Cosmological Recombination

In this thesis we focus on studying the physics of cosmological recombination and how the details of recombination affect the Cosmic Microwave Background (CMB) anisotropies. We present a detailed calculation of the spectral line distortions on the CMB spectrum arising from the Lyα and two-photon transitions in the recombination of hydrogen (H), as well as the corresponding lines from helium (He). The peak of these distortions mainly comes from the Lyα transition and occurs at about 170μm, which is the Wien part of the CMB. The detection of this distortion would provide the most direct supporting evidence that the Universe was indeed once a plasma. The major theoretical limitation for extracting cosmological parameters from the CMB sky lies in the precision with which we can calculate the cosmological recombination process. Uncertainty in the details of hydrogen and helium recombination could effectively increase the errors or bias the values of the cosmological parameters derived from microwave anisotropy experiments. With this motivation, we perform a multi-level calculation of the recombination of H and He with the addition of the spin-forbidden transition for neutral helium (He i), plus the higher order two-photon transitions for H and among singlet states of He i. Here, we relax the thermal equilibrium assumption among the higher excited states to investigate the effect of these extra forbidden transitions on the ionization fraction xe and the CMB angular power spectrum Cl. We find that the inclusion of the spin-forbidden transition results in more than a percent change in xe, while the higher order non-resonance two-photon transitions give much smaller effects compared with previous studies. Lastly we modify the cosmological recombination code recfast by introducing one more parameter to reproduce recent numerical results for the speed-up of helium recombination. Together with the existing hydrogen ‘fudge factor’, we vary these two parameters to account for the remaining dominant uncertainties in cosmological recombination. By using a Markov Chain Monte Carlo method with Planck forecast data, we find that we need to determine the parameters to better than 10% for He i and 1% for H, in order to obtain negligible effects on the cosmological parameters.