On DESI's DR2 exclusion of $Λ$CDM

Abstract: The DESI collaboration, combining their Baryon Acoustic Oscillation (BAO) data with cosmic microwave background (CMB) anisotropy and supernovae data, have found significant indication against $\Lambda$CDM cosmology. The significance of the exclusion of $\Lambda$CDM can be interpreted entirely as significance of the detection of the $\boldsymbol{w_a}$ parameter that measures variation of the dark energy equation of state. We emphasize that DESI's DR2 exclusion of $\Lambda$CDM is quoted in the articles for a combination of BAO and CMB data with each of three different and overlapping supernovae datasets (at 2.8-sigma for Pantheon+, 3.8-sigma for Union3, and 4.2-sigma for DESY5). We show that one can neither choose amongst nor average over these three different significances. We demonstrate how a principled statistical combination yields a combined exclusion significance of 3.1-sigma. Further we argue that, based on available knowledge, and faced with these competing significances, the most secure inference from the DESI DR2 results is the 3.1-sigma level exclusion of $\Lambda$CDM, obtained from combining DESI+CMB alone, while omitting supernovae.