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UNIONS cosmic shear constrains S_8 to 0.891 with 0.79 sigma agreement to Planck in harmonic space.

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T0 review · grok-4.3

2026-06-30 21:19 UTC pith:347TYR43

load-bearing objection UNIONS supplies the first cosmic-shear S8 from its r-band data via standard pseudo-C_ell in a non-tomographic setup, with the result sitting inside existing scatter but tied to the A_IA prior. the 1 major comments →

arxiv 2605.13543 v3 pith:347TYR43 submitted 2026-05-13 astro-ph.CO

UNIONS-3500 Weak Lensing: IV. 2D cosmological constraints in harmonic space

S. Guerrini , L. W. K. Goh , F. Hervas-Peters , C. Daley , M. Kilbinger , A. Wittje , C. Murray , L. Baumont
show 9 more authors
S. Fabbro H. Hildebrandt M. J. Hudson L. van Waerbeke A. H. Wright T. de Boer J.-C. Cuillandre E. Magnier A. W. McConnachie
This is my paper
classification astro-ph.CO
keywords cosmic shearweak lensingUNIONS surveyharmonic spaceS8 parameterintrinsic alignmentscosmological constraints
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

The paper performs a non-tomographic cosmic shear analysis on UNIONS r-band data using a harmonic-space pseudo-C_ell estimator to constrain cosmological parameters in flat Lambda CDM. Astrophysical effects including baryonic feedback and intrinsic galaxy alignments are modeled, with the intrinsic alignment amplitude sampled from a prior based on direct measurements. The resulting S_8 value is reported with uncertainties and shown to be consistent with Planck CMB results and other weak lensing surveys. The analysis checks robustness to scale cuts and finds agreement between harmonic and configuration space statistics via lognormal simulations.

Core claim

Using the pseudo-C_ell estimator on UNIONS data in a non-tomographic setup, the analysis obtains S_8 ≡ σ_8 √(Ω_m / 0.3) = 0.891^{+0.057}_{-0.084}, consistent at 0.79 sigma with Planck and 0.87 to 1.51 sigma with other weak lensing surveys. Results are robust to analysis choices, with 2.18 sigma agreement between configuration and harmonic space from lognormal simulations. The largest uncertainty source is the degeneracy between S_8 and the intrinsic alignment amplitude A_IA.

What carries the argument

The pseudo-C_ell harmonic-space estimator of the cosmic shear signal, applied non-tomographically while marginalizing over baryonic feedback and the intrinsic alignment amplitude A_IA drawn from a direct-measurement prior.

Load-bearing premise

The prior distribution on the intrinsic alignment amplitude A_IA is accurate enough that its degeneracy with S_8 does not shift the reported central value or quoted uncertainty.

What would settle it

An independent measurement of the intrinsic alignment amplitude A_IA that lies outside the range assumed in the prior would move the inferred S_8 value beyond the reported error bars.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • The S_8 constraint remains stable when different scale cuts and modelling choices are applied.
  • Lognormal simulations confirm 2.18 sigma agreement between the harmonic-space and configuration-space statistics.
  • The current 2D analysis provides the foundation for upcoming tomographic and 3x2-point cross-correlation studies that exploit UNIONS overlap with spectroscopic surveys.

Where Pith is reading between the lines

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

  • Tighter direct measurements of A_IA could reduce the dominant uncertainty in S_8 without changing the analysis pipeline.
  • The reported consistency with Planck leaves open whether larger future datasets will maintain or reduce the mild tension seen with some other weak lensing results.
  • Extending the same harmonic-space pipeline to tomographic bins could test whether redshift-dependent effects alter the present S_8 central value.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

1 major / 1 minor

Summary. The manuscript presents a non-tomographic cosmic shear analysis of the UNIONS survey in harmonic space via the pseudo-C_ℓ estimator. Within flat ΛCDM, baryonic feedback and intrinsic alignments are modeled; PSF systematics are verified to be subdominant; scale cuts and modeling choices are tested; and lognormal simulations are used to check consistency between configuration-space and harmonic-space statistics at the 2.18σ level. The primary result is S_8 ≡ σ_8 √(Ω_m / 0.3) = 0.891^{+0.057}_{-0.084}, reported as consistent with Planck at 0.79σ and with other weak-lensing surveys at 0.87–1.51σ. The degeneracy between S_8 and A_IA (sampled from a direct-measurement prior) is identified as one of the dominant uncertainty sources.

