This talk is based on our recent paper (arXiv:0706.2294), where we study 
specifically if a thermal gas of closed strings could arise as a consequence 
of the decay of unstable D-branes. This then provides a specific initial 
condition scenario for the thermal string gas in string gas cosmology at 
a finite origin of time. In my talk I will first briefly review some basic 
elements of string gas cosmology. Then I will give a long introduction to 
unstable D-branes. Finally, I will argue how low-dimensional unstable 
D-branes time evolve to thermal string gas.