Dynamical Dark Energy from DESI DR2 and DES Year 6 BAO, Supernovae and Cosmic Chronometers: A 3σ Preference Without Decisive Bayesian Evidence

Abstract Baryon acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) and the Dark Energy Survey (DES) have suggested a preference for dynamical dark energy with w0 > −1 and wa < 0. I derive cosmological constraints in the ΛCDM, wCDM and w0waCDM models from the DESI DR2 and DES Y6 BAO measurements, combined with the full Planck cosmic microwave background (CMB) likelihood, two recent Type Ia supernova compilations (Union3.1 and DES-Dovekie), and 38 cosmic chronometer H(z) measurements that provide a largely model-independent probe of the late-time expansion. For ΛCDM, the full combination yields H0 = 68.41 ± 0.30 km s−1 Mpc−1, Ωm = 0.301 ± 0.004 and rdrag = 147.55 ± 0.20 Mpc. In the wCDM model, the equation of state is consistent with a cosmological constant once the CMB is included (w = −0.992 ± 0.028). In the dynamical w0waCDM model, the data prefer an evolving equation of state, w0 = −0.740 ± 0.082 and wa = −0.893 ± 0.285, deviating from (w0, wa) = ( − 1, 0) at the 3σ level for both supernova samples, in agreement with recent DESI DR2 analyses. A Bayesian comparison qualifies this: the fit improves over ΛCDM (Δχ2 ≃ −12), yet the Bayes factor between w0waCDM and ΛCDM is inconclusive (ln B ≃ −0.5), and wCDM is disfavoured (ln B ≃ −4), leaving a clear preference only for the evolving over the constant equation of state (ln B ≃ +3.6). The data thus support a mild but persistent fit-level preference for an evolving equation of state, robust to the supernova compilation, that is not yet matched by decisive Bayesian evidence.