Peer review The Atacama Cosmology Telescope - DR6 Constraints on Extended Cosmological Models Blake Sherwin Cambridge

This report presents a structural critique of “The Atacama Cosmology Telescope: DR6 Constraints on Extended Cosmological Models” (arXiv:2503.14454). While the observational analysis and statistical treatment are technically sound, the manuscript’s central claim—namely, that it tests foundational assumptions of cosmology—is shown to lack formal justification. A minimal mathematical criterion for a physical theory is introduced: a theory must specify a closed evolution rule E:S→S E:S→S that determines the dynamics of admissible states without reliance on externally introduced, unconstrained parameters. The extended models analyzed in the ACT DR6 study fail this criterion. They do not define alternative dynamical laws but rather implement parametric deformations of the fixed ΛCDM evolution rule. The additional parameters are not derived from internal consistency conditions, symmetry principles, or generative constraints; they are freely introduced and subsequently fit to data. Because the extended frameworks are constructed to recover ΛCDM continuously in the limit of vanishing deformation parameters, they are structurally protected against falsification. Statistical bounds on such parameters therefore quantify numerical compatibility within a fixed dynamical structure but do not constitute tests of independent physical theories. The critique advanced here is structural, not methodological: the issue is not empirical precision but the absence of dynamical closure and generative necessity. Under a strict formal definition of theory, the work does not test alternative cosmological foundations, but only the stability of ΛCDM under unconstrained parametric perturbations.