Are Dynamical Dark-Energy Cosmologies Old Enough?

Abstract

Recent analysis combining baryon acoustic oscillation measurements with cosmic microwave background and Type Ia supernova data show an apparent preference for a time-dependent dark-energy equation of state within $w_0w_a$ parameterizations. Here we test the robustness of this result against an independent cosmological consistency requirement, namely that the expansion history must provide sufficient time to accommodate the oldest observed stellar populations. Motivated by recent stellar and globular-cluster age determinations, we impose progressively stronger lower bounds, $t_0 \geq 13.6$, $13.75$, $13.8$, and $13.85$ Gyr, on the expansion histories allowed by DESI DR2+CMB+Pantheon+ data. We find a systematic displacement of the posterior in the $(w_0,w_a)$ plane toward the $Λ$CDM prediction, $(w_0,w_a)=(-1,0)$, as the minimum allowed age is increased. For $t_0 \geq 13.85$ Gyr, the Mahalanobis separation from $Λ$CDM decreases from $2.56σ$ to $1.72σ$, bringing the $Λ$CDM prediction inside the 95\% credible region. Our results show that the current evidence for dynamical dark energy is sensitive to independent cosmic-age information and that stellar chronometry provides a direct robustness test of the DESI-favored expansion histories. Requiring sufficiently old cosmologies can appreciably reduce the statistical preference for departures from the standard $Λ$CDM model.