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Abstract: In this thesis we focus on studying the physics of cosmological recombinationand how the details of recombination affect the Cosmic Microwave BackgroundCMB anisotropies.
We present a detailed calculation of the spectral linedistortions on the CMB spectrum arising from the Lyman-alpha and the lowesttwo-photon transitions in the recombination of hydrogen H, and thecorresponding lines from helium He.
The peak of these distortions mainlycomes from the Lyman-alpha transition and occurs at about 170 microns, which isthe Wien part of the CMB.
The major theoretical limitation for extractingcosmological parameters from the CMB sky lies in the precision with which wecan calculate the cosmological recombination process.
With this motivation, weperform a multi-level calculation of the recombination of H and He with theaddition of the spin-forbidden transition for neutral helium He I, plus thehigher order two-photon transitions for H and among singlet states of He I.
Wefind that the inclusion of the spin-forbidden transition results in more than apercent change in the ionization fraction, while the other transitions givemuch smaller effects.
Last we modify RECFAST by introducing one more parameterto reproduce recent numerical results for the speed-up of helium recombination.Together with the existing hydrogen `fudge factor-, we vary these twoparameters to account for the remaining dominant uncertainties in cosmologicalrecombination.
By using a Markov Chain Monte Carlo method with Planck forecastdata, we find that we need to determine the parameters to better than 10% forHe I and 1% for H, in order to obtain negligible effects on the cosmologicalparameters.

Author: Wan Yan Wong



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