Comparing LambdaCDM, wCDM, and w0waCDM models with DESI DR2 BAO: Redshift-Resolved Diagnostics and the Role of rd

TL;DR

This study reanalyzes DESI DR2 BAO data to compare dark energy models, finding that the preference for dynamical dark energy diminishes when fixing the sound horizon, emphasizing the importance of early-universe anchoring.

Contribution

It introduces a robustness test fixing the sound horizon to isolate its impact on dark energy model preferences in BAO data analysis.

Findings

All models fit BAO data well with similar chi2 values.

Weak preference for LCDM over more complex models.

Fixing rd reduces evidence for dynamical dark energy.

Abstract

We reanalyze DESI DR2 baryon acoustic oscillation (BAO) measurements to compare LambdaCDM, wCDM, and w0waCDM. Using DM/rd, DH/rd in seven redshift bins, we reconstruct the covariance and run Markov Chain Monte Carlo in Omegam, h rd, w0, wa. In the BAO-only case, all models fit well (tilde chi2 simeq 0.8 - 1.05). Model-selection metrics show at most weak preference for LCDM; the slightly lower chi2 of w0waCDM is offset by complexity, and the pivoted equation of state is consistent with -1 (omegap = -0.899 pm 0.087 at zp simeq 0.34). These results agree with the DESI DR2 analysis. To assess the role of early-universe information, we add a Gaussian prior on rd from Planck DR3 rather than using the full CMB likelihood. Fixing rd isolates the BAO-ruler calibration and yields no significant evidence for dynamical dark energy. The key discriminator is which early-time anchor is held fixed; anchoring theta-ast can raise Omegam in w0waCDM, increasing r-ast and DA(z-ast) to keep theta-ast constant, thereby mimicking late-time evolution, whereas anchoring rd does not. We therefore advocate a robustness test comparing fixed-rd and fixed theta-ast analyses; under the former, DESI DR2 BAO remain fully consistent with LambdaCDM. This work introduces a controlled fixed-rd robustness test that isolates the role of sound-horizon anchoring from the full CMB likelihood. We demonstrate quantitatively that the reported ~3-sigma preference for dynamical dark energy is not reproduced under fixed-rd anchoring.