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

TL;DR

This study evaluates how different basis choices and prior assumptions influence dark energy constraints from DESI BAO data, finding that combined basis fits yield stable, consistent results supporting a cosmological constant.

Contribution

It provides a detailed analysis of the impact of basis and prior variations on BAO-based dark energy constraints, establishing a benchmark for future surveys.

Findings

Ratio-only fits increase degeneracy and can mislead about dark energy dynamics.

Joint fits with combined basis restore parameter consistency.

Model diagnostics show only moderate support for LambdaCDM.

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.