Evolving dark energy or supernovae systematics?

ABSTRACT Recent results from the Dark Energy Spectroscopic Instrument (DESI) collaboration have been interpreted as evidence for evolving dark energy. However, this interpretation is strongly dependent on which Type Ia supernova (SN) sample is combined with DESI measurements of baryon acoustic oscillations and observations of the cosmic microwave background radiation. The strength of the evidence for evolving dark energy ranges from ${\sim} 3.9 \sigma$ for the Dark Energy 5 year (DES5Y) SN sample to ${\sim} 2.5 \sigma$ for the Pantheon+ sample. The cosmology inferred from Pantheon+ sample alone is consistent with the Planck lambda cold dark matter ($\Lambda$CDM) model and shows no preference for evolving dark energy. In contrast, the the DES5Y SN sample favours evolving dark energy and is discrepant with the Planck$\Lambda$CDM model at about the $3\sigma$ level. Given these difference, it is important to question whether they are caused by systematics in the SN compilations. A comparison of SN common to both the DES5Y and Pantheon+ compilations shows evidence for an offset of ${\sim} 0.04$ mag between low and high redshifts. Systematics of this order can bring the DES5Y sample into good agreement with the Planck$\Lambda$CDM cosmology and Pantheon+. I comment on a recent paper by the DES collaboration that rejects this possibility.