The Bayesian view of DESI DR2 with unimpeded: Evidence and tension in a combined analysis with CMB and supernovae across cosmological models

TL;DR

This paper applies a Bayesian framework to reanalyze DESI DR2 data, revealing how model preferences depend on calibration choices and tensions with other datasets, and emphasizing the importance of tension quantification.

Contribution

It introduces a fully Bayesian reanalysis of DESI DR2 using nested sampling, demonstrating how model preferences are affected by calibration and dataset tensions.

Findings

Bayesian analysis weakens or reverses model preferences compared to traditional methods.

The earlier DES-SN5YR calibration sustains a significant preference for dynamical dark energy.

Tensions between datasets influence model selection and are crucial for interpretation.

Abstract

We apply the $\texttt{unimpeded}$ framework to perform a fully Bayesian reanalysis of the DESI DR2 data, using nested sampling with $\texttt{PolyChord}$ to compute evidences for $\Lambda$CDM and seven extensions across combinations of DESI DR1/DR2, Planck CMB, supernovae (Pantheon+, Union3, DES-SN5YR, DES-Dovekie), and DES-Y1 weak lensing. The Bayesian Ockham's razor penalises extended models, yielding weaker or opposite preferences compared to $\Delta\chi^2$-based analyses. For DESI DR2 BAO combined with Planck CMB alone, the DESI collaboration's $3.1\sigma$ frequentist preference for $w_0w_a$CDM is eliminated entirely: we obtain ${\ln B = -0.57{\scriptstyle\pm0.26}}$, modestly favouring $\Lambda$CDM. Adding DES-Dovekie, the recalibration of DES-SN5YR, maintains this concordance (${\ln B = -0.30{\scriptstyle\pm0.19}}$). However, when the earlier DES-SN5YR calibration is included instead, the DESI collaboration's $4.2\sigma$ result survives the Bayesian Ockham penalty as a $3.07{\scriptstyle\pm0.10},\sigma$ preference (${\ln B = +3.32{\scriptstyle\pm0.27}}$). That this signal persists despite the Ockham penalty makes the role of tension quantification essential: our analysis traced the preference to the earlier DES-SN5YR calibration, which introduced a $2.95{\scriptstyle\pm 0.04},\sigma$ conflict with DESI DR2 within $\Lambda$CDM -- a tension that stands out from the grid -- reduced to $1.96{\scriptstyle\pm 0.04},\sigma$ with the DES-Dovekie recalibration. With DES-Dovekie, the Bayesian evidence for dynamical dark energy vanishes.