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

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) ( D V / r d , D M / D H ) , which separates the isotropic BAO scale from the scale-free Alcock–Paczynski ratio. This work compares Λ Λ CDM, ω CDM ω CDM , and ω ₀ω ₐCDM ω 0 ω a CDM models to evaluate how the choice of data basis and the width of the prior on ω ₐ ω a affect dark-energy inference. Ratio-only fits ( DM/DH D M / D H ) amplify the (ω ₀,\,ω ₐ) ( ω 0 , ω a ) degeneracy and can produce large apparent shifts in point estimates without genuine evidence for dynamical dark energy. Joint fits using (DV/rd, DM/DH) ( 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, ω ₚ=ω (aₚ) -0.9± 0.1 ω p = ω ( a p ) ≃ - 0.9 ± 0.1 at zₚ 0.34 z p ≃ 0.34 , remains stable and