Aspects of Quasi-Single Field Inflation

Abstract

A simple extension of single-field slow roll inflation is called quasi-single field inflation (QSFI). In quasi-single field inflation, the inflaton is coupled to one or more scalar fields with masses of order Hubble constant during inflation, H, called isocurvatons. Depending on the interactions between the inflaton and the isocurvatons, the observable consequences of such a theory can vary dramatically. This thesis is primarily concerned with analyzing how these interactions affect cosmological observables. We begin by discussing QSFI with one isocurvaton. In particular, we study the non-perturbative limit of a kinetic interaction coupling the inflaton to the isocurvaton. In this non-perturbative limit, the kinetic interaction results in isocurvaton mode functions that oscillate yet decay quickly after horizon crossing. This oscillatory, decaying behavior is reflected in the primordial non-Gaussianity and other observables. Then, we study the perturbative limit of the aforementioned kinetic interaction. Instead of quickly decaying, oscillating mode functions as in the non-perturbative limit, the mode functions in the perturbative limit decay slowly and do not oscillate. Due to this slow decay, the primordial bispectrum is enhanced in the squeezed limit. This enhanced squeezed limit can result in large contributions to galactic halo correlations at long wavelengths. We explore these long wavelength enhancements to galactic halo correlations in detail. We then discuss how quantum loops of isocurvatons in QSFI can significantly contribute to galactic halo correlations at long wavelengths. In fact, we show that loops can give the most important contributions at long wavelengths in certain models. Finally, we consider theories with multiple isocurvatons. Such theories may consist of interactions coupling the isocurvatons to eachother. These interactions can result in slowly decaying yet oscillating late-time behavior for the isocurvaton mode functions. This combines the two characteristic features from the non-perturbative (oscillating) and perturbative (slowly-decaying) single isocurvaton QSFI models discussed above. These features could result in an oscillatory bispectrum that is enhanced in the squeezed limit. Oscillations in the galactic halo power spectrum at long wavelengths may then be indicative of such multi-isocurvaton models.