There has been much discussion, including some here in this newgroup,
regarding the "tension" between various measurements of the Hubble
constant. In general, using the CMB (a somewhat more indirect
measurement, but probably better understood) tends to give a somewhat
lower value, while more "local" measurements, such as type Ia
supernovae, tend to give higher values. Using time delays in gravitational-lens systems also tends to give higher values, though
perhaps somewhat lower than supernovae, but the uncertainties are
larger. Some have expressed hope that other methods would be able to
pin things down, but unless all methods but one agree, it will be
difficult to say which are wrong. Reducing the uncertainties won't help---indeed, it would make matters worse---as long as the values
themselves continue to disagree.

Gravitational lensing is well understood, but a basic problem are
degeneracies in the lens model; several models can give, for a given
measured time delay between the lensed images, the same other
observables, and a different resulting value for the Hubble constant.
There have been attempts to break this degeneracy with other
observations, but these are often fraught with their own uncertainties.

On Wednesday, Angela Ng discussed a method to use quasar reverberation
mapping to effectively cancel out the lens model. Read about the
details at https://arxiv.org/abs/1912.03386. Another idea, presented
on Thursday by Max Foxley-Marrable, is to use a type Ia supernova as the
lensed source. Not only will this provide a very precise measurement of
the time delay, but the known absolute magnitude can break the lens-model degeneracy, since usually the absolute magnitude is unknown and hence degenerate with the lensing amplification. I can't find anything about
this on the web (too new), but the idea seemed clear.

There was also a session on biases in the community regarding hiring and
so on. The session was friendlier and level-headed than I expected
(perhaps indicating some bias, or at least prejudice, on my part),
though most of the discussion concentrated on how to avoid one's own
biases one knows about, rather than unknown biases or the problem of
working around biases held by people who see no reason to change the
status quo. In general, of course, such sessions are to some extent
preaching to the choir (or cheering for the pep squad), though I found
it interesting.

For a long time various anomalies in the CMB have been discussed. They
don't go away with better data, and the consensus seems to be that they
are real in the sense that there is no obvious problem with the data,
but what is behind them is completely unclear, including the question
whether they are related to tension in the Hubble constant. Suggestions
range from solar-system effects to a non-trivial topology of the
Universe. The latter was discussed on Wednesday by Glenn Starkman.
This is discussed a bit about 2/3 of the way through this lecture: https://m.youtube.com/watch?v=SmQQq-9fX7k.

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