Non-parametric reconstructions of dynamical dark energy via flexknots

ABSTRACT Recent cosmological surveys have provided unprecedented data sets that can be used to reconstruct the history of the dark energy equation of state. In this work, a free-form ‘flexknot’ parametrization is employed to represent $w(a)$ as a linear spline between free-moving nodes, the number of which may vary. By combining Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillation measurements together with Pantheon+ or Dark Energy Survey 5-year (DES5Y) supernovae, the functional posteriors of $w(a)$ reveal an unexpected W-shaped structure. While the Bayesian evidence may still favour Lambda cold dark matter ($\Lambda$CDM), similar results are obtained with both cobaya and a pared-down pipeline, which suggests the structure is indeed present in the data. The tension R-statistic and suspiciousness have been marginalized over models, and demonstrate that while the reconstructions from DESI and Pantheon+ agree, DESI and DES5Y do not. We conclude that, while there is no smoking gun for dynamical dark energy, the structure unearthed in this work is generally too complex to be captured by the restrictive wCDM or Chevallier–Polarski–Linder parametrizations.