A Regime-Activated Lensing Response Improves the Fit to KiDS-1000 Cosmic Shear and Reframes the Interpretation of S₈

Lagret. Her er abstracten:Weak gravitational lensing measurements are commonly interpreted within the ΛCDM framework using the parameter combination S₈ = σ₈√(Ωm/0.3) as an effective descriptor of shear amplitude. This implicitly assumes that lensing responds directly to matter clustering with no additional scale- or redshift-dependent response factor. We test a phenomenological alternative in which the lensing response includes an explicit, regime-activated term Σ(k,z). Using the KiDS-1000 cosmic shear dataset, we find that a late-time, small-scale lensing response significantly improves the likelihood relative to ΛCDM at fixed cosmology, is robust to scale cuts, cannot be reproduced by simple amplitude rescaling, and exhibits a clear tomographic signature localized to low-redshift bins. When cosmological parameters are allowed to vary on a grid, the model prefers lower growth amplitude but higher effective shear amplitude once the response term is included, indicating that part of the apparent S₈ tension arises from parameterization rather than direct observational conflict. All results are maximum-likelihood evaluations, not full posterior inference, and require cross-survey replication before definitive conclusions can be drawn.