Does DESI DR2 challenge $\Lambda$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 $\Lambda$CDM-anchored $\Omega_m$ inferred from Planck and SNe Ia (Pantheon$^{+}$, Union3, DES-SN5YR): for LRG1 the tensions are $2.42\sigma$, $1.91\sigma$, $2.19\sigma$, and $2.99\sigma$; for LRG3+ELG1 they are $2.60\sigma$, $2.24\sigma$, $2.51\sigma$, and $2.96\sigma$. Across redshift bins DR2 shows improved agreement relative to DR1, with the $\Omega_m$ tension dropping from $2.20\sigma$ to $1.84\sigma$. Nevertheless, DR2 alone is not decisive against $\Lambda$CDM, and the apparent deviation is driven mainly by LRG1 and LRG2. In a $\omega_0\omega_a$CDM fit using all tracers we find a posterior mean with $w_0>-1$, consistent with dynamical dark energy and nominally challenging $\Lambda$CDM. Removing LRG1 and/or LRG2 restores $\Lambda$CDM concordance ($\omega_0\to-1$); moreover, $\omega_0^{\mathrm{(LRG2)}}>w_0^{\mathrm{(LRG1)}}$, indicating that LRG2 drives the trend more strongly. Model selection via the natural-log Bayes factor $\ln\mathrm{BF}\equiv\ln(Z_{\Lambda\mathrm{CDM}}/Z_{\omega_0\omega_a\mathrm{CDM}})$ yields weak evidence for $\Lambda$CDM when LRG1, LRG2, or both are removed, and is inconclusive for the full sample. Hence the data do not require the extra $\omega_a$ freedom, and the apparent $\omega_0>-1$ preference should be interpreted cautiously as a reflection of the $\omega_0$$\omega_a$ degeneracy with limited per-tracer information.