Sensitivity of calibration-free DESI DR2 BAO--supernova constraints to a distance-duality tilt

Abstract

Recent measurements from DESI baryon acoustic oscillations (BAO) have reported a moderate preference for dynamical dark energy, challenging the $\Lambda$CDM paradigm. Nonetheless, such a trend could reflect a tension between the shape and the offset of the Hubble diagrams predicted by current BAO and Type Ia supernovae (SN) data, which may underpin a possible breakdown of the cosmic distance duality relation linking luminosity distances $D_{\mathrm{L}}$ and angular-diameter distances $D_{\mathrm{A}}$. We explore this possibility by parametrizing deviations from this relation as $\eta (z)\equiv D_{\mathrm{L}}/[{(1+z)}^2{D}_{\mathrm{A}}]=\alpha {(1+z)}^{\epsilon }$, and investigate how the preference for a Chevallier-Polarski-Linder (CPL) dynamical dark energy is modified once $\epsilon \ne 0$ is allowed. Using DESI DR2 and uncalibrated SN data (Pantheon$+$, DES-Dovekie, Union3, and Union3.1), we find that allowing $\epsilon$ to vary freely in both $\Lambda$CDM and CPL reduces the CPL preference with respect to $\Lambda$CDM by a larger amount than the addition of spatial curvature ${\Omega }_k$; only Union3.1 exhibits comparable reductions. We also find that the preference reduction is robust to alternative $\eta (z)$ parametrizations, and that the $\eta (z)$ constraints remain stable under the removal of BAO tracers or low-redshift cutoffs for Pantheon$+$, whereas the DES-Dovekie preference largely disappears once $z<0.1$ SN are removed. In a separate calibrated analysis, illustrative Planck sound-horizon and SH0ES absolute-magnitude priors give $\alpha \simeq 0.942\pm 0.014$, quantifying the normalization mismatch under those assumptions. We also discuss photon-loss and varying-$G$ interpretations: ordinary positive opacity cannot explain the negative central tilts $\epsilon$ found for a $\Lambda$CDM background, while local gravity tests strongly constrain the illustrative unscreened varying-$G$ scenario.

arXiv commentV2: 16 pages, 5 figures, minor clarifications included. V1: 15 pages, 5 figures. The numerical analysis files for the production of the results and figures of V2 can be found in the following GitHub repository: https://github.com/Rivillion/Sensitivity_test_distance_duality