From Raw Data to Cosmology with the Atacama Cosmology Telescope

Abstract

Measurements of the cosmic microwave background (CMB) are one of the main tools used to constrain the cosmological parameters we use to describe the nature and evolution of our universe. As we study the CMB in greater detail, in addition to learning new and exciting things about the universe, we also become increasingly sensitive to various systematics arising from our instruments or our data processing techniques, so great care must be taken to ensure these do not bias our results. In this thesis, we present the analysis of data from the Atacama Cosmology Telescope (ACT), an experiment used to measure both the temperature and polarization of the CMB. This includes work done for ACT's recent data release (DR4) as well as progress made since then. We describe the process of transforming raw data into maps of the sky, and then angular power spectra which are used as inputs into a likelihood to constrain cosmology. As part of this procedure, we characterize the beams of the instrument, which determine how it responds to different scales on the sky. This is critical for all science analyses, including the accurate measurement of power spectra. During the map-making step, we make additional maps with the specific purpose of checking for systematic effects which may bias our data. Also, once we've outlined the power spectrum pipeline, we reproduce ACT DR4 spectra using publicly available code, and in the process explore how different choices made in the analysis influence the final results. Finally, we investigate the possibility of using the data collected by ACT during the day. This data has not been used for cosmological analyses so far due to the specific challenges it presents, mainly that the beams undergo time-dependent deformations due to heat from the Sun distorting the telescope structure. We explore different methods to mitigate and correct for these daytime beams.