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

Seokcheon Lee

TL;DR

The paper investigates whether tension in the matter density prior $Ω_{m0}$ biases dark-energy constraints when combining DESI DR2 BAO with Pantheon+ SNe. By generating 1000 mock DESI DR2 BAO realizations under a fiducial $Λ$CDM cosmology and varying the $Ω_{m0}$ prior mean, the authors show a linear bias in the inferred $ω_0$ and $ω_a$ that tracks the prior and can mimic non-$Λ$CDM results observed in real data. They demonstrate that while the prior tightens uncertainties, it also shifts the posterior away from the true model, suggesting that current apparent deviations from $Λ$CDM may be artifacts of prior consistency rather than new physics. The work highlights the importance of consistent priors in multi-probe cosmology and calls for methods that reconcile dataset tensions, potentially resolving current cosmological discrepancies without new physics.

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 $Λ$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 $Λ$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-$Λ$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-$Λ$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.

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

TL;DR

The paper investigates whether tension in the matter density prior biases dark-energy constraints when combining DESI DR2 BAO with Pantheon+ SNe. By generating 1000 mock DESI DR2 BAO realizations under a fiducial CDM cosmology and varying the prior mean, the authors show a linear bias in the inferred and that tracks the prior and can mimic non-CDM results observed in real data. They demonstrate that while the prior tightens uncertainties, it also shifts the posterior away from the true model, suggesting that current apparent deviations from CDM may be artifacts of prior consistency rather than new physics. The work highlights the importance of consistent priors in multi-probe cosmology and calls for methods that reconcile dataset tensions, potentially resolving current cosmological discrepancies without new physics.

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 () from the standard CDM model predictions (). A notable aspect of these results is the role of 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 , , and causes significant biases in the estimated dark energy parameters when the prior mean deviates from its true underlying value. Specifically, applying an prior mean of 0.33 (consistent with some SNe-only constraints) to mock data, assuming a true CDM universe (), yields biased estimates such as and . This systematic shift, driven by the prior, moves the estimated parameters towards the non-CDM region, offering a qualitative resemblance to outcomes reported in current combined DESI DR2 BAO + Pantheon+ SNe analyses (e.g., , ). Our findings suggest that these observed non-CDM parameters may largely arise from statistical biases due to prior tensions between datasets. This study proposes a potential resolution to current cosmological tensions without necessarily invoking new physics.

Paper Structure

This paper contains 10 sections, 4 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Methodology
  3. Fiducial Model and BAO Sensitivities
  4. Mock Data and Covariance Matrix Construction
  5. Results and Discussion
  6. Interplay of Sensitivities and Covariance Structure
  7. Bias in Parameter Estimates
  8. Uncertainty Propagation
  9. Remark on mock design and observable redundancy
  10. Conclusion

Figures (3)

  • Figure 1: Absolute sensitivities of BAO observables with respect to $\Omega_{m0}$, $\omega_0$, and $\omega_a$ across redshift. The fiducial cosmology is $\Omega_{m0} = 0.3$, $\omega_0 = -1.0$, $\omega_a = 0.0$.
  • Figure 2: Estimated $\omega_0$ and $\omega_a$ as a function of the $\Omega_{m0}$ prior mean for different combinations of DESI DR2 BAO observables. Points show the mean of best-fit (or posterior-mean) values across 1000 mock realizations; error bars denote the standard deviation across realizations (not single-mock posterior widths). Red dashed lines show the true $\Lambda$CDM values, and the vertical green dotted line indicates the true $\Omega_{m0} = 0.3$.
  • Figure 3: Propagated $1\sigma$ uncertainties on $\omega_0$ and $\omega_a$ as a function of $\Omega_{m0}$ prior mean. Each panel shows standard deviations from 1000 mock realizations for different BAO observables.