| Summary: | The study of gravitational waves is one of the most exciting areas of modern physics. As a relatively young observational field it promises a great deal of new and important information about the Universe in which we find ourselves. This thesis investigates the behaviour of gravitational waves in cosmology. As such, it starts with details of: the standard model of cosmology, LambdaCDM; relevant observational techniques; techniques for data analysis and the theoretical and observational details of cosmological gravitational waves.
The background details motivate further investigation of how best to constrain primordial gravitational waves using the cosmic microwave background. I present reproductions and improvements on two existing techniques, valid for short- and long-wavelength gravitational waves respectively, before developing an approach applicable for all wavelengths. This new approach is thoroughly explored, consistency checks are performed and a new constraint on primordial gravitational waves in a previously unconstrained frequency region is presented.
As a result of calculations of the evolution of gravitational waves in the new approach, attention then turns to the Hubble tension. I review existing attempts to alleviate the Hubble tension before focusing on and constraining the axion model which has emerged as one of the more successful models for addressing the H_0 problem. A model-independent approach is introduced that can be used to determine which modifications to the background expansion of the Universe are necessary to reduce the Hubble tension. This approach will be useful for comparison of models that aim to alleviate the Hubble tension by modifying the expansion history, such as the gravitational wave model introduced here.
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