V.D34 — BAO standard ruler

The BAO standard ruler is the comoving sound horizon at the recombination orbit depth n_rec: r_s(n_rec) = R_d[integral c_s(n) dell/dn dn], where c_s(n) is the baryon-photon fluid sound speed. The inputs -- baryon-to-photon ratio, photon energy density, kappa(D;1) = 1 - iota_tau -- are all derived from iota_tau.