Assessing the Robustness of the CPL Parametrization to Basis and Prior Variations: Insights from DESI DR2 BAO Data

Abstract

This work reexamines cosmological parameter constraints from the DESI Data Release 2 baryon acoustic oscillation (BAO) measurements using the distance-basis representation (D_V/r_d, D_M/D_H), which separates the isotropic BAO scale from the scale-free Alcock-Paczynski ratio. We compare LambdaCDM, wCDM, and w_0w_aCDM models to evaluate how the choice of data basis and the width of the prior on w_a affect dark-energy inference. Ratio-only fits (D_M/D_H) amplify the (w_0, w_a) degeneracy and can produce large apparent shifts in point estimates without genuine evidence for dynamical dark energy. Joint fits using (D_V/r_d, D_M/D_H) restore parameter consistency and show that these shifts mainly trace the degeneracy ridge. The pivoted equation of state, w_p = w(a_p) \simeq -0.9 \pm 0.1 at z_p \simeq 0.34, remains stable and consistent with a cosmological constant within 1sigma. Model-selection diagnostics (AIC, BIC, and Bayes factors) provide only moderate support for LambdaCDM, indicating no significant evidence for an evolving w(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.