How massive would the neutrinos have to be so that relic neutrino from the big bang would account for all dark matter?
That's their current temperature. They would have been much hotter when the CMB was emitted (thousands of degrees).Wikipedia offers "It is estimated that today the CνB has a temperature of roughly 1.95 K". How cool do they have to be to contribute to the dark matter effect?
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.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?
Wikipedia offers "It is estimated that today the CνB has a temperature of roughly 1.95 K". How cool do they have to be to contribute to the dark matter effect?
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.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).
Maybe dark matter does not exist? Why is this never discussed?
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...
How can you discard Dark Energy ? (~70% of the whole universe according to observations)
"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.
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).
Yes indeed. However this "mass vs temperature" thing (which I used to explain the HDM/CDM), can be quite misleading in some cases... eg. MACHOs can contribute to DM, and axions can be very light (mass from a few micro-eV to milli-eV) and yet be cold ... (of course the axions are created in "rest").
The evidence does not support his conclusion. Modified gravity models to account for observations have generally failed. I'm pretty sure that there has not been any gravity model that has been able to explain the Bullet Cluster without dark matter, for example (the one that claims to makes use of extra neutrinos, i.e. another form of dark matter).Maybe dark matter does not exist? Why is this never discussed?
[Mentor's note: Removed reference to unacceptable source]