Consider an interstellar medium of one hydrogen atom/cm$^3$ with $500$ photons/cm$^3$ coming from CMB. This density of particles is very weak according to quantum decoherence. So, according to Schrödinger equation, the wave function of each atom could be quite flat/spread.
Is there an equilibrium coherence/decoherence due to Schrödinger equation and CMB?
In other words, is the wave function of each atom stable (on average)?
Can (the squared modulus of) this wave function be almost equidistributed in one cm$^3$?
Can then the interstellar matter be correlated?
If relevant, we can ask the same questions for an intergalactic medium of one hydrogen atom/m$^3$.