Is Dark Energy Increasing or Decreasing in the Late Universe?

Maryam Aghaei Abchouyeh; Maurice H. P. M. van Putten

doi:10.1016/j.jheap.2026.100709

paper

Is Dark Energy Increasing or Decreasing in the Late Universe?

arXiv:2509.23168 · doi:10.1016/j.jheap.2026.100709

Abstract

The concordance cosmological model $Λ$ CDM assumes dark energy to be a constant, consistent with early-time observations, evidenced by Planck- $Λ$ CDM analyses. However, in the face of late-time tensions, the nature of dark energy remains a central open problem. Modern precision cosmology offers a potential new window in the $w (a) CDM$ framework, which provides a model-independent prescription for its unknown equation of state $w (a)$ . A confrontation of $w (a) CDM$ with data generally constitutes a nonlinear inference problem. We find that $w_{0} w_{a}$ estimates posterior to a fully non-linear $w (a)$ analysis are stabilized by the {\it Baryon Acoustic Oscillation} (BAO) constraint on $c_{M} = Ω_{m, 0} h^{2}$ , inherited for instance from Planck- $Λ$ CDM analysis of the CMB. It produces $w_{0} w_{a}$ estimates that are invariant under constraint-preserving variations in $Ω_{m, 0}$ . In contrast, the early-linearization $w_{0} w_{a}$ CDM shows pronounced correlation with $Ω_{m, 0}$ even when preserving $c_{M}$ . We quantify this correlation resulting from the non-commutativity of $w_{0} w_{a}$ estimation and linearization in $w (a)$ CDM. This discrepancy is demonstrated in controlled mock-data experiments. Applied to cosmic chronometer data, $w_{0} w_{a}$ estimates from correlation-free late linearization of $w (a)$ CDM analysis favor $w_{0} < - 1$ , whereas $w_{0} w_{a}$ CDM favors $w_{0} > - 1$ . If the correlation between $w_{0}$ and $Ω_{m, 0}$ in $w_{0} w_{a}$ CDM is interpreted as arising from linearization effects rather than a physical origin, application to DESI DR2 may shift $w_{0}$ downward, potentially extending to $w_{0} < - 1$ , corresponding to increasing dark energy at the present epoch. Alternatively, if the correlation is of hitherto unseen physical origin, the $w_{0} w_{a}$ CDM parametrization is self-consistent and no such correction to the DESI inference may be required.

arXiv comment18 pages, 9 figs

Is Dark Energy Increasing or Decreasing in the Late Universe?

Abstract

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