Why Parsimony Matters: UAT/UCP, the 7% Parameter, and Bayesian Evidence Against ΛCDM with Cosmic Chronometers

<jats:p>Conceptual note comparing the Unified Applicable Time (UAT) and Unified Causal Principle (UCP) framework against the standard ΛCDM model using 31 cosmic chronometer H(z) measurements (Moresco et al. 2016, 2020). UAT/UCP contains a single free parameter (the 7% thermal calibration margin) constrained by the existence of cosmic structures, while ΛCDM uses six. Despite a slightly higher raw χ², UAT is decisively favoured by the Akaike Information Criterion (ΔAIC = -4.20), the Bayesian Information Criterion (ΔBIC = -11.37), and the approximate Bayes factor (B ≈ 294, decisive on the Jeffreys scale). The note clarifies that UAT/UCP is not an extension of ΛCDM and cannot be fairly implemented in standard Boltzmann codes; it must be judged on its own terms. The accompanying Python script (bayesian_comparison.py) is provided for full reproducibility.</jats:p>