Dark Energy Survey Year 6 Results

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/arcmin2) 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 ΛCDM model and find S8≡σ8(Ωm/0.3)0.5=0.798+0.014−0.015 (marginalized mean with 68% CL) when using the non-linear alignment model (NLA) and S8=0.783+0.019−0.015 with the tidal alignment and tidal torque model (TATT), providing 1.8% and 2.5% uncertainty on S8. 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 1.1σ (1.7σ) and in S8 at 2.0σ (2.3σ) 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 S8 constraint up to 1σ. For our fiducial model choices, the resultant uncertainties in S8 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.

Image

Posteriors on cosmological parameters from DES Y6 and a combination of CMB experiments (Planck 2018, ACT DR6 and SPT-3G, TT+TE+EE+lowE no lensing; green). The purple and orange contours show our fiducial results with two alternative models for intrinsic alignment (NLA and TATT). In the full parameter space, the difference between DES Y6 cosmic shear and the CMB is 1.1σ for NLA and 1.7σ for TATT.

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See also: Recorded webinar on DES Y6 Cosmology Results (1/22/2026)