Seems to me that what I imagine slow neutrinos to be, has all the qualities of dark matter. Passes through regular matter without making a fuss, has mass and therefore exerts gravity, does not emit or reflect light.
Sure, we’ve only ever observed fast neutrinos but from what I understand, we are only able to observe them because they are moving so fast and thus have high energy (relatively). What would an enormous number of neutrinos orbiting the galactic center ‘look’ like? Are we sure this isn’t what dark matter is?
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Yes, there are many theoretical astrophysicists who suspect that neutrinos may compose dark matter, or at least a large portion of it.
However, there are also some arguments to suggest that neutrinos can not account for the entire mass of dark matter that is believed to exist, maybe only 30-60% of it.
(My personal theory is that the “fabric of space-time” itself may actually be exhibiting properties of mass on a large cosmological scale)
Neutrinos only interact through the weak force (and of course the gravitational force).
If a neutrino does not have enough energy (is not travelling fast enough), it basically cannot react with anything. It can sometimes impart its kinetic energy (bouncing off the nucleus of an atom) but that is extremely rare (since the weak force only operates at extremely small distances, and since neutrinos have a low mass and therefore don’t tend to be very slow).
Another possible interaction is a neutrino and an anti-neutrino annihilating with each other. This would produce a pair of (sometimes 3) photons. These annihilations are predicted to happen with greater frequency in places that have a denser concentration of neutrinos, for example the core of planets. Keep in mind though that neutrinos don’t “fall” like ordinary matter. Since ordinary matter is largely transparent to neutrinos, and since they don’t interact much with each other, a neutrino is more likely to assume an orbit around (or within) a center of mass. So they are like a gas cloud, except the cloud essentially exerts zero pressure.
In the very early Universe neutrinos would’ve interacted with normal matter sufficiently that they would’ve been at the same temperature as normal matter, however since they decoupled the only significant cooling of cosmic neutrino background would be due to redshift which means it would still be too ‘hot’ to be cold dark matter.
However it is difficult to observe neutrinos and it may be possible that some unobserved (though possibly already theorized) interactions exist which could yet mean they are still a candidate for making up the majority of the dark mass.
That’s an interesting thought: you could have neutrinos in an orbit with some part of the orbit inside the mass of the thing it is orbiting. I wonder if such an orbit is stable. From the perspective of a neutrino on an elliptical orbit, with the periapsis below surface area, the mass of the body it is orbiting would appear to change as it passes below the surface.
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Are you using ‘hot’ as a synonym for fast?
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Yes, “hot” in this context means the same thing as “fast”.
Would neutrino-antineutrino interactions have already provided a cosmological signature?
[QUOTE=Zach29]
Yes, there are many theoretical astrophysicists who suspect that neutrinos may compose dark matter, or at least a large portion of it.
However, there are also some arguments to suggest that neutrinos can not account for the entire mass of dark matter that is believed to exist, maybe only 30-60% of it.
[/quote]
The idea that Standard Model neutrinos are a significant piece of dark matter is well ruled out. The neutrinos are there, but they can’t account for more than 5% of dark matter. (Note that that’s an upper limit, so they might account for an even smaller fraction.)
Obligatory xkcd. The large corpus of evidence for dark matter is not just at cosmological scales. The introduction of one or more new particle species (i.e., dark matter) is a remarkably efficient explanation for all the disparate observations.
There might be some crossed wires here. There is no appreciatable build-up of relic neutrinos in planetary cores. There can be build-up of dark matter, but only if the particle(s) are sufficiently heavy and have a sufficient interaction cross section (they must interact to shed energy), and if the gravitating body is sufficiently massive (stars >> planets).
In a scenario where dark matter can build up an excess in a gravitational well, one can look for dark matter / dark antimatter annihilation products. Neutrinos are a common annihilation product to look for since they can escape the star.
Not in the sense I think you mean. The cosmological relic neutrinos are crazy low energy and have crazy low interaction cross sections and have crazy low densities. I was curious enough to see what these crazies multiply up to, and in very rough numbers I estimate one such event per year per galactic volume. The output of such an event would be microwave photons, which the universe is famously afroth with.
(Annihilation to photons is a heavily suppressed process for neutrinos to begin with given that neutrinos are neutral particles and thus do not directly couple to photons.)
I should add: There are models of dark matter that invoke “sterile neutrinos”, but these are not the usual neutrinos and would be new particles beyond the Standard Model.
Sterile neutrinos are pretty much theoretical duct tape, anyway. That is to say, when you’ve got a thing that you’ve invested a lot in, but it starts falling apart, you try to duct-tape it back together before you throw it away. Except it doesn’t really work very well, so you try to fix it with more duct tape, until your thing is composed almost entirely of tape, and it still needs to be thrown away. Well, that happens with scientific theories sometimes, too, and all of the talk of sterile neutrinos appears to be an example of it.
Pasta, this could be possible if the dark matter was not compressible (i.e. had extremely long wavelengths). In this case you would only notice gravitational effects at extremely long distance scales.
I think Pasta’s point is that we notice effects at less than extremely long distance scales.
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Compressible? Long wavelength? This does not compute, and it doesn’t seem to have anything to do with your statement about the “fabric of space-time”.
In any case, as Saffer notes, if you have a solution that only produces effects at cosmological distance scales, then you don’t have a solution. The observational evidence for dark matter lies in numerous physical phenomena occurring at a range of spatial and temporal scales.