A Change is as Good as a Test: Observational Tests of Extensions to the Concordance Cosmological Model

Abstract

In this thesis I examine some extensions to the concordance model of cosmology and their observational consequences. In particular, I look at how vector perturbations sourced by cosmic strings could generate a B-mode signal in weak lensing surveys. For a string network close to the current observational upper bound, this signal could be detected within the next 5-10 years. I also examine model independent modified gravity and forecast the constraints that could be obtained with the combination CMB+weak lensing+galaxy cluster counts. For the specific example of Planck (CMB+cluster catalogue) and DES(weak lensing), deviations from General Relativity could be constrained to within 5% in the next 3-4 years. The forecast for cluster counts relies on mass functions calibrated for ∧CDM+GR cosmologies, so I then investigate how reliable the predictions are in the case of modified gravity. At first glance, it appears that the mass function formalism is reliable and, with a little cheating, can even work in the case where modified gravity has a screening scale.