Are neutrinos definitely ruled out as Dark Matter?

[nikkkom]

Ordinary Standard Model neutrinos _are_ dark matter particles, in a sense that they have mass, and they only weakly interact with everything else.

IIRC they are not considered to be a satisfactory candidate because nearly all neutrinos in the Universe are relativistic, whereas we know that observed dark matter is "clumpy" and thus must be composed of non-relativistic particles.

Why we think that neutrinos are relativistic? (1) all neutrino-generating processes today are generating relativistic neutrinos, and (2) vast majority of neutrinos should be primordial, from ~1s after Big Bang, when neutrinos decoupled. They must have temperature of ~2K now, which is ~0.2meV, but with their low masses they must have (nearly) relativistic velocities even today.

However, argument (2) assumes that we do not miss anything and that there are no unknown or unanticipated process which was generating "cold" neutrinos. Let's for the sake of argument assume that there was some such process, and there is a subset of CvB neutrinos which is actually much colder than 2K.

Is this possibility ruled out?
IIRC low-energy neutrinos have low cross-sections, and CvB neutrinos were never directly observed (neither their density nor their velocity). Do I remember this wrong?

 

[Vanadium 50]

Yes, you could do that. but it requires giving up thermodynamics - it means neutrinos are giving up energy by spontaneously cooling. Most people don't want to do that.

 

[mfb]

We know the (approximate) density of primordial neutrinos. They do not contribute notably to the total energy density - and if you cool them down in some way they contribute even less.

 

[nikkkom]

We know the (approximate) density of primordial neutrinos.

Do you mean "we know it theoretically"?

 

[nikkkom]

Yes, you could do that. but it requires giving up thermodynamics - it means neutrinos are giving up energy by spontaneously cooling.

Axion DM theory does have a mechanism which generates "supercooled" axions, which end up as cold dark matter - without giving up thermodynamics. However, axion searches are coming up empty so far.

Let's say an unknown mechanism generated supercooled neutrinos. How can this be experimentally detected or disproved?

 

[mfb]

Do you mean "we know it theoretically"?

There is no direct measurement of the relic neutrino density today, but the CMB structure is influenced by the density. We don't just have a theoretical prediction, we also have an experiment confirmation. See e. g. Neutrino Physics from the Cosmic Microwave Background and Large Scale Structure (1309.5383).

More neutrinos would need some production mechanism that happened later. Decays of other dark matter particles for example. You can construct such a scenario, of course, but it gets obscure: You need very light dark matter particles produced in large number, they have to cool down quickly, and then decay to slow neutrinos later. Observable difference to just the original dark matter particles in cosmology: Null. PTOLEMY is a proposed experiment to detect the cosmic neutrino background, it would find those.

 

[stoomart]

IIRC they are not considered to be a satisfactory candidate because nearly all neutrinos in the Universe are relativistic, whereas we know that observed dark matter is "clumpy" and thus must be composed of non-relativistic particles.

Sorry to be a layman chiming in on an I-level thread, but could neutrinos be considered "clumpy" in the sense of "intersection density"? I imagine the density would be much greater for points in the neighborhood of neutrino sources (inside galaxies) than those in intergalactic space.

 

[nikkkom]

These regions of higher that average density of neutrinos will not last: astrophysical sources of neutrinos generate relativistic neutrinos, which means they escape to infinity.

 

[stoomart]

Agreed, but active galaxies seem like they are a beehive of neutrinos, especially with evidence that our SMBH produces very high-energy neutrinos (per the paper: Neutrino Lighthouse at Sagittarius A*).

 

[nikkkom]

So what?
SMBHs powering quasars also produce a lot of light. Does this light create a significant overdensity?

 

[stoomart]

So what?
SMBHs powering quasars also produce a lot of light. Does this light create a significant overdensity?

SMBHs aside, it makes sense for the radiation density inside galaxies to be much higher than outside, I'm just applying that same concept to neutrinos. To speculate a bit further, I suspect galaxies would come unglued as the density of dark matter drops, leading to its constituents being ejected from the system.

 

[nikkkom]

SMBHs aside, it makes sense for the radiation density inside galaxies to be much higher than outside

Compared to matter density (even ordinary "baryonic" matter density only), radiation density is negligible.

 

[snorkack]

What are the present lower bound and upper bound on the rest mass of the lowest mass neutrino state?