Significance. If the central result holds, the work supplies an independent northern-sky S_8 constraint from a new photometric dataset and demonstrates the practical use of harmonic-space estimators for UNIONS data. The explicit lognormal-simulation cross-check between statistics is a constructive element that supports reproducibility. The analysis is positioned as the first step toward tomographic and 3×2-point extensions that exploit UNIONS spectroscopic overlap.

major comments (1)
  1. [Abstract / Results] Abstract and results section: Because the analysis is performed without tomography, the pseudo-C_ℓ signal receives comparable contributions from the lensing kernel and from intrinsic alignments, producing a strong degeneracy between S_8 and A_IA. The quoted posterior therefore depends on the accuracy of the external prior on A_IA. No explicit robustness test (e.g., re-running the chains with a widened or shifted prior on A_IA and reporting the change in the S_8 central value and credible interval) is described, leaving open the possibility that the reported asymmetric errors or central value are sensitive to the precise choice of that prior.
minor comments (1)
  1. [Abstract] The abstract states that PSF systematic contributions were verified not to affect the results; the corresponding quantitative test (e.g., the amplitude of the residual B-mode or the shift in S_8 when the PSF model is varied) should be shown explicitly in the methods or appendix.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and for recognizing the value of our non-tomographic harmonic-space analysis. We address the single major comment below.

read point-by-point responses

  1. Referee: [Abstract / Results] Abstract and results section: Because the analysis is performed without tomography, the pseudo-C_ℓ signal receives comparable contributions from the lensing kernel and from intrinsic alignments, producing a strong degeneracy between S_8 and A_IA. The quoted posterior therefore depends on the accuracy of the external prior on A_IA. No explicit robustness test (e.g., re-running the chains with a widened or shifted prior on A_IA and reporting the change in the S_8 central value and credible interval) is described, leaving open the possibility that the reported asymmetric errors or central value are sensitive to the precise choice of that prior.

    Authors: We agree that an explicit robustness test to the A_IA prior would strengthen the presentation. The manuscript already notes that the S_8–A_IA degeneracy is one of the dominant sources of uncertainty and adopts a prior derived from direct measurements, but we did not quantify the sensitivity of the posterior to variations in that prior. In the revised manuscript we will add this test by re-running the chains with a widened prior (e.g., doubling the standard deviation while keeping the same central value) and report the resulting shifts in the S_8 median and credible interval. revision: yes

Circularity Check

0 steps flagged

Standard data fit to pseudo-C_ℓ with external priors; derivation self-contained

full rationale

The paper performs a standard Bayesian fit of cosmological parameters (including S8) to measured pseudo-C_ℓ spectra, marginalizing over nuisance parameters with priors taken from external direct measurements (A_IA) and standard ΛCDM modeling. No equation or result is shown to reduce by construction to a quantity defined solely from the paper's own fitted values or prior self-citations; the reported posterior is obtained from the likelihood on the data. Validation steps such as lognormal simulations and scale-cut tests are independent checks rather than redefinitions of the target quantity. No self-citation load-bearing, ansatz smuggling, or uniqueness theorem from the same authors is invoked to force the central result.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The analysis rests on the standard LambdaCDM framework and external priors for nuisance parameters; no new entities are postulated.

