There are two distinct ideal changes for any of the thermodynamic processes of physical reality. One is called adiabatic the other is called isothermal. Both are important for understanding the physical concept of free energy which is a measure of the ability of a physical thermodynamic system to do useful work. For all adiabatic processes the physical system undergoing thermodynamic changes do not gain or lose heat energy. For all isothermal processes the physical system undergoing thermodynamic changes do so at constant absolute temperature. As the largest physical thermodynamic system the universe is neither adiabatic nor isothermal. Ideally, it is not adiabatic mainly because of the need of a resolution for the existence of dark matter and energy. Ideally, it is not isothermal since the universal accelerated expansion is done at the expense of decreasing absolute temperature of the cosmic microwave background radiation.

However, for the thermodynamic processes of the quantum vacuum fluctuations of zero-point energies it must be both adiabatic and isothermal. It is adiabatic because the vacuum does not gain or lose energy for many billions of years since the big bang singularity where and when both matter and energy were then created. It is also isothermal since the absolute zero temperature of the vacuum can neither be reached nor altered by artificial means. Nonetheless, the ultimate caveat is that the zero-point energies can hardly be set free unless cold fusion can be initialized by laser irradiation of deuterons.