I Can there be structures made from neutrinos with angular momentum?

[Suekdccia]

TL;DR Summary

Can there be structures made from neutrinos that can have large amounts of angular momentum (similar to a neutron star or a rotating black hole)?

Would it be possible to eventually have structures made from neutrinos somewhere in the universe, as it is indicated in this question (https://physics.stackexchange.com/questions/80390/are-neutrino-stars-theoretically-possible), like halos of neutrino gas surrounding the center of galaxies?

Also, would these haloes eventually collapse into a "central" point as the orbit around the galaxy decays due to gravitational waves emission?

And if it would be possible, could they have high amounts of angular momentum? Even if they collapse, would the high angular momentum be conserved?

 

[Orodruin]

No, not really. Neutrinos are way too light for this to be possible. Even the lowest energy neutrinos in the universe - the cosmic neutrino background - simulations end up with only ever so slight overdensities due to galactic gravitation.

 

[Suekdccia]

No, not really. Neutrinos are way too light for this to be possible. Even the lowest energy neutrinos in the universe - the cosmic neutrino background - simulations end up with only ever so slight overdensities due to galactic gravitation.

But eventually neutrinos would lose their kinetic energy in the future. Couldn't they be attracted by gravity to form structures then? Also, right handed neutrinos are a candidate for dark matter. So, couldn't left-handed neutrinos also form similar structures?

 

[Orodruin]

But eventually neutrinos would lose their kinetic energy in the future. Couldn't they be attracted by gravity to form structures then? Also, right handed neutrinos are a candidate for dark matter. So, couldn't left-handed neutrinos also form similar structures?

In order to form a structure, you need not only be cold enough, but you also need a way to radiate away excess energy. This is essentially why dark matter is not believed to clump more. Left handed neutrinos are both light, move fast, and have no real way of getting rid of excess energy to form structures by themselves. I would be surprised if tgis would occur at any significant level even in the distant future, but if you can find any published reference discussing ot I’d be happy to take a look.

 

[Suekdccia]

I would be surprised if tgis would occur at any significant level even in the distant future

Even at enormous future scales? Also, perhaps I didn't make it clear in the question, but I'm not claiming that these structures would be common or significantly abundant. Just asking if they are possible, even if they would be rather rare

According to this paper (https://pubs.aip.org/aip/acp/article/1666/1/140004/907455/Neutrinos-and-dark-matter) standard model neutrinos could form hot dark matter structures. However observations and simulations suggest that there is more cold dark matter than hot dark matter. The active neutrinos of the standard model, having very low mass (and therefore very high speeds) are therefore unlikely to account for all dark matter. But this means that at least *some* of the dark matter could be possibly made from these kind of neutrinos (even if not the majority of it)

 

[Vanadium 50]

It sounds like you aren't asking a question, you are arguing a position.

If your new question is "if a trillion trillion years from now could their be two neutrinos bound by mutual attraction anywhere in the universe?" I guess so. If that doesn't "count" because of one reason or another, this becomes a guessing game - how big is a "structure", when is the future, how big a slice of the universe we consider, etc...

 

[Suekdccia]

It sounds like you aren't asking a question, you are arguing a position.

If your new question is "if a trillion trillion years from now could their be two neutrinos bound by mutual attraction anywhere in the universe?" I guess so. If that doesn't "count" because of one reason or another, this becomes a guessing game - how big is a "structure", when is the future, how big a slice of the universe we consider, etc...

Sorry, I didn't mean to lecture anyone. I just wanted to point out that there are studies that indicate that even with standard model neutrinos, some structures could be formed (perhaps in form of neutrino halos or as "hot" dark matter) in contrast to what post #2 seems to suggest (that it's impossible for neutrinos to aggregate)

Considering this, my questions basically are 3:

Is it physically possible that neutrinos may aggregate to form gravitationally bound "structures" (even in the form of haloes or diffuse bodies) for example around galaxies (even if they would be relatively rare)? I mean, no microscopic structures but macroscopical ones

Would these structures eventually collapse into a central point? Or would they stay like that, orbiting the cental point indefinetely?

Could these structures have high amounts of angular momentum? (Similarly to the angular momentum of a rotating black hole or a neutron star)

 

[Suekdccia]

if you can find any published reference discussing ot I’d be happy to take a look.

Sorry I didn't mean to argue a position without further discussion, I just tried to point out that the article that I referenced indicated that standard model neutrinos could form structures corresponding to hot dark matter (which would in turn be a part of the total dark matter content, which would be much higher). Have you look into it? Do you think it's wrong?

 

[Orodruin]

Do you think it's wrong?

