Distant objects in the Universe look the same as nearby ones, in the sense that a redshift 8 quasar looks like a point source just the same as a nearby M dwarf star looks like a point source. This must, somehow, strongly constrain the scattering properties of the Universe. The Universe is very transparent, but there definitely is scattering and absorption (by dust, by gas, by rocks); why don't we see this as some kind of “blurring” of distant sources? I discussed this today in the cafeteria at Flatiron with Charlie Epstein (Flatiron) and Marsha Berger (NYU).
My question was: What can we learn about scattering in the Universe from this fact that distant point sources look the same as nearby ones. They didn't have a simple answer. Epstein, however, said that, in the linear continuum limit, all that matters is epsilon and mu, so maybe we should just think about epsilon and mu fields that depend on position and frequency. Interesting thought. He also pointed out that pulsing sources should be dispersed (and they are!), and that dispersion should go up with distance (and it does!).
