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What is negative pressure?

  1. Feb 28, 2010 #1
    I read somewhere that negative pressure is responsible for repulsive gravity that is increasing the expansion of the universe. Can repulsive gravity exist? and if so how is it related to negative pressure?

    I also heard that the negative gravitational energy compensates for all other energies in the universe making the total energy in the universe zero.
     
  2. jcsd
  3. Feb 28, 2010 #2

    mgb_phys

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    There is no evidence for repulsive gravity, there are possible measurements that suggest the normal law of gravity isn't quite right on large scales (gravity is weaker than it shoudl be) but nothing conclusive.

    This isn't 'negative gravity' it's just how you do the calculation. If you start with the mass in one point and lift the masses outward it's easier to make that energy negative, so when the mass is attracted back inward, that movement is postive - but it's purely a question of convention. It was once thought that the energy of the gravity expansion balanced the mass-energy of the rest if the universe - that isn't true.
     
  4. Feb 28, 2010 #3
    On what premise have we discredited this? Did we disprove this somehow?
     
  5. Feb 28, 2010 #4
    It sounds like the OP is referring to dark energy which, if I understand correctly, can be represented as negative pressure in GR's source tensor, has a repulsive consequence of accelerating intergalactic expansion, and still figures prominently in the mainstream picture.
     
  6. Feb 28, 2010 #5

    mgb_phys

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    ok - I thought they mean the pioneer anomoly
     
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