 

[nikkkom]

What are the present lower bound and upper bound on the rest mass of the lowest mass neutrino state?

https://arxiv.org/pdf/1309.5383.pdf
Neutrino Physics from the Cosmic Microwave Background and Large Scale Structure

 

[mfb]

Sorry to be a layman chiming in on an I-level thread, but could neutrinos be considered "clumpy" in the sense of "intersection density"? I imagine the density would be much greater for points in the neighborhood of neutrino sources (inside galaxies) than those in intergalactic space.

All the neutrinos emitted in the last 13 billion years combined have a negligible contribution to the overall energy density (10^-5.5). Only those produced in the last ~100,000 years are closer to galaxies, which is 5 orders of magnitude smaller than the overall contribution. Completely negligible.

What are the present lower bound and upper bound on the rest mass of the lowest mass neutrino state?

See the previous post for lower bounds. For upper bounds, the best limits are from cosmology, but unfortunately they are often model-dependent. Something between 130 and 200 meV for the sum of the three mass eigenstates, depending on what you consider. Not far away from the lower limits!

 

[stoomart]

All the neutrinos emitted in the last 13 billion years combined have a negligible contribution to the overall energy density (10^-5.5). Only those produced in the last ~100,000 years are closer to galaxies, which is 5 orders of magnitude smaller than the overall contribution. Completely negligible.

I agree these numbers are quite damning for neutrinos being considers as a valid candidate for dark matter. However I must say that after researching neutrinos, their properties sure make then look/swim/quack like a duck (WIMP). It seems the main issue with neutrinos being considered as WIMPs is their mass range would need to be ~100 GeV, which I wonder if that's not impossible considering this statement from the paper I referenced in post #9:

Most events are downward-going, because upward-going NUs suffer absorption by the Earth.

 

[mfb]

It seems the main issue with neutrinos being considered as WIMPs is their mass range would need to be ~100 GeV, which I wonder if that's not impossible considering this statement from the paper I referenced in post #9:

Mass and energy are not the same thing. The high-energetic neutrinos are still light. And they are extremely rare. Most of the neutrino energy density (excluding the cosmic neutrino background) comes from core-collapse supernovas, with neutrino energies in the MeV range.
Cosmic energy inventory

Pointing to increasingly smaller contributions to the total energy density won't lead to anything.

 

[nikkkom]

It seems the main issue with neutrinos being considered as WIMPs is their mass range would need to be ~100 GeV

Not really. To perform the role of DM, particles need to have a specific (small) average _velocily_ early in the history of the Universe. The mass of the particles doesn't matter.

IOW: 100 GeV particles with average velocities of, say, 100 km/s would work *exactly the same*, as far as galaxy formation is concerned, as 1 eV particles with average velocities of 100 km/s. (The mass density of the particles should also be the same, of course).

The problem with SM neutrinos is that current theories predict that primordial ones in CvB have wrong velocity: they are relativistic at least for the few several billions of years.

 

[Vanadium 50]

Let's say an unknown mechanism generated supercooled neutrinos.

Axions act as they do because they have axion-like interactions and are bosons. Neutrinos act as they do because they have neutrino-like interactions and are fermions. Sure, if you give neutrinos non-neutrino like properties they can be dark matter: but this is a trivial statement. How do you know that a meter under the surface the moon isn't made of green cheese? You sort of don't, but nobody seriously considers this.

 

[nikkkom]

Sure, if you give neutrinos non-neutrino like properties they can be dark matter

The thing is, you don't have to change their properties. Unlike all other particles of the SM, neutrinos as-is can be dark matter.

 

[Orodruin]

The thing is, you don't have to change their properties. Unlike all other particles of the SM, neutrinos as-is can be dark matter.

Not if you want to produce them in the early Universe.

 

[Vanadium 50]

But you do have to change their properties. They are too hot.

 

[Chronos]

Neutrinos are still pretty hot to be considered a serious candidate as a major component of DM. Were this not true we should see a consistent rise in galactic DM content with redshift, which is not observed. In fact, low z galaxies tend to exhibit more DM than their high z counterparts. Without active replacement of primordial neutrinos by more recent processes, it certainly appears galactic DM fractions should tend to decrease with z due to continuous escape from local gravity wells. There is little doubt neutrinos are one component of the galactic DM budget, but, hardly the dominant component. It is difficult to avoid requiring a colder particle to do the heavy lifting to fit the currently observed DM distribution profiles. Naturally this; assumes DM is particle based - a premise under fire from the continuing failure of searches to identify a viable candidate. Of course, we are hampered by the bias of looking under a street light for the quarter dropped at night next to a parking meter 2 blocks away.

