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B Why gravitational force is attractive? (and why dark matter is repulsive?)

  1. Sep 15, 2016 #1
    Why gravitational force is attractive?
    • Some where it was written that gravitons are hypothetical particles which mediates the force of gravitation, and it has a spin of 2 , and Quantum field theory had proved that any particle of spin 2 characteristic will always mediate attractive force. But this theory is not yet proved .
    • And it was written somewhere that there is an exceptional case for gravitational force to be attractive. Ie dark energy is a case of gravitational repulsion and it is responsible for the expansion of universe. And it is due to the negative pressure effect of dark energy, what is this?
    • What if the existence of gravitons particle is proved, then how this theory will explain dark energy.
    • Some people say that gravitational force is always attractive because mass can never be negative, why we are talking about negativity or positivity of mass , why we are comparing it to the electromagnetic force(just like positve and negative charges repels and there is no negative mass therefore no repulsion)
     
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  3. Sep 15, 2016 #2

    phinds

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    "it was written somewhere" is not an acceptable citation here. if we knew what dark energy was we wouldn't be calling it "dark energy", we'd be calling it by a name that is descriptive of what it is.
    • Since we don't know what dark energy is, how could we possibly know how gravitons relate to it?
      "Some people say" is not an acceptable citation here. I believe it was thought for a while that anti particles (the positron, for example) would have negative mass but that is not the case.
     
  4. Sep 15, 2016 #3
    Ok , dark energy is called as dark energy because we don't know much about it, I was asking if existence of gravitons is proved to be right. And quanum theory had proved that if gravitons exist then they will always attract things(because they have spin-2) then how it will explain the expansion of universe.
     
  5. Sep 15, 2016 #4

    phinds

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    Look, we CALL the reason for the expansion dark energy because we don't know what it is. How can we know what relationship gravitions will have with something when we don't know what that something is? Gravitrons may have nothing to do with the expansion of the universe.
     
  6. Sep 15, 2016 #5
    Thanks; your answer made me satisfied, how can I relate something to something, about which we dont know anything.
     
  7. Sep 15, 2016 #6

    Chronos

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    The usual approach is to describe it phenomenologically, as batman said, it's not who you are underneath, it's what you do that defines you.
     
  8. Sep 15, 2016 #7

    Orodruin

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    It won't. Dark energy is not needed for the universe to expand. You could have a universe without dark energy (until recently, in cosmological terms, the dark energy component was negligible) and it would still expand.

    You need it to obtain accelerated expansion.
     
  9. Sep 15, 2016 #8

    phinds

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    Yeah, @parshyaa, I mispoke when I said we call the reason for the expansion dark energy. Orodruin is right, of course. We call the reason for the ACCELERATION of the expansion dark energy.
     
  10. Sep 15, 2016 #9
    In your title you have "dark matter is repulsive", it isn't but perhaps you are referring to dark energy?
     
  11. Sep 15, 2016 #10
    Yes you are right,I don't know who edited my question , my question was only why gravitational force is attractive?
     
  12. Oct 6, 2016 #11

    Dale

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    A nonsense post and a bunch of follow-up posts have been deleted. I remind our forum regulars to click the report button for nonsense rather than the reply button.
     
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