Hybrid and multifield inflation

Abstract

In this thesis I study the generation of density perturbations in two classes of inflationary models: hybrid inflation and multifield inflation with non-minimal coupling to gravity. In the case of hybrid inflation, we developed a new method of treating these perturbations that does not rely on a classical trajectory for the fields. A characteristic of the spectrum is the appearance of a spike at small length scales, which could conceivably seed the formation of black holes that can evolve to become the supermassive black holes found at the centers of galaxies. Apart from numerically calculating the resulting spectrum, we derived an expansion in the number of waterfall fields, which makes the calculation easier and more intuitive. In the case of multifield inflation, we studied models where the scalar fields are coupled non-minimally to gravity. We developed a covariant formalism and examined the prediction for non-Gaussianities in these models, arguing that they are absent except in the case of fine-tuned initial conditions. We have also applied our formalism to Higgs inflation and found that multifield effects are too small to be observable. We compared these models to the early data of the Planck satellite mission, finding excellent agreement for the spectral index and tensor to scalar ratio and promising agreement for the existence of iso-curvature modes.