Lecture notes on thermodynamics of ideal string gases and its application in cosmology

Understand

In these lecture notes I give a pedagogical introduction to the thermodynamics of ideal string gases.

• The computation of thermodynamic quantities in the canonical ensemble formalism will be shown in detail with explicit examples.
• Attention will be given mainly to the thermodynamical consequences of string degrees of freedom, where I will especially address i) the Hagedorn temperature, a critical temperature above which the canonical ensemble description breaks down, which can be the onset point of some instability of the string gas; ii) the phase structure arising from compactification, embodied in the moduli-dependence of the Helmholtz free energy, which corrects the tree-level vacuum and can provide mechanism for moduli stabilization.
• Then I will briefly explain the implementation of string gas thermodynamics in cosmology, showing a simple example which gives rise to a radiation-dominated early universe.