DESI and SNe: Dynamical Dark Energy, $Ω_m$ Tension or Systematics?

Abstract: Dark Energy Spectroscopic Instrument (DESI) observations, when combined with Cosmic Microwave Background (CMB) and Type Ia supernovae (SNe), have led to statistically significant dynamical dark energy (DDE) claims. In this letter we reconstruct the $\Lambda$CDM parameter $\Omega_m$ from the $w_0 w_a$CDM cosmologies advocated by the DESI collaboration. We identify i) a mild increasing $\Omega_m$ trend at high redshift and ii) a sharp departure from $\Lambda$CDM at low redshift. The latter, which is statistically significant, is driven by SNe that are $1.9 \sigma- 2.5 \sigma$ discrepant with DESI full-shape galaxy clustering in overlapping redshift ranges. We identify a low redshift subsample of the Dark Energy Survey (DES) SNe sample that is discrepant with DESI at $3.4 \sigma$ despite both observables probing the same effective redshift. SNe and BAO/full-shape modeling should not disagree on $\Omega_m$ at the same effective redshift. This ``$\Omega_m$ tension'' most likely points to unexplored systematics. In general, any \textit{bona fide} DDE signal should be confirmed independently across observables, so the DDE claims are premature.