The impact of Ωm0 prior bias on cosmological parameter estimation: reconciling DESI DR2 BAO and Pantheon + SNe Data

ABSTRACT Recent cosmological parameter analyses combining DESI DR2 Baryon Acoustic Oscillation (BAO) data with external probes, such as Pantheon + Supernovae (SNe) observations, have reported deviations of the dark energy equation-of-state parameters ($\omega _0, \omega _a$) from the standard Lambda Cold Dark Matter ($\Lambda$CDM) model predictions ($\omega _0=-1, \omega _a=0$). A notable aspect of these results is the role of $\Omega _{\mathrm{ m}0}$ prior information from SNe, which is known to exhibit tension with BAO-only constraints. In this study, we rigorously investigate this effect through a statistical analysis using 1000 mock DESI DR2 BAO data realizations. We demonstrate that the strong degeneracy between $\omega _0$, $\omega _a$, and $\Omega _{\mathrm{ m}0}$ causes significant biases in the estimated dark energy parameters when the $\Omega _{m0}$ prior mean deviates from its true underlying value. Specifically, applying an $\Omega _{m0}$ prior mean of 0.33 (consistent with some SNe-only constraints) to mock data, assuming a true $\Lambda$CDM universe ($\Omega _{\mathrm{ m}0}=0.30, \omega _0=-1, \omega _a=0$), yields biased estimates such as $\omega _0 \approx -0.82 \pm 0.06$ and $\omega _a \approx -0.82 \pm 0.4$. This systematic shift, driven by the $\Omega _{m0}$ prior, moves the estimated parameters towards the non-$\Lambda$CDM region, offering a qualitative resemblance to outcomes reported in current combined DESI DR2 BAO + Pantheon + SNe analyses (e.g. $\omega _0 = -0.888^{+0.055}_{-0.064}$, $\omega _a = -0.17 \pm 0.46$). Our findings suggest that these observed non-$\Lambda$CDM parameters may largely arise from statistical biases due to $\Omega _{\mathrm{ m}0}$ prior tensions between data sets. This study proposes a potential resolution to current cosmological tensions without necessarily invoking new physics.