Abstract
The standard cosmological model successfully describes many observations from
widely different epochs of the Universe, from primordial nucleosynthesis to the
accelerating expansion of the present day. However, as the basic cosmological
parameters of the model are being determined with increasing and unprecedented
precision, it is not guaranteed that the same model will fit more precise
observations from widely different cosmic epochs. Discrepancies developing
between observations at early and late cosmological time may require an
expansion of the standard model, and may lead to the discovery of new physics.
The workshop "Tensions between the Early and the Late Universe" was held at the
Kavli Institute for Theoretical Physics on July 15-17 2019 (More details of the
workshop (including on-line presentations) are given at the website:
https://www.kitp.ucsb.edu/activities/enervac-c19) to evaluate increasing
evidence for these discrepancies, primarily in the value of the Hubble constant
as well as ideas recently proposed to explain this tension. Multiple new
observational results for the Hubble constant were presented in the time frame
of the workshop using different probes: Cepheids, strong lensing time delays,
tip of the red giant branch (TRGB), megamasers, Oxygen-rich Miras and surface
brightness fluctuations (SBF) resulting in a set of six new ones in the last
several months. Here we present the summary plot of the meeting that shows
combining any three independent approaches to measure H$_0$ in the late
universe yields tension with the early Universe values between 4.0$\sigma$ and
5.8$\sigma$. This shows that the discrepancy does not appear to be dependent on
the use of any one method, team, or source. Theoretical ideas to explain the
discrepancy focused on new physics in the decade of expansion preceding
recombination as the most plausible. This is a brief summary of the workshop.
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