No, I think your interpretation of it is wrong. It is talking about the large scale structure formation, not about structures made of neutrinos.

 

[Suekdccia]

No, I think your interpretation of it is wrong. It is talking about the large scale structure formation, not about structures made of neutrinos.

Perhaps I'm misuing the word "structure". Let's see if I can clarify what I mean...

I don't necessarily mean compact structures like a "neutrino star", but more like diffuse halos surrounding large scale structures (I asked about halos surrounding galaxies before, but it's true that if we are considering large scale structures, we should be rather talking about clusters of galaxies).

For example, in the OP I linked a similar question to my own (https://physics.stackexchange.com/questions/80390/are-neutrino-stars-theoretically-possible). The most voted answer proposes that rather than "neutrino stars", there could be "halos of degenerate neutrino gas around galactic clusters" and it links to this other paper (https://academic.oup.com/mnras/article/434/3/2679/1046015) which discusses the possibility of the presence of neutrino halos around large scale structures. In the introduction, it indicates:

The Λ cold dark matter (ΛCDM) paradigm has been embraced after it was realized that the most natural DM candidate, the neutrino, seems incapable to explain the formation of large-scale structures such as galaxies and galaxy clusters. ΛCDM has been successful in describing the cosmic microwave background and large-scale structures. It is adaptable to many other situations, though often not predictive. However, despite decades and dozens of searches, the CDM particle has not been established (Aprile et al. 2012), also not at the Large Hadron Collider (Aad et al. 2011), so one may wish to keep an open eye at other scenarios.

It has been put forward by Gibson (1996) that in the early Universe the role of turbulence and viscosity in the protoplasma and after the transition to neutral gas is more important than commonly assumed. Gravitational hydrodynamics alone is capable to explain large-scale structure formation without CDM trigger, in a three-step top-down scenario; see also Nieuwenhuizen, Gibson & Schild (2009). First, the plasma undergoes a viscous fragmentation at redshift z = 5100, creating voids of 40 Mpc comoving size. After the decoupling of photons the gas condenses in Jeans clumps of 600 000 solar masses and they in their turn fragment in 200 billion micro brown dwarfs of Earth weight. This picture explains a wealth of observations (Schild 1996; Nieuwenhuizen et al. 2009; Nieuwenhuizen, van Heusen & Liska 2012) and motivates to re-open our minds for the possibility of free-streaming DM, like neutrino hot DM.

Which apparently indicates that there could be halos formed by light neutrinos with high speeds* (which I guess that could increase in size as neutrinos lose kinetic energy over time, but please correct me if I'm wrong on this)

And in this paper (https://arxiv.org/pdf/astro-ph/9707285), in section 1.7.6. the author proposes that mixed dark matter models (with both cold and hot dark matter) do explain better our observations as we would need less-clustered dark matter to address them concerning large scale structure formation. It specifically mentions simulations of extended hot dark matter halos.

Also here (https://franciscovillaescusa.github.io/neutrinos.html) the author shows some simulations of halos of light neutrinos that become more aggregated once their speed becomes low enough and compares it with cold dark matter.


Considering all of this, couldn't these types of "structures" (If the word "structures" is the right term) of light neutrino halos be physically possible (even more, when the neutrinos will lose kinetic energy with time)? I mean, couldn't there be diffuse and less clustered groups of matter consisting of standard model neutrinos that would become a bit more clustered as they lose kinetic energy over time?


Or even in that case all of this is wrong and no such structures could possibly ever form? Then why do these authors even mention hot dark matter halos as a possibility?

*please note that the author talks about models of free streaming neutrinos (or hot dark matter), but here (https://adlibitum.oats.inaf.it/seminari/nieuwenhuizen.pdf) he nuances that they are free streaming up until being trapped by a galaxy cluster

 

[PeterDonis]

According to this paper (https://pubs.aip.org/aip/acp/article/1666/1/140004/907455/Neutrinos-and-dark-matter) standard model neutrinos could form hot dark matter structures.

No, the paper actually says the opposite:

"The disagreement of the actual galaxy distribution with the predictions of the hot dark matter paradigm is apparent from the simulation, thus excluding Standard Model neutrinos as the dominant component of dark matter" (p. 140004-1).

The actual possibility the paper discusses is that non-Standard Model particles making up dark matter could decay into Standard Model neutrinos which we could then detect.

 

[Suekdccia]

No, the paper actually says the opposite:

"The disagreement of the actual galaxy distribution with the predictions of the hot dark matter paradigm is apparent from the simulation, thus excluding Standard Model neutrinos as the dominant component of dark matter" (p. 140004-1).

