Cosmic Chronometers Grade the Histories: Differential Ages Favour the Slow Track, Weakly and Independently

Cosmic chronometers measure the expansion rate directly, from the differential ages of passively evolving galaxies, without a distance ladder and without assuming a cosmological model beyond the metric. This paper scores four expansion histories against ten published chronometer measurements between redshifts 0.179 and 1.965, and additionally fits the expansion rate's normalisation to the chronometers alone under the Planck shape. The Planck history scores a chi-squared of 4.68 for ten points; the evolving-dark-energy variant scores 4.83; the small-ruler combination scores 6.74; and the SH0ES rate imposed on the Planck shape scores 9.22. The chronometer-only fit gives a Hubble constant of 68.38 ± 2.13 km/s/Mpc — consistent with the Planck value and about two sigma below the SH0ES value. The chronometers thus lean the same way as the microwave background, but with errors too large to arbitrate: at their present precision they disfavour only the most naive combination (ladder rate, standard shape), and they accept every history that adjusts its shape. Because their systematics — stellar population synthesis, metallicity, star-formation histories — share nothing with either disputing anchor, even this weak, independent lean carries information about where the tension's resolution is unlikely to live. This study is computed from published values; it is not a measurement.