You might wish to explore the following article from October 13, 2020, and note that one of the authors is Simon Birrer of Stanford University, who is pursuing the same investigation into the Hubble constant as Wendy Freedman from the University of Chicago:
Cosmology Intertwined IV: The Age of the Universe and its Curvature
A precise measurement of the curvature of the Universe is of primeval importance for cosmology since it could not only confirm the paradigm of primordial inflation but also help in discriminating between different early Universe scenarios. The recent observations, while broadly consistent with a spatially flat standard Λ Cold Dark Matter (ΛCDM) model, are showing tensions that still allow (and, in some cases, even suggest) a few percent deviations from a flat universe. In particular, the Planck Cosmic Microwave Background power spectra, assuming the nominal likelihood, prefer a closed universe at more than 99% confidence level. While new physics could be in action, this anomaly may be the result of an unresolved systematic error or just a statistical fluctuation. However, since a positive curvature allows a larger age of the Universe, an accurate determination of the age of the oldest objects provides a smoking gun in confirming or falsifying the current flat ΛCDM model.
With the long standing Hubble constant disagreement, and σ8 − S8 tension, there are some anomalies in the Planck 2018 cosmological results that deserve further investigations. Between them the most significant from the statistical point of view, is the preference at 3.4σ for a closed Universe. Moreover, the Planck dataset also suggest an indication at more than 2σ for Modified Gravity. This disagreement with the predictions for a flat universe of the standard model is connected with the higher, anomalous, lensing contribution in the Cosmic Microwave Background (CMB) power spectra, characterized by the AL parameter, that is strongly degenerate with Ωk. A closed universe also solves a well-know tension above 2σ between the low
unresolved systematics in the Planck 2018 data, or can be simply due to a statistical fluctuation.
Contact Information:
Eleonora Di Valentino (JBCA, University of Manchester, UK) [
eleonora.divalentino@manchester.ac.uk]
See:
https://arxiv.org/pdf/2008.11286.pdf
See:
https://www.sciencedirect.com/science/article/abs/pii/S2212686420304799?via=ihub
See:
https://arxiv.org/pdf/2110.09562.pdf
Unaccounted systematic uncertainties, in particular, those with Cepheids and supernovae, have been proposed as causes for the H0 tension. But this class of explanation appears to be less favorable with a variety of new data sets coming from various sources confirming the tension at the 4.4 - 6σ level.
More recently it has been proposed that systematic uncertainties related to the choice of Cepheid color-luminosity calibration method could affect the ability of the distance ladder to measure the value of H0 to the required pre- cision. So far a large array of both early and late universe modifications to ΛCDM have been proposed: they are summarized in two recent thorough reviews. While there has been no clear preferred solution to date, the work of Knox & Millea points out that early universe solutions are “less unlikely.” However, it seems that early universe solutions could unfortunately fail to agree with large scale structure observations as shown in and make the S8 tension even more prominent.
The S tension, while not as statistically significant (ranging between 1.5 - 2.5σ) has received a lot of attention as well. Most proposed solutions introduce some form of self interactions in the dark sector in an attempt to erase structure in the late universe. Other proposals include, but are not limited to, dark matter-neutrino interactions, modifications to gravity, or neutrino self-interactions. The apparent correlation between the two tensions indicates that one tension cannot be addressed without the other.
One notable early universe solution for the H0 tension is Early Dark Energy (EDE), an early period of dark energy domination that reduces the size of the acoustic horizon and thus increases the value of H0 inferred from CMB measurements. To achieve this, the model introduces a scalar field that behaves like a cosmological constant at high redshifts (z > 3000) and then gets diluted at the same rate as radiation or faster as the universe expands. Unfortunately, agood fit of the model to the CMB power spectra requires a higher value of matter density at recombination than ΛCDM. This enhances structure formation at late times and increases the value of S8.
On the other hand, the introduction of Decaying Dark Matter (DDM) has been investigated as a candidate to solve both tensions simultaneously. More specifically, late time decays of a massive cold parent particle. Implications of the ACT data set for EDE can be found in two recent works.
Despite all the efforts invested, it appears that a single modification of ΛCDM in either the early or late universe has yet been successful in solving both tensions at the same time.
There appear to be many variables to consider when trying to get to the bottom of the Hubble Constant. I urge everyone to keep looking and reading about this interesting issue in cosmology and physics.
Hartmann352
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