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Some Aspects of Brane Inflation

Gary Shiu, S. -H. Henry Tye

TL;DR

This work argues that brane inflation grounds the inflaton potential in higher-dimensional gravity, alleviating fine-tuning by tying the potential to bulk/brane dynamics. A velocity-dependent inter-brane term modifies the scalar perturbations through a factor $Z(\phi)$, while tensor modes remain governed by the bulk, altering the tensor-to-scalar ratio and the spectral tilt. The authors derive general expressions for $n-1$ and its running in the presence of $Z$, and analyze two explicit potentials—the interpolating and inverse-power forms—demonstrating red-tilted spectra and potential observable signatures in CMB and large-scale structure. Overall, brane dynamics predicts distinctive signatures that could differentiate brane inflation from standard 4D scenarios, via changes in $T/S$, $dn/d\ln k$, and scale-dependent tilt connected to bulk-brane physics.

Abstract

The inflaton potential in four-dimensional theory is rather arbitrary, and fine-tuning is required generically. By contrast, inflation in the brane world scenario has the interesting feature that the inflaton potential is motivated from higher dimensional gravity, or more generally, from bulk modes or string theory. We emphasize this feature with examples. We also consider the impact on the spectrum of density perturbation from a velocity-dependent potential between branes in the brane inflationary scenario. It is likely that such a potential can have an observable effect on the ratio of tensor to scalar perturbations.

Some Aspects of Brane Inflation

TL;DR

This work argues that brane inflation grounds the inflaton potential in higher-dimensional gravity, alleviating fine-tuning by tying the potential to bulk/brane dynamics. A velocity-dependent inter-brane term modifies the scalar perturbations through a factor , while tensor modes remain governed by the bulk, altering the tensor-to-scalar ratio and the spectral tilt. The authors derive general expressions for and its running in the presence of , and analyze two explicit potentials—the interpolating and inverse-power forms—demonstrating red-tilted spectra and potential observable signatures in CMB and large-scale structure. Overall, brane dynamics predicts distinctive signatures that could differentiate brane inflation from standard 4D scenarios, via changes in , , and scale-dependent tilt connected to bulk-brane physics.

Abstract

The inflaton potential in four-dimensional theory is rather arbitrary, and fine-tuning is required generically. By contrast, inflation in the brane world scenario has the interesting feature that the inflaton potential is motivated from higher dimensional gravity, or more generally, from bulk modes or string theory. We emphasize this feature with examples. We also consider the impact on the spectrum of density perturbation from a velocity-dependent potential between branes in the brane inflationary scenario. It is likely that such a potential can have an observable effect on the ratio of tensor to scalar perturbations.

Paper Structure

This paper contains 7 sections, 52 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Dynamics of the Inflaton
  3. Brane Inflation
  4. Examples
  5. An Interpolating Potential
  6. Inverse Power Potential
  7. Discussion

Figures (1)

  • Figure 1: The tilt of the density perturbation spectral index $n-1$ for $d_{\perp}=6$ as a function of the number of e-foldings $N$ from the end of inflation. The solid line corresponds to the full potential of the form $1- \zeta z^4/{4r^{d_{\perp}-2}}$. The set of points correspond to $-3/N$, the tilt for the approximate potential $A- C z^4/4$.