Stringent constraint on the CCC + TL cosmology with <i>H</i> ( <i>z</i> ) measurements

ABSTRACT Recently, the Covarying Coupling Constants and Tired Light (CCC + TL) hybrid model was proposed to explain the unexpectedly small angular diameters of high-redshift galaxies observed by the James Webb Space Telescope (JWST) that are challenging to reconcile with the Lambda cold dark matter ($\Lambda$CDM) model. In this work, we test the CCC + TL model against model-independent Hubble parameter [$H(z)$] measurements obtained from cosmic chronometers. It turns out that the parameter set optimized for the type-Ia supernova (SN Ia) data set within the CCC + TL model fails to reproduce the $H(z)$ data, but the $\Lambda$CDM model works well. Statistical comparison using the $\Delta \chi ^2$ strongly favours $\Lambda$CDM over CCC + TL for the $H(z)$ data, with $\Delta \chi ^2 = 61.52$. Crucially, the CCC + TL framework exhibits a severe internal tension, where the SN Ia-optimized speed-of-light variation index $\alpha$ is rejected by the $H(z)$ data set with a likelihood ratio of $\mathcal {R} \approx 1.7 \times 10^{-14}$. Our result suggests that the tension posed by JWST observations of compact high-z galaxies may originate from the intrinsic properties and evolution of galaxies in the early universe.