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Inflationary String Theory?

C. P. Burgess

TL;DR

The paper surveys efforts to realize inflation within string theory, focusing on moduli stabilization in Type IIB flux compactifications and two main inflationary avenues: racetrack inflation via a two-exponential superpotential and brane-antibrane inflation in warped throats. It shows that racetrack models can yield slow-roll inflation near a saddle with a red tilt $n_s \approx 0.95$ and little running, while providing a landscape of vacua that allows eternal inflation in some regions; brane-antibrane scenarios require substantial tuning and can predict a blue tilt $n_s \sim 1.03-1.08$, with potential cosmic-string signatures at the end of inflation. The work highlights the delicate balance between moduli stabilization, uplift to de Sitter, and inflationary dynamics, suggesting a two-stage or highly constrained inflationary history may be necessary within string theory. Overall, it demonstrates that string-motivated effective field theories can realize inflation, but systematic exploration of moduli, reheating, and higher-order corrections remains crucial for robust predictions.

Abstract

The inflationary paradigm provides a robust description of the peculiar initial conditions which are required for the success of the Hot Big Bang model of cosmology, as well as of the recent precision measurements of temperature fluctuations within the cosmic microwave background. Furthermore, the success of this description indicates that inflation is likely to be associated with physics at energies considerably higher than the weak scale, for which string theory is arguably our most promising candidate. These observations strongly motivate a detailed search for inflation within string theory, although it has (so far) proven to be a hunt for a fairly elusive quarry. This article summarizes some of the recent efforts along these lines, and draws some speculative conclusions as to what the difficulty finding inflation might mean.

Inflationary String Theory?

TL;DR

The paper surveys efforts to realize inflation within string theory, focusing on moduli stabilization in Type IIB flux compactifications and two main inflationary avenues: racetrack inflation via a two-exponential superpotential and brane-antibrane inflation in warped throats. It shows that racetrack models can yield slow-roll inflation near a saddle with a red tilt and little running, while providing a landscape of vacua that allows eternal inflation in some regions; brane-antibrane scenarios require substantial tuning and can predict a blue tilt , with potential cosmic-string signatures at the end of inflation. The work highlights the delicate balance between moduli stabilization, uplift to de Sitter, and inflationary dynamics, suggesting a two-stage or highly constrained inflationary history may be necessary within string theory. Overall, it demonstrates that string-motivated effective field theories can realize inflation, but systematic exploration of moduli, reheating, and higher-order corrections remains crucial for robust predictions.

Abstract

The inflationary paradigm provides a robust description of the peculiar initial conditions which are required for the success of the Hot Big Bang model of cosmology, as well as of the recent precision measurements of temperature fluctuations within the cosmic microwave background. Furthermore, the success of this description indicates that inflation is likely to be associated with physics at energies considerably higher than the weak scale, for which string theory is arguably our most promising candidate. These observations strongly motivate a detailed search for inflation within string theory, although it has (so far) proven to be a hunt for a fairly elusive quarry. This article summarizes some of the recent efforts along these lines, and draws some speculative conclusions as to what the difficulty finding inflation might mean.

Paper Structure

This paper contains 5 sections, 7 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Modulus Fixing
  3. Racetrack Inflation
  4. Brane-Antibrane Inflation
  5. Discussion

Figures (3)

  • Figure 1: Plot for a racetrack type potential illustrating the local minima between which a saddle point exists on which inflation can start. Units are $M_p=1$.
  • Figure 2: Plot for the same potential as for Fig. \ref{['F1']}, showing a close-up of the saddle point which lies between the two local minima. Units are $M_p=1$.
  • Figure 3: Plot showing the inflationary potential as a function of the inter-brane separation, $\psi$, and the internal-space volume, $\sigma$. Also shown are several inflaton trajectories starting from different initial field values. Units are $M_p=1$.