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UBC Theses and Dissertations

Cosmological tests of gravity Narimani, Ali


General Relativity (GR) has long been acclaimed for its elegance and simplicity, and has successfully passed many stringent observational tests since it was introduced a century ago. However, there are two regimes in which the theory has yet to be fully challenged. One of them is in the neighbourhood of very strong gravitational fields, and the other is the behaviour of gravity on cosmological scales. While strong field gravity has challenged theorists because of the desire to find consistency between GR and quantum mechanics, cosmology has motivated extensions to GR via the empirical discoveries of dark matter and dark energy. In this thesis, we study a diverse range of modifications to GR and confront them with observational data. We discuss how a generic theory of modified gravity can be parameterized for studies within cosmology, and we introduce a general parameterization that is simpler than those that have been previously considered. This parameterization is then applied to investigate a specific theory, known as ``gravitational aether''. The gravitational aether theory was created to solve one of the theoretical inconsistencies that exists between GR and quantum mechanics, namely the fact that vacuum fluctuations appear not to gravitate. Cosmology is unique in testing this theory, and we find that the gravitational aether solution is excluded when all of the available cosmological data are combined. Nevertheless, a generalization of this theory provides a consistent way to describe the strength of coupling between pressure and gravity, and we present the most accurate measurements of this coupling parameter. In addition, we discuss the constraints that can be placed on modified gravity models using the latest data from cosmic microwave background (CMB) anisotropies, combined with several other probes of large-scale structure. Currently the most accurate CMB anisotropy measurements come from the Planck 2015 CMB power spectra, which we use, along with other cosmological data sets, to perform an extensive study of modified theories of gravity. We find that GR remains the simplest model that can explain all of the data. We end with a discussion of the prospects for future experiments that can improve our understanding of gravity.

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