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Assessing the Robustness of the CPL Parametrization to Basis and Prior Variations: Insights from DESI DR2 BAO Data

Seokcheon Lee

TL;DR

This study reexamines DESI DR2 BAO data using non-redundant distance bases to test the CPL parametrization's robustness to basis choice and $\omega_a$ priors. By conducting BAO-only analyses with $h r_d$ treated as a free combination, the authors show that apparent evidence for dynamical dark energy arises from degeneracy geometry and prior volume rather than new physics. The pivoted equation of state $w_p = w(a_p)$ remains robustly around $-0.9$ at $z_p \approx 0.34$ across bases, consistent with a cosmological constant ($w=-1$) within $1\sigma$. Model-selection diagnostics yield only moderate support for $\Lambda$CDM relative to CPL extensions, emphasizing the need for absolute-scale anchoring and degeneracy-aware interpretation in BAO-only analyses for DESI and future surveys.

Abstract

We reexamine cosmological parameter constraints from the DESI Data Release 2 baryon acoustic oscillation (BAO) measurements using the distance-basis representation (D_V/r_d, D_M/D_H), which separates the isotropic BAO scale from the scale-free Alcock-Paczynski ratio. We compare LambdaCDM, wCDM, and w_0w_aCDM models to evaluate how the choice of data basis and the width of the prior on w_a affect dark-energy inference. Ratio-only fits (D_M/D_H) amplify the (w_0, w_a) degeneracy and can produce large apparent shifts in point estimates without genuine evidence for dynamical dark energy. Joint fits using (D_V/r_d, D_M/D_H) restore parameter consistency and show that these shifts mainly trace the degeneracy ridge. The pivoted equation of state, w_p = w(a_p) \simeq -0.9 \pm 0.1 at z_p \simeq 0.34, remains stable and consistent with a cosmological constant within 1sigma. Model-selection diagnostics (AIC, BIC, and Bayes factors) provide only moderate support for LambdaCDM, indicating no significant evidence for an evolving w(a). These findings clarify the interplay among basis choice, absolute-scale anchoring, and degeneracy geometry in BAO-only dark-energy analyses, providing a benchmark for future DESI and next-generation surveys.

Assessing the Robustness of the CPL Parametrization to Basis and Prior Variations: Insights from DESI DR2 BAO Data

TL;DR

This study reexamines DESI DR2 BAO data using non-redundant distance bases to test the CPL parametrization's robustness to basis choice and priors. By conducting BAO-only analyses with treated as a free combination, the authors show that apparent evidence for dynamical dark energy arises from degeneracy geometry and prior volume rather than new physics. The pivoted equation of state remains robustly around at across bases, consistent with a cosmological constant () within . Model-selection diagnostics yield only moderate support for CDM relative to CPL extensions, emphasizing the need for absolute-scale anchoring and degeneracy-aware interpretation in BAO-only analyses for DESI and future surveys.

Abstract

We reexamine cosmological parameter constraints from the DESI Data Release 2 baryon acoustic oscillation (BAO) measurements using the distance-basis representation (D_V/r_d, D_M/D_H), which separates the isotropic BAO scale from the scale-free Alcock-Paczynski ratio. We compare LambdaCDM, wCDM, and w_0w_aCDM models to evaluate how the choice of data basis and the width of the prior on w_a affect dark-energy inference. Ratio-only fits (D_M/D_H) amplify the (w_0, w_a) degeneracy and can produce large apparent shifts in point estimates without genuine evidence for dynamical dark energy. Joint fits using (D_V/r_d, D_M/D_H) restore parameter consistency and show that these shifts mainly trace the degeneracy ridge. The pivoted equation of state, w_p = w(a_p) \simeq -0.9 \pm 0.1 at z_p \simeq 0.34, remains stable and consistent with a cosmological constant within 1sigma. Model-selection diagnostics (AIC, BIC, and Bayes factors) provide only moderate support for LambdaCDM, indicating no significant evidence for an evolving w(a). These findings clarify the interplay among basis choice, absolute-scale anchoring, and degeneracy geometry in BAO-only dark-energy analyses, providing a benchmark for future DESI and next-generation surveys.

Paper Structure

This paper contains 8 sections, 24 equations, 2 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Effect of the Increased Matter Density in CPL Posteriors
  3. DESI DR2 BAO Data and the $\omega_{0}\omega_{a}\mathrm{CDM}$ Model
  4. Background model
  5. Mapping between BAO fitting parameters and distances
  6. Methodology
  7. Results
  8. Conclusion

Figures (2)

  • Figure 1: Posterior distributions for $\Omega_{m0}$, $h r_d$, $w_0$, and $w_a$ in $\omega_{0}\omega_{a}\mathrm{CDM}^{(25)}$ from Scenario A using $(D_V/r_d,\,D_M/D_H)$. Shaded regions indicate 68% and 95% credible contours.
  • Figure 2: Posterior distributions for $\Omega_{m0}$, $w_0$, and $w_a$ in $\omega_{0}\omega_{a}\mathrm{CDM}^{(25)}$ from Scenario B using $D_M/D_H$ only. The extremely low reduced $\tilde{\chi}^2$ in the wide-prior run ($\tilde{\chi}^2\!\simeq\!0.42$) indicates mild overfitting of a low-dimensional dataset, not genuine physical improvement.