Assessing the robustness of the CPL parametrization to basis and prior variations: insights from DESI DR2 BAO data

초록

This work reexamines cosmological parameter constraints from the DESI Data Release 2 baryon acoustic oscillation (BAO) measurements using the distance-basis representation (DV/rd,DM/DH), which separates the isotropic BAO scale from the scale-free Alcock–Paczynski ratio. This work compares ΛCDM, ωCDM, and ω0ωaCDM models to evaluate how the choice of data basis and the width of the prior on ωa affect dark-energy inference. Ratio-only fits (DM/DH) amplify the (ω0,ωa) degeneracy and can produce large apparent shifts in point estimates without genuine evidence for dynamical dark energy. Joint fits using (DV/rd,DM/DH) restore parameter consistency and show that these shifts mainly trace the degeneracy ridge. The pivoted equation of state, ωp=ω(ap)≃-0.9±0.1 at zp≃0.34, remains stable and consistent with a cosmological constant within 1σ. Model-selection diagnostics (AIC, BIC, and Bayes factors) provide only moderate support for ΛCDM, indicating no significant evidence for an evolving ω(a). These findings clarify the interplay among basis choice, absolute-scale anchoring, and degeneracy geometry in BAO-only dark-energy analyses, providing a benchmark for future DESI and next-generation surveys.

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