Today Sicheng Lin (NYU) defended his PhD dissertation on the connections between galaxies and the dark-matter field in which they live. He worked on elaborations of what's known as “halo occupation”, “abundance matching” and the like. At the end, I asked my standard questions about how the halo occupation fits into ideas we have about gravity, and the symmetries of physical law. After all, “haloes” aren't things that exist in the theory of gravity. And yet, the model is amazingly successful at explaining large-scale structure data, even down to tiny details. That led to a very nice and very illuminating discussion of all the things that could matter to galaxy clustering and dark-matter over-densities, including especially time-scales. An important dissertation in an important area: I learned during the defense that the DESI project has taken more than one million spectra in it's “science verification” phase. Hahaha! It makes all my work from 1994 to 2006-ish seem so inefficient!
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