Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Multifield DBI Inflation and Non-Gaussianities

Min-xin Huang, Gary Shiu, Bret Underwood

TL;DR

This work analyzes multifield DBI inflation in warped-brane scenarios, focusing on how isocurvature modes influence the power spectrum and non-Gaussianity in the small sound speed limit. Using the $\delta N$ formalism and a two-field, highly radial trajectory, it shows that isocurvature contributions to $P_\zeta$ are suppressed by $c_s$, yielding effectively single-field behavior for the power spectrum. The leading non-Gaussianity remains $f_{NL}\sim1/c_s^2$, with a subleading multifield term producing a distinctive folded-triangle bispectrum, providing a potential observational signature. Overall, the results suggest multifield effects are subdominant in small-$c_s$ DBI inflation, simplifying model-building and guiding interpretations of CMB constraints in brane inflation scenarios.

Abstract

We analyze the trajectories for multifield DBI inflation, which can arise in brane inflation models, and show that the trajectories are the same as in typical slow roll inflation. We calculate the power spectrum and find that the higher derivative terms of the DBI action lead to a suppression of the contribution from the isocurvature perturbations. We also calculate the bispectrum generated by the isocurvature perturbation, and find that it leads to distinctive features.

Multifield DBI Inflation and Non-Gaussianities

TL;DR

This work analyzes multifield DBI inflation in warped-brane scenarios, focusing on how isocurvature modes influence the power spectrum and non-Gaussianity in the small sound speed limit. Using the formalism and a two-field, highly radial trajectory, it shows that isocurvature contributions to are suppressed by , yielding effectively single-field behavior for the power spectrum. The leading non-Gaussianity remains , with a subleading multifield term producing a distinctive folded-triangle bispectrum, providing a potential observational signature. Overall, the results suggest multifield effects are subdominant in small- DBI inflation, simplifying model-building and guiding interpretations of CMB constraints in brane inflation scenarios.

Abstract

We analyze the trajectories for multifield DBI inflation, which can arise in brane inflation models, and show that the trajectories are the same as in typical slow roll inflation. We calculate the power spectrum and find that the higher derivative terms of the DBI action lead to a suppression of the contribution from the isocurvature perturbations. We also calculate the bispectrum generated by the isocurvature perturbation, and find that it leads to distinctive features.

Paper Structure

This paper contains 7 sections, 34 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Multifield Equations of Motion
  3. Trajectories in Multifield Brane Inflation
  4. The Power Spectrum of Multifield DBI Inflation
  5. The inflationary perturbation
  6. Multifield Non-Gaussianity
  7. Discussion

Figures (4)

  • Figure 1: The trajectory of a multiple field inflationary system can be decomposed into an "adiabatic" field $\sigma$ with components along the trajectory and an "entropy" field $s$ orthogonal to the trajectory.
  • Figure 2: Multifield models have a number of different trajectories, depending on the curvature of the potential for the fields. The sharpness of the turns is controlled by the ratio of the curvatures.
  • Figure 3: The shape of non-Gaussianity in multifield DBI inflation is shown through a plot of ${\mathcal{A}}(k_1,k_2,1)/(k_1k_2)$ in (\ref{['eq:NGShape']}). Notice that in the folded triangle limit $k_1=k_2=k_3/2$ the bispectrum is negative, and constitutes a distinctive signature of multifield DBI inflation in the small $c_s$ limit. The presence of opposite signs of the non-Gaussianity may give rise to interesting observational effects.
  • Figure 4: The negative part of the non-gaussianity in the folded triangle limit is shown.