On the Realization of Assisted Inflation

TL;DR

The paper addresses the fine-tuning problem in inflation by exploring assisted inflation with multiple scalar fields. It generalizes the mechanism from exponential to chaotic potentials, showing that decoupled field sectors enable a single effective field with suppressed couplings, while cross-couplings undermine the approach. A concrete 5D KK scenario is analyzed, where a large number of KK modes can yield an effectively mild 4D quartic coupling, allowing chaotic inflation without severe tuning, provided initial conditions and mass scales satisfy certain constraints. The work highlights both the potential and the limitations of dimensionally lifted, multi-field inflation, demonstrating that large extra dimensions can realize assisted inflation when cross-couplings are controlled.

Abstract

We consider conditions necessary for a successful implementation of so-called assisted inflation. We generalize the applicability of assisted inflation beyond exponential potentials as originally proposed to include standard chaotic (λφ^4 or m^2 φ^2) models as well. We also demonstrate that in a purely 4-dimensional theory, unless the assisted sector is in fact decoupled, the additional fields of the assisted sector actually impede inflation. As a specific example of an assisted sector, we consider a 5-dimensional KK model for which the extra dimension may be somewhat or much larger than the inverse Planck scale. In this case, the assisted sector (coming from a KK compactification) eliminates the need for a fine-tuned quartic coupling to drive chaotic inflation. This is a general result of models with one or more "large" extra dimensions.