Data Analysis in Global 21cm Experiments: Physically Motivated Bayesian Modelling Techniques

Abstract

21cm cosmology is a field in which the absorption and emission from the cosmic radio background by neutral hydrogen gas is used to probe cosmology and astrophysics of early epochs of the universe. In particular, this process is one of the most promising methods of measuring the Cosmic Dawn, when the first stars formed, and the Epoch of Reionisation, making it a key objective of modern radio cosmology. This thesis primarily investigates the application of Bayesian data analysis techniques to global 21cm cosmology, to aid in overcoming two of the most prominent difficulties in detecting a sky-averaged ('global') 21cm signal: the presence of foregrounds around four orders of magnitude brighter than the signal and systematic distortions that arise from chromaticity of the antenna's gain pattern. Therefore, in this thesis, the impact that these difficulties can have on experiments is investigated through simulations and the efficacy with which existing data analysis techniques in the field can manage them is quantified. Following this, a new data analysis pipeline is developed, utilising Bayesian processes, that is designed to overcome limitations with existing techniques. The primary concept of the pipeline developed in this thesis is to perform continuous physically motivated simulations of observations using parametrised models of the sky and antenna to explain and fit for systematic distortions in a physically understood manner. Throughout this thesis, this pipeline is developed and tested in simulations to quantify its performance and limitations. The core of this work was published in Anstey et al. (2021). This thesis also discusses the additional technique of utilising time- and antenna-dependencies in data, coupled with the developed pipeline, to improve the Bayesian modelling process, which is being written in Anstey et al. (in prep.), as well as a method by which simulated observations in the developed pipeline could be used to help guide the design of a global 21cm experiment, published in Anstey et al. (2022). The techniques developed in this thesis are generally applicable to any global 21cm experiment. However, they were developed with the primary intent of being utilised in the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) (de Lera Acedo et al. 2022).