<p>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. </p>

<p>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. </p>

<p>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.</p>

CC BY-SA 4.0
2019-05-31T17:02:58.453Z
*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.
PM 2Ring
CC BY-SA 4.0
2019-05-31T17:05:52.563Z
Given ridiculously enough time (heat death ~ infinite time scale), I would still think this would be the ultimate fate. On any earthly timescales this is and would be extremely rare of course.
tuomas
CC BY-SA 4.0
2019-05-31T17:14:30.903Z
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.
PM 2Ring
CC BY-SA 4.0
2019-05-31T17:17:29.833Z
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
tuomas
CC BY-SA 4.0
2019-05-31T17:22:54.380Z
The vast majority of the neutrinos emitted in the last 13.8 billion years are stilll whizzing around,so the universe would have to last an awfully long time for them all to be absorbed. But that would only take care of neutrinos now in existence; in the next 10 or 20 billion years,countless more neutrinos would be emitted,so the universe will always be choc-a- bloc with neutrinos. As for the expansion & eventual fate,I favour the Big Crunch. Anything else has to have a one-off,difficult-to-explain special cause,God for instance,& that makes me feel uneasy.
Michael Walsby
CC BY-SA 4.0
2019-05-31T17:56:07.933Z
@Michael FWIW, it's highly likely that the majority of the neutrinos in the universe today are the neutrinos in the [cosmic neutrino background](https://en.wikipedia.org/wiki/Cosmic_neutrino_background), which decoupled from the rest of matter when the universe was still very young. Those neutrinos have been severely redshifted, so they are a lot slower than the neutrinos produced by stellar fusion and supernovae.
PM 2Ring
CC BY-SA 4.0
2019-06-01T02:39:02.700Z
@MichaelWalsby Don't you mean big bounce?  Big Crunch is still a one-off; it just ends up rather more compact in the end.
zibadawa timmy
CC BY-SA 4.0
2019-06-01T08:39:50.743Z
Yes,that's what I mean. It solves the problem of a one-off cause. An oscillating universe which perpetually recycles itself solves a lot of problems. There are those who say the universe is infinite,& that raises more problems. An infinite universe would have infinite mass & would require infinite energy to accelerate it. Where would all this energy come from & how could the universe accelerate forever if it were constantly fighting against the gravitational drag of infinite mass?
Michael Walsby
CC BY-SA 4.0
2019-06-01T10:01:39.000Z
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.
tuomas
CC BY-SA 4.0
2019-06-01T11:18:16.900Z
Thomas,if you don't want to stray off topic,why not answer my original question?
Michael Walsby
CC BY-SA 4.0
2019-06-01T11:31:13.230Z
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.
tuomas
CC BY-SA 4.0
2019-05-31T16:57:24.117Z
2019-05-31T16:57:24.117Z
tuomas
<p>Unless the neutrinos encounter something, yes they'll continue to travel forever. Because they have mass, they move through time(1). Thus, they will continue to move in the direction of time and space infinity (relative to where they started).</p>
<p>They will never catch up to the photons, though. Unlike mass objects, they have no definite location until they bump into something with mass.</p>
<p>If they don't, they more than just continue forever. They go farther than mass objects, to <a href="https://www.yourdictionary.com/null-infinity" rel="nofollow noreferrer">null infinity</a>.</p>
<p>Those photons don't feel time pass, and so never experience anything but their own creation.</p>
<p>===================</p>
<p>(1) Feynman Australia lectures and his easy pieces book</p>

CC BY-SA 4.0
2024-06-19T01:57:01.760Z
2024-06-19T02:02:56.373Z
2024-06-19T02:02:56.373Z
Miss Understands
<p>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.</p>

CC BY-SA 4.0
2019-05-31T17:08:40.830Z
What do you mean by "orbit the universe", exactly?
PM 2Ring
CC BY-SA 4.0
2019-05-31T17:28:02.187Z
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.
Michael Walsby
CC BY-SA 4.0
2019-05-31T17:39:09.680Z
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?
PM 2Ring
CC BY-SA 4.0
2019-05-31T22:22:38.170Z
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.
Michael Walsby
CC BY-SA 4.0
2019-06-01T08:57:14.503Z
My question seems to have got an interesting debate going,& that's good. Without such debate we could find ourselves in the same sort of situation that existed for a 1000 years after Ptolemy proposed his purely imaginary epicycles to explain the motion of Mars as it orbited the Earth. We need to question our assuptions from time to time.
Michael Walsby
CC BY-SA 4.0
2019-06-01T09:00:20.943Z
Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/94381/discussion-between-pm-2ring-and-michael-walsby).
PM 2Ring
CC BY-SA 4.0
2019-06-01T23:40:37.447Z
-1. Asking "is it possible they could orbit the universe indefinitely?" assumes the Universe has a centre of mass, but if it did (and it would need to be incredibly massive if it's to capture almost massless extremely fast neutrinos), we certainly would have deduced its presence from observation of galactic redshifts.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2019-06-02T11:26:15.800Z
I haven't said they orbit the universe,only asked if it's possible,& if not,where do they go? A particle which doesn't interact & travels close to speed c will cover an extremely long distance in 50 billion years! Regaridng the universe,it certainly has mass,& all other masses have centres,so why not the universe? Before we go looking for the missing mass,we need to know how much mass there is right under our noses,then we will have an idea of how much is still unaccounted for. Space must be teeming with neutrinos & other hard to detect particles,so how much is in a cubic light year of space?
Michael Walsby
<cosmology>
Where do Neutrinos Go if They Don't Interact?
Michael Walsby
# Where do Neutrinos Go if They Don't Interact?

