Does DESI DR2 challenge $Λ$CDM paradigm ?

Abstract

Although debate on DESI DR1 systematics remains, DESI DR2 is consistent with DR1 and strengthens its trends. In our analysis, the LRG1 point at $z_{\mathrm{eff}}=0.510$ and the LRG3+ELG1 point at $z_{\mathrm{eff}}=0.934$ are in tension with the $Λ$CDM-anchored $Ω_m$ inferred from Planck and SNe Ia (Pantheon$^{+}$, Union3, DES-SN5YR): for LRG1 the tensions are $2.42σ$, $1.91σ$, $2.19σ$, and $2.99σ$; for LRG3+ELG1 they are $2.60σ$, $2.24σ$, $2.51σ$, and $2.96σ$. Across redshift bins DR2 shows improved agreement relative to DR1, with the $Ω_m$ tension dropping from $2.20σ$ to $1.84σ$. Nevertheless, DR2 alone is not decisive against $Λ$CDM, and the apparent deviation is driven mainly by LRG1 and LRG2. In a $ω_0ω_a$CDM fit using all tracers we find a posterior mean with $w_0>-1$, consistent with dynamical dark energy and nominally challenging $Λ$CDM. Removing LRG1 and/or LRG2 restores $Λ$CDM concordance ($ω_0\to-1$); moreover, $ω_0^{\mathrm{(LRG2)}}>w_0^{\mathrm{(LRG1)}}$, indicating that LRG2 drives the trend more strongly. Model selection via the natural-log Bayes factor $\ln\mathrm{BF}\equiv\ln(Z_{Λ\mathrm{CDM}}/Z_{ω_0ω_a\mathrm{CDM}})$ yields weak evidence for $Λ$CDM when LRG1, LRG2, or both are removed, and is inconclusive for the full sample. Hence the data do not require the extra $ω_a$ freedom, and the apparent $ω_0>-1$ preference should be interpreted cautiously as a reflection of the $ω_0$$ω_a$ degeneracy with limited per-tracer information.