Abstract:

In this thesis, we consider both theoretical and experimental aspects of the cosmic microwave background (CMB) anisotropy for [cursive l] > 500. Part one addresses the process by which the universe first became neutral, its recombination history. The work described here moves closer to achieving the precision needed for upcoming small-scale anisotropy experiments. Part two describes experimental work with the Atacama Cosmology Telescope (ACT), designed to measure these anisotropies, and focuses on its electronics and software, on the site stability, and on calibration and diagnostics.

Cosmological recombination occurs when the universe has cooled sufficiently for neutral atomic species to form. The atomic processes in this era determine the evolution of the free electron abundance, which in turn determines the optical depth to Thomson scattering. The Thomson optical depth drops rapidly (cosmologically) as the electrons are captured. The radiation is then decoupled from the matter, and so travels almost unimpeded to us today as the CMB. Studies of the CMB provide a pristine view of this early stage of the universe (at around 300,000 years old), and the statistics of the CMB anisotropy inform a model of the universe which is precise and consistent with cosmological studies of the more recent universe from optical astronomy.

The recombination history is the largest known uncertainty in the standard cosmological model for CMB anisotropy formation. Here we investigate the formation of neutral helium during cosmological recombination and find that it is significantly different than was previously understood. We show that several new effects produce a modest (~ 0.5% at [cursive l] = 3000) change in the CMB anisotropy that should be included in the analysis of data from small-scale anisotropy experiments. These small scales contain further information from the primary CMB and from interactions of the CMB with intervening matter as it travels from the recombination era to us today. This information will improve constraints on the spectral slope of primordial perturbations, the baryon density, and possibly dark energy. The methods described in the second half of this thesis are part of the ACT analysis pipeline.