2023/11/22 by Licia Verde, Nils Schöneberg, Verde, Licia +3 · 1 voice · 16 citations
Computer Science · Physics and Astronomy · #Astronomy and Astrophysical Research #Computational Physics and Python Applications #Cosmology and Gravitation Theories #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences
paper · pdf · doi:10.48550/arxiv.2311.13305
openalex publication_date 2023/11/22 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/01
The Hubble parameter H0, is not a univocally-defined quantity: it relates redshifts to distances in the near Universe, but is also a key parameter of the ΛCDM standard cosmological model. As such, H0 affects several physical processes at different cosmic epochs, and multiple observables. We have counted more than a dozen H0'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 ΛCDM for two decades; the one we might have found makes us wish we could put the genie back in the bottle.