The Cosmic Microwave Background (CMB) is an invaluable source of information, playing a large role in progressing cosmology to a new era of precision. The CMB could contain traces of primordial gravitational waves (PGWs) in the form of B-mode polarisation. The detection of B-mode polarisation could shed light on the initial conditions of the universe. Measuring B-mode polarisation is no simple task however, with the signal expected to be many orders of magnitude smaller than contamination from galactic foregrounds. This means all sources of contamination, noise, and systematic effects in our observations must be managed carefully. Continuously rotating half-wave plates (CRHWPs) are an increasingly popular tool for their ability to mitigate noise and systematic effects. They have already been adopted by a range of experiments, and several forthcoming experiments. Most notably for this thesis Simons Observatory (SO, Ade et al., 2019) will also use a CRHWP. A CRHWP modulates incoming polarised signals to higher frequency space, away from the effects of low frequency noise. A CRHWP can also allow a detector which is only sensitive to polarisation in a single direction to independently measure both the Q and U Stokes polarisation states. Use of a CRHWP necessitates demodulation during the data analysis stage. In this thesis, we create a self-contained pipeline for the simulation of sky scans with a CRHWP in an SO-like setup. We create 4 demodulation pipelines, and compare the efficacy of each when recovering a B-mode signal in five distinct noise regimes. We extend this to include the effect of non-ideality in a HWP; we have modelled three types of non-ideality and repeated this investigation. Finally, we have put constraints on the non-idealities for recovery of the B-mode signal at the SO target precision of sigma(r) = 0.003. We find that of the methods investigated, a frequency-domain approach is the most effective. However, in the low-E regime where the primordial B-mode is expected to peak, both the frequency and time domain approaches perform equally well. This provides a powerful consistency check for future CMB experiments. The effects of non-ideality do not change this conclusion. Differencing detectors is also found not to have any significant benefit, other than reducing the amount of data which needs to be stored. If detectors are not differenced, the combination of data can be left to a later stage in the analysis which may be beneficial. Finally, we find that for SO to recover the B-mode at its target precision, the non-idealities in the HWP have the following constraints: the signal lost through the half-wave plate cannot exceed 0.053%; the departure between the expected and actual phase shifts introduced by the half-wave plate cannot exceed 3.7 degrees in either direction; and the amplitude of any cross polarisation cannot exceed 0.29% that of the signal of interest.
Date of Award | 1 Aug 2024 |
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Original language | English |
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Awarding Institution | - The University of Manchester
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Supervisor | Daniel Thomas (Supervisor) & Michael Brown (Supervisor) |
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- data analysis
- half-wave plate
- data pipelines
- astrophysics
- cosmology
- physics
- astronomy
Use of Continuously Rotating Halfwave Plates in Cosmic Microwave Background Experiments
Rashid, M. (Author). 1 Aug 2024
Student thesis: Phd