Local non-Gaussianity from rapidly varying sound speeds

TL;DR

This paper investigates how rapidly changing sound speeds during multi-field inflation can generate significant local non-Gaussianity, providing analytic expressions and specific model examples.

Contribution

It derives analytic formulas for non-Gaussianity with varying sound speeds in multi-field inflation, extending beyond slow variation assumptions.

Findings

Rapidly varying sound speeds can produce large local non-Gaussianity.

Analytic expressions valid beyond slow variation are obtained.

Two-field models in warped throats show significant non-Gaussianity at the end of inflation.

Abstract

We study the effect of non-trivial sound speeds on local-type non-Gaussianity during multiple-field inflation. To this end, we consider a model of multiple-field DBI and use the deltaN formalism to track the super-horizon evolution of perturbations. By adopting a sum separable Hubble parameter we derive analytic expressions for the relevant quantities in the two-field case, valid beyond slow variation. We find that non-trivial sound speeds can, in principle, curve the trajectory in such a way that significant local-type non-Gaussianity is produced. Deviations from slow variation, such as rapidly varying sound speeds, enhance this effect. To illustrate our results we consider two-field inflation in the tip regions of two warped throats and find large local-type non-Gaussianity produced towards the end of the inflationary process.