  We investigate the thermodynamical properties of quantum fields in curved
spacetime. Our approach is to consider quantum fields in curved spacetime as a
quantum system undergoing an out-of-equilibrium transformation. The
non-equilibrium features are studied by using a formalism which has been
developed to derive fluctuation relations and emergent irreversible features
beyond the linear response regime. We apply these ideas to an expanding
universe scenario, therefore avoiding assumptions on the relation between
entropy and quantum matter. We provide a fluctuation theorem which allows us to
understand particle production due to the expansion of the universe as an
entropic increase. Our results pave the way towards a different understanding
of the thermodynamics of relativistic and quantum systems in our universe.