 

[nikkkom]

But you do have to change their properties. They are too hot.

Yes, naturally, there is this problem. (If there wouldn't be any problems with the idea "DM is neutrinos", it would be widely accepted, which it is not)

What I find interesting is that this appears to be the _only_ problem this idea has.

There would be a serious problem if there would be evidence that DM particles have masses, spins, or interactions incompatible with them being neutrinos, but no such observations exist yet.

 

[nikkkom]

I also want to note that talk about "SM neutrino" is imprecise. Strictly speaking, SM neutrinos are massless. And we know that real neutrinos have mass, so SM must be extended one way or another to match this fact.

I just found a paper from 2005 which describes the simplest SM extension (it merely adds three right-handed neutrinos with Majorana masses): https://arxiv.org/abs/hep-ph/0505013

The paper chooses new free parameters such that they have lightest (and thus stable on cosmological scales) RH neutrino with 2-5 keV mass. It becomes their DM particle. Two heavier RH neutrinos are chosen to have masses above 1 GeV so that they decay before BB baryogenesis and thus not spoil element abundancies.

Active neutrinos' masses are m1 ~= O(0.01meV), m2 = 9.05+-0.2 meV, m3 = 48+-6 meV.

Note that these active masses basically match the fresh experimental constraints from the 2014 paper I cited earlier, https://arxiv.org/pdf/1309.5383.pdf:

Not bad.

 

[mfb]

What I find interesting is that this appears to be the _only_ problem this idea has.

There is also no known mechanism that would produce them in sufficient number. Adding things is challenging with the constraints on the number of light neutrinos from Z decays and cosmology.

PTOLEMY would be a great tool to actually see the slow neutrinos.

 

[Orodruin]

I also want to note that talk about "SM neutrino" is imprecise. Strictly speaking, SM neutrinos are massless. And we know that real neutrinos have mass, so SM must be extended one way or another to match this fact.

I just found a paper from 2005 which describes the simplest SM extension (it merely adds three right-handed neutrinos with Majorana masses): https://arxiv.org/abs/hep-ph/0505013

The paper chooses new free parameters such that they have lightest (and thus stable on cosmological scales) RH neutrino with 2-5 keV mass. It becomes their DM particle. Two heavier RH neutrinos are chosen to have masses above 1 GeV so that they decay before BB baryogenesis and thus not spoil element abundancies.

Active neutrinos' masses are m1 ~= O(0.01meV), m2 = 9.05+-0.2 meV, m3 = 48+-6 meV.

Note that these active masses basically match the fresh experimental constraints from the 2014 paper I cited earlier, https://arxiv.org/pdf/1309.5383.pdf:

View attachment 114339

Not bad.

The type of dark matter that you have in the nuMSM is generally not referred to as "neutrino dark matter", but as "sterile neutrino dark matter". It is mainly composed of a type of neutral fermion that essentially does not interact with the SM. Its existence is unverified.

 

[Vanadium 50]

What I find interesting is that this appears to be the _only_ problem this idea has.

Well, if you only have one problem, "Let's give neutrinos new properties inconsistent with what we see in the lab" is a pretty big one. And it's not the only one - if you slow down neutrinos, you need more of them for the same energy density. More neutrinos screws up BBN.

 

[Orodruin]

More neutrinos screws up BBN.

More radiation screws up BBN. If you could add a neutrino component that was non-relativistic during BBN it would not change much. The problem of producing said component remains.

 

[Vanadium 50]

I agree there is a production problem.

The BBN issue is second order, so it's small. Worse, the effect is lumped in with n_f, so it's hard to see. I think it's of order a percent, so we expect n_f = 3.03 or so for 3 families of neutrinos. The neutrino density is ~100/flavor/cm^3, and taking 0.2 eV for the sum of the neutrino masses (a guess on the high side), that means its 20 eV/cm^3. The dark matter density is around 600 MeV/cm^3, so to make neutrinos work, you need to increase their density by 30 million. Not only does this impose, as you say, a serious production problem, even second order effects get large.