(Asked by: username_0 on 2019-05-31)

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.

**Tags:** cosmology

> **Comments:**
> **username_1**: What do you mean by "orbit the universe", exactly?  
> **username_0**: 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.  
> **username_1**: 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?  
> **username_0**: 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.  
> **username_0**: My question seems to have got an interesting debate going,& that's good. Without such debate we could find ourselves in the same sort of situation that existed for a 1000 years after Ptolemy proposed his purely imaginary epicycles to explain the motion of Mars as it orbited the Earth. We need to question our assuptions from time to time.  
> **username_1**: Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/94381/discussion-between-pm-2ring-and-michael-walsby).  
> **username_2**: -1. Asking "is it possible they could orbit the universe indefinitely?" assumes the Universe has a centre of mass, but if it did (and it would need to be incredibly massive if it's to capture almost massless extremely fast neutrinos), we certainly would have deduced its presence from observation of galactic redshifts.  
> **username_0**: I haven't said they orbit the universe,only asked if it's possible,& if not,where do they go? A particle which doesn't interact & travels close to speed c will cover an extremely long distance in 50 billion years! Regaridng the universe,it certainly has mass,& all other masses have centres,so why not the universe? Before we go looking for the missing mass,we need to know how much mass there is right under our noses,then we will have an idea of how much is still unaccounted for. Space must be teeming with neutrinos & other hard to detect particles,so how much is in a cubic light year of space?  

---

## 2 answers

### Answer by username_3 on 2019-05-31. Score: 3

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.

> **Comments:**
> **username_1**: *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.  
> **username_3**: Given ridiculously enough time (heat death ~ infinite time scale), I would still think this would be the ultimate fate. On any earthly timescales this is and would be extremely rare of course.  
> **username_1**: 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.  
> **username_3**: 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  
> **username_0**: The vast majority of the neutrinos emitted in the last 13.8 billion years are stilll whizzing around,so the universe would have to last an awfully long time for them all to be absorbed. But that would only take care of neutrinos now in existence; in the next 10 or 20 billion years,countless more neutrinos would be emitted,so the universe will always be choc-a- bloc with neutrinos. As for the expansion & eventual fate,I favour the Big Crunch. Anything else has to have a one-off,difficult-to-explain special cause,God for instance,& that makes me feel uneasy.  
> **username_1**: @Michael FWIW, it's highly likely that the majority of the neutrinos in the universe today are the neutrinos in the [cosmic neutrino background](https://en.wikipedia.org/wiki/Cosmic_neutrino_background), which decoupled from the rest of matter when the universe was still very young. Those neutrinos have been severely redshifted, so they are a lot slower than the neutrinos produced by stellar fusion and supernovae.  
> **username_4**: @MichaelWalsby Don't you mean big bounce?  Big Crunch is still a one-off; it just ends up rather more compact in the end.  
> **username_0**: Yes,that's what I mean. It solves the problem of a one-off cause. An oscillating universe which perpetually recycles itself solves a lot of problems. There are those who say the universe is infinite,& that raises more problems. An infinite universe would have infinite mass & would require infinite energy to accelerate it. Where would all this energy come from & how could the universe accelerate forever if it were constantly fighting against the gravitational drag of infinite mass?  
> **username_3**: 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.  
> **username_0**: Thomas,if you don't want to stray off topic,why not answer my original question?  
> **username_3**: 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.  

---

### Answer by username_5 on 2024-06-19. Score: 0

Unless the neutrinos encounter something, yes they'll continue to travel forever. Because they have mass, they move through time(1). Thus, they will continue to move in the direction of time and space infinity (relative to where they started).

They will never catch up to the photons, though. Unlike mass objects, they have no definite location until they bump into something with mass.

If they don't, they more than just continue forever. They go farther than mass objects, to [null infinity](https://www.yourdictionary.com/null-infinity).

Those photons don't feel time pass, and so never experience anything but their own creation.

===================

(1) Feynman Australia lectures and his easy pieces book


---

