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, ${\omega }_0^{(LRG2)}>w_0^{(LRG1)}$, 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.

arXiv comment13 pages, 6 figures, Published in The Astrophysical Journal

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