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Comment by tlogan | Hacker News Reader

Question for experts:

If sterile neutrinos exist, would they be the best candidate for dark matter? Additionally, would it be impossible to detect them since they do not interact with anything and possess only mass?

EDIT:

My reasoning is as follows:

- Low-energy neutrinos might not be relativistic. For example, relic neutrinos from the Big Bang, which have a temperature of around 1.95 K, would travel at approximately 4.5% of the speed of light.

- I've also read that sterile neutrinos are hypothesized to have a large mass (though I'm not sure).

So "slow neutrino" + "not so light like normal neutrino" => dark matter

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All neutrinos are relativistic (moving at almost speed of light) and our current model assumes dark matter is "cold" (moving at relatively slow speeds).

Thank you. I updated my question with my reasoning.

But I assume that in order for these sterile neutrinos to be dark matter they need to be really slow ...

The known (non-sterile) neutrinos are very light, but there's no particular reason a hypothetical sterile neutrino would be.You can cook up sterile neutrinos with very large masses if you want to

https://arxiv.org/abs/1303.6912

There is excellent evidence for then-relativistic cosmic neutrinos from the cosmic microwave background acoustic oscillations. As with the CMB, these neutrinos cooled into a "relic" field with the metric expansion.

A cooling gas of relativistic neutrinos becomes non-relativistic when the average neutrino momentum becomes comparable to their rest mass. The present cosmic neutrino background (CNB) temperature is about 1.9 kelvins corresponding to ~ 1.7e-04 eV/c^2 which is smaller than some neutrino masses.

This is still "hot" compared to Cold Dark Matter, and CNB also only a small fraction of the required CDM energy density.

The lightest cosmic neutrino background mass eigenstates could be relativistic at present, so how the CNB is split between (FLRW) radiation and dark matter is ripe for research. It will mostly be "hot" DM because of the heavier, colder species. But not hot enough to leave clusters of galaxies, unlike neutrinos produced by present-day astrophysics (and our own laboratory experiments and nuclear power generation).

https://en.wikipedia.org/wiki/Cosmic_neutrino_background

https://bigthink.com/starts-with-a-bang/cosmic-neutrino-back...

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