Maartens, RoyViljoen, Jan-Albert2022-02-082024-05-142022-02-082024-05-142022https://hdl.handle.net/10566/15000Philosophiae Doctor - PhDIn recent history there have been several advances in cosmology, which has significantly shaped our understanding of the Universe. The current leading theory is called ΛCDM, which can successfully model the expansion of the Universe from a primordial state and describe the dynamics of its contents, thereby resulting in the large-scale structure present today. The model is based on general relativity, that describes gravitational interaction as the curvature of a four-dimensional manifold called space-time. However, despite the many successes of ΛCDM, there are a number of things that need further investigation. The Cosmic Microwave Background (CMB) is the oldest observable radiation in the Universe, and this cosmological relic contains a detectable structure. The process leading up to the CMB determines the initial conditions of ΛCDM, but is still poorly understood. It is widely accepted that inflation was responsible for the rapid expansion after the Big Bang, although this is yet to be verified experimentally. The distribution of the primordial potential is imprinted on ultra-large scales of the matter distribution, which offers an important insight into uncovering this mystery. In addition to the primordial Universe, there are other concepts that still puzzle us in ΛCDM itself. The fact that we have been unable to directly detect and explain these dark components (that make up around 96% of the Universe) has prompted several theorists to consider alternative cosmological models. Therefore, testing general relativity and ΛCDM is still an essential part of cosmological research. A key observational discriminant between general relativity and modified theories of gravity is the rate at which the large-scale structure grows from small perturbations. The relativistic effects (or light-cone effects) expected in general relativity also offer an independent test of the gravitational model.enCosmologyRadiationGalaxiesNeutral hydrogenUniversity of the Western Cape