In the astrophysics seminar today, Castorina (SISSA) gave a nice talk about the influence of neutrinos on the evolution of large-scale structure. The nice thing is that even the known neutrino species have to have an observable effect (and do; see Planck); every dark-matter halo (concentration) should have another neutrino halo around it. Immediately after the talk we argued about the detectability of this neutrino halo; it could in principle be detected with weak lensing, but it is hard; detecting the neutrinos more directly is even harder. I predicted (and Castorina didn't have an immediate answer) that there should be large variance (especially at low masses) in the fraction of the mass of each condensed dark-matter halo that is in neutrinos. My prediction—which is qualitative and ill-thought-out—is based on intuitions about dynamics with multiple fluids with different initial velocity distributions. Maybe there are some collapsed objects that are neutrino dominated! They would be rare, but maybe exist somewhere?
Nick Konidaris (Caltech) appeared out of nowhere to join us for lunch. He is working on inexpensive spectroscopic follow-up systems for surveys like LSST and SKA. We discussed various things, including the fact that extended emission is just as well detected by small telescopes as large, that there are software vs hardware trade-offs, and that many of the gut decisions we make in designing experiments could be made objectively. We tentatively agreed to try to write a short, pedagogical note about the first of these.