Does DESI DR2 challenge ΛCDM paradigm?

Abstract

Although the debate about the systematic errors of DESI DR1 is still open, recent DESI DR2 is consistent with DESI DR1 and further strengthens the results of DESI DR1. In our analysis, both the LRG1 point at zeff = 0.510 and the LRG3+ELG1 point at zeff = 0.934 are in tension with the ΛCDM-anchored value of Ωm inferred from Planck and the Type Ia supernovae compilations Pantheon+, Union3, and DES-SN5YR. For luminous red galaxy 1 (LRG1) the tensions are 2.42σ, 1.91σ, 2.19σ, and 2.99σ, respectively; for LRG3+emission line galaxy 1 (ELG1) they are 2.60σ, 2.24σ, 2.51σ, and 2.96σ, respectively. From low to high redshift bins, DESI DR2 shows improved consistency relative to DESI DR1: the Ωm tension decreases from 2.20σ to 1.84σ. However, DESI DR2 alone does not provide decisive evidence against the ΛCDM model, and the apparent signal is largely driven by specific tracers, LRG1 and LRG2. In the ω0ωaCDM analysis, including all tracers yields a posterior mean with ω0 > −1, which aligns with scenarios of dynamical dark energy as a potential explanation and suggests that the DESI DR2 challenges the ΛCDM paradigm. While removing LRG1 and/or LRG2 fully restores ΛCDM concordance (i.e., ω0 → −1), we also find ω0(LRG1)>ω0(LRG2) , indicating LRG1 drives the apparent dynamical dark energy trend more strongly. Model selection using the natural log Bayes factor lnBF≡ln(ZΛCDM/Zω0ωaCDM) shows weak evidence for ΛCDM when LRG1, LRG2, or both are removed, and it is inconclusive for the full sample; thus, the data do not require the extra ωa freedom, and the apparent ω0 > −1 preference should be interpreted cautiously as a manifestation of the ω0─ωa degeneracy under limited per tracer information.