What question did this study set out to answer?

The central aim is to analyze the clustering amplitude parameter S₈ under controlled stress test conditions.

February 5, 2026Open Access

S₈ as an Interface-Dependent Stress Test: Residual Migration and Asymmetric Closure Across ACT and BAO

Key Points

The central aim is to analyze the clustering amplitude parameter S₈ under controlled stress test conditions.
Conducted a controlled stress test of S₈ using ACT DR6 CMB lensing and BAO likelihoods.
Imposed constrained S₈ priors to track redistribution of χ² contributions across interfaces.
Explored relationships between local consistency and global closure failure.
S₈ exhibited different behavior compared to H₀ under stress conditions.
Residuals showed asymmetric migration between observational interfaces.
No configuration achieved simultaneous closure across all likelihood components.

Abstract

This technical note presents a controlled interface-dependent stress test of the clustering amplitude parameter S₈ using ACT DR6 CMB lensing and BAO likelihoods. Rather than proposing resolutions to the S₈ tension or introducing new physical mechanisms, the analysis is explicitly diagnostic. By imposing constrained S₈ priors and tracking the redistribution of χ² contributions across observational interfaces, the study probes whether local consistency can coexist with the failure of global closure. The results show that S₈ behaves differently from H₀ under comparable stress conditions: residuals migrate asymmetrically between interfaces, and no single configuration achieves simultaneous closure across all likelihood components. These findings support an interpretation of cosmological tensions as interface-dependent stress tests rather than indicators of missing parameters or new physics, and contribute to a structured diagnostic analysis of inferential limits in contemporary cosmology.

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