Phase Transitions During Inflation and the CMB Hemispherical Power Asymmetry

Planck and WMAP have observed a ~ 10% hemispherical asymmetry of the CMB power, which cannot be explained in the context of conventional inflation models with statistically isotropic Gaussian density perturbations. As an approach to understanding this, the project will investigate the dynamics of phase transitions which occur during inflation, in particular the spectrum of perturbations of a scalar field undergoing a transition. (Note that this is a second scalar field, not the inflaton.) It has previously been shown that such transitions can account for scalar field perturbations with a hemispherical power asymmetry across a horizon volume. The project will study the asymmetry in the context of plateau inflation models, in particular Starobinsky inflation and non-minimally coupled inflation models, which are known to be naturally compatible with the Planck values for the spectral index and tensor-to-scalar ratio. Once the asymmetry in the scalar field perturbation is understood, models will be constructed to translate the scalar field perturbation into a curvature perturbation with a hemispherical power asymmetry. This is challenging, as the model should not produce a corresponding asymmetry in the mean CMB temperature. The study will serve to test whether scalar field based models can generate a hemispherical CMB power asymmetry, identify the general conditions required to do so, and, if unsuccessful, determine the reasons why such models cannot be constructed.