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Intergalactic space is 2.7K (https://en.wikipedia.org/wiki/Outer_space) Interstellar space is a little warmer. My understanding is that these will decrease over time.

The phase diagrams of helium I can find vary a little bit; eg

helium phase diagram

but they all show a superfluid state at 0 pressure and a sufficiently low temperature. (Helium 4 at 2.17K (http://ltl.tkk.fi/research/theory/helium.html)

This suggests that eventually, the helium in the intergalactic, and later, the interstellar medium will condense.

My questions:

  1. Is this prediction accurate?
  2. What effect will this have on someone observing space at that time?
  3. What other interesting effects would this have?
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    $\begingroup$ It seems like the average distance between atoms would preclude having a condensed phase. It is a good question though $\endgroup$
    – RC_23
    Commented Jan 10, 2023 at 17:42
  • $\begingroup$ Condensation requires the atomic wave functions to overlap. Thus, there's a /minimum/ pressure for it to happen. Somebody can do the back-of-the-envelope, but I'd bet outer space is waaaaaay too low-density for that. $\endgroup$
    – rfl
    Commented Jan 10, 2023 at 20:05
  • $\begingroup$ If two He atoms of low enough energy eventually encounter each other, will they condense together and stay together, barring a higher energy interaction? Or do you need more interactions on a shorter time scale for the phase transition to occur? $\endgroup$
    – Alex K
    Commented Jan 10, 2023 at 23:29
  • $\begingroup$ @rfl would that be just a collision as in an ideal gas? Or do ideal gas approximations not really work in that limit? $\endgroup$
    – Alex K
    Commented Feb 26, 2023 at 15:40
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    $\begingroup$ Ideal gas doesn't work at all anymore: Ideal gas means elastic collissions of point particles only. Here, it's all about quantum wavefunctions overlapping. Don't think of it so much as two Fermions sticking together to form a Boson; while some talk about Cooper pairs like that, it's misleading, as Cooper pairs (and by extensions Helium paired up in intergalactic space) aren't localized. But why don't you simply do the back-of-the-envelope to calculate the deBroglie wavelength of Helium, and compare it to the mean distance in intergalactic space ;) $\endgroup$
    – rfl
    Commented Feb 26, 2023 at 15:54

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The "temperature of space" is a little misleading. The Cosmic Microwave Background indeed has a temperature of around 2.7 Kelvin. The baryonic matter, including helium, contained in the ISM and IGM has temperatures between $10^5$ and $10^7$ Kelvin. The CMB and baryonic matter are not in equilibrium. I suspect that the thermal coupling is weak enough that the time needed to come to equilibrium is longer than the age of the universe, but a more knowledgable cosmologist might correct me. The helium may eventually radiate enough heat to reach temperatures low enough to condense, but then the density is still too low.

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  • $\begingroup$ That's really helpful! Can you elaborate a little on how the igm would reach equilibrium (the primary mechanisms involved)? Black body radiarion doesn't seem like it would do anything in this case. $\endgroup$
    – Alex K
    Commented Feb 28, 2023 at 2:25

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