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arxiv: 2605.16425 · v1 · pith:H3ZJ266Fnew · submitted 2026-05-14 · 🌌 astro-ph.HE

Polarization Birefringence and Waveform Systematics in GW231123

Tonghua Liu , Chenggang Shao , Kai Liao This is my paper

Pith reviewed 2026-05-20 20:53 UTC · model grok-4.3

classification 🌌 astro-ph.HE
keywords gravitational wavespolarization birefringenceGW231123waveform systematicsparity violationbinary black holesgravitational wave propagationhigh-mass mergers
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The pith

GW231123 yields no waveform-independent evidence for polarization birefringence, with the single positive result traced to modeling degeneracies.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper tests whether the short high-mass binary black hole event GW231123 carries a frequency-dependent rotation of its polarization basis that would signal parity violation during propagation. Instead of directly sampling a distance-normalized coefficient, the authors sample the band-differential rotation between 20 Hz and 448 Hz and derive the corresponding coefficient for comparison with standard parametrizations. They repeat the analysis with three waveform families that differ in how they treat spin, precession, and higher modes. Only one family returns a positive Bayes factor for birefringence; the other two favor general relativity, and the posteriors on the derived coefficient remain consistent with zero in all cases. A reader would care because any claimed detection of new gravitational physics must survive changes in waveform assumptions before it can be treated as robust.

Core claim

We analyze GW231123 using IMRPhenomXPHM, IMRPhenomXO4a, and NRSur7dq4. Sampling the band-differential rotation δ_br with a uniform prior on [-π, π], we obtain posteriors on the derived birefringence coefficient that are consistent with general relativity, giving 90 percent upper limits of 0.378, 0.097, and 0.273 respectively. The directly sampled δ_br remains broad with a 90 percent interval of roughly 2.8 rad. The Bayes factors are waveform-dependent, and we find no waveform-robust evidence for parity-violating propagation; the positive result for IMRPhenomXO4a is interpreted as a waveform-dependent birefringence-like response driven by the mass-ratio–distance–spin degeneracy of this short,

What carries the argument

Band-differential rotation δ_br = Δ(448 Hz) − Δ(20 Hz), which quantifies the accumulated polarization-plane rotation across the detector band and is converted to the standard propagation coefficient β_br^derived for comparison with existing bounds.

If this is right

  • Upper limits on the derived birefringence coefficient remain consistent with general relativity for every waveform family tested.
  • The accumulated rotation across the analysis band stays only weakly constrained at |δ_br|_90 ≈ 2.8 rad.
  • Tests of parity violation in short high-mass binary black hole signals require cross-checks with multiple waveform families to guard against false positives.
  • Mass-ratio–distance–spin degeneracies in high-mass events can generate apparent birefringence signatures inside specific waveform models.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar parameter degeneracies could mimic other modified-gravity signatures in high-mass gravitational-wave events, requiring multi-waveform validation for any claimed deviation.
  • Tighter constraints on high-mass waveform modeling would reduce the chance that systematics are misread as propagation effects.
  • A catalog-wide analysis that enforces waveform consistency could place stronger limits on frequency-dependent birefringence than single-event studies allow.

Load-bearing premise

That a birefringence-like signature appearing in only one waveform family must be attributed to modeling systematics rather than to a real frequency-dependent rotation that the other families simply fail to capture.

What would settle it

A future event or re-analysis in which all three waveform families return consistent positive evidence for birefringence with overlapping posteriors on δ_br would indicate genuine parity-violating propagation rather than a modeling artifact.

Figures

Figures reproduced from arXiv: 2605.16425 by ChengGang Shao, Kai Liao, Tonghua Liu.

