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Abstract

  • ▪  The Hubble parameter, , is not an univocally defined quantity: It relates redshifts to distances in the near Universe, but it is also a key parameter of the ΛCDM standard cosmological model. As such, affects several physical processes at different cosmic epochs and multiple observables. We have counted more than a dozen s that are expected to agree if () there are no significant systematics in the data and their interpretation and () 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 s1 Mpc1) and high (73 km s1 Mpc1) camps. 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 ( ≳ 1,100) 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 ΛCDM for two decades; the one we might have found makes us wish we could put the genie back in the bottle.

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/content/journals/10.1146/annurev-astro-052622-033813
2024-06-07
2024-06-18
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  • Article Type: Review Article
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