String Gas Cosmology: Basics and Brandenberger-Vafa Scenario

Abstract

In this chapter we address the first truly stringy cosmological model, the string gas model, started by Brandenberger and Vafa. The reason for the model starts with a very simple observation: If inflation lasts for more than 70 e-folds (which admittedly is not necessary, we need more than 50–60 e-folds, but restricting $N_e<70$ would not be needed for experimental reasons, only for calculational reasons, which seems unreasonable), then current cosmological scales, $\sim H_0^{-1}$, that leave an imprint in the CMBR, were less than Planck size $l_\mathrm{Pl}$ at the initial time. But then we cannot trust the effective field theory of inflation, and we would need a full quantum gravity theory, like string theory.