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

TL;DR

This DES Year 6 cosmic shear study delivers high-precision cosmological constraints within a flat $\Lambda$CDM framework by combining an advanced shape catalog, SOMPZ-based redshifts, and end-to-end calibration that jointly accounts for shear, redshift biases, and astrophysical systematics. It employs two intrinsic-alignment models (NLA and TATT) and conservative scale cuts to mitigate baryonic and non-linear power-spectrum uncertainties, reporting $S_8$ values of $0.798^{+0.014}_{-0.015}$ (NLA) and $0.783^{+0.019}_{-0.015}$ (TATT) with $1.8\%$ and $2.5\%$ precision, respectively. The results are broadly consistent with Planck+ACT+SPT and other low-$z$ probes, though slight tensions appear in certain parameter directions that depend on IA modeling; Bayesian evidence favors the simpler NLA model, while acknowledging that IA modeling and baryon feedback can modestly shift $S_8$ by up to about $1\sigma$. This analysis showcases robust internal validation and methodical handling of systematics, providing a solid foundation for next-generation cosmic-shear programs like LSST, Euclid, and the Roman Space Telescope.

Abstract

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.

Figures (14)

• Figure 1: Estimated Y6 source redshift distributions, divided into four tomographic bins. The shaded curves and band show the mean and standard deviation of 100 $n(z)$ realisations drawn from the posterior from our Y6 TATT analysis. In the lower panel we show the corresponding lensing kernels (see Equation \ref{['eq:gz']}).
• Figure 2: Measured cosmic shear two-point correlations from DES Y6. Each panel shows a redshift bin combination (as labelled) and the upper/lower triangles represent the two types of correlation $\xi_+$ and $\xi_-$. In each case, we overlay the data with a theory prediction at the best-fit parameters from our two fiducial analyses (NLA in orange and TATT in purple). The lower panel of each correlation function shows the residual of the measured and theoretical best-fit data vector $\xi^{\rm data}_{\pm} - \xi^{\rm theory}_{\pm}$ scaled by the uncertainty in the covariance matrix. The shaded grey regions are scales excluded from the fits due to baryonic feedback and other uncertainties on very small or large scales. The goodness of fit to the $\Lambda$CDM model calculated as $p-$values are 0.06 and 0.13 for NLA and TATT, respectively.
• Figure 3: 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 orange and purple contours represent our fiducial results with NLA and TATT analysis choices (described in Section \ref{['sec:method']}). In all cases, we show $68\%$ and $95\%$ confidence levels. In the full parameter space, the difference between DES Y6 cosmic shear and CMB is 1.1$\sigma$ for NLA and 1.7$\sigma$ for TATT.
• Figure 4: DES Y6 cosmic shear (orange) compared with DESI DR2 BAO result (brown, dashed), CMB lensing (blue, dashed), the combination of our fiducial Y6 cosmic shear with NLA analysis with DESI BAO (pink), and the combination of CMB lensing (Planck + ACT + SPT) with DESI BAO (green). This tests the consistency of cosmological constraints probed with galaxy lensing and CMB lensing, when anchored to the same $\Omega_{\rm m}$, as constrained by the BAO.
• Figure 5: DES Y6 results (filled orange; NLA and purple; TATT) compared with other lensing survey results - DECADE+DES Y3 (dashed green, decade), KiDS-Legacy (dash-dot pink, kids-legacy), and HSC Y3 (dotted blue, li23).