1974
-
[25]
D. J. Gross and J. H. Sloan, Nucl. Phys. B 291, 41 (1987)
1987
-
[26]
R. R. Metsaev and A. A. Tseytlin, Nucl. Phys. B 293, 385 (1987)
1987
-
[27]
Kanti, N
P. Kanti, N. E. Mavromatos, J. Rizos, K. Tamvakis and E. Winsta nley, Phys. Rev. D 54, 5049 (1996)
1996
-
[28]
Torii, H
T. Torii, H. Yajima and K. i. Maeda, Phys. Rev. D 55, 739 (1997)
1997
-
[29]
Z. K. Guo, N. Ohta and T. Torii, Prog. Theor. Phys. 120, 581 (2008) 27
2008
-
[30]
Pani and V
P. Pani and V. Cardoso, Phys. Rev. D 79, 084031 (2009)
2009
-
[31]
Kleihaus, J
B. Kleihaus, J. Kunz and E. Radu, Phys. Rev. Lett. 106, 151104 (2011)
2011
-
[32]
P. Pani, C. F. B. Macedo, L. C. B. Crispino and V. Cardoso, Phys . Rev. D 84, 087501 (2011)
2011
-
[33]
Ayzenberg, K
D. Ayzenberg, K. Yagi and N. Yunes, Phys. Rev. D 89, 044023 (2014)
2014
-
[34]
Ayzenberg and N
D. Ayzenberg and N. Yunes, Phys. Rev. D 90, 044066 (2014)
2014
-
[35]
Kleihaus, J
B. Kleihaus, J. Kunz and S. Mojica, Phys. Rev. D 90, 061501 (2014)
2014
-
[36]
Maselli, P
A. Maselli, P. Pani, L. Gualtieri and V. Ferrari, Phys. Rev. D 92, 083014 (2015)
2015
-
[37]
Kleihaus, J
B. Kleihaus, J. Kunz, S. Mojica and E. Radu, Phys. Rev. D 93, 044047 (2016)
2016
-
[38]
J. L. Bl´ azquez-Salcedo, C. F. B. Macedo, V. Cardoso, V. Fer rari, L. Gualtieri, F. S. Khoo, J. Kunz and P. Pani, Phys. Rev. D 94, 104024 (2016)
2016
-
[39]
P. V. P. Cunha, C. A. R. Herdeiro, B. Kleihaus, J. Kunz and E. Ra du, Phys. Lett. B 768, 373 (2017)
2017
-
[40]
Zhang, M
H. Zhang, M. Zhou, C. Bambi, B. Kleihaus, J. Kunz and E. Radu, P hys. Rev. D 95, 104043 (2017)
2017
-
[41]
J. L. Bl´ azquez-Salcedo, F. S. Khoo and J. Kunz, Phys. Rev. D 96, 064008 (2017)
2017
-
[42]
Pierini and L
L. Pierini and L. Gualtieri, Phys. Rev. D 103, 124017 (2021)
2021
-
[43]
Pierini and L
L. Pierini and L. Gualtieri, Phys. Rev. D 106, 104009 (2022)
2022
-
[44]
J. L. Bl´ azquez-Salcedo, F. S. Khoo, B. Kleihaus and J. Kunz, P hys. Rev. D 111, no.2, L021505 (2025)
2025
-
[45]
Kleihaus and J
B. Kleihaus and J. Kunz, Phys. Rev. Lett. 86, 3704 (2001)
2001
-
[46]
Kleihaus, J
B. Kleihaus, J. Kunz and F. Navarro-Lerida, Phys. Rev. D 66, 104001 (2002)
2002
-
[47]
Kleihaus, J
B. Kleihaus, J. Kunz and F. Navarro-Lerida, Phys. Rev. D 69, 064028 (2004)
2004
-
[48]
Sch¨ onauer and R
W. Sch¨ onauer and R. Weiß, J. Comput. Appl. Math. 27, 279 (19 89) 279; M. Schauder, R. Weiß and W. Sch¨ onauer, The CADSOL Program Package , Univer- sit¨ at Karlsruhe, Interner Bericht Nr. 46/92 (1992)
1992
-
[49]
J. L. Bl´ azquez-Salcedo, F. S. Khoo, J. Kunz and L. M. Gonz´ alez-Romero, Phys. Rev. D 109, 064028 (2024)
2024
-
[50]
F. S. Khoo, B. Azad, J. L. Bl´ azquez-Salcedo, L. M. Gonz´ alez -Romero, B. Kleihaus, J. Kunz and F. Navarro-L´ erida, Phys. Rev. D 109, 084013 (2024)
2024
-
[51]
Holoborodko, Advanpix 5.1.0.15432, http://www.advanpix.com
P. Holoborodko, Advanpix 5.1.0.15432, http://www.advanpix.com
-
[52]
F. L. Juli´ e, L. Pompili and A. Buonanno, Phys. Rev. D 111, no.2, 024016 (2025)
2025
-
[53]
T. P. Sotiriou and S. Y. Zhou, Phys. Rev. Lett. 112 (2014) 251102 28
2014
-
[54]
T. P. Sotiriou and S. Y. Zhou, Phys. Rev. D 90 (2014) 124063
2014
-
[55]
J. F. M. Delgado, C. A. R. Herdeiro and E. Radu, JHEP 04, 180 (2020)
2020
-
[56]
Sullivan, N
A. Sullivan, N. Yunes and T. P. Sotiriou, Phys. Rev. D 103, no.12, 124058 (2021)
2021
-
[57]
A. K. W. Chung and N. Yunes, Phys. Rev. Lett. 133, 181401 (2024)
2024
-
[58]
A. K. W. Chung and N. Yunes, Phys. Rev. D 110, 064019 (2024)
2024
-
[59]
F. S. Khoo, J. L. Bl´ azquez-Salcedo, B. Kleihaus and J. Kunz, [a rXiv:2412.09377 [gr-qc]]. 29
Reviewed August 11, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.