Running Non-Gaussianities in DBI Inflation

TL;DR

This paper analyzes non-Gaussianities in the simplest IR DBI inflation model, focusing on the three-point function of the curvature perturbation and its running. It shows that a small sound speed $c_s=\gamma^{-1}$ from relativistic inflaton motion yields sizable equilateral-type non-Gaussianity, with $f_{NL}$ scaling as $f_{NL} \sim 0.32\,\gamma^2$ in the equilateral limit and $f_{NL} \sim 0.036\beta^2 N_e^2$ for the IR model, and a running $n_{NG}-1\simeq -2/N_e$ that encodes the background geometry. The work contrasts IR with UV DBI inflation and slow-roll models, highlighting how the running and shape depend on the warp geometry and how COBE/Planck-era normalization and tilt constraints influence parameter ranges. Overall, the paper identifies running non-Gaussianity as a potential probe of internal-space geometry and warp factors in string-inspired inflation models and discusses observational prospects and theoretical caveats.

Abstract

We study the non-Gaussianity in the simplest infrared (IR) model of the DBI inflation. We show that the non-Gaussianity in such a model is compatible with the current observational bound, and is within the sensitivity of future experiments. We also discuss the scale dependence of the non-Gaussianity. In the DBI inflation, such a feature can be used as a probe to the properties of the background geometry of the extra dimensions or internal space.