SBi3PCF: cosmology with Dark Energy Survey Year 3 cosmic shear

We present the first cosmic shear data analysis of the integrated 3-point correlation function, a higher-order weak lensing statistic, and the first simulation-based inference analysis of a three-point cosmic shear correlation function on. The new analysis still awaits peer review, but it already lays out the central claim clearly.

It matters because cosmology operates at the edge of what current instruments can measure, where systematic errors and model assumptions are never trivial. Small discrepancies between independent measurements have historically pointed toward missing physics rather than simple calibration errors, and the ongoing tension in the Hubble constant is a live example of how a persistent disagreement between methods can reshape the theoretical landscape. Each new dataset that approaches this territory with independent systematics adds real information to a problem that has resisted easy resolution for more than a decade. We present the first cosmic shear data analysis of the integrated 3-point correlation function (i3PCF), a higher-order weak lensing statistic, and the first simulation-based. Using SBi3PCF, our previously developed SBI framework for the i3PCF, we jointly constrain the $w$CDM parameter space with the cosmic shear 2-point correlation functions $ξ_\pm$.

Our forward model is built on the CosmoGridV1 N-body simulations and includes the survey mask, shape noise, intrinsic alignment, baryonic feedback, photometric redshift. The analysis was carried out blind, with all choices frozen, and only unblinded once coverage tests, tests of the accuracy of the learned likelihoods, and internal consistency.

For the joint $ξ_\pm$ + $ζ_\pm$ analysis we obtain $Ω_m = 0.272^{+0.038}_{-0.033}$, $S_8 = 0.784^{+0.024}_{-0.021}$ and $w_0 = -1.04^{+0.25}_{-0.19}$. Adding $ζ_\pm$ improves the figure of merit in the $Ω_m$-$S_8$-$w_0$ parameter space by a factor of $1.59$ over $ξ_\pm$ alone, as predicted by simulated i3PCF analyses.

This gain comes from third-order information on angular scales $\geq 15'$ and aperture radii $\geq 50'$, a regime where baryonic effects are subdominant. The constraints are consistent with the other DES Y3 analyses of the same catalogue and with independent lensing analyses, and differ by at most $2.

The relevance goes beyond one dataset because even small shifts in measured parameters can matter when the field is testing the limits of the standard cosmological model. The Lambda-CDM framework describes the observable universe with remarkable economy, but its success rests on two components, dark matter and dark energy, whose physical nature remains entirely unknown. Any credible measurement that tightens or loosens the constraints on those components moves the entire theoretical enterprise forward, regardless of whether the immediate result looks dramatic on its own terms.

This shows that $ζ_\pm$ can be analysed on cosmological survey data with a fully simulation-based approach, an important step towards Stage-IV surveys. Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy.

Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to see whether the effect survives when independent surveys, different calibration strategies and tighter control of systematic uncertainties enter the picture. Programmes such as Euclid, DESI and the Rubin Observatory will deliver datasets over the next several years that cover the same parameter space with largely independent methods. If the current signal persists through those tests, its theoretical implications will become impossible to set aside. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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