In a dark energy dominated universe, it seems that all the particles get away from each other and that the final state will be one with one or zero particles per horizon. This sounds very intuitive, but it is based on classical physics and GR. Particles have wavefunctions and this is whar confuses me in the context of expansion (and leaving the horizon). In the very far future, due to expansion particles will interact rarely and therefore there will be no decoherence really, all of the wavefunctions will spread. However, due to spreading, it would be possible that their wavefunctions interact and decohere again (paradoxically) or overlap and form something like a Bose Einstein condensate. Both options are of course strange so this serves rather as a reduction an absurdum. My real question is, quantum mechanically, what can we expect regarding the behavior of particles and their lack of interactions? Does it make sense to call them particles or will they really be spread out wavefunctions? If the final state is one of one particle per horizon, will the wavefunction of the particle cover all of the horizon? Thanks for the patience.