# Will the significant discrepancies in Hubble parameter determinations be resolved by 2030 within the ΛCDM standard cosmological model?

The Hubble "constant", $H_0$, is basically the current expansion rate of the universe (the expansion rate varies with time). Two main ways to determine the value of $H_0$ are based on different approaches contrasting early universe vs late universe methodologies. The issue is that the values determined by these two independent methods have now widened to the point where there is a significant statistical difference of approximately 4 to 5 sigma between the two, despite the increasing precision of each method's results over time. This difference is now widely considered among experts as having become a problem for the ΛCDM standard model of cosmology (or even, among some, as a crisis). Continuing observational projects and theoretical work have been dedicated at attempts to understand and resolve the discrepancy.

A mid-July 2019 workshop at the Kavli Institute for Theoretical Physics at UCSB was convened to bring together both experimental and theoretical researchers in the field to review and assess the current state of affairs and identify promising next steps at resolution. The coordinators for this event drafted a paper Tensions between the Early and the Late Universe summarizing the event proceedings. An image from this paper plotting the different values determined by the various methods is at this link.

Videos and slides of the talks are available at this conference website link. There are also a number of excellent recent science media articles about this issue, e.g. (in descending published date order) by Natalie Wolchover, Emily Conover, Josh Sokol, and Davide Castelvecchi. There is also a twitter hashtag devoted to the workshop activity with, e.g., some of the participants tweeting their live reactions during the workshop.

Some examples of areas under investigation for a possible eventual resolution include: (a) identifying and correcting systematic errors in the various determination methods, (b) an early dark energy injection prior to recombination, (c) nonstandard neutrino physics, (d) gravity modifications, and in general (e) searching for a discovery of new physics that modifies or replaces the current ΛCDM standard model of cosmology. As an overview for possible ways forward, a new paper, The Hubble Hunter's Guide, attempts "to consider the broadest possible set of potential cosmological solutions to reconcile" the opposing observations.

By 01-Jan-2030, will the source(s) of the current tension in H0 results be resolved without a need to replace the ΛCDM standard cosmological model?

A criteria that was suggested for determining consensus on a resolution is to poll researchers working on this issue near the question close date, asking what sigma they ascribe to the discrepancy, assuming ΛCDM. A positive resolution results if the mean of replies by at least 5 polled cosmologists is < 2 sigma. A mean of > 4 sigma resolves as negative; between 2 and 3 is an indeterminate resolution.

Footnote: Examples of early universe methods - Planck, DES+BAO+BBN versus late time methods - SH0ES, CCHP, H0LiCOW, MIRAS, Megamasers, Surface Brightness Fluctuations. See this graphic for an illustrative plot of the differences. More information on these methods is in the conference summary paper. Note that by 2030, H_0 determinations from gravitational wave standard sirens is likely to have become a robust late time method.

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