free parameters (2)
  • A_IA
    Amplitude of intrinsic galaxy alignments; sampled from a prior obtained from direct measurements and identified as one of the largest uncertainty sources.
  • baryonic feedback parameters
    Astrophysical systematic effects modeled but not enumerated in the abstract.
axioms (2)
  • domain assumption Standard LambdaCDM cosmology
    Analysis performed in the context of the standard LambdaCDM cosmology.
  • domain assumption Non-tomographic 2D analysis is sufficient
    The work explicitly uses a non-tomographic analysis.

pith-pipeline@v0.9.1-grok · 5958 in / 1519 out tokens · 42107 ms · 2026-06-30T21:19:50.988780+00:00 · methodology

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read the original abstract

The Ultraviolet Near Infrared Optical Northern Survey (UNIONS) is a photometric survey in the northern sky. The quality of the data in the $r$ band provides precise shape measurements to measure the growth of structures using cosmic shear. This work aims to constrain cosmological parameters using a harmonic-space estimator of the cosmic shear signal, known as pseudo-$C_\ell$, in a non-tomographic analysis. We perform our analysis in the context of the standard $\Lambda$CDM cosmology. We model astrophysical systematic effects such as baryonic feedback and intrinsic alignments of galaxies. We verify that the point spread function systematic contribution does not affect our results. We assess the impact of different scale cuts and modelling choices on the constraints. We find $S_8 \equiv \sigma_8 \sqrt{\Omega_{\rm m}/0.3} = 0.891^{+0.057}_{-0.084}$, consistent at the $0.79 \, \sigma$ level with \emph{Planck} and between $0.87$ to $1.51 \, \sigma$ with other weak lensing surveys. Our results are robust to analysis choices, and we use lognormal simulations to assess the consistency between configuration and harmonic space results, finding a $2.18 \, \sigma$ agreement between the two statistics. The degeneracy between $S_8$ and the amplitude of the intrinsic alignment, $A_{\rm IA}$, sampled from a prior obtained from direct measurements, is one of the largest sources of uncertainty. This work is part of the first cosmological analysis of the UNIONS survey using cosmic shear and paves the way for future tomographic and $3 \times 2$ point cross-correlation analyses, exploiting the unique overlap of UNIONS with deep spectroscopic surveys in the northern hemisphere.

Figures

Figures reproduced from arXiv: 2605.13543 by A. H. Wright, A. Wittje, A. W. McConnachie, C. Daley, C. Murray, E. Magnier, F. Hervas-Peters, H. Hildebrandt, J.-C. Cuillandre, L. Baumont, L. van Waerbeke, L. W. K. Goh, M. J. Hudson, M. Kilbinger, S. Fabbro, S. Guerrini, T. de Boer.

