Scale dependences of local form non-Gaussianity parameters from a DBI isocurvature field

TL;DR

This paper derives how the spectral indices and runnings of local non-Gaussianity parameters from a DBI isocurvature field are suppressed by the sound speed, linking it to Lorentz boost effects.

Contribution

It introduces a method to compute spectral indices and their runnings for non-Gaussianity parameters in DBI models, highlighting the suppression effect due to sound speed.

Findings

Spectral indices of $f_{NL}$ and $g_{NL}$ are suppressed by the sound speed $c_s$.

The suppression effect is interpreted via Lorentz boost in the moving brane frame.

Provides analytical expressions for indices and their runnings in DBI isocurvature models.

Abstract

We derive the spectral indices and their runnings of local form $f_{NL}$ and $g_{NL}$ from a DBI isocurvature field and we find that the indices are suppressed by the sound speed $c_s$. This effect can be interpreted by the Lorentz boost from the viewpoint in the frame where brane is moving.