Light Propagation in a Weakly Perturbed Expanding Universe

I investigate the theoretical aspects of light propagation through the universe in the framework of a weakly perturbed Robertson-Walker model. Assuming that the metric perturbations are small, which is a valid approximation almost everywhere in our uni-verse, I derive the general equations of light propagation using the geodesic equations in a perturbed space-time. These equations are used to study the temperature and spatial distribution of light, with the goal of comparing the theoretical predictions to the actual observations. The first part of the thesis is dedicated to the study of temperature distribution in the cosmic microwave background (CMB). I start with primary contributions using the linear theory analysis, which is an excellent approximation if the anisotropies were generated when the universe recombined at a redshift around 1100. I present a treatment of perturbations based on a tight-coupling approximation, which simplifies the equations significantly and enables one to identify the physical processes and parameters, while at the same time preserving an accuracy of 10-20 % when compare