String Gas Cosmology: Progress and Problems

TL;DR

String Gas Cosmology proposes an early universe model based on string theory principles, suggesting a quasi-static phase with string gases that produce scale-invariant fluctuations, offering an alternative to inflation.

Contribution

The paper reviews the development, predictions, and challenges of String Gas Cosmology as an alternative early universe model based on string theory.

Findings

Predicts an almost scale-invariant spectrum of fluctuations

Provides an alternative to inflation for structure formation

Identifies key problems facing the model

Abstract

String Gas Cosmology is a model of the evolution of the very early universe based on fundamental principles and key new degrees of freedom of string theory which are different from those of point particle field theories. In String Gas Cosmology the universe starts in a quasi-static Hagedorn phase during which space is filled with a gas of highly excited string states. Thermal fluctuations of this string gas lead to an almost scale-invariant spectrum of curvature fluctuations. Thus, String Gas Cosmology is an alternative to cosmological inflation as a theory for the origin of structure in the universe. This short review focuses on the building blocks of the model, the predictions for late time cosmology, and the main problems which the model faces.