The S8 Tension

The parameter S(8) quantifies how homogeneous the entire Universe is in terms of matter density, with lower values being more homogeneous than larger values. At higher values, matter is more concentrated in clumps and webs of high matter density, while comparative cosmic voids are bigger and more deep. At lower values, the amount of matter in a volume of space doesn't vary as much across the universe.

S(8) appears to vary between the early-universe and late universe, even though in the paradigmatic ΛCDM model of cosmology, which has been battered by numerous contradictions with astronomy observations, this parameter should remain the same. This tension has also been parallel to the Hubble tension, causing many astrophysicists to suspect that  they have a common cause.

The S8 tension between the early-universe and late universe, however, may be substantially a function of systemic measurement errors, rather than a real phenomena, as a new review article observes.

The parameter S(8)≡σ(8)*(Ωm/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 ``S8 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 2.4σ--2.7σ (DESY6), whereas KiDS Legacy results demonstrate statistical consistency at <1σ (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 ΛCDM cannot be excluded.

Ioannis Pantos, Leandros Perivolaropoulos, "Status of the S8 Tension: A 2026 Review of Probe Discrepancies" arXiv:2602.12238 (February 12, 2026).