In thermodynamic books I read free expansion does NOT change the temperature of the gas as no work is done against any medium. Therefore initial internal energy equals to final and as internal energy only the function of temperature the temperature does not go down. I keep reading after the Big Bang the universe expanded and cooled down. But why? the gas did work against a medium which our universe was in? Why the universe cooled down then? Wasn't that an adiabatic process?