The free Bose gas on Robertson-Walker spacetimes

In this letter we define a new state on the Weyl algebra of the Klein-Gordon field on Robertson-Walker spacetimes. This state approximates a thermal equilibrium state and opens the possibility to do quantum statistical mechanics on expanding universes. We explain why this state is believed to be a physically relevant state in the framework of algebraic quantum field theory. The methods used in the proof are shortly reviewed. We also explain how to describe the evolution of the state. It is shown that the inverse temperature of the state changes with the scale parameter in the Robertson-Walker metric.