Detectability of avoided crossings in black hole ringdowns
Pith reviewed 2026-05-20 17:51 UTC · model grok-4.3
The pith
Avoided crossings in black hole quasinormal modes produce interference that may be detectable even when the individual frequencies cannot be resolved.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Quasinormal modes of black holes can exhibit avoided crossings in which specific frequencies approach each other while their amplitudes are enhanced and acquire nearly opposite phases, leading to characteristic interference. Resolving such closely spaced modes through black hole spectroscopy is observationally challenging. Using Bayesian analysis with three waveform models, the work shows that inferring the separate complex frequencies and amplitudes depends on their separation and on the choice of template. Resolving the individual frequencies is difficult even under optimistic conditions, yet collective waveform signatures associated with avoided crossings may still be identified through a
What carries the argument
Avoided crossing between two quasinormal modes, in which frequencies approach, amplitudes enhance, and phases become nearly opposite to produce interference in the ringdown waveform.
If this is right
- Individual quasinormal mode frequencies remain difficult to resolve even under optimistic conditions.
- Collective waveform signatures tied to avoided crossings can be recovered with complementary waveform descriptions.
- Detection requires that avoided-crossing modes dominate the ringdown and that slower modes contribute negligibly or can be removed.
- The choice of waveform template affects how well frequencies and amplitudes are inferred near an avoided crossing.
Where Pith is reading between the lines
- Observers may need waveform templates that encode collective interference rather than isolated mode frequencies when close pairs are present.
- Time-domain and frequency-domain analyses could be combined to isolate avoided-crossing effects in real gravitational-wave data.
- The same interference logic might apply to other resonant pairs in black-hole perturbation spectra beyond the specific cases studied.
Load-bearing premise
The assumption that the avoided-crossing modes dominate the observed ringdown signal and that contamination from more slowly damped modes is negligible or can be removed.
What would settle it
A measured ringdown waveform in which the avoided-crossing modes are expected to dominate yet no collective interference signature appears after slower modes are subtracted.
Figures
read the original abstract
Quasinormal modes (QNMs) of black holes can exhibit avoided crossings (ACs), in which specific QNM frequencies approach each other while their amplitudes are enhanced and acquire nearly opposite phases, leading to characteristic interference. Resolving such closely spaced modes through black hole spectroscopy is observationally challenging. In this paper, we investigate the detectability of nearly degenerate QNMs in the presence of an AC within a Bayesian framework using three waveform models. We examine how the inference of the complex frequencies and amplitudes depends on the separation between the two QNM frequencies and on the choice of template waveform. We find that resolving the individual QNM frequencies is difficult even under optimistic conditions. On the other hand, collective waveform signatures associated with ACs may still be identified through complementary waveform descriptions, provided that the AC-related modes dominate the observed ringdown signal and contamination from more slowly damped modes is negligible or can be removed.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper examines the detectability of avoided crossings (ACs) among quasinormal modes (QNMs) in black hole ringdown signals. Using a Bayesian framework applied to three waveform models on controlled injections, it finds that resolving the individual (closely spaced) QNM frequencies remains difficult even under optimistic conditions. It concludes that collective waveform signatures associated with ACs may nevertheless be identifiable through complementary descriptions, provided the AC-related modes dominate the observed signal and contamination from more slowly damped modes is negligible or can be removed.
Significance. If the central claims hold, the work usefully distinguishes the practical limits of standard black-hole spectroscopy from the potential utility of collective AC signatures. The explicit use of multiple waveform models and forward-modeling inference is a strength that allows assessment of template dependence. The conditional positive result could guide analysis strategies for ringdown data from gravitational-wave detectors, provided the dominance/contamination premise can be validated.
major comments (2)
- [Abstract] Abstract (final sentence): The claim that collective AC signatures 'may still be identified ... provided that the AC-related modes dominate the observed ringdown signal and contamination from more slowly damped modes is negligible or can be removed' is load-bearing for the positive detectability conclusion. The manuscript applies the three models only to controlled injections; no explicit robustness tests against realistic contamination amplitudes, removal procedures, or varying dominance ratios are described. This leaves the practical applicability of the collective-signature route unsecured.
- [Bayesian framework] Bayesian framework description: The abstract and main text report qualitative findings on frequency/amplitude inference but omit specifics on prior choices, sampling algorithms, convergence diagnostics, or quantitative model-comparison metrics (e.g., Bayes factors or evidence values). These omissions prevent independent verification of the reported difficulty in resolving individual QNMs.
minor comments (2)
- [Figures] Figure captions and legends would benefit from explicit indication of which waveform model corresponds to each curve when comparing the three templates.
- [Notation] Notation for the complex QNM frequencies and amplitudes should be introduced once with a clear table or equation reference to avoid ambiguity across sections.
