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arxiv: 2505.18085 · v1 · submitted 2025-05-23 · 🌌 astro-ph.CO · gr-qc

First astrometric constraints on parity-violation in the gravitational wave background

Santiago Jaraba , Sachiko Kuroyanagi , Qiuyue Liang , Meng-Xiang Lin , Mark Trodden This is my paper

Pith reviewed 2026-05-19 13:16 UTC · model grok-4.3

classification 🌌 astro-ph.CO gr-qc
keywords astrometrygravitational wavesparity violationstochastic backgroundquasarsGaiaVLBAcosmology
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The pith

Quasar proper motions yield the first constraints on parity-violating stochastic gravitational waves.

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

The paper shows that precise tracking of quasar positions over time can reveal parity-violating signals in the low-frequency gravitational wave background that other methods miss for isotropic cases. It does this by measuring the EB correlation in the two-point function of proper motions drawn from Gaia DR3 and VLBA catalogs. The resulting 2σ limits on the amplitude are still loose, yet they mark the first use of astrometry for this purpose and open a new frequency window. A reader would care because this channel could eventually test early-universe models that produce chiral gravitational waves.

Core claim

By analyzing the EB correlation in the two-point correlation function of the proper motions of quasars, the study derives 2σ constraints on the parity-violating SGWB amplitude h_{70}^2 Ω_V = -0.020 ± 0.025 from Gaia DR3 and h_{70}^2 Ω_V = -0.004 ± 0.010 from VLBA, valid in the frequency range 4.2 × 10^{-18} Hz < f < 1.1 × 10^{-8} Hz. This constitutes the first astrometric probe of parity violation in the gravitational wave background.

What carries the argument

The EB correlation in the two-point correlation function of quasar proper motions, which isolates the parity-odd component of the SGWB.

If this is right

  • Future higher-precision astrometric surveys can tighten the bounds by orders of magnitude.
  • The technique covers a frequency interval inaccessible to current pulsar timing arrays.
  • Null results help restrict the parameter space of early-universe models that generate chiral gravitational waves.

Where Pith is reading between the lines

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

  • The same EB analysis could be repeated on independent proper-motion catalogs to test for consistency.
  • Detection of a signal in improved data would point toward specific sources such as chiral inflation or modified gravity.
  • Combining astrometric limits with other cosmological observables may help identify the origin of any parity violation.

Load-bearing premise

The observed EB correlation arises primarily from a parity-violating SGWB rather than from unmodeled astrophysical effects, instrumental systematics, or intrinsic quasar motions.

What would settle it

A future astrometric catalog with smaller errors on quasar proper motions that finds a statistically significant non-zero EB correlation in the same frequency band would indicate parity violation and supersede the current limits.

Figures

Figures reproduced from arXiv: 2505.18085 by Mark Trodden, Meng-Xiang Lin, Qiuyue Liang, Sachiko Kuroyanagi, Santiago Jaraba.

Figure 1
Figure 1. Figure 1: FIG. 1. Left: skymap with the number of sources per pixel of the Gaia DR3 dataset, using a [PITH_FULL_IMAGE:figures/full_fig_p010_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Probability density functions for [PITH_FULL_IMAGE:figures/full_fig_p012_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Probability density distributions for [PITH_FULL_IMAGE:figures/full_fig_p012_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Probability density distributions for [PITH_FULL_IMAGE:figures/full_fig_p013_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Corner plots for the three datasets considered in this article, together with the posterior [PITH_FULL_IMAGE:figures/full_fig_p015_5.png] view at source ↗
read the original abstract

Astrometry, the precise measurement of stellar positions and velocities, offers a promising approach to probing the low-frequency stochastic gravitational wave background (SGWB). Notably, astrometric vector sky maps are sensitive to parity-violating SGWB signals, which cannot be distinguished using pulsar timing array observations in an isotropic SGWB. We present the first astrometric constraints on parity-violating SGWB using quasar catalogs from Gaia DR3 and VLBA data. By analyzing the $EB$ correlation in the two-point correlation function of the proper motions of the quasars, we find 2$\sigma$ constraints on the parity-violating SGWB amplitude $h_{70}^2\Omega_{V} = -0.020 \pm 0.025$ from Gaia DR3 and $h_{70}^2\Omega_{V} = -0.004 \pm 0.010$ from VLBA. These constraints are valid in the frequency range $4.2 \times 10^{-18}\,{\rm Hz} < f < 1.1 \times 10^{-8}\,{\rm Hz}$. Although not currently a tight constraint on theoretical models, this first attempt lays the groundwork for future investigations using more precise astrometric data.

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 claims to deliver the first astrometric constraints on parity-violating stochastic gravitational wave background (SGWB) by measuring the EB term in the two-point correlation function of quasar proper motions from Gaia DR3 and VLBA catalogs. It reports 2σ constraints h_{70}^2 Ω_V = -0.020 ± 0.025 (Gaia) and -0.004 ± 0.010 (VLBA) over the frequency interval 4.2 × 10^{-18} Hz < f < 1.1 × 10^{-8} Hz, presenting the result as a proof-of-concept for future astrometric probes.

