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third "livepost" from the Texas Symposium on Relativistic Astrophysics
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 lectu https://m.youtube.com/watch?v=SmQQq-9fX7k. |
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