The actual possibility the paper discusses is that non-Standard Model particles making up dark matter could decay into Standard Model neutrinos which we could then detect.

if I read it correctly it indicates that hot dark matter (standard model neutrinos) cannot be the dominant form of dark matter, in the sense that it could be part of it, but not the majority of it. Dark matter would be mostly formed then by non-standard neutrinos, or put in another way, that cold dark matter is the dominant one, and that could decay into standard neutrinos. Is this wrong?

 

[PeterDonis]

if I read it correctly it indicates that hot dark matter (standard model neutrinos) cannot be the dominant form of dark matter, in the sense that it could be part of it, but not the majority of it.

"Could be part of it" is just handwaving for "we can't detect any effects of it". In other words, it's pure speculation in the absence of any evidence. Which makes it no better than Carl Sagan's invisible dragon in the garage.

Dark matter would be mostly formed then by non-standard neutrinos

The paper considers the hypothesis that dark matter must be mostly made of non-Standard Model particles--with "mostly" meaning "all that we can actually detect the effects of". But "non-Standard Model particles" is more general than "non-standard neutrinos". The non-Standard Model particles don't have to be some kind of neutrinos.

cold dark matter is the dominant one

Yes, but that is already known to be the case regardless of what the cold dark matter ends up being made of.

that could decay into standard neutrinos

That is another hypothesis that the paper considers.

 

[Suekdccia]

"Could be part of it" is just handwaving for "we can't detect any effects of it". In other words, it's pure speculation in the absence of any evidence. Which makes it no better than Carl Sagan's invisible dragon in the garage.


The paper considers the hypothesis that dark matter must be mostly made of non-Standard Model particles--with "mostly" meaning "all that we can actually detect the effects of". But "non-Standard Model particles" is more general than "non-standard neutrinos". The non-Standard Model particles don't have to be some kind of neutrinos.


Yes, but that is already known to be the case regardless of what the cold dark matter ends up being made of.


That is another hypothesis that the paper considers.

So it is just utterly impossible that neutrino halos may form? Even considering the links from #10 post?

Is it physically impossible (or not known) that neutrinos that will lose enough kinetic energy in the future bounding and clustering in structures (even if diffuse ones)?

 

[PeterDonis]

So it is just utterly impossible that neutrino halos may form? Even considering the links from #10 post?

Is it physically impossible (or not known) that neutrinos that will lose enough kinetic energy in the future bounding and clustering in structures (even if diffuse ones)?

Asking if something is "impossible" is a useless question in an area of physics in which there is still much research being done.

Given our best current upper bounds for Standard Model neutrino masses, and the extreme weakness of Standard Model neutrino interactions, it is extremely unlikely that they could have formed significant gravitationally clumped structures given the current age of the universe. That is what @Oroduin told you way back in post #2. That is the best we can say given the actual evidence we currently have.

Once you start talking about speculative hypotheses involving non-Standard Model particles (whether neutrinos or otherwise), there is really nothing useful that can be said other than "this is an open area of research". We have no evidence of any non-Standard Model particles and what theoretical constraints we think we have are too weak to be much help.

 

[Suekdccia]

Asking if something is "impossible" is a useless question in an area of physics in which there is still much research being done.

Given our best current upper bounds for Standard Model neutrino masses, and the extreme weakness of Standard Model neutrino interactions, it is extremely unlikely that they could have formed significant gravitationally clumped structures given the current age of the universe. That is what @Oroduin told you way back in post #2. That is the best we can say given the actual evidence we currently have.

Once you start talking about speculative hypotheses involving non-Standard Model particles (whether neutrinos or otherwise), there is really nothing useful that can be said other than "this is an open area of research". We have no evidence of any non-Standard Model particles and what theoretical constraints we think we have are too weak to be much help.

I understand that it is very unlikely that neutrinos may form gravitationally clumped structures given the current age of the universe, but I was rather asking about far future scenarios (where neutrinos would not move fast)... Couldn't they cluster as they have a non-zero mass (once they have lost nearly all of their kinetic energy in the far future)?

 

[Vanadium 50]

You are going to ask the same question over and over again until you get the answer you want, aren't you?

Iif a trillion trillion years from now could their be two neutrinos bound by mutual attraction anywhere in the universe? I guess so. If that doesn't "count" because of one reason or another, this becomes a guessing game - how big is a "structure", when is the future, how big a slice of the universe we consider, etc...

 

[PeterDonis]

I was rather asking about far future scenarios

No references that you have given deal with this at all. If you cannot find any references, then we have no valid basis for discussion.

 

[PeterDonis]

Due to the lack of any valid references as a basis for discussion, this thread is closed.