A Bayesian Perspective on Evidence for Evolving Dark Energy

TL;DR

This work re-evaluates claims of evolving dark energy by performing Bayesian model comparison between $\text{ΛCDM}$ and $w_0w_a$CDM across DESI DR2 BAO, Planck CMB, and SN data. Using nested sampling to compute the Bayesian evidence, the authors find $\ln B = -0.57 \pm 0.26$ for DESI DR2 + CMB (favoring ΛCDM) but $\ln B = +3.32 \pm 0.27$ for the DESI DR2 + CMB + DES-Y5 triplet (favoring $w_0w_a$CDM, with $3.07 \pm 0.10\,\sigma$ significance). A five-metric tension analysis identifies a significant, low-dimensional DESI DR2 vs DES-Y5 conflict within ΛCDM, which is mitigated in $w_0w_a$CDM by distributing the tension across a larger parameter space. The results, corroborated by independent geometric analyses, suggest the apparent evidence for dynamic dark energy is largely driven by cross-dataset tensions and should be interpreted with caution regarding new physics.

Abstract

The DESI collaboration reports a significant preference for a dynamic dark energy model ($w_0w_a$CDM) over the cosmological constant ($Λ$CDM) when their data are combined with other frontier cosmological probes. We present a direct Bayesian model comparison using nested sampling to compute the Bayesian evidence, revealing a contrasting conclusion: for the key combination of the DESI DR2 BAO and the Planck CMB data, we find the Bayesian evidence modestly favours $Λ$CDM (log-Bayes factor $\ln B = -0.57{\scriptstyle\pm0.26}$), in contrast to the collaboration's 3.1$σ$ frequentist significance in favoring $w_0w_a$CDM. Extending this analysis to also combine with the DES-Y5 supernova catalogue, our Bayesian analysis reaches a significance of $3.07{\scriptstyle\pm0.10}\,σ$ in favour of $w_0w_a$CDM. By performing a comprehensive tension analysis, employing five complementary metrics, we pinpoint the origin: a significant ($\approx 2.95σ$), low-dimensional tension between DESI DR2 and DES-Y5 that is present only within the $Λ$CDM framework. The $w_0w_a$CDM model is preferred precisely because its additional parameters act to resolve this specific dataset conflict. The convergence of our findings with independent geometric analyses suggests that the preference for dynamic dark energy is primarily driven by the resolution of inter-dataset tensions, warranting a cautious interpretation of its statistical significance.

Figures (1)

• Figure 1: Posterior comparisons in $w_0w_a$CDM showing the full cosmological parameter space. Left: DESI DR2 alone (black dashed) and pairwise combinations with CMB (purple), Pantheon+ (blue), Union3 (orange), and DES-Y5 (green). Right: Triplet combinations with DESI BAO + CMB combined with Pantheon+ (blue), Union3 (orange), and DES-Y5 (green). The differing constraints on $w_0$ and $w_a$ reflect the varying levels of tension between DESI BAO and each additional dataset. Figures produced with anestheticanesthetic.