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What evidence is there that dark matter isn't one of the known types of neutrinos?

If it were, how would this be measurable?

ripper234
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4 Answers4

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Dark matter can be hot, warm or cold. Hot means the dark matter particles are relativistic (kinetic energy on the order of the rest mass or much higher), cold means they are not relativistic (kinetic energy much less than rest mass) and warm is in between. It is known that the total amount of dark matter in the universe must be about 5 times the ordinary (baryonic) matter to explain the CMB as measured by WMAP.

However, cold dark matter must be a very significant component of the universe to explain the growth of structures from the small fluctuations in the early universe that grew to become galaxies and stars (see this reference). Thus cold dark matter is also required to explain the currently measured galactic rotation curves.

Now, the neutrino oscillation experiments prove that neutrinos have a non-zero rest mass. However, the rest masses must still be very small so they could only contribute to the hot dark matter. The reason they can only be hot dark matter is because it is assumed that in the early hot, dense universe, the neutrinos would have been in thermal equilibrium with the hot ordinary matter at that time. Since the neutrino's rest mass is so small, they would be extremely relativistic, and although the neutrinos would cool as the universe expands, they would have still been very relativistic at the time of structure formation in the early universe. Thus, they can only contribute to hot dark matter in terms of the early growth of structure formation. [Because of the expansion of the universe since then, the neutrinos should have cooled so much that they are non-relativistic today.]

According to this source:

Current estimates for the neutrino fraction of the Universe’s mass–energy density lie in the range 0.1% <∼ ν <∼ a few %, under standard assumptions. The uncertainty reflects our incomplete knowledge of neutrino properties.

So most cosmic neutrinos are probably less than 10% of the total dark matter in the universe. In addition most of the rest (of the non-neutrino) 90% of dark matter must also be cold dark matter - both in the early universe and even now.

