BAO miscalibration cannot rescue late-time solutions to the Hubble tension

Abstract

Baryon Acoustic Oscillation (BAO) measurements play a key role in ruling out post-recombination solutions to the Hubble tension. However, because the data compression leading to these measurements assumes a fiducial $\hat{I}›$CDM cosmology, their reliability in testing late-time modifications to $\hat{I}›$CDM has at times been called into question. We play devil's advocate and posit that fiducial cosmology assumptions do indeed affect BAO measurements in such a way that low-redshift acoustic angular scales (proportional to the Hubble constant $H_0$) are biased low, and test whether such a rescaling can rescue post-recombination solutions. The answer is no. Firstly, strong constraints on the shape of the $z\lesssim 2$ expansion history from unanchored Type Ia Supernovae (SNeIa) prevent large deviations from $\hat{I}›$CDM. In addition, unless $\hat{I}{\text{c◯}}_m$ is significantly lower than $0.3$, the rescaled BAO measurements would be in strong tension with geometrical information from the Cosmic Microwave Background. We demonstrate this explicitly on several dark energy (DE) models ($w$CDM, CPL DE, phenomenologically emergent DE, holographic DE, $\hat{I}{›}_s$CDM, and the negative cosmological constant model), finding that none can address the Hubble tension once unanchored SNeIa are included. We argue that the $\hat{I}{›}_s$CDM sign-switching cosmological constant model possesses interesting features which make it the least unpromising one among those tested. Our results demonstrate that possible fiducial cosmology-induced BAO biases cannot be invoked as loopholes to the Hubble tension "no-go theorem", and highlight the extremely important but so far underappreciated role of unanchored SNeIa in ruling out post-recombination solutions.

42 pages, 22 sub-figures arranged into 15 figures, many references. v2: additional references added, very minor changes to the legends of Figures 5 and 10. Version accepted for publication in PRD