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Imagine we have a cold region of the universe, almost devoid of matter and radiation. Or perhaps in a future universe where the CMB has "cooled" down to sufficiently low "temperatures"

Could there be long lived macroscopic Bose-Einstein and Fermionic states of matter there? Could matter composing these states clump together by itself (by its own gravitarional attraction)? Or perhaps if we had at first some matter that would attract the gas gravitationally so that the gas is already clumped (even if the matter that caused it to clump eventually disappeared or decayed)?

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It wouldn't survive for long as you need a trapping potential to keep the gas together.

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  • $\begingroup$ Could matter composing these states clump together by itself (by its own gravitarional attraction)? Or perhaps if we had at first some matter that would attract the gas gravitationally so that the gas is already clumped (even if the matter that caused it to clump eventually disappeared or decayed)? @VincentThacker $\endgroup$
    – vengaq
    Commented Jun 16 at 10:04
  • $\begingroup$ Of course this happens, but at galactic scales. Namely, gravity is the weakest interaction out of the four fundamental interactions. The gravitational attraction between atoms in a macroscopic BEC is negligible in probably every scenario. For example, the escape velocity from a rubidium atom at a distance of an angstrom is on the order of $10^{-23}~$m/s (I did a quick calculation, you can check it with Newtonian gravity). To give you an idea: atoms in a magneto-optical trap have velocities on the order of cm/s. $\endgroup$
    – Jesse
    Commented Jun 17 at 13:36
  • $\begingroup$ so in galactic scales we could have composite clumped Bose-Einstein and Fermionic condensates matter? @Jesse $\endgroup$
    – vengaq
    Commented Jun 20 at 10:07
  • $\begingroup$ @vengaq No, I meant that matter will clump together due to gravity at such scales. It will not be a Bose-Einstein or fermionic condensate where all particles have the same ground state wave function. $\endgroup$
    – Jesse
    Commented Jun 20 at 11:35
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Fermions do not form a condensate, since they can not accumulate in one quantum state. Sometimes people call pairs of fermions condensed, such as in superconducting materials, but I assume that this is not what you meant. However, fermions form a state of quantum degeneracy.

Neutron stars are considered to be in quantum degeneracy due to the strong gravitational attraction.

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  • $\begingroup$ Thank you. Any other case of states of quantum degeneracy that can exist in outer space? @kai90 $\endgroup$
    – vengaq
    Commented Jul 5 at 9:50

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