Figure 1
Figure 1. Figure 1: shows the final calibrated distribution n(z). 3. Methods This work aims to extract cosmological constraints from the measurements of the angular power spectrum of the non￾tomographic cosmic shear field inferred from the UNIONS data. This section describes the estimation of the angular power spec￾trum from the data, the theoretical modelling of the power spec￾trum, and the covariance matrix used in the mult… view at source ↗
Figure 2
Figure 2. Figure 2: The cosmic shear power spectrum from the UNIONS weak lensing sample. Data points are computed with NaMaster using a catalogue-based estimator (see Sect. 3.1). Error bars are obtained using the Gaussian covariance estimator of NaMaster with added non-Gaussian contributions from OneCovariance (see Sect. 3.3). The solid red line corresponds to the best fit ob￾tained using our fiducial setup described in Sect.… view at source ↗
Figure 3
Figure 3. Figure 3: Correlation matrix of the non-tomographic cosmic shear power spectrum. The covariance matrix is estimated using two theory prescriptions, iNKA and OneCovariance. It is com￾pared to the covariance estimated from GLASS mocks. Top pan￾els: Gaussian parts of the theory covariance compared to the simulation covariance. Bottom: Full covariance, including the non-Gaussian part of OneCovariance. The bottom right p… view at source ↗
Figure 5
Figure 5. Figure 5: shows the EB and BB power spectra measured on the non-tomographic UNIONS data, using the procedure de￾scribed in Sect. 3.1. The covariance is estimated using iNKA (see Sect. 3.3 for details). For the EB power spectrum, we obtain a χ 2 of 20 for 32 degrees of freedom, leading to a probability-to￾exceed (PTE) of 0.95. For the BB power spectrum, we measure a χ 2 of 41 for 32 degrees of freedom, amounting to a… view at source ↗
Figure 4
Figure 4. Figure 4: Comparison of the error bars of the theory prescriptions and the covariance estimated from GLASS mocks. Top panel: er￾ror bars on the diagonal in solid lines. The dashed lines cor￾respond to the second diagonal. Bottom: Relative error com￾pared to the fiducial covariance matrix used for the analysis, specifically the iNKA Gaussian part and the OneCovariance non-Gaussian part. There is a discrepancy of 15% … view at source ↗
Figure 6
Figure 6. Figure 6: 1D and 2D posteriors on S 8 ≡ σ8(Ωm/0.3)0.5 and Ωm obtained using our fiducial analysis setup in harmonic space (this work, blue contours), configuration space (Goh et al. 2026, orange contours). This is compared to constraints from Planck (pink contours). The consistency between configuration and har￾monic space constraints on S 8 is discussed in Sect. 5.3. Our re￾sults are consistent with Planck at the 0… view at source ↗
Figure 7
Figure 7. Figure 7: Comparison of 1D marginal posterior distributions over the parameters S 8 ≡ σ8(Ωm/0.3)0.5 , σ8, and Ωm from UNIONS (this work), other experiments and consistency tests. (i) Constraints obtained from the harmonic space (this work) and configuration space (Goh et al. 2026) analyses of UNIONS data are shown in red and orange, respectively. (ii) Comparison with constraints obtained from other experiments, incl… view at source ↗
Figure 8
Figure 8. Figure 8: Distribution of χ 2 values obtained on the GLASS mocks at each evaluated best-fit. The vertical dashed line shows the χ 2 value obtained on the data for our fiducial analysis setup. The PTE is computed as the fraction of mocks with a χ 2 larger than the one obtained on the data. −0.100 −0.075 −0.050 −0.025 0.000 0.025 0.050 0.075 0.100 ∆S8 = S8,config − S8,harm 0 2 4 6 8 10 12 Density PTE = 1.5 × 10−2 Nσ =… view at source ↗
Figure 9
Figure 9. Figure 9 [PITH_FULL_IMAGE:figures/full_fig_p011_9.png] view at source ↗

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Forward citations

Cited by 4 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Analytical covariances for catalogue-based pseudo-$C_\ell$s

    astro-ph.CO 2026-07 conditional novelty 7.0

    A new analytic method computes disconnected covariance matrices for catalogue-based pseudo-Cℓ power spectra by smoothing source positions and treating self-pair shot noise exactly.

  2. Assessing the large-scale angular clustering of UNIONS Lyman Break Galaxies via cross-correlations

    astro-ph.CO 2026-07 conditional novelty 6.0

    UNIONS LBGs yield robust cross-power spectra with Planck CMB lensing and DESI/Quaia quasars at amplitudes matching theory, despite systematics that render the auto-spectrum unusable.

  3. Assessing the large-scale angular clustering of UNIONS Lyman Break Galaxies via cross-correlations

    astro-ph.CO 2026-07 conditional novelty 6.0

    UNIONS Lyman-break galaxy auto-clustering is unusable at large scales due to imaging systematics, but LBG x CMB-lensing and LBG x quasar cross-spectra are measured robustly, with amplitudes consistent with predictions.

  4. Revisiting the 'Lensing is Low' Problem with UNIONS

    astro-ph.CO 2026-06 unverdicted novelty 5.0

    New UNIONS galaxy-galaxy lensing data around CMASS galaxies indicates no significant lensing is low problem, with joint HOD fits to GGL and GC favoring a slightly lower matter power spectrum amplitude than Planck.

Reference graph

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2 extracted references · 2 canonical work pages · cited by 3 Pith papers

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