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New to the board. Gravity questions

  1. May 9, 2008 #1

    My name is Tom. I am just learning about space and physics and how the universe works.
    I have a question and I am not sure if I am posting it in the correct place or not so please let me know if I am posting it wrongly.

    My question is about gravities relationship with the expansion of the universe.

    The question is that could empty space be curved?

    By this I mean either:
    1) Can there be a gravitational field that permiates the universe as a whole. Maybe from the sum of small ( negligable effects ) multiplied over the numerous massive of the universe, dark matter, or using brane theory ... a source from outside our brane.
    2) That space just has a natural 4d curve to it.

    Now for number one above. If all of the matter in the universe has an effect on every single point in the universe ( even if it is negligable ) then at each point all forces would roughly even out so the direction of the gravitational pull would not exist, however the effect of the red shift would work.

    Now the questions remains how much gravity would be needed to create the redshift needed for this effect. Since the curvature near a massive object greatly offsets the effects of universal expansion, I think the amount of curvature throughout space needed to produce the redshift should be within reason.

    My question is how can I calculate the amount of gravity that would be needed?
    How can I test this?
  2. jcsd
  3. May 22, 2008 #2


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    This is a good question. The answer is a resounding no. Empty space cannot be curved. In the theory of General Relativity the presence of energy causes curvature in spacetime that leads to the effects that we call gravity. In empty space there is no energy hence no curvature.

    To continue the answer from about, spacetime doesn't have a 'natural' curve to it, as I say it takes the presence of energy (of which matter is one example) to curve spacetime.

    Note that redshift is produced by a combination of the receding motion of distance galaxies (the doppler shift) and the gravitational effect of the energy in the Universe acting on photons as they travel.

    Now there is something important to realise. It might seem like in an infinite universe with matter spread out evenly then the effect of gravity of everything will cancel out and gravity plays no role. It turns out that this is not the case, not because of some strange effect of relativity, even in Newtonian gravity the same things happens. So the dynamics of the Universe is determined by the density of material in the Universe, even if it is dispersed evenly.
  4. May 22, 2008 #3


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    Wallace raises a point worth considering. In an observationally finite universe, CMB uneveness [anistropy] is guaranteed. In an observationally infinite universe, perfect gaussian distribution is guaranteed.
  5. Jun 5, 2008 #4
    Wouldn't ZPE effect "empty" space and give it a non-zero amount of energy?
  6. Jun 5, 2008 #5


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    Yes. But truly empty space has zero zero point energy, i.e. the cosmological constant is zero. If the CC is not zero, then the space is not 'empty' from the point of view of cosmology.
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