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byThagg ( 9904 ) writes:
Back when it was thought that neutrinos were massless, it was impossible to believe that there were huge masses of neutrinos surrounding galaxies, as they would have to travel at the speed of light. But now that we know that neutrinos have mass, maybe they could travel a lot more slowly, slow enough to be captured by a galaxy.
Think about it; there are a huge amount of neutrinos created every microsecond in every star in every galaxy, and they hardly interact with anything. They've been accumulating since the big bang.
What happened to the early photons? Those created as the universe first became transparent initially were very high energy indeed, but as the universe has expanded they've lost energy, to the point that they correspond to a temperature of just 3 degrees kelvin. What happens to neutrinos of a similar vintage?
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byBaloroth ( 2370816 ) writes:
Same thing happened to the neutrinos as happened to the photons. They cooled down. Currently, the neutrino background is ~1.7K, I believe (they're a bit cooler than photons as photons decoupled from matter much later in the early universe than neutrinos did). Neutrinos are, on cosmological scales, treated mostly the same way photons are (they behave in a similar fashion). In any case, the current energy in neutrinos is about ~60% of that in photons, and photons are about 4 orders of magnitude below the energy in dark matter.
We can also predict how the universe would evolve if neutrinos made up the bulk of dark matter. Since it didn't evolve that way, dark matter has to be something else.
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byevilviper ( 135110 ) writes:
Could dark matter be super low-energy neutrinos?
Nope.
Or at least, they could still only account for a small fraction of observed dark-matter.
http://www.astro.princeton.edu/~dns/MAP/Bahcall/node6.html [princeton.edu]
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