FrankH
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    I've no idea from this answer why there cannot be enough cold neutrinos to explain galactic rotation curves – lurscher Nov 27 '11 at 05:26
  • for instance, how can we know for sure that there aren't more cold neutrinos if their cross section decrease substantially at non-relativistic ranges. There might be 10, 100 or 10 million more than what standard assumptions would led us to believe – lurscher Nov 27 '11 at 05:54
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    @lurscher - sorry I screwed up and had the same link in the 2nd and 3rd paragraphs. I corrected that just now, so please click on the link in the 2nd paragraph to read why the neutrinos would only be hot DM. Your speculation that the cross section might be much higher than what current electroweak theory would say it is might be true, but then that means neutrinos are not what we currently think they are. All we can say is that our current understanding of neutrinos means that they could only contribute to hot DM and that they could not give the cold DM needed for structure formation. – FrankH Nov 27 '11 at 06:34
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    Ok, makes sense as long as we believe neutrinos don't have additional forces that make them lump together at low energies. – lurscher Nov 27 '11 at 14:53
  • Neutrino HDM is ruled out by large-scale structure observations. – user12345 May 18 '13 at 12:04
  • @user12345 - Are you talking about some new result recently published? I do already say in the answer that CDM is "needed to explain the growth of structures...". Are you saying something different than that? – FrankH May 19 '13 at 15:58
  • I was referring to the top-down structure formation (from blinis) that would occur if $\nu$s were HDM, as opposed to the bottom-up formation from CDM that results in the structures we see. It's not new, but they are well and truly ruled-out, unless $\nu$ are like 10 eV and we don't know anything :D – user12345 May 19 '13 at 19:40
  • Also, I believe that $\nu$ oscillations only 'prove' a mass difference, not a mass for all three species. But, that's being a bit pedantic, sorry. – user12345 May 19 '13 at 19:42
  • @user12345 no problem, but the three measured mass differences imply that at least the two heaviest neutrinos have mass. Probably all three have mass... – FrankH May 20 '13 at 00:47
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    Yeah, it'd be a pretty weird Universe otherwise – user12345 May 22 '13 at 20:25
  • It is not correct to state that "It is known that the total amount of dark matter in the universe must be about 5 times the ordinary (baryonic) matter to explain the CMB as measured by WMAP." This is conjectured; it is not "known". – Harry Macpherson Oct 11 '15 at 12:56
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    @HarryMacpherson You are wrong. There is a HUGE amount of evidence behind the Standard Model of Cosmology and the standard model says DM = 5 x Ordinary Matter. See https://en.wikipedia.org/wiki/Lambda-CDM_model . The CMB alone is a huge part of the evidence, but there are many other sources also. That amount of evidence is FAR beyond the level of "conjecture". – FrankH Oct 12 '15 at 01:51
  • No, @FrankhH, it's you that's wrong. You say "There is a HUGE amount of evidence behind the Standard Model of Cosmology and the standard model says DM = 5 x Ordinary Matter." That is not a basis for saying "it is known" that the universe contains a lot of dark matter. Dark matter is being sought but has not yet been found. Its existence is conjectured. There are several other models that do not require dark matter. – Harry Macpherson Oct 17 '15 at 12:27
  • @HarryMacpherson : The exact particle that is the DM particle is unknown and is being searched for and it's properties have not been determined. However, it is known that the DM must exist and that is must be particles. That is what there is a huge amount of evidence for. – FrankH Oct 19 '15 at 00:16
  • @FrankH - It is not known that dark matter "must" exist, nor that it exists. Even if there were a huge amount of evidence for it (which there isn't - what there is is a huge combined amount of speculation plus evidence), that would not make it "known" that it exists. The greater the search for it without finding it, the more the weight of evidence diminishes. You do not seem to be applying the scientific method of forming hypotheses and then trying to disprove them. – Harry Macpherson Oct 31 '15 at 12:15
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    @HarryMacpherson - There are nine general classes of evidence for DM: 1. Galaxy rotation curves, 2. Velocity dispersions of galaxies, 3. Galaxy clusters and gravitational lensing, 4. Cosmic microwave background, 5. Sky surveys and baryon acoustic oscillations, 6. Type Ia supernovae distance measurements, 7. Lyman-alpha forest, 8. Structure formation, 9. Bullet Cluster. Each of these classes includes MANY individual measurements. If that is NOT overwhelming evidence for DM then I don't know what could possibly convince you. I give up. You can get the last word in. I quit. – FrankH Nov 01 '15 at 03:32
  • @FrankH - Dark matter has not been observed and therefore its existence is speculation - hypothetical - however much evidence there is for it. I'll give the last word to the same Wikipedia article you got those points from, by citing the alternative hypotheses which would explain the same evidence that the dark matter hypothesis does: 1) Mass in extra dimensions, 2) Topological defects, 3) Modified gravity, 4) Fractality of Spacetime. – Harry Macpherson Nov 01 '15 at 22:47
  • The first link is broken again. I'd really like to see the logic against cold neutrinos summarized here. – quuxman Feb 13 '16 at 11:29
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    @quuxman - I removed the broken link and added what I remember was the argument of what the link would have said. – FrankH Feb 14 '16 at 08:06
  • Neutrinos are not relativistic today. – ProfRob Feb 19 '16 at 20:46
  • @RobJeffries - Do you have a reference that states this? I do not believe that we know this for a fact. You could be right if all neutrino masses are heavy enough, but we have no constraints on the mass of the lightest neutrino. So that neutrino could still be extremely relativistic. – FrankH Feb 20 '16 at 05:15
  • @RobJeffries - Further, even if all the neutrinos today are non-relativistic, they could still have been relativistic in the early universe when cold dark matter was needed to cause the growth of structures. – FrankH Feb 20 '16 at 05:19
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    @FrankH Yes, neutrinos are hot dark matter because they were highly relativistic at an epoch when the horizon contained the masses of the cosmic structures one is referring to. – ProfRob Feb 20 '16 at 08:29
  • @FrankH The most massive neutrino must be more than 0.04 eV. The sum of all three has been estimated as between 0.3 and $<2$ eV. Current neutrino temperature 1.95K. So $kT/m_{\nu}c^2 \sim 10^{-3}$, so not highly relativistic. It doesn't really matter what happens to the lightest one since the flavours oscillate. – ProfRob Feb 20 '16 at 08:42
  • @RobJeffries - thanks for the education. I will correct my answer... – FrankH Feb 21 '16 at 23:35
  • @RobJeffries: "Neutrinos are not relativistic today." -- Do you mean that primordial neutrinos (those produced during or shortly after the Big Bang) are not relativistic today? Aren't all the neutrinos we can currently detect (from the Sun, from nuclear reactors, briefly from SN 1987A, etc.) relativistic -- mostly because we're not currently able to detect neutrinos with lower energies? – Keith Thompson Nov 07 '17 at 00:21
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    @KeithThompson Yes, I was sloppy. I mean that primordial neutrinos are not now highly relativistic. Neutrinos from stars, supernovae etc. are. – ProfRob Nov 07 '17 at 00:31
  • @RobJeffries: Cool stuff, thanks. More info: https://en.wikipedia.org/wiki/Cosmic_neutrino_background – Keith Thompson Nov 07 '17 at 01:30
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Hot dark matter could be partly neutrinos - but they (probably) don't interact enough to have been resposnible for initial galaxy formation.

