Primordial fluctuations and non-Gaussianities in multi-field DBI inflation

TL;DR

This paper investigates multi-field DBI inflation models, revealing that adiabatic and entropy modes share a sound speed, with entropy modes dominating in amplitude at small sound speeds, significantly impacting observable spectra and non-Gaussianities.

Contribution

It demonstrates that in multi-field DBI inflation, adiabatic and entropy modes propagate with a common sound speed, and entropy modes dominate at small sound speeds, affecting cosmological observables.

Findings

Adiabatic and entropy modes share a common sound speed.

Entropy modes are amplified more than adiabatic modes at small sound speeds.

Non-Gaussianity shape remains similar to single-field DBI case.

Abstract

We study Dirac-Born-Infeld (DBI) inflation models with multiple scalar fields. We show that the adiabatic and entropy modes propagate with a common effective sound speed and are thus amplified at the sound horizon crossing. In the small sound speed limit, we find that the amplitude of the entropy modes is much higher than that of the adiabatic modes. We show that this could strongly affect the observable curvature power spectrum as well as the amplitude of non-Gaussianities, although their shape remains as in the single-field DBI case.