Figure 1
Figure 1. Figure 1: FIG. 1. Detector-frame and source-frame mass posteriors for the birefringent runs compared with the corresponding public [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Marginalized posteriors for the derived propagation coefficient [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Posterior correlations of [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Bayes factors for the birefringent model relative to the [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
read the original abstract

GW231123 is a short, massive binary-black-hole event whose source properties show strong waveform dependence. We use this event to test gravitational-wave polarization birefringence, modeled as a frequency-dependent rotation of the tensor-polarization basis. Instead of sampling a distance-normalized coefficient directly, we sample the band-differential rotation $\delta_{\rm br}=\Delta(448\,\mathrm{Hz})-\Delta(20\,\mathrm{Hz})$ with prior $[-\pi,\pi]$, and report the derived coefficient $\beta_{\rm br}^{\rm derived}$ for comparison with standard propagation parametrizations. We analyze three waveform families: IMRPhenomXPHM (XPHM), IMRPhenomXO4a (XO4a), and NRSur7dq4. The derived posteriors are consistent with the general relativity value, giving $90\%$ upper limits $|\beta_{\rm br}^{\rm derived}|_{90}=0.378,\,0.097,\,0.273$ for XPHM, XO4a, and NRSur7dq4, respectively. The directly sampled $\delta_{\rm br}$ posterior remains broad, with $|\delta_{\rm br}|_{90}\simeq2.8\,\mathrm{rad}$, so the accumulated rotation across the analysis band is weakly constrained. The Bayes factors are waveform dependent: $\ln\mathcal{B}_{\rm br/GR}=-1.26\pm0.30$, $+3.64\pm0.28$, and $-0.86\pm0.29$, respectively. We therefore find no waveform-robust evidence for parity-violating propagation. The positive XO4a result is better interpreted as a waveform-dependent birefringence-like response associated with the mass-ratio--distance--spin degeneracy of this short high-mass event.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript analyzes the short, high-mass binary black hole event GW231123 to test for gravitational-wave polarization birefringence, modeled as a frequency-dependent rotation of the tensor polarization basis. The authors sample the band-differential rotation δ_br = Δ(448 Hz) − Δ(20 Hz) with a uniform prior [−π, π] rather than a distance-normalized coefficient, derive β_br^derived for comparison with standard parametrizations, and analyze the event with three waveform families (IMRPhenomXPHM, IMRPhenomXO4a, NRSur7dq4). Posteriors for β_br^derived are consistent with the GR value (90% upper limits 0.378, 0.097, 0.273), the directly sampled δ_br posterior is broad (|δ_br|_90 ≃ 2.8 rad), and Bayes factors are waveform-dependent (ln B_br/GR = −1.26, +3.64, −0.86). The positive XO4a result is attributed to a mass-ratio–distance–spin degeneracy inducing a spurious birefringence-like response; the overall conclusion is that there is no waveform-robust evidence for parity-violating propagation.

Significance. If the central claim and reinterpretation hold, the work usefully illustrates how waveform modeling choices can produce apparent beyond-GR signals in short, high-mass events through parameter degeneracies, reinforcing the need for multi-family analyses in tests of GR. The direct sampling of δ_br, explicit reporting of derived quantities, and use of three distinct waveform approximants are strengths that improve transparency. The result is a cautionary demonstration rather than a strong new constraint, but it is still valuable for the field provided the degeneracy attribution is substantiated.

major comments (2)
  1. [§4] §4 (Bayes factor and posterior results for XO4a): The claim that ln B_br/GR = +3.64 arises exclusively from a mass-ratio–distance–spin degeneracy that produces a spurious birefringence-like response in IMRPhenomXO4a but not in XPHM or NRSur7dq4 is load-bearing for the 'no waveform-robust evidence' conclusion. No quantitative support is provided, such as posterior correlation plots between δ_br and the degenerate parameters or injection-recovery tests showing differential bias across families. Without this, the alternative that XO4a captures a real frequency-dependent rotation missed by the other models cannot be excluded.
  2. [Methods] Methods section (derivation of β_br^derived from sampled δ_br): The paper samples δ_br directly and then reports a derived β_br^derived for comparison with standard parametrizations. The explicit mapping or independence of this derivation step from the sampled parameter should be shown (e.g., via an equation or appendix) to confirm it does not introduce hidden fitting or circularity, consistent with the modest circularity concern in the stress-test note.
minor comments (2)
  1. [Abstract] Abstract and §2: The frequency points 20 Hz and 448 Hz are introduced without immediate justification; a brief statement of why these band edges were chosen (e.g., detector sensitivity or signal content) would improve clarity.
  2. [Tables/Figures] Figure captions and Table 1: Ensure all waveform-specific Bayes factors and 90% limits are tabulated together with their uncertainties for easy cross-comparison.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments, which have helped clarify the presentation of our results. We respond to each major comment below and indicate the revisions we will make to the manuscript.

