Super-Horizon Scale Dynamics of Multi-Scalar Inflation

We consider the dynamics of a multi-component scalar field on super-horizon scales in the context of inflationary cosmology. We present a method to solve the perturbation equations on super-horizon scales, i.e., in the long wavelength limit, by using only the knowledge of spatially homogeneous background solutions. In doing so, we clarify the relation between the perturbation equations in the long wavelength limit and the background equations. Then as a natural extension of our formalism, we provide a strategy to study super-horizon scale perturbations beyond the standard linear perturbation theory. Namely we reformulate our method so as to take into account the nonlinear dynamics of the scalar field. Study of multi-scalar inflation is a current topic in cosmology, motivated mainly by the fact that supergravity theories suggest the existence of many flat directions in the scalar field potential 1). Observationally we will be able to determine the accurate spectrum of primordial density fluctuations in the near future, e.g., by the next-generation MAP and PLACK satellites 2). At that time, to test a variety