The Impact of $\Omega_{m0}$ Prior Bias on Cosmological Parameter Estimation: Reconciling DESI DR2 BAO and Pantheon+ SNe Data Combination Results

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 ($\oo, \oa$) from the standard $\Lambda$CDM model predictions ($\oo=-1, \oa=0$). A notable aspect of these results is the role of $\Omo$ 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 $\oo$, $\oa$, and $\Omo$ causes significant biases in the estimated dark energy parameters when the $\Omo$ prior mean deviates from its true underlying value. Specifically, applying an $\Omo$ prior mean of 0.33 (consistent with some SNe-only constraints) to mock data, assuming a true $\Lambda$CDM universe ($\Omo=0.30, \oo=-1, \oa=0$), yields biased estimates such as $\oo \approx -0.82 \pm 0.06$ and $\oa \approx -0.82 \pm 0.4$. This systematic shift, driven by the $\Omo$ 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., $\oo = -0.888^{+0.055}_{-0.064}$, $\oa = -0.17 \pm 0.46$). Our findings suggest that these observed non-$\Lambda$CDM parameters may largely arise from statistical biases due to $\Omo$ prior tensions between datasets. This study proposes a potential resolution to current cosmological tensions without necessarily invoking new physics.