Dark Energy Survey Year 6 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing

TL;DR

DES Year 6 presents a comprehensive cosmological analysis using 3×2pt correlations (cosmic shear, galaxy clustering, and galaxy–galaxy lensing) from ~Y6 DES data in flat ΛCDM and wCDM. The work employs a forward-modeling pipeline with CosmoSIS, HMCode2020 for baryonic effects, non-Limber corrections, and a sophisticated redshift calibration scheme (SOMPZ+WZ with Balrog transfer functions and redshift-mode marginalization). It delivers the tightest DES-only large-scale-structure constraints to date, finds a mild full-parameter tension with CMB results (Δχ² and Δp-values discussed across ΛCDM and wCDM, with S8-projections up to ≈2.6σ), and demonstrates the added value of jointly combining DES 3×2pt with DES’s own SN, BAO, and cluster data, as well as external probes, to produce the strongest overall cosmological constraints to date (including ∑mν<0.14 eV at 95% CL in ΛCDM). The results show no compelling evidence for deviations from a cosmological constant and illustrate how late-time LSS constraints complement early-universe CMB measurements, while delivering sub-percent precision on key parameters such as S8 and Ωm when combined with external data. The analysis also provides a robust framework for blinding, internal consistency checks, and cross-probe tension assessments that will guide future Stage-IV surveys.

Abstract

We present cosmology results combining galaxy clustering and weak gravitational lensing measured in the full six years (Y6) of observations by the Dark Energy Survey (DES) covering $\sim$5000 deg$^2$. We perform a large-scale structure analysis using three two-point correlation functions (3$\times$2pt): (i) cosmic shear from 140 million source galaxy shapes, (ii) galaxy clustering of 9 million lens galaxy positions, and (iii) galaxy-galaxy lensing from their cross-correlation. We model the data in flat $Λ$CDM and $w$CDM cosmologies. The combined analysis yields $S_8\equiv σ_8 (Ω_{\rm m}/0.3)^{0.5} = 0.789^{+0.012}_{-0.012}$ and matter density $Ω_{\rm m} = 0.333^{+0.023}_{-0.028}$ in $Λ$CDM (68\% CL), where $σ_8$ is the clustering amplitude. These constraints show a (full-space) parameter difference of 1.8$σ$ from a combination of cosmic microwave background (CMB) primary anisotropy datasets from Planck 2018, ACT-DR6, and SPT-3G DR1. Projected only into $S_8$ the difference is $2.6σ$. In $w$CDM the Y6 3$\times$2pt results yield $S_8 = 0.782^{+0.021}_{-0.020}$, $Ω_{\rm m} = 0.325^{+0.032}_{-0.035}$, and dark energy equation-of-state parameter $w = -1.12^{+0.26}_{-0.20}$. For the first time, we combine all DES dark-energy probes: 3$\times$2pt, SNe Ia, BAO and Clusters. In $Λ$CDM this combination yields a $2.8σ$ parameter difference from the CMB. When combining DES 3$\times$2pt with other low-redshift datasets (DESI DR2 BAO, DES SNe Ia, SPT clusters), we find a 2.3$σ$ parameter difference with CMB. A joint fit of Y6 3$\times$2pt, CMB, and those low-redshift datasets produces the tightest $Λ$CDM constraints to date: $S_8 = 0.806^{+0.006}_{-0.007}$, $Ω_{\rm m} = 0.302^{+0.003}_{-0.003}$, $h = 0.683^{+0.003}_{-0.002}$, and $\sum m_ν< 0.14$ eV (95\% CL). In $w$CDM, this combination yields $w = -0.981^{+0.021}_{-0.022}$.

Figures (17)

• Figure 1: DES footprint in equatorial coordinates. The $\sim 5000 \deg^2$wide-field survey footprint is shown as a black outline, with overplotted convergence map using the Wiener filter reconstruction method y3-massmapping. The white circles, scaled to approximately one full DECam field-of-view, show the supernovae field locations. Other current and planned surveys are shown as well. The South Pole Telescope footprint includes the SPTpol and SPT-SZ surveys described in Bocquet2024.
• Figure 2: Estimated redshift distributions of the Y6 source (top) and lens (bottom) samples. Different colors represent different tomographic bins. The middle panel shows the lensing efficiency (Equation \ref{['eq:lensing_eff']}) for each source bin. In each case, the solid lines and band represent the mean and standard distribution of 100 $n(z)$ reconstructions, drawn from the posterior of our fiducial $3\times2$pt linear bias analysis. Note that lens bin 2 is unshaded because it is excluded from our fiducial analysis (see Appendix \ref{['sec:unblinding_details']} for details).
• Figure 3: Angular galaxy clustering auto-correlation function as a function of angular separation $\theta$ for different lens redshift bins. Upper panels show measurements (black points) with best-fit models: $\Lambda$CDM linear galaxy bias (black solid), $\Lambda$CDM nonlinear bias (green dashed), and $w$CDM linear bias (pink dotted). Lower panels show residuals in units of the expected standard deviation. Gray shaded regions indicate excluded scales: lighter gray for linear bias cuts, darker gray for nonlinear bias cuts. Lens bin 2 is entirely shaded because it is excluded from the fiducial analysis (see Appendix \ref{['sec:unblinding_details']}). Triangle markers indicate the residuals are larger than 3$\sigma$.
• Figure 4: Galaxy-galaxy lensing correlation function $\gamma_\mathrm{t}$ as a function of angular separation $\theta$ for different lens-source bin combinations $(l^i, s^j)$. Upper panels show measurements (black points) with best-fit models: $\Lambda$CDM linear galaxy bias (black solid), $\Lambda$CDM nonlinear bias (green dashed), and $w$CDM linear bias (pink dotted). Lower panels show residuals. Gray shaded regions indicate excluded scales: lighter gray for linear bias cuts, darker gray for nonlinear bias cuts. Lens bin 2 is entirely shaded because it is excluded from the fiducial analysis (see Appendix \ref{['sec:unblinding_details']}). Note that this figure does not reflect the point-mass marginalization procedure, which is applied directly to the inverse covariance matrix during likelihood evaluation and cannot be visualized through the covariance matrix elements shown here.
• Figure 5: Cosmic shear correlation functions $\xi_+(\theta)$ (left panels) and $\xi_-(\theta)$ (right panels) for different source redshift bin combinations $(i,j)$. Upper panels show measurements (black points) with best-fit models: $\Lambda$CDM linear galaxy bias (black solid), $\Lambda$CDM nonlinear bias (green dashed), and $w$CDM linear bias (pink dotted). Lower panels show residuals in units of the expected standard deviation. Gray shaded regions indicate excluded scales.