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String Gas Cosmology

Thorsten Battefeld, Scott Watson

TL;DR

The paper critically surveys String Gas Cosmology (SGC), deriving the effective action from string theory and examining moduli stabilization via string gases. It argues that purely winding modes cannot stabilize extra dimensions at late times, but that enhanced symmetry states and quantum moduli trapping offer robust mechanisms for stabilization, potentially tying into dark matter candidates. It connects these dynamics to late-time cosmology, including possible observational imprints and a framework for moduli-space dynamics on a string landscape. The work also emphasizes the limitations of the low-energy, moduli-space approximation and highlights the need to integrate time-dependent string dynamics and, potentially, inflation to develop testable predictions.

Abstract

We present a critical review and summary of String Gas Cosmology. We include a pedagogical derivation of the effective action starting from string theory, emphasizing the necessary approximations that must be invoked. Working in the effective theory, we demonstrate that at late-times it is not possible to stabilize the extra dimensions by a gas of massive string winding modes. We then consider additional string gases that contain so-called enhanced symmetry states. These string gases are very heavy initially, but drive the moduli to locations that minimize the energy and pressure of the gas. We consider both classical and quantum gas dynamics, where in the former the validity of the theory is questionable and some fine-tuning is required, but in the latter we find a consistent and promising stabilization mechanism that is valid at late-times. In addition, we find that string gases provide a framework to explore dark matter, presenting alternatives to $Λ$CDM as recently considered by Gubser and Peebles. We also discuss quantum trapping with string gases as a method for including dynamics on the string landscape.

String Gas Cosmology

TL;DR

The paper critically surveys String Gas Cosmology (SGC), deriving the effective action from string theory and examining moduli stabilization via string gases. It argues that purely winding modes cannot stabilize extra dimensions at late times, but that enhanced symmetry states and quantum moduli trapping offer robust mechanisms for stabilization, potentially tying into dark matter candidates. It connects these dynamics to late-time cosmology, including possible observational imprints and a framework for moduli-space dynamics on a string landscape. The work also emphasizes the limitations of the low-energy, moduli-space approximation and highlights the need to integrate time-dependent string dynamics and, potentially, inflation to develop testable predictions.

Abstract

We present a critical review and summary of String Gas Cosmology. We include a pedagogical derivation of the effective action starting from string theory, emphasizing the necessary approximations that must be invoked. Working in the effective theory, we demonstrate that at late-times it is not possible to stabilize the extra dimensions by a gas of massive string winding modes. We then consider additional string gases that contain so-called enhanced symmetry states. These string gases are very heavy initially, but drive the moduli to locations that minimize the energy and pressure of the gas. We consider both classical and quantum gas dynamics, where in the former the validity of the theory is questionable and some fine-tuning is required, but in the latter we find a consistent and promising stabilization mechanism that is valid at late-times. In addition, we find that string gases provide a framework to explore dark matter, presenting alternatives to CDM as recently considered by Gubser and Peebles. We also discuss quantum trapping with string gases as a method for including dynamics on the string landscape.

Paper Structure

This paper contains 21 sections, 106 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Dynamics of Strings in Time-Dependent Backgrounds
  3. Cosmology with String Gases
  4. Assumptions of the string gas approach
  5. Energy and pressure of a string gas
  6. Classical dynamics of string gases
  7. Initial Conditions and the Dimensionality of Space-time
  8. Cosmological Evolution in the Presence of a String Gas
  9. Summary of 10d Dynamics and Moduli Stabilization
  10. $4D$ Dynamics and the Effective Potential
  11. Quantum dynamics of string gases
  12. Quantum dynamics of string gases
  13. Late time cosmology and observations
  14. Dark Matter
  15. General setup
  16. ...and 6 more sections

Figures (1)

  • Figure 1: This graph shows the primary results of Watson:2003gf, where stabilization of the string frame radion (green or light line) in the presence of string winding and momentum modes was demonstrated. The damping of the oscillations relied crucially on the dilaton running slowly to weak coupling (red or dark line).