Designing concordant distances in the age of precision cosmology: The impact of density fluctuations

Discrepancies between distance measurements and $Λ$CDM predictions reveal notable features in the distance-redshift relation, possibly suggesting the presence of an evolving dark energy component. Given the central role of the Friedmann-Lemaître-Robertson-Walker (FLRW) metric in modeling cosmological distances, we investigate here whether these features instead point to a possible departure from the fundamental FLRW symmetries. Exploiting the transverse and line-of-sight distances provided by baryonic acoustic oscillations (BAO) observations, we demonstrate that observed distances hint at a slight but systematic preference for an anisotropic expansion rate emerging regardless of the dark energy model considered. Leveraging this non-FLRW feature, we investigate an inhomogeneous extension of the $Λ$CDM model that naturally provides an anisotropic expansion rate. Our analysis demonstrates that models featuring spherical overdensities can explain BAO, supernova, and cosmic microwave background data, providing fits statistically indistinguishable from those obtained with a phantom dark energy scenario. When Pantheon+ data is considered, our analysis challenges the FLRW framework at $2.8σ$ and yields scenarios that can be interpreted as subtle but non-negligible deviations from the FLRW metric. When DESY5 supernovae are considered instead, deviations are notably more significant, yielding scenarios that mildly violate the Copernican principle and exclude the FLRW assumption at $5.2σ$. Overall, our results motivate a more in-depth investigation of whether the perfectly homogeneous and isotropic FLRW paradigm can still be assumed to accurately predict cosmological distances in the era of precision cosmology.