Departmental Oral Examination (Thesis Title: "Cosmological Tests of Gravity")

Event Date and Time: 
Mon, 2016-05-30 11:00 - 13:00
Room 309, Hennings Building
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 

General Relativity (GR) has successfully passed many observational tests and has long been acclaimed for its simplicity and correctness. However, there are two areas where the theory has yet to be fully challenged. One of them is in strong field gravity, and the other one is gravity on cosmological scales. While strong field gravity has challenged theorists because of issues of consistency between GR and quantum mechanics,cosmology has motivated extensions to GR via the observational discoveries of dark matter and dark energy.

In this thesis, we study a diverse range of modified theories of gravity and test them with observational data. These theories have been proposed to test GR on the cosmological scales while attempting to solve one of the theoretical or observational problems of GR. We discuss how a generic theory of modified gravity can be parametrized for studies within cosmology, and introduce a general parameterization that is simpler than those that have been previously considered. This parametrization is then applied to investigate a specific theory, known as “gravitational aether". The gravitational aether theory is aimed at solving one of the theoretical inconsistencies that exists between GR and quantum mechanics. We explain why cosmology is unique in testing this theory, and present results that prove the gravitational aether solution is excluded when all of the available cosmological data are combined. We also show that a generalization of this theory provides a consistent way to describe the strength of coupling between pressure and gravity, and present the most accurate measurements of this parameter.

Finally, we discuss the constraints that can be placed on modified gravity modelsusing the latest cosmological data from the Planck satellite combined with probes of large-scale structure. Planck currently provides the most accurate measurements of the anisotropies in the cosmic microwave background (CMB) temperature. We use Planck 2015 CMB power spectra, along with other cosmological data sets, to perform an extensive study of the modified theories of gravity. The thesis ends with a discussion of the prospects for future experiments to improve the measurements of cosmological parameters, and hence constraints on modified theories of gravity.

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