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 (BAO) and observations of the cosmic microwave background (CMB) radiation. The strength of the evidence for evolving dark energy ranges from ~3.9 sigma for the Dark Energy 5 year (DES5Y) SN sample to ~2.5 sigma for the Pantheon+ sample. The cosmology inferred from Pantheon+ sample alone is consistent with the Planck LCDM 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 LCDM 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 ~0.04 mag. between low and high redshifts. Systematics of this order can bring the DES5Y sample into good agreement with the Planck LCDM cosmology and Pantheon+. I comment on a paper by the DES collaboration that rejects this possibility.