In quantum field theory and curved spacetime, even though everything empties out classically, quantum mechanically, there is still a nonzero temperature. In classical mechanics, the temperature of a box of gas or something like that is the average kinetic energy of all the molecules up to some constant factor. But in quantum mechanics, it's a little bit different because in the classical box of gas, even if the macro state is constant, right? Even if you're in equilibrium, so all of your gas is perfectly smooth everywhere, the micro state is not constant. At a constant temperature, at a fixed temperature, the molecules are moving around.