A Lockdown Perspective on the Hubble Tension (with comments from the
  SH0ES team)
  This is a transcript of a talk that I gave in Cambridge on 17th July 2020 on
the `Hubble tension'. I review the SH0ES analyses by Riess and collaborators
and point out some internal inconsistencies, including a discrepancy between
the relative distances inferred from Cepheids of two of the primary geometric
distance anchors, the Large Magellanic Cloud (LMC) and NGC 4258. I then ask
`what would it take to make SH0ES compatible with early time measurements?'.
The answer is a systematic bias of 0.1 - 0.15 mag in the intercept of the
Cepheid period-luminosity relations of SH0ES galaxies. Such a bias resolves the
Hubble tension, the tension between the distance anchors, and the difference
between SH0ES and the tip of the red giant branch (TRGB) distance ladder, as
measured and calibrated by Freedman and collaborators. I show that the
difference between the TRGB and SH0ES values of H0 is caused mainly by a
systematic calibration offset. In the short term, observational efforts should
be focussed on improving the calibrations of the distance anchors and nearby
galaxies, rather than trying to measure distance moduli to more supernovae host
galaxies. I argue that an independent distance estimate to NGC 4258 is
particularly critical. With such observations, it should be possible, on a
relatively short timescale, to establish definitively whether the Hubble
tension really exists.
