precise supernovae measurements

During a mental-health day (that is, no research or indeed any work at all), I did come in to see Saurabh Jha (Rutgers) give an excellent seminar on type Ia supernovae as tools for precise cosmological tests. He spoke about this year's Nobel Prize but also lots of technical details about the precision of supernovae observations and how that can be verified empirically—that is, without any working theory other than the cosmological principle (which can also be tested, of course!). It is a rare talk that can be so technical and absolutely engrossing at the same time. One of the most interesting ideas in the talk is that observation of even a single type Ia supernova behind a massive galaxy cluster can put a strong and unique constraint on the mass distribution because it directly tests the gravitational magnification. It breaks the mass sheet degeneracy that remains after shear fitting. That's sweet.