Dark Matter and neutrinos

It's not a matter of amount, but of temperature. Neutrinos are light enough that they still have quite a lot of kinetic energy while galaxies are first starting to form. This means that they are simply moving too fast to become captured by gravitational potential wells.

A universe where neutrinos were dark matter would look very, very different from the one we observe.

 

That's their current temperature. They would have been much hotter when the CMB was emitted (thousands of degrees).

I don't know just how cold they had to be.

 

Why is it a matter of temperature?
It depends on both mass and temperature. Too heavy neutrinos would contribute to CDM.
However what Dark Matter are you refering to? Hot or Cold dark matter?

 

Hot DM= relativistic at the time of decoupling
Cold DM= non-relativistic at the time of decoupling

 

The sterile neutrino, an oft cited DM candidate, would be considered warm dark matter. It has a putative mass in the 10 Kev range. Cosmological models based on warm dark matter appear plausible.

 

Yes, it's true that if there were another neutrino that was much heavier than the electron, mu, and tau neutrinos, it could make up the dark matter particle. But the three neutrino types that we have detected can't do the job because they are too light.

 

Neutrinos are dark matter, in a sense that they represent massive (meaning not-massless), almost non-interacting particles. But since it seems that their rest masses are in milli-eV's, it means that at their current temperature (as you said, ~2K) their _velocity_ is very high, and they move too fast to enter an orbit around even the largest concentrations of mass. They are, orbitally speaking, hyperbolic. Their density fluctuations are becoming less pronounced with time, they "smear out".

Thus, this particular class of dark matter particles can't explain galaxy formation, a process which lead to the "dark matter hypothesis". This hypothesis needs much slower moving dark matter particles.

Dark matter particles can be slower moving at the same temp if they are more massive. Or, there can be an exotic mechanism which somehow drains them of their velocity (for example, http://en.wikipedia.org/wiki/Axion theory has a light, but very "cold", slow moving dark matter particles).

 

I don't think that there's any sort of exotic mechanism which drains axions of their velocity. It's just that most WIMP models assume thermal production, which means high velocities for lighter particles. Axions aren't produced thermally, but as a result of a phase transition in the early universe. Because they are produced much earlier than with other dark matter models, they end up at a much lower temperature, and so have low velocity despite their small mass.

The physics is essentially identical between this effect and the effect that causes neutrinos to be slightly cooler than photons (~2K vs. ~3K): as one form of matter becomes non-relativistic, the particles and anti-particles annihilate, dumping the energy that was in those particles into photons. This heats up the photons, and anything else that interacts with them. Any form of matter that was non-interacting at that time doesn't pick up this extra heat, and so ends up colder.

For example, neutrino decoupling happened at around 1MeV, a temperature at which protons are already non-relativistic, but electrons are still relativistic. So the energy from electron-positron annihilation after neutrino decoupling heats up the photons a little bit, but the neutrinos are unaffected.

Edit: I decided to double-check, and I guess I'm mistaken. The axion velocity is so low not because of normal temperature damping, but because their production mechanism produces them with essentially no velocity. This article goes into a little bit of detail on the subject:
http://web.mit.edu/redingtn/www/netadv/specr/345/node3.html

 

http://en.m.wikipedia.org/wiki/Hot_dark_matter

 

In fact, I heard of another way of explaining Hot/Cold DM... which relates the particles with their ability of forming structures....
CDM has formed structures so far [eg exist in galaxy halos etc]...whereas HDM is still freely-streaming the universe...

 

Maybe dark matter does not exist? Why is this never discussed?

[Mentor's note: Removed reference to unacceptable source]

 

It is discussed. There have been many debates also on the topic ( MOND vs DM) ...
Several observations however, imply that there is something missing... For me DM is much more interesting as a candidate, and can solve more problems than just the observed (especially in the particle physics standard model)...e.g. the axions except for being candidates of CDM, can also solve the strong CP-problem...SUSY with neutralinos also has its advantages... and so on...
but this question, could be a distinct thread (if it's not already out there)

 

Dark matter is a tough one. Dark energy can be discarded, and there are plenty of models that still fit the observations (except one must discard the almost dogmatic Copernican Principle). But dark matter, though less significant overall, is a harder metaphysical substance to discard, because ultimately the only way to get rid of it may be to go back to the aether without universal gravity. In that case we start from where we left off over 100 years ago!

 

How can you discard Dark Energy ? (~70% of the whole universe according to observations)
I am not sure I understand the comment on the aether against "universal" (??) gravity. There is no aether...

 

You can discard dark energy by going to an LTB model, or the expanding wave model (Smoeller), or some of the Bianchi branch models as studied by Ellis, etc. My point was that discarding dark matter is actually more difficult than discarding dark energy, because the options are less satisfying to modern physics (i.e., aether).

 

"Ability to form structures" and "low velocity" in this case are directly related. Fast-moving dark matter particles are hyperbolic. Slow-moving ones are in bound orbits around clumps of mass.

 

The only evidence for Dark Energy is acceleration of uniform expansion. If someone comes up with another explanation of it, then Dark Energy hypothesis is no longer needed.