Abstract: |
The primary cosmic microwave background anisotropies have proven to be among the most useful cosmological probes, due to a combination of impressive observational advances and the simplicity of the underlying theory -- the CMB is a linear perturbation on a homogeneous and isotropic background. The most difficult part of the theoretical prediction for the primary CMB anisotropies has turned out to be the modeling of the cosmic recombination epoch: the transition from an ionized to neutral Universe, roughly 400,000 years after the Big Bang, which is the "surface" that we actually see when we look at the CMB. Even though the early Universe contained just three elements, computing its recombination history at the sub-percent level needed for Planck involves a maze of forbidden transitions, line escape processes, and radiative transfer in the far ultraviolet where the continuum opacity is dominated by multi-photon transitions in hydrogen (rather than by metals or dust). I will conclude by evaluating our current understanding of the recombination epoch and prospects for more direct tests of recombination models. |