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It is said that neutrinos can travel for billons of years at almost the speed of light without interacting with anything. That being the case,what do they do - is it possible they could orbit the universe indefinitely? They certainly can't leave it.

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    $\begingroup$ What do you mean by "orbit the universe", exactly? $\endgroup$ – PM 2Ring May 31 '19 at 17:08
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    $\begingroup$ I mean what I say,but I'm just asking if it's possible. If I knew what becomes of countless hordes of neutrinos which almost never interact,I wouldn't need to ask. It's pretty obvious they will leave our galaxy far behind. $\endgroup$ – Michael Walsby May 31 '19 at 17:28
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    $\begingroup$ Sorry, I still don't know what it means for something to orbit the universe. The universe is most probably infinite. How do you orbit an infinite thing? $\endgroup$ – PM 2Ring May 31 '19 at 17:39
  • $\begingroup$ What is the evidence for an infinite universe? An infinite universe would have infinite mass & that might create problems when it eventually contracts into a Big Crunch. I know Big Crunches are out of fashion these days,but we can't rule it out. To say the universe is infinite is a very big assumption,& extraordinary claims require extraordinary evidence.At almost the speed of light,a neutrino could cover an immense distance in 50 billion years. $\endgroup$ – Michael Walsby May 31 '19 at 22:22
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – PM 2Ring Jun 1 '19 at 9:00
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It depends on the ultimate fate of universe, as with everything else in the universe. Fundamentally, neutrinos are not much different from “ordinary matter”. Yes, they do interact only via weak force and gravity, but this only quantitatively changes the frequency of interaction, but does not change the game qualitatively. As long as there’s non-zero chance for them to interact, and as long as there is non-zero density, there is non-zero probability they will eventually interact with another particle (or black hole), and given infinite timeframe, it will happen for sure.

Accelerating expansion of universe does however complicate things a bit, but again, eventually there is bound to be some random fluctuation which will knock the neutrino off from its track.

Eventually neutrinos would meet anti-neutrinos and annihilate each other. Since the chances for this are really low, I would suspect neutrino-antineutrino collisions being the very last blink of this universe - given heat death is the ultimate fate of this existence.

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    $\begingroup$ Eventually neutrinos would meet anti-neutrinos and annihilate each other. Probably not. You can fire a beam of neutrinos and antineutrinos at each other and nothing will happen, unless the particles in the beams have outrageously high energy. $\endgroup$ – PM 2Ring May 31 '19 at 17:02
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    $\begingroup$ Neutrino + antineutrino cannot directly produce photons, they'd have to create a Z boson (or maybe more than 1), and those guys are really heavy. Unless the neutrino & antineutrino have huge kinetic energy, the reaction can't happen. OTOH, I guess it can still happen via quantum tunneling, if you wait long enough. But I expect that black holes would eat them before that has a chance to happen. $\endgroup$ – PM 2Ring May 31 '19 at 17:14
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    $\begingroup$ Yes, I fully agree. I was thinking more of the universal time scales beyond black holes (ie after even SMBHs have evaporated), there’s not much else left in the universe than time and neutrino-antineutrino annihilations... the gist being here: if it can happen, it will happen. It will take ridiculously long time frames though $\endgroup$ – tuomas May 31 '19 at 17:17
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    $\begingroup$ Most of the observations agree with contemporary models, which in turn do imply universe that is infinite both in space and time. While this might not be preferred metaphysically for all, this is simply how science works: observations and theoretical models are based on best available facts, not opinions nor beliefs. Also this discussion is utterly off-topic for this question and answer. $\endgroup$ – tuomas Jun 1 '19 at 10:01
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    $\begingroup$ I think I did. What ultimately happens to neutrinos / antineutrinos is not much different to what happens to electrons / positrons, only the cut-off between different outcomes (depending on eg expansion acceleration rate of the universe) might be different, as neutrinos are electrically neutral and very light, it will be way more difficult for them to be bound together than electron/positron pairs. Now exactly whether all of them, most of them or almost none of them will ultimately annihilate, depends on facts beyond our current measurement abilities, I believe. $\endgroup$ – tuomas Jun 1 '19 at 11:31

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