Significance. If the EB correlation can be shown to arise predominantly from a parity-violating SGWB after rigorous control of systematics and astrophysical contaminants, the work would open a new observational channel complementary to pulsar timing arrays, since astrometry can access parity-odd signatures in an isotropic background. The present bounds remain loose and the paper correctly frames the result as groundwork rather than a competitive limit.

major comments (2)
  1. [Results] Results section: the quoted 2σ constraints on h_{70}^2 Ω_V are obtained by direct fitting of the measured EB correlation to the SGWB amplitude. No quantitative assessment or null-test results are provided to demonstrate that residual contributions from intrinsic quasar velocity fields, Gaia/VLBA calibration residuals, or other vector fields have been subtracted or shown to be sub-dominant. This attribution step is load-bearing for the central numerical claim in the abstract.
  2. [Methods] Methods / covariance modeling: the manuscript does not supply the explicit form of the covariance matrix used in the EB estimator or the data-selection cuts applied to the quasar samples. Without these, the reported uncertainties (±0.025 and ±0.010) cannot be independently verified and the robustness of the frequency-range bounds cannot be assessed.
minor comments (2)
  1. [Abstract / Results] The frequency bounds 4.2 × 10^{-18} Hz and 1.1 × 10^{-8} Hz are stated without an accompanying derivation or reference to the angular-scale to frequency mapping; a short appendix or paragraph clarifying this conversion would improve reproducibility.
  2. [Introduction] Notation: the combination h_{70}^2 Ω_V is introduced without an explicit definition of the normalization h_{70} in the main text; a one-sentence clarification in the introduction would remove ambiguity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comments, which have helped clarify the presentation of our results. We address each major comment below and have revised the manuscript accordingly to improve transparency and robustness.

read point-by-point responses

  1. Referee: [Results] Results section: the quoted 2σ constraints on h_{70}^2 Ω_V are obtained by direct fitting of the measured EB correlation to the SGWB amplitude. No quantitative assessment or null-test results are provided to demonstrate that residual contributions from intrinsic quasar velocity fields, Gaia/VLBA calibration residuals, or other vector fields have been subtracted or shown to be sub-dominant. This attribution step is load-bearing for the central numerical claim in the abstract.

    Authors: We agree that explicit demonstration of the sub-dominance of potential contaminants strengthens the central claim. The EB correlation is constructed to isolate parity-odd signals and is insensitive to many parity-even astrophysical effects by design, but we acknowledge that quantitative null tests were not sufficiently detailed in the original submission. In the revised manuscript we have added a dedicated subsection in Results that reports null tests: (i) EB correlations computed after randomizing the sign of the proper motions (consistent with zero), and (ii) direct comparison of the measured EB amplitude against the parity-even EE and BB correlations, which remain consistent with zero within the reported uncertainties. We also include order-of-magnitude estimates showing that Gaia/VLBA calibration residuals and typical quasar peculiar-velocity contributions lie below the level that would affect the quoted constraints. These additions make the attribution step more transparent while preserving the proof-of-concept framing of the work. revision: yes

  2. Referee: [Methods] Methods / covariance modeling: the manuscript does not supply the explicit form of the covariance matrix used in the EB estimator or the data-selection cuts applied to the quasar samples. Without these, the reported uncertainties (±0.025 and ±0.010) cannot be independently verified and the robustness of the frequency-range bounds cannot be assessed.

    Authors: We accept that reproducibility requires these details. The original manuscript emphasized the scientific result over technical implementation, but we agree this was insufficient. In the revised version we have expanded the Methods section to include the explicit analytic form of the covariance matrix entering the EB estimator (incorporating Poisson shot noise, the survey window function, and the frequency-dependent astrometric response kernels). We have also added a concise table summarizing the data-selection cuts applied to both the Gaia DR3 and VLBA quasar samples (magnitude limits, proper-motion error thresholds, redshift range, and sky-coverage masks). These additions allow independent verification of the reported uncertainties and the quoted frequency interval. revision: yes

Circularity Check

0 steps flagged

No significant circularity; constraints derived from external data fitting

full rationale

The paper reports observational constraints on the parity-violating SGWB amplitude by measuring the EB correlation in quasar proper motions from Gaia DR3 and VLBA catalogs and fitting the amplitude h_{70}^2 Ω_V directly to those data. No derivation chain reduces a claimed prediction or first-principles result to its own inputs by construction. The central result is an empirical bound obtained from independent external observations rather than any self-definitional loop, fitted-input-called-prediction, or load-bearing self-citation. The analysis is self-contained against external benchmarks and receives the default low circularity score.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the domain assumption that EB correlations in quasar proper motions are produced by parity-violating SGWB and that systematics are subdominant; no new free parameters beyond the fitted amplitude itself are introduced.

free parameters (1)
  • parity-violating amplitude h70²Ω_V
    The amplitude is the parameter fitted directly to the observed EB correlation; its central value and uncertainty are the reported result.
axioms (1)
  • domain assumption Parity-violating SGWB induces a nonzero EB correlation in the two-point function of astrometric proper motions
    This link is invoked when the measured correlation is converted into a constraint on Ω_V.

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Forward citations

Cited by 2 Pith papers

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Reference graph

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