read point-by-point responses

  1. Referee: [§4] §4 (Bayes factor and posterior results for XO4a): The claim that ln B_br/GR = +3.64 arises exclusively from a mass-ratio–distance–spin degeneracy that produces a spurious birefringence-like response in IMRPhenomXO4a but not in XPHM or NRSur7dq4 is load-bearing for the 'no waveform-robust evidence' conclusion. No quantitative support is provided, such as posterior correlation plots between δ_br and the degenerate parameters or injection-recovery tests showing differential bias across families. Without this, the alternative that XO4a captures a real frequency-dependent rotation missed by the other models cannot be excluded.

    Authors: We agree that the attribution of the positive Bayes factor for XO4a requires stronger quantitative support to rule out the possibility of a genuine signal captured only by that model. In the revised manuscript we will add posterior correlation plots for the XO4a analysis that explicitly show the correlations between δ_br and the mass ratio, luminosity distance, and component spins. We will also include results from injection-recovery tests performed with GR signals whose parameters are drawn from the GW231123 posterior; these tests demonstrate that XO4a recovers a spurious non-zero δ_br preference while the other two families recover values consistent with zero. The expanded discussion in §4 will present these diagnostics and thereby substantiate the degeneracy interpretation. revision: yes

  2. Referee: [Methods] Methods section (derivation of β_br^derived from sampled δ_br): The paper samples δ_br directly and then reports a derived β_br^derived for comparison with standard parametrizations. The explicit mapping or independence of this derivation step from the sampled parameter should be shown (e.g., via an equation or appendix) to confirm it does not introduce hidden fitting or circularity, consistent with the modest circularity concern in the stress-test note.

    Authors: We thank the referee for highlighting the need for greater transparency in the post-processing step. The conversion from the directly sampled δ_br to the reported β_br^derived is a deterministic linear rescaling that follows from the assumed frequency dependence of the birefringence rotation and the conventional definition of the propagation coefficient β. In the revised manuscript we will insert the explicit mapping equation into the Methods section and add a short appendix that derives the relation step by step, confirming that no additional fitting or re-sampling is involved. This change removes any ambiguity about circularity. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation self-contained via direct sampling and cross-waveform comparison

full rationale

The paper directly samples the band-differential rotation δ_br from the GW data under three independent waveform models and computes Bayes factors against GR. The reported β_br^derived is a post-sampling algebraic transformation used solely for literature comparison and does not feed back into the likelihood or force the Bayes factors. The central null conclusion follows from the observed waveform dependence of the Bayes factors themselves (negative for XPHM and NRSur7dq4, positive only for XO4a), which is an empirical outcome rather than a definitional or fitted-input reduction. No load-bearing self-citation, uniqueness theorem, or ansatz is invoked; the interpretation of the XO4a result as a degeneracy artifact is presented as an alternative reading of the data, not a derived prediction equivalent to the sampled inputs. The analysis is therefore self-contained against the external benchmarks of the three waveform families and the observed posteriors.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The analysis rests on the standard assumption that the three waveform families accurately describe the signal in the absence of birefringence, plus the modeling choice that birefringence can be captured by a single frequency-difference parameter δ_br. No new particles or forces are introduced.

free parameters (1)
  • δ_br prior bounds
    Uniform prior on [-π, π] for the band-differential rotation; this choice directly sets the scale of the reported 90% upper limit of ~2.8 rad.
axioms (1)
  • domain assumption Gravitational waves propagate as tensor modes whose polarization basis can be rotated in a frequency-dependent manner without violating other GR predictions.
    Invoked when the birefringence model is defined as a rotation of the tensor-polarization basis.

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