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Time: 21:40-22:00 (UTC+8), 4 February 2026, Wednesday
Host: Dong Lai
Location: Online
Join Tencent Meeting：https://meeting.tencent.com/dm/rXLIkv2nzsHN
Meeting ID: 192651862 (no password）
Abstract:
A persistent discrepancy in modern cosmology, known as the $S_8$ tension, is that weak gravitational lensing consistently infers a lower clustering amplitude than the cosmic microwave background. This mismatch may reflect unaccounted observational systematics, complex astrophysical processes, or new physics in the dark sector like dynamical dark energy or warm dark matter. A key complication is that weak lensing inherently mixes contributions from multiple physical scales and cosmic epochs along the line of sight. Therefore, the origin of the tension may lie in scale-dependent effects, rather than in a breakdown of the cosmological model itself. To disentangle this projection-induced degeneracy, a different inference framework is required. The Bernardeau-Nishimichi-Taruya (BNT) transform provides an elegant way to reorganize lensing kernels, isolating contributions from distinct redshifts and physical scales. We extend this formalism into a practical analysis pipeline that maps angular ($\ell$-space) weak-lensing observables onto well-defined three-dimensional ($k$-space) modes. This approach yields unbiased and statistically competitive constraints on $S_8$ that are robust to changes in nonlinear modeling assumptions. Applying this method to the KiDS-Legacy cosmic shear data, we remove contributions from nonlinear scales with $k > 0.33\;\mathrm{Mpc}^{-1}$ and obtain $S_8 = 0.717^{+0.047}_{-0.046}$ from the resulting physically truncated data vector. This large-scale-only $S_8$ constraint is $1.14\sigma$ lower than the constraint derived from all scales, and $1.80\sigma$ lower than the constraint obtained after removing large-scale modes of $k < 0.33\;\mathrm{Mpc}^{-1}$. This suggests that, at the current data quality, nonlinear astrophysical feedback does not significantly bias $S_8$. While the data do not yet allow us to determine whether this scale dependence originates from observational systematics or cosmology, the observed trend is nonetheless counterintuitive when compared to most proposed explanations of the $S_8$ tension.
Biography:
Shiming Gu is a John I. Watters Graduate Research Fellow in Cosmology at the University of British Columbia, where he is expected to receive his Ph.D. in 2026 under the supervision of Ludovic van Waerbeke. He was also a RUB International Research Scholar at the German Centre for Cosmological Lensing from 2023 to 2024.