Einstein’s theory of general relativity provides cosmologists with the current best framework to describe the Universe. Nevertheless, the theory has observational and theoretical limitations. In...Show moreEinstein’s theory of general relativity provides cosmologists with the current best framework to describe the Universe. Nevertheless, the theory has observational and theoretical limitations. In turn, scientists have come to develop modified theories of gravity. The thesis compares the theory of general relativity with a particular class of modified theories called scalar-tensor theories, which incorporates a scalar field that couples to matter. Using f(R) theory, a sub-class of scalar-tensor theories, we develop a theoretical understanding of how certain observational differences emerge from a given gravitational framework. After doing so we use EFTCAMB to simulate various gravitational theories and compute their predicted luminosity distance power spectrum. This tool tracks the variance of the inferred luminosity distance fluctuations emerging from independent gravitational waves and supernova events. The fluctuations arise from the anisotropies present in the Universe, which have their evolution and dynamics directly dependent on the gravitational framework probed. More explicitly, the thesis investigates a general class of effective field theory models, k-Mouflage and Generalised Brans-Dicke models. The interference power spectrum was unique to modified gravitational theories making its detection a smoking gun result for the existence of modified theories. Even so, it remains that the signals present are not substantial enough to be detected in the foreseeable future. More encouragement comes from constructing the gravitational wave luminosity distance power spectrum since it exhibits amplitudes of larger values. Nevertheless, most theories investigated showed small deviations from general relativity, rendering them difficult to constrain in the foreseeable future using this tool as well. From this we conclude that although the luminosity distance power spectrum has the potential to be a revolutionary tool in fundamental physics and cosmology, its use in upcoming surveys to help constrain theories, let alone delineate them, seems to be unrealistic for the foreseeable future.Show less
