Galileon models form a class of models where an additional scalar field is added to the Lagrangian describing the general theory of relativity. The addition of the scalar field causes a wide array...Show moreGalileon models form a class of models where an additional scalar field is added to the Lagrangian describing the general theory of relativity. The addition of the scalar field causes a wide array of phenomena within our universe to change. Among those phenomena are both the expansion of the universe and the formation of large scale structures. We will study how they are both changed within a subclass of the Galileon models, called the Galileon ghost condensate models. First, we explore the parameter space of the model to find the values that give rise to non-singular evolu- tions of the expansion of the universe. Then, we examine how the large scale structures would form within those universes. To do that we use the spherical collapse model, in which the evolution of a spherical overden- sity is tracked. The spherical overdensity models how a relatively small perturbation leads to the formation of dark matter halos. We will show that the Galileon ghost condensate models still allow for a large degree of freedom within the spherical collapse, which would allow further research to constrain its parameter space.Show less
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
The Covariant Galileon Model is an extension of General Relativity constructed by adding an extra scalar degree of freedom to it. The mathematical background of the model is therefore also found in...Show moreThe Covariant Galileon Model is an extension of General Relativity constructed by adding an extra scalar degree of freedom to it. The mathematical background of the model is therefore also found in differential geometry. The equations of motion of the model can be derived from its Lagrangian. Using the ADM formalism and tools from differential geometry the EFT functions of the model are then found. Numerical solutions to the model are given for two different sets of parameters and for variations of the the present day matter density and the Hubble constant at the start of the simulation. From these it is concluded that a more thorough Monte Carlo simulation of the model is a useful tool for further analysis of the model. Furthermore more research is needed for a better interpretation of the found solutions to the model.Show less
