Primordial non-Gaussianity from the DBI Galileons

TL;DR

This paper investigates primordial non-Gaussianity in inflation models inspired by DBI Galileons, calculating the bispectrum and constraining model parameters based on non-Gaussianity amplitude and tensor-to-scalar ratio.

Contribution

It extends the analysis of non-Gaussianity to DBI Galileon models, showing how the equilateral non-Gaussianity estimator applies and deriving new bounds on model parameters.

Findings

$f_{NL}^{equil}$ ranges between -0.32/$c_s^2$ and -0.16/$c_s^2$ for DBI Galileons.

Large non-Gaussianities occur when the sound speed $c_s$ is much less than 1.

In G-inflation, $f_{NL}^{equil}$ relates to the tensor-to-scalar ratio as $4.62 r^{-2/3}$.

Abstract

We study primordial fluctuations generated during inflation in a class of models motivated by the DBI Galileons, which are extensions of the DBI action that yield second order field equations. This class of models generalises the DBI Galileons in a similar way with K-inflation. We calculate the primordial non-Gaussianity from the bispectrum of the curvature perturbations at leading order in the slow-varying approximations. We show that the estimator for the equilateral-type non-Gaussianity, $f_{\rm NL} ^{equil}$, can be applied to measure the amplitude of the primordial bispectrum even in the presence of the Galileon-like term although it gives a slightly different momentum dependence from K-inflation models. For the DBI Galileons, we find $-0.32 /c_s^2 < f_{\rm NL} ^{equil} < -0.16/c_s^2$ and large primordial non-Gaussianities can be obtained when $c_s$ is much smaller than 1 as in the usual DBI inflation. In G-inflation models, where a de Sitter solution is obtained without any potentials, the non-linear parameter is given by $f_{\rm NL}^{equil} = 4.62 r^{-2/3}$ where $r$ is the tensor to scalar ratio, giving a stringent constraint on the model.