Novel Calibration Techniques to Enable Next-Generation Cosmic Microwave Background Observations

Abstract

Cosmic Microwave Background (CMB) experiments like Advanced ACTPol (AdvACT) and Simons Observatory (SO) seek to uncover the physics of the early Universe, just ~10^-32 seconds after its formation, and its evolution since. With just 6 parameters, the Lambda-CDM model of cosmology describes our Universe exceptionally well, but leaves some questions unanswered. The CMB can answer questions left unanswered by Lambda-CDM, including whether the early Universe underwent a period of inflation, if there are particles beyond the Standard Model, the sum of the neutrino masses, and the nature of dark matter and dark energy.

However, reaching the level of precision necessary to measure the signals in the CMB that can answer these questions is extremely difficult and requires unprecedented sensitivity, which presents a number of instrumental challenges. Additionally, measurements can be contaminated by polarized foregrounds such as dust and synchrotron emission. To recover the CMB signal, we must characterize and remove these polarized foregrounds with high precision, which requires multiple frequency bands and highly stringent instrument calibration. Previous experiments have never needed to calibrate to such extreme levels, but future experiments seeking to improve CMB measurements will require sub-percent level uncertainties in detector bandpass characterization. This will necessitate new calibration techniques.

In this thesis, I present my work towards achieving this unprecedented level of precision in detector bandpass calibration for AdvACT, SO, and future experiments such as CMB-S4, through analyzing current measurements and using novel techniques to improve Fourier transform spectrometer (FTS) bandpass calibration, including characterizing the FTS transfer function, design improvements, and improved coupling optics. I will discuss the viability of these methods for future CMB experiments. This work will improve the frequency calibration needed to remove polarized foregrounds from CMB maps, which will enable CMB measurements that will advance our understanding of the fundamental physics of the Universe.