Challenges for String Gas Cosmology

TL;DR

The paper surveys the conceptual problems of inflationary cosmology—such as the amplitude and trans-Planckian issues, singularities, and the cosmological constant puzzle—and presents String Gas Cosmology (SGC) as a string-theory-inspired alternative aimed at a non-singular early universe. It develops the SGC framework using a string-frame dilaton-gravity description with a gas of string states (momentum, winding, and oscillatory modes) and highlights how T-duality and winding-mode dynamics can hinder expansion while allowing three dimensions to grow via winding annihilation. Progress on moduli stabilization is reviewed, with radii potentially fixed at the self-dual radius when momentum and winding modes balance, though dilaton stabilization remains a central challenge. The paper discusses remaining questions, including achieving consistent entropy production and a viable inflationary or alternative mechanism within SGC, and calls for moving beyond heuristic treatments toward a more complete background-string-theory analysis.

Abstract

In spite of the phenomenological successes of the inflationary universe scenario, the current realizations of inflation making use of scalar fields lead to serious conceptual problems which are reviewed in this lecture. String theory may provide an avenue towards addressing these problems. One particular approach to combining string theory and cosmology is String Gas Cosmology. The basic principles of this approach are summarized.

Figures (1)

• Figure 1.: Space-time diagram (sketch) showing the evolution of scales in inflationary cosmology. The vertical axis is time, and the period of inflation lasts between $t_i$ and $t_R$, and is followed by the radiation-dominated phase of standard big bang cosmology. During exponential inflation, the Hubble radius $H^{-1}$ is constant in physical spatial coordinates (the horizontal axis), whereas it increases linearly in time after $t_R$. The physical length corresponding to a fixed comoving length scale labelled by its wavenumber $k$ increases exponentially during inflation but increases less fast than the Hubble radius (namely as $t^{1/2}$), after inflation.