2026/08/02 by Youri Carloni, Orlando Luongo, Marek Biesiada
Physics and Astronomy · #astro-ph.CO #gr-qc
15 pages, 7 figures, 4 tables
arxiv created 2026/08/02 · arxiv updated 2026/08/04
We introduce a density-level pivot construction for the Chevallier-Polarski-Linder (CPL) parameterization by defining the normalized dark energy density fp≡ f\rm DE(ap) and the equation of state ωp≡ ω(ap) at an optimized pivot scale factor ap. This reparameterization leaves the underlying CPL cosmology unchanged and allows the two models to be compared using parameters with a direct physical interpretation at the epoch where the data are most sensitive. Accordingly, following the DESI DR2 results, we compare a newly proposed fa fbCDM parameterization, based on a second-order Taylor expansion of the normalized dark energy density, with the standard w0waCDM model. In particular, we constrain the original and pivoted parameterizations using compressed cosmic microwave background (CMB), DESI DR2 baryon acoustic oscillations (BAO), cosmic chronometers (CC), and Pantheon+ Type Ia supernovae data, with the SH0ES prior imposed on H0. We find that applying the same density-level pivot prescription to the w0waCDM model substantially reduces the correlation between its dark energy parameters and provides tighter and more stable constraints. The statistical comparison shows that this model remains favored over the Taylor expansion of the dark energy density, even when both models are analyzed in the optimized parameter basis. Moreover, the pivoted CPL parameterization accurately reproduces the background evolution of quintessence models, providing a reliable phenomenological approximation to the underlying dark energy dynamics, better than the fafbCDM model. We conclude that changing the parameter basis improves the performance of the w0waCDM model, which emerges as the most suitable framework to describe the dark energy sector within the class of models considered here.