Dark Energy Survey Year 6 Results: Cosmological Constraints from Cosmic Shear

We present legacy cosmic shear measurements and cosmological constraints using six years of Dark Energy Survey imaging data. From these data, we study ~140 million galaxies (8.29 galaxies/arcmin$^2$) that are 50% complete at i=24.0 and extend beyond z=1.2. We divide the galaxies into four redshift bins, and obtain cosmic shear measurement with a signal-to-noise of 83, a factor of 2 higher than the Year 3 analysis. We model the uncertainties due to shear and redshift calibrations, and discard measurements on small angular scales to mitigate baryon feedback and other small-scale uncertainties. We consider two fiducial models to account for the intrinsic alignment (IA) of the galaxies. We conduct a blind analysis in the context of the $Λ$CDM model and find $S_8 \equiv σ_8(Ω_m/0.3)^{0.5}=0.798^{+0.014}{-0.015}$ (marginalized mean with 68% CL) when using the non-linear alignment model (NLA) and $S{8} = 0.783^{+0.019}_{-0.015}$ with the tidal alignment and tidal torque model (TATT), providing 1.8% and 2.5% uncertainty on $S_8$. Compared to constraints from the cosmic microwave background from Planck 2018, ACT DR6 and SPT-3G DR1, we find consistency in the full parameter space at 1.1$σ$ (1.7$σ$) and in $S_8$ at 2.0$σ$ (2.3$σ$) for NLA (TATT). The result using the NLA model is preferred according to the Bayesian evidence. We find that the model choice for IA and baryon feedback can impact the value of our $S_8$ constraint up to $1σ$. For our fiducial model choices, the resultant uncertainties in $S_8$ are primarily degraded by the removal of scales, as well as the marginalization over the IA parameters. We demonstrate that our result is internally consistent and robust to different choices in calibrating the data, owing to methodological improvements in shear and redshift measurement, laying the foundation for next-generation cosmic shear programs.

💡 Research Summary

This paper presents the first cosmic‑shear analysis of the full six‑year Dark Energy Survey (DES) data set, comprising roughly 140 million galaxies (8.29 gal arcmin⁻²) with a 50 % completeness limit at i = 24.0 mag and extending beyond redshift z ≈ 1.2. The galaxies are divided into four tomographic redshift bins (0.2 < z < 0.43, 0.43 < z < 0.63, 0.63 < z < 0.90, 0.90 < z < 1.30). Two‑point shear correlation functions ξ₊(θ) and ξ₋(θ) are measured with a signal‑to‑noise ratio of 83, a factor of two improvement over the previous Year‑3 (Y3) analysis.

To control systematic errors, the shape catalog is built using Metacalibration combined with extensive image simulations, achieving multiplicative bias m and additive bias c at the 10⁻³ level. Photometric redshifts are obtained by a Bayesian combination of several independent methods (BPZ, SOM‑based, clustering redshifts, lensing ratios) and jointly calibrated with shear using realistic image simulations that account for blending effects.

Small angular scales where baryonic feedback and non‑linear power‑spectrum uncertainties dominate are removed (θ ≲ 2′, corresponding to k ≳ 3 h Mpc⁻¹). This scale cut is the dominant source of information loss, degrading the final S₈ precision by roughly 30 %.

Two intrinsic‑alignment (IA) models are considered. The first, the Non‑Linear Alignment (NLA) model, introduces an IA amplitude A_IA and redshift evolution η_IA applied to the non‑linear matter power spectrum. The second, the Tidal Alignment and Tidal Torque (TATT) model, adds two extra IA parameters (A₁, A₂) to capture both alignment and torque contributions. Bayesian evidence slightly favors NLA, but both models are statistically acceptable.

Within a flat ΛCDM framework (six parameters: Ω_m, σ₈, h, n_s, Ω_b, τ) the analysis yields:

• NLA: S₈ ≡ σ₈(Ω_m/0.3)⁰·⁵ = 0.798 ⁺⁰·⁰¹⁴₋₀·₀₁₅ (1.8 % uncertainty)
• TATT: S₈ = 0.783 ⁺⁰·⁰¹⁹₋₀·₀₁₅ (2.5 % uncertainty)

These results are consistent with external CMB measurements: compared to Planck 2018 the full parameter space overlaps at 1.1 σ (NLA) and 1.7 σ (TATT), while the S₈ values differ by 2.0 σ and 2.3 σ respectively. Similar levels of agreement are found with ACT DR6 and SPT‑3G DR1.

The dominant residual uncertainties arise from (1) the removal of small‑scale data, (2) marginalization over IA parameters, and (3) the choice of baryonic‑feedback prescription. Switching IA models can shift S₈ by up to ≈1 σ, underscoring the need for improved physical IA modeling in future surveys.

Extensive validation is performed: a blind analysis protocol prevents confirmation bias; mock catalogs and full‑image simulations test the pipeline; internal consistency checks (e.g., varying scale cuts, alternative IA priors) demonstrate robustness. The results also feed into the DES “3 × 2 pt” analysis (cosmic shear + galaxy‑galaxy lensing + galaxy clustering) presented in companion papers, and set the methodological foundation for upcoming Stage‑IV surveys such as LSST, Euclid, and the Roman Space Telescope.

In summary, DES Y6 delivers a high‑precision measurement of the matter clustering amplitude, S₈≈0.79, with systematic uncertainties now dominated by astrophysical modeling rather than instrumental calibration. This work represents a significant step toward the sub‑percent precision required for next‑generation weak‑lensing cosmology.