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String Cosmology

James M. Cline

TL;DR

The paper surveys how string theory informs cosmology, focusing on two pillars: a landscape-based explanation for a small cosmological constant via flux discretuum and eternal inflation, and a suite of string-inspired inflationary models driven by brane dynamics and moduli stabilization. It details the Bousso-Polchinski mechanism, warped flux compactifications (notably the KS throat), and the KKLT stabilization paradigm, then systematically analyzes brane-antibrane inflation, the η problem, and several proposed cures including DBI inflation, shift-symmetric constructions, and racetrack scenarios. It also connects theory to observables, deriving power-spectrum relations, exploring non-Gaussian signatures, cosmic strings, and reheating challenges in warped geometries, highlighting both distinctive predictions and current obstacles. Overall, the work underscores how string theory yields rich, testable cosmological frameworks while admitting unresolved issues like moduli stabilization effects on inflation and reheating efficiency, motivating further theoretical and observational work.

Abstract

A pedagogical introduction to aspects of string cosmology, including the landscape (BPBT) solution to the cosmological constant problem, brane-antibrane inflation, warped compactification, the KKLMMT model, the eta problem of SUGRA models, DBI inflation, Kahler modulus and racetrack inflation, the D3-D7 model, cosmic superstrings, and the problem of reheating. Also includes basic methods for phenomenology of multifield models with nonstandard kinetic terms.

String Cosmology

TL;DR

The paper surveys how string theory informs cosmology, focusing on two pillars: a landscape-based explanation for a small cosmological constant via flux discretuum and eternal inflation, and a suite of string-inspired inflationary models driven by brane dynamics and moduli stabilization. It details the Bousso-Polchinski mechanism, warped flux compactifications (notably the KS throat), and the KKLT stabilization paradigm, then systematically analyzes brane-antibrane inflation, the η problem, and several proposed cures including DBI inflation, shift-symmetric constructions, and racetrack scenarios. It also connects theory to observables, deriving power-spectrum relations, exploring non-Gaussian signatures, cosmic strings, and reheating challenges in warped geometries, highlighting both distinctive predictions and current obstacles. Overall, the work underscores how string theory yields rich, testable cosmological frameworks while admitting unresolved issues like moduli stabilization effects on inflation and reheating efficiency, motivating further theoretical and observational work.

Abstract

A pedagogical introduction to aspects of string cosmology, including the landscape (BPBT) solution to the cosmological constant problem, brane-antibrane inflation, warped compactification, the KKLMMT model, the eta problem of SUGRA models, DBI inflation, Kahler modulus and racetrack inflation, the D3-D7 model, cosmic superstrings, and the problem of reheating. Also includes basic methods for phenomenology of multifield models with nonstandard kinetic terms.

Paper Structure

This paper contains 22 sections, 126 equations, 15 figures.

Table of Contents

  1. Dark Energy
  2. The problem of scales
  3. The string theory landscape
  4. The Bousso-Polchinski (Brown-Teitelboim) mechanism
  5. Caveats to the Landscape approach
  6. Inflation
  7. Brane-antibrane inflation
  8. Warped Compactification
  9. Warped brane-antibrane inflation
  10. The $\eta$ problem
  11. Solutions (?) to the $\eta$ problem
  12. Superpotential corrections
  13. Tuning the length of the throat
  14. Multibrane inflation
  15. DBI inflation (D-celleration)
  16. ...and 7 more sections

Figures (15)

  • Figure 2: (a) Scalar field potential which is a toy model for the landscape. (b) A part of the potential illustrating the failure of tunneling, à la Coleman-DeLuccia.
  • Figure 3: Intrinsic probability distribution for values of $\Lambda$ coming from minima of the landscape potential.
  • Figure 4: Lattice of allowed 4-form flux integers, and a hyperspherical shell of values consistent with canceling a large negative $\Lambda_{\rm bare}$ to get a small $\Lambda_{\rm obs}$.
  • Figure 5: Nucleation of bubble of new $\Lambda$ whose wall is an M2-brane.
  • Figure 6: Parallel D3 brane and $\overline{\hbox{D3}}$ antibrane.
  • ...and 10 more figures