A tale of many $H_0$

TL;DR

The paper discusses the multiple definitions of the Hubble constant, their discrepancies, and explores whether these differences indicate systematic errors or fundamental issues with the standard cosmological model.

Contribution

It highlights the existence of multiple $H_0$ measurements, analyzes their discrepancies, and discusses potential implications for cosmology and the standard model.

Findings

$H_0$ measurements cluster around two values: 68 and 73 km/s/Mpc.

Discrepancies may stem from data systematics or fundamental model issues.

The debate involves early universe physics and possible modifications to the cosmological model.

Abstract

The Hubble parameter $H_0$, is not a univocally-defined quantity: it relates redshifts to distances in the near Universe, but is also a key parameter of the $\Lambda$CDM standard cosmological model. As such, $H_0$ affects several physical processes at different cosmic epochs, and multiple observables. We have counted more than a dozen $H_0$'s which are expected to agree if a) there are no significant systematics in the data and their interpretation and b) the adopted cosmological model is correct. With few exceptions (proverbially confirming the rule) these determinations do not agree at high statistical significance; their values cluster around two camps: the low (68 km/s/Mpc) and high (73 km/s/Mpc) camp. It appears to be a matter of anchors: the shape of the Universe expansion history agrees with the model, it is the normalizations that disagree. Beyond systematics in the data/analysis, if the model is incorrect there are only two viable ways to "fix" it: by changing the early time ($z\gtrsim 1100$) physics and thus the early time normalization, or by a global modification, possibly touching the model's fundamental assumptions (e.g., homogeneity, isotropy, gravity). None of these three options has the consensus of the community. The research community has been actively looking for deviations from $\Lambda$CDM for two decades; the one we might have found makes us wish we could put the genie back in the bottle.