So according to physics, because of the expansion of the universe, there will be a time in the future, where all that exists (maybe…quantum fields) will be so far away from each other that it will be impossible for them to communicate with each other, interact and exchange energies. But those "individual pockets" of energy will still be there, because energy is conserved.
Question #1
So how can we talk about a "dead" universe in the future when the same amount of energy that exists now will also exist and be "floating around" this so-called "dead" universe in the future ?
I guess we could say that the same amount of energy will still be there, but because the carriers of that energy won't be able to communicate with each other it will be some sort of "passive" energy that never interacts with/changes into anything else, so for lack of a better term, we could call that "passive" state of energy a "dead" state.
But because of the law of conservation of energy, the universe cannot continue to expand indefinitely. At some point it must run out of "expansion" energy because there is only a certain amount of energy in the universe and we know that part of it is already used for stuff other than "expansion" so the universe cannot create new "expansion" energy, ex nihilo, ad infinitum.
Question #2
And if the universe were to stop expanding wouldn't those "pockets of energy" floating around eventually start finding each other once again and start interacting with each other once again, in which case we wouldn't be talking about a "dead" universe anymore ?
Best Answer
First, the “dead” terminology is just a label, so don’t take it too seriously. It is fine to call a universe where nothing interesting ever happens “dead”, even if it still contains energy and particles. There is no implication that “dead” means 0 energy.
Second, the energy of the universe is not even well defined, let alone conserved. While energy conservation applies locally everywhere in the universe, globally there is no unique way to add up the energy here and there to get a total energy for the whole universe.
This means that the argument that “the universe cannot create new expansion energy, ex nihilo, ad infinitum” is not justified. That total energy cannot even be computed, let alone compared to past or future values.
However, you can discuss the energy density of the universe. The cosmological constant means that there is a fixed energy density due to spacetime itself. A constant energy density over an expanding volume could be interpreted as an increasing total amount of energy (although doing so runs into the same definitional problems mentioned above)