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DBI Inflation in the Tip Region of a Warped Throat

Steven Kecskemeti, John Maiden, Gary Shiu, Bret Underwood

TL;DR

This work investigates DBI inflation in the tip region of warped throats, where the warp factor becomes nearly constant. By employing a generalized warp factor form and the Hamilton-Jacobi framework, it shows that 60 or more e-folds can be generated in the tip region, but this typically comes with large non-Gaussianities due to a small sound speed. In the Klebanov-Strassler throat, tip-region inflation tends to clash with observational bounds on non-Gaussianity, whereas other throat geometries may alleviate these constraints. The study highlights how the detailed throat geometry imprints on observables and emphasizes the need to consider tip-region dynamics for realistic reheating and post-inflation phenomenology.

Abstract

Previous work on DBI inflation, which achieves inflation through the motion of a $D3$ brane as it moves through a warped throat compactification, has focused on the region far from the tip of the throat. Since reheating and other observable effects typically occur near the tip, a more detailed study of this region is required. To investigate these effects we consider a generalized warp throat where the warp factor becomes nearly constant near the tip. We find that it is possible to obtain 60 or more e-folds in the constant region, however large non-gaussianities are typically produced due to the small sound speed of fluctuations. For a particular well-studied throat, the Klebanov-Strassler solution, we find that inflation near the tip may be generic and it is difficult to satisfy current bounds on non-gaussianity, but other throat solutions may evade these difficulties.

DBI Inflation in the Tip Region of a Warped Throat

TL;DR

This work investigates DBI inflation in the tip region of warped throats, where the warp factor becomes nearly constant. By employing a generalized warp factor form and the Hamilton-Jacobi framework, it shows that 60 or more e-folds can be generated in the tip region, but this typically comes with large non-Gaussianities due to a small sound speed. In the Klebanov-Strassler throat, tip-region inflation tends to clash with observational bounds on non-Gaussianity, whereas other throat geometries may alleviate these constraints. The study highlights how the detailed throat geometry imprints on observables and emphasizes the need to consider tip-region dynamics for realistic reheating and post-inflation phenomenology.

Abstract

Previous work on DBI inflation, which achieves inflation through the motion of a brane as it moves through a warped throat compactification, has focused on the region far from the tip of the throat. Since reheating and other observable effects typically occur near the tip, a more detailed study of this region is required. To investigate these effects we consider a generalized warp throat where the warp factor becomes nearly constant near the tip. We find that it is possible to obtain 60 or more e-folds in the constant region, however large non-gaussianities are typically produced due to the small sound speed of fluctuations. For a particular well-studied throat, the Klebanov-Strassler solution, we find that inflation near the tip may be generic and it is difficult to satisfy current bounds on non-gaussianity, but other throat solutions may evade these difficulties.

Paper Structure

This paper contains 12 sections, 77 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Overview of DBI Inflation
  3. Hamilton-Jacobi Approach
  4. DBI Inflation in a Generic Warped Throat
  5. Inflationary Observables
  6. Warped Compactifications
  7. AdS$_5$ Throat
  8. Mass Gap
  9. IR DBI Inflation
  10. Discussion
  11. The KS Throat
  12. Non-Gaussianities

Figures (1)

  • Figure 1: Plotted are the warp factors for different throat geometries as a function of $\tau$: the long dashed line is an AdS$_5$ geometry, the thick line is the warp factor for a KS throat with a log-correction as in Kofman:2005yzKKLMMTHerzog:2001Klebanov:2000nc, the thin red line is the mass gap approximation, and the short dashed line is the exact KS warp factor. Inset: The region near the tip, $\tau\sim 0$ is enlarged to show the differences in the warp factors. Notice that the mass gap approximation models the flattening of the KS warp factor.