On DESI’s DR2 exclusion of ΛCDM

ABSTRACT The Dark Energy Spectroscopic Instrument (DESI) collaboration, combining their baryon acoustic oscillation (BAO) data with cosmic microwave background (CMB) anisotropy and supernovae data, have found significant indication against the Lambda cold dark matter ($\Lambda$CDM) cosmology. This can also be interpreted as the significance of the detection of the $w_a$ parameter that measures variation of the dark energy equation of state. DESI’s DR2 article quotes exclusion of $\Lambda$CDM for combinations of BAO and CMB data with each of three different and overlapping supernovae compilations (at 2.8σ for Pantheon+ , 3.8σ for Union3, and 4.2σ 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 3.1σ. Further we argue that, faced with these competing significances, the most secure inference from the DESI DR2 results is the 3.1σ level exclusion of $\Lambda$CDM obtained from combining DESI + CMB alone, omitting supernovae.