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edpell
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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).edpell said: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.ChrisVer said: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 referring to? Hot or Cold dark matter?
edpell said: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.nikkkom said: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).
John_QPublic said:Maybe dark matter does not exist? Why is this never discussed?
ChrisVer said: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...
ChrisVer said:How can you discard Dark Energy ? (~70% of the whole universe according to observations)
nikkkom said:"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.
John_QPublic said: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).
ChrisVer said: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).John_QPublic said:Maybe dark matter does not exist? Why is this never discussed?
[Mentor's note: Removed reference to unacceptable source]
The current understanding of dark matter is that it is a type of matter that does not interact with light or other forms of electromagnetic radiation, making it invisible to traditional telescopes. It is thought to make up around 85% of the total mass of the universe and is essential for explaining the observed gravitational effects on galaxies and galaxy clusters.
Neutrinos are subatomic particles that have no electric charge and interact only weakly with other particles, making them difficult to detect. They are created through various natural processes, such as nuclear reactions in the sun and supernovae explosions.
Some theories suggest that neutrinos could potentially account for a portion of dark matter due to their small mass and weak interactions. However, current research and observations suggest that neutrinos make up only a small fraction of the total dark matter in the universe, if any at all.
The current estimate for the mass of neutrinos is very small, ranging from 0.05 to 0.320 electron volts (eV). This is much less than the mass of other particles, such as protons and electrons, and makes it difficult for neutrinos to account for all of the dark matter in the universe.
Based on current research and observations, it is unlikely that neutrinos can account for all of the dark matter in the universe. If they were to account for all of it, they would need to have a mass of at least 3 eV, which is significantly larger than the current estimate. Therefore, it is more likely that dark matter is made up of other yet undiscovered particles or phenomena.