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Quantity of free neutrons in universe.

  1. Aug 20, 2011 #1
    How many free neutrons in the universe are there decaying at any given time?
  2. jcsd
  3. Aug 21, 2011 #2


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    Science Advisor

    Not very many. Since the neutron lifetime is only about 30 min, an equivalent question is, "How many are being produced?"

    Solar flares produce showers of neutrons, which might come from high-energy p-He4 collisions. Neutrons apparently from cosmic sources have been detected, but they are probably the result of the collision of high-energy cosmic neutrinos with local matter.
  4. Aug 23, 2011 #3
    Well, what I was curious about was how the universe has changed in proton:neutron ratio since the big bang. Seems to me that if neutrons decay at a given rate and protons stay relatively stable, then over the last 13.8 billion years or so that the universe has become more proton rich. I would think that there are a lot of processes in the universe that would be creating a large quantity of free neutrons that would decay, but I don't know this for a certainty. But if this is true and not my misunderstanding, then what would be the difference in protons:neutrons today vs post big bang?
  5. Aug 23, 2011 #4


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    Staff: Mentor

    Actually, because most of the matter that formed initially was hydrogen and helium, more neutrons exist now than before due to the fusion of matter inside of stars. If you start solely from Hydrogen, then more than half of the new matter is made of Neutrons, which originally were formed from protons that used to be Hydrogen nuclei.
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