Status of the $S_8$ Tension: A 2026 Review of Probe Discrepancies

TL;DR

This review analyzes the ongoing $S_8$ tension between early and late-universe measurements, using a new combined CMB framework and compiling recent survey results to assess systematic effects and potential new physics.

Contribution

It introduces a unified 2026 baseline analysis of $S_8$ combining multiple CMB datasets and compares recent survey results to evaluate the tension and its possible origins.

Findings

Combined CMB analysis yields $S_8=0.836^{+0.012}_{-0.013}$ with reduced uncertainties.

DES Year 6 results show a $2.4$--$2.7\sigma$ tension in $S_8$, while KiDS results are consistent within $<1\sigma$.

Survey-specific systematics likely contribute to discrepancies, but new physics cannot be ruled out.

Abstract

The parameter $S_8 \equiv \sigma_8 (\Omega_m/0.3)^{0.5}$ quantifies the amplitude of matter density fluctuations. A persistent discrepancy exists between early-universe CMB observations and late-universe probes. This review assesses the ``$S_8$ tension'' against a new 2026 baseline: a unified ``Combined CMB'' framework incorporating Planck, ACT DR6, and SPT-3G. This combined analysis yields $S_8 = 0.836^{+0.012}_{-0.013}$, providing a higher central value and reduced uncertainties compared to Planck alone. Compiling measurements from 2019--2026, we reveal a striking bifurcation: DES Year 6 results exhibit a statistically significant tension of $2.4\sigma$--$2.7\sigma$ in $S_8$ \citep{DESY6}, whereas KiDS Legacy results demonstrate statistical consistency at $<1\sigma$ \citep{Wright2025}. We examine systematic origins of this dichotomy, including photometric redshift calibration, intrinsic alignment modeling, and shear measurement pipelines. We further contextualize these findings with cluster counts (where eROSITA favors high values while SPT favors low), galaxy-galaxy lensing, and redshift-space distortions. The heterogeneous landscape suggests survey-specific systematic effects contribute substantially to observed discrepancies, though new physics beyond $\Lambda$CDM cannot be excluded.