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

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 $w = -0.981^{+0.021}_{-0.022}$.

💡 Research Summary

The Dark Energy Survey (DES) presents its final six‑year (Y6) cosmological analysis, covering roughly 5,000 square degrees of the southern sky. Using the full depth of the survey, the collaboration constructs three complementary two‑point statistics—cosmic shear from 140 million source galaxy shapes, galaxy clustering from 9 million lens galaxies, and galaxy‑galaxy lensing from their cross‑correlation—collectively referred to as the “3×2pt” analysis. The data span redshifts up to z ≈ 2 and are processed through a rigorous pipeline that includes state‑of‑the‑art point‑spread‑function modeling, photometric‑redshift calibration, masking, image simulations, and extensive mock catalogs. A strict blinding protocol is applied to avoid confirmation bias.

In a flat ΛCDM framework, the combined 3×2pt measurements yield S₈ ≡ σ₈(Ω_m/0.3)^0.5 = 0.789 ± 0.012 and Ω_m = 0.333^{+0.023}_{‑0.028} (68 % confidence). Compared with the previous Year‑3 analysis, the constraining power in the Ω_m–σ₈ plane improves by a factor of two, thanks to higher source density, deeper redshift coverage, and refined modeling of non‑linear clustering and galaxy bias. When compared to a joint CMB dataset (Planck 2018, ACT‑DR6, SPT‑3G DR1), the full‑parameter difference is 1.8 σ, while the projection onto the S₈ axis shows a 2.6 σ tension, echoing the well‑known “σ₈‑tension” between low‑z large‑scale‑structure probes and the early‑Universe CMB measurements.

Extending the analysis to wCDM, where the dark‑energy equation‑of‑state parameter w is allowed to deviate from –1, the Y6 3×2pt results give w = –1.12^{+0.26}{‑0.20}, S₈ = 0.782^{+0.021}{‑0.020}, and Ω_m = 0.325^{+0.032}_{‑0.035}. The value of w remains consistent with a cosmological constant, indicating no significant evidence for dynamical dark energy in this dataset.

For the first time, DES combines all its internal probes—3×2pt, Type Ia supernovae, baryon acoustic oscillations, and galaxy clusters—into a unified analysis. In ΛCDM this four‑probe combination yields a 2.8 σ discrepancy with the CMB, while a hybrid combination of DES 3×2pt with external low‑redshift data (DESI DR2 BAO, DES SNe Ia, SPT clusters) reduces the tension to 2.3 σ. A joint fit that includes Y6 3×2pt, the full CMB suite, and the aforementioned low‑z datasets produces the tightest constraints to date: S₈ = 0.806^{+0.006}{‑0.007}, Ω_m = 0.302 ± 0.003, H₀ = 0.683^{+0.003}{‑0.002}, and an upper limit on the sum of neutrino masses Σ m_ν < 0.14 eV (95 % CL). In wCDM, the same combination yields w = –0.981^{+0.021}_{‑0.022}, showing no statistically significant departure from ΛCDM.

These results represent the most precise low‑redshift cosmological constraints to date, demonstrating the power of multi‑probe analyses and the importance of meticulous systematic control. The DES Y6 analysis sets a benchmark for upcoming surveys such as LSST, Euclid, and the Roman Space Telescope, both in terms of statistical precision and methodological rigor. The modest residual tension with CMB measurements continues to motivate further investigation into possible new physics, improved modeling of small‑scale effects, or unaccounted systematic uncertainties.