The ∼5σ mismatch between the value of the Hubble parameter
measured by SH0ES and the one inferred from the inverse distance ladder (IDL)
constitutes the biggest tension afflicting the standard model of cosmology,
which could be pointing to the need of physics beyond ΛCDM. In this
paper we study the background history required to solve the H0 tension if we
consider standard prerecombination physics, paying special attention to the
role played by the data on baryon acoustic oscillations (BAO) employed to build
the IDL. We show that the anisotropic BAO data favor an ultra-late-time
(phantom-like) enhancement of H(z) at z≲0.2 to solve the tension,
accompanied by a transition in the absolute magnitude of supernovae of Type Ia
M(z) in the same redshift range. The effective dark energy (DE) density must
be smaller than in the standard model at higher redshifts. Instead, when
angular BAO data (claimed to be less subject to model dependencies) is employed
in the analysis, we find that the increase of H(z) starts at much higher
redshifts, typically in the range z∼0.6−0.9. In this case, M(z) could
experience also a transition (although much smoother) and the effective DE
density becomes negative at z≳2. Both scenarios require a violation of
the weak energy condition (WEC), but leave an imprint on completely different
redshift ranges and might also have a different impact on the perturbed
observables. They allow for the effective crossing of the phantom divide.
Finally, we employ two alternative methods to show that current data from
cosmic chronometers do not exclude the violation of the WEC, but do not add any
strong evidence in its favor neither. Our work puts the accent on the utmost
importance of the choice of the BAO data set in the study of the possible
solutions to the H0 tension.Comment: 20 pages, 13 figures, 3 table