| Summary: | In this thesis we focus on studying the physics of cosmological recombination and how the details of recombination affect the Cosmic Microwave Back
ground (CMB) anisotropies. We present a detailed calculation of the spectral
line distortions on the CMB spectrum arising from the Ly α 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 Ly α transition and occurs at about 170 μm, 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 cosmologi
cal recombination process. Uncertainty in the details of hydrogen and helium
recombination could effectively increase the errors or bias the values of the cos
mological 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 C. 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 introduc
ing 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 HeT and 1% for H, in order to obtain negligible effects on
the cosmological parameters.
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