Current dark energy equation of state and its present rate of change are not well determined

Determine the present-day value of the dark energy equation-of-state parameter w_DE(z=0) and its present rate of change with redshift, using analyses that marginalize over higher-order terms in the Taylor expansion of w_DE(a), for the data combination of DESI DR1 BAO, Pantheon+ Type Ia supernovae, and Planck CMB shift parameters.

Background

The paper analyzes the common Chevallier-Polarski-Linder (CPL) parametrization of the dark energy equation of state and extends it by including higher-order terms in a Taylor expansion in (1−a). When only the first-order CPL truncation is used, the data appear to suggest deviations from a cosmological constant. However, when higher-order terms are allowed and marginalized over, the evidence for such deviations diminishes markedly.

Under this extended analysis, the authors explicitly state that current data do not determine the present value of w_DE nor its current rate of change with sufficient precision to claim that vacuum energy is excluded. This highlights an unresolved measurement problem in model-agnostic characterizations of dark energy when higher-order expansion terms are properly accounted for.