I discussed with Lauren Anderson (Flatiron) our project to use photometry and parallax to transfer spectroscopic labels to stars without spectroscopy (and, first, to de-noise the spectroscopic labels). This got me confused about how to explain the project to spectroscopists and non-spectroscopists alike: We have a way to use Gaia parallaxes to put logg values onto stars, but making no use whatsoever of stellar structure or evolution models, nor even scalings. Not even in the training set of labels! Indeed, I think we have a way to measure stellar masses with no use of physical models of stellar structure. I called Hans-Walter Rix (MPIA) to discuss further.
At lunch time there was an excellent brown-bag talk on light scalar dark matter by Ken Van Tilburg (NYU). He made beautiful, simple arguments about computing the properties of light scalar dark matter, and also very simple arguments about limiting the mass scale. When the dark matter gets very light, it becomes like a field of radio waves, but with a strange dispersion relation (because the particle rest mass isn't zero). This leads to highly observable effects. Huge interesting regions of parameter space are unexplored, experimentally, but there are prospects for both astrophysical and laboratory tests. There is an interesting regime at the massive end, where occupation numbers get small and the dark matter could even show macroscopic wave–particle duality effects. Overall it was highly educational, and a perfect example of the interdisciplinarity of the CCPP.