String Gas Cosmology: Progress and Problems
  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.
