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Could Dark Matter be composed of neutrons?

  1. Jan 2, 2015 #1
    I was thinking about the properties of dark matter - how it doesn't seem to interact with any of the forces of the universe except gravity and I was thinking about how neutrinos also don't have any charge and they don't interact with any other forces except the weak force and gravity. I thought how it would probably be difficult to observe weak force interactions in dark matter at a cosmic scale and then it occurred to me - neutrons also don't interact with the EM force!

    I know that neutrons are normally very short lived by themselves and quickly decay (into hydrogen atoms?) but what if there was portions of space where this wasn't the case. I know that neutron stars are made of highly compressed neutrons so they interact quite energetically with "regular" matter, but what would uncompressed normal-density (or just higher density) matter look like if it was only comprised of neutrons?

    Am I correct that it neutrinos which interact with free neutrons to decay into hydrogen or a proton? If so, if there were sections of space where the neutrino saturation level were significantly lower could neutrons exist stably for long periods of time? If so, this seems like this matter would have the exact properties we are looking for in dark matter.

    Any thoughts? If I have my facts wrong can someone correct me?


  2. jcsd
  3. Jan 2, 2015 #2


    Staff: Mentor

    Yes, they do. Neutrons don't have any electric charge, but they have a magnetic moment.

    A free neutron decays into a proton, electron, and antineutrino. The proton and electron won't normally form a hydrogen atom, at least not right away, because the electron will be too energetic to be captured by the proton.

    Then the laws of physics would have to be different in those portions of space. All the evidence we have indicates that there are no such portions of space.

    The normal reaction of free neutrons, as above, produces an antineutrino; it does not require the presence of a neutrino, and so is not affected by the density of neutrinos. There is, IIRC, a related reaction where a neutron can absorb a neutrino and produce a proton and electron, but it is far too rare to have any effect on the stability of free neutrons.
  4. Jan 2, 2015 #3


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    This is not true. Neutrons can interact via the EM force, as Peterdonis said.

    Uncompressed matter cannot be made of neutrons, so your question doesn't have an answer.

    An electron anti-neutrino is emitted during the decay process of a free neutron, but no neutrino interacts with the neutron prior to decay.
  5. Jan 2, 2015 #4


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    Please be aware that personal theories are not allowed per PF rules. It's better to ask questions than to make up scenarios and then ask if those scenarios are correct.
  6. Jan 2, 2015 #5
    Thank you for the clarification and feedback. Of course neutrons interact with the EM force - they are made of quarks! *bonks head*

    I'll refrain from putting out crazy theories in the future and just ask questions.
  7. Jan 2, 2015 #6


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    A good rule of thumb is that if something seems obvious to consider as a solution to an unsolved problem, it probably has been tried and found to fail.

    Locking the thread as the question has been answered.
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