Martin Beckett
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  • Am I mistaken to conclude that they'd also be unable to explain galaxy rotation curves? Those suggest a halo distribution of dark matter around the centres of galaxies. Neutrino's wouldn't have that spatial distribution, neither when formed in the big bang or in later nuclear reactions. – MSalters Nov 21 '11 at 09:53
  • @MSalters: That is, in essence, the reason people distinguish between hot and cold dark matter. To explain both the structure of the cosmos and the rotation curves it has to cool enough to collect in/around galaxies. – dmckee --- ex-moderator kitten Jul 09 '12 at 17:57
  • @MSalters Your question is worthy of further discussion. The current estimates of neutrino mass make them non-relativistic now, and capable of being captured by galaxies and clusters. Depending on the exact neutrino mass one could have neutrino density enhancements of factors of 10 around large galaxies. – ProfRob Feb 20 '16 at 09:15
  • However, this enhancement would still not provide anywhere near the mass required to explain galaxy rotation curves. – ProfRob Feb 20 '16 at 09:17
  • "they (probably) don't interact enough" due to high velocities? – SRS Jun 13 '18 at 09:58
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Neutrinos from the big bang have been redshifted to ~2K = ~0.0002 eV, which is considerably lower than the current best upper bound on neutrino rest mass (0.1eV). We have no way to directly detect the flux of neutrinos at this low energy and the indirect methods at deducing it are tentative at best. So primordial neutrinos might indeed be a significant component of Cold/Warm dark matter. We don't know.

Jonathan Ray
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  • When you refer to "neutrinos from the big bang," what history do you have in mind for them? Would they have gone through some period of thermal equilibrium and then become decoupled? If so, then wouldn't their abundance be constrained by known particle physics? –  Sep 12 '13 at 15:03
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    Said upper bound would be extremely high because of neutrinos' extremely low probability of interacting with anything. And such an upper bound should be taken with a large grain of salt until someone unifies QM with relativity. I recall that it would take a light year of lead to block about half of any neutrino flux. Coincidentally, the mass of a cubic light year of lead at 11.3g/cm^3 would be within an order of magnitude of the total mass of the observable universe. I.e. a neutrino would have to cross most of the universe to be reabsorbed, on average. – Jonathan Ray Dec 23 '14 at 22:30
  • but you wouldn't need a cubic light year of lead to block any given neutrino, just one light year times the cross-sectional area of the neutrino. although i guess what you are saying is, if you integrate across all primordial neutrinos, you have to have a cubic light year. – Michael Feb 04 '15 at 19:52
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    But the same calculations that give you the temperature tell you how many there should be and hence $\Omega_{\nu}\sim 0.003$. – ProfRob Feb 19 '16 at 20:51
  • Yeah, this answer isn't right, the calculation yields $\Omega h^2 \simeq \frac{ \sum_i m_i}{93\text{ MeV}}$, see any review on this topic – innisfree Oct 18 '19 at 04:46
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Cold neutrinos which clumped together would form a Fermi-Dirac condensate. Unlike electrons in an atom there would be no mutual repulsion and the quantum numbers could increase truly "astronomically". For a large concentrate all but the early neutrino contributors would be far from cold. Such a concentrate would behave like a huge heavy ball of unobservable, very rareified liquid which is exacty what you see in a barred spiral galaxy, the bar is in the liquid where g varies as r and the spiral arms are outside, subject to the inverse square law. Cold neutrinos may have been around since the early universe but another source could be black holes where they may pour out like Hawkinge radiation or as a result of accretion disc annihilation at the event horizon. Either way they would be very cold by the time they had crawled away from the hole.

Paul E G Cope
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    The electron electron repulsion doesn't change the atomic occupation number in any qualitative way, it only makes the atoms a little bigger than they would be otherwise, not by a factor of 10. The neutrinos are not assumed cold here, they would have to be moving absurdly slowly for that to happen. Neutrinos are not Hawking emitted until the black hole gets as small the compton wavelength of the neutrino, which isn't infinite (neutrinos are massive). – Ron Maimon Jul 09 '12 at 03:48