Simulated Author's Rebuttal
We thank the referee for their thorough review and constructive feedback on our manuscript. We appreciate the recognition of the strengths of our approach using multiple waveform models. Below, we provide point-by-point responses to the major comments and outline the revisions we intend to make.
read point-by-point responses
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Referee: [Abstract] Abstract (final sentence): The claim that collective AC signatures 'may still be identified ... provided that the AC-related modes dominate the observed ringdown signal and contamination from more slowly damped modes is negligible or can be removed' is load-bearing for the positive detectability conclusion. The manuscript applies the three models only to controlled injections; no explicit robustness tests against realistic contamination amplitudes, removal procedures, or varying dominance ratios are described. This leaves the practical applicability of the collective-signature route unsecured.
Authors: We agree that the positive conclusion regarding collective signatures is conditional and that our current analysis relies on controlled injections without explicit tests for contamination. To address this, we will revise the manuscript to include a more detailed discussion in the conclusions section on the implications of contamination and the assumptions required for the collective signatures to be detectable. We will also modify the abstract to better reflect the idealized conditions of our study while maintaining the conditional nature of the claim. revision: yes
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Referee: [Bayesian framework] Bayesian framework description: The abstract and main text report qualitative findings on frequency/amplitude inference but omit specifics on prior choices, sampling algorithms, convergence diagnostics, or quantitative model-comparison metrics (e.g., Bayes factors or evidence values). These omissions prevent independent verification of the reported difficulty in resolving individual QNMs.
Authors: We thank the referee for pointing this out. Upon review, we realize that while some details are present in the methods section, they are not sufficiently detailed. In the revised manuscript, we will expand Section 2.2 to explicitly describe the prior distributions used for the QNM frequencies and amplitudes, the nested sampling algorithm employed (with specific settings), convergence diagnostics such as the evidence tolerance, and we will include quantitative metrics like Bayes factors in a new table or figure to support the model comparisons and the difficulty in resolving individual modes. revision: yes
Circularity Check
No circularity: analysis uses forward modeling and Bayesian inference on controlled injections
full rationale
The paper performs numerical injections of ringdown waveforms containing avoided crossings into noise, then applies Bayesian inference using three distinct waveform templates to assess parameter recovery. The reported difficulty in resolving individual QNM frequencies follows directly from the posterior widths obtained in these controlled simulations. The conditional statement that collective AC signatures may be identifiable is explicitly qualified by the dominance/contamination premise and does not reduce any derived quantity to a fitted input by construction. No self-definitional equations, fitted-input predictions, or load-bearing self-citations appear in the derivation chain; the results remain externally falsifiable against the injected signals.
Axiom & Free-Parameter Ledger
Reference graph
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2DS model The 2DS model consists of a simple superposition of two damped sinusoids, as shown in Eq. (21a), given by h(t) =A 1e−t/τ1ei(2πf1t+ϕ1) +A 2e−t/τ2ei(2πf2t+ϕ2),(23) whereAi,fi,τi, andϕi denote the real-valued amplitude, frequency, damping time, and initial phase of thei-th QNM(i∈{1, 2}), respectively. The waveform convention is chosen to be consist...
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[2]
2AC model The 2AC model also consists of two damped modes, but incorporates the characteristic enhancement and the de- structive interference associated with an AC. This model corresponds to Eq. (21b) and is written as h(t) = A δωe−t/τ1ei(2πf1t+ϕA) −A(1 +αeiϕαδω) δω e−t/τ2ei(2πf2t+ϕA),(24) where A, α, ϕA, andϕαare real-valued model parame- ters. Here,δω:=...
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This approximation defines the effective EP model described by Eq.(21c)
EP model The 2AC model can be further approximated by a single effective double-pole QNM with frequencyf and damping timeτ. This approximation defines the effective EP model described by Eq.(21c). The EP waveform introduced in Eq. (21c) is reparameterized as h(t) = ( Ce iϕC + iDeiϕDt ) e−t/τei2πft,(25) where C and ϕC denote the amplitude and phase of the ...
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Controlled injections of nearly degenerate QNM pairs To evaluate the detectability of AC signatures in ring- down signals, we consider controlled injections of nearly degenerate QNM pairs whose complex-frequency separa- tions are systematically varied. In realistic situations, not only the two QNMs associated with the AC but also the excitation of other m...
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Here we inject these two modes and assume that the lower overtones have been removed beforehand
Application to avoided crossing in GR As an example of the AC in GR, we apply the three waveform models to an idealized ringdown signal com- posed of the(ℓ,m,n) = (2, 2, 5)and(2 , 2, 6)QNMs of a Kerr black hole with dimensionless spinj = 0.9and mass M = 60M⊙. Here we inject these two modes and assume that the lower overtones have been removed beforehand. ...
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Figure 4 shows the posterior distributions inferred using the three waveform models
We setα=M with M = 60M⊙, corresponding to α∼0.296 ms, and chooseϕA =ϕα= 0. Figure 4 shows the posterior distributions inferred using the three waveform models. The initial phase parame- ters are not shown since they are not relevant to the present discussion. The posteriors of the frequencies and damping times obtained with the 2DS and 2AC mod- els are qu...
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discussion (0)
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