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

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.