tranparency: the monopole term

Worked on the paper I am writing with More on the transparency of the Universe, showing that the consistency of baryon acoustic feature (not oscillation) measurements with supernovae type Ia measurements provides a non-trivial constraint on Lorentz invariance and transparency. Right now this is not super precise, but it is highly complementary to measurements of absorption (presumably by dust) in lines of sight near galaxies, because there is no model-independent way to integrate the absorption signal correlated with galaxies to the mean, global value—what I would call the monopole term.