Is the low clustering amplitude from weak lensing a real discrepancy?
In plain words
Weak gravitational lensing, the slight distortion of distant galaxy shapes by intervening matter, measures how clumpy matter is today. Some surveys find it less clumpy than the cosmic microwave background predicts and others do not.
Precise statement
With S8 = sigma8 (Omega_m / 0.3)^0.5, combined Planck, ACT DR6 and SPT-3G give $S8 = 0.836 (+0.012, -0.013)$ in LCDM. KiDS-Legacy agrees within $1\ \sigma$ while DES Y6 is lower by $2.4\ \text{to}\ 2.7\ \sigma$. Determine whether late-time S8 measured to 1 percent differs from the CMB prediction at more than $3\ \sigma$ after photometric-redshift, intrinsic-alignment and shear calibration are validated.
What would settle it
Euclid and Rubin LSST cosmic shear with spectroscopic redshift calibration, cross-checked with CMB lensing tomography.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
2026 status: KiDS-Legacy consistent with the CMB, DES Y6 at $2.4\ \text{to}\ 2.7\,\sigma$ tension.
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