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The expansion of the universe and gravity: One question.

  1. Dec 5, 2011 #1

    I am not a physicist or mathematician.

    I read that the universe is expanding and most pundits are 100% certain of this. The expansion seems to be much more than astral bodies moving away from each other. The actual space and fabric of the universe is expanding.

    Newton felt space was just space whereas Einstein felt space had substance and hence was significantly altered by the presence of mass. This alteration in the fabric of space creates gravity. The alteration in the fabric of space also curves light.

    As far as I know the force of gravity is more or less constant. I would think the attraction between the earth and the moon and between the Sun and the Earth is more or less the same over long periods.

    One question:

    If the force of gravity is related to the curvature of space and if space is expanding. Should there be a dilution of whatever constitutes the fabric of space and hence a gradual decline in the force of gravity?
    Last edited: Dec 5, 2011
  2. jcsd
  3. Dec 5, 2011 #2


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    The attractive force varies with the distance between the objects. As such the force is constantly changing and never steady.

    You are correct. On a universal scale the energy density that contributes to gravity is decreasing, resulting in an accelerated expansion as gravity weakens between objects. AKA this means that as space expands normal matter and dark matter become more and more isolated into galaxy groups with large spaces of intergalactic voids almost completely empty. As expansion continues these voids grow larger and the gravity within them grows smaller. So on a universal scale gravity contributes less and less as time goes on.

    From wikipedia: http://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe

  4. Dec 5, 2011 #3
    Sure, there is variation form instant to instant and the mass of the objects changes from instant to instant. I assume that changes in mass affect the curvature of space and gravity is never steady. However, over the long run it could be seen as a constant. Perhaps the dilution of space is so slow that we cannot measure the reduction in gravity.


    How will this affect the orbit of the sun through the galaxy (I read it takes 225 million years to complete one orbit)?

    Will the Earth orbit grow larger over time creating colder temperatures? I assume we will evolve to accept such a change since we evolve to live in the present Earth conditions.

    Is the dilution of space responsible for the expansion of the universe?

    Thanks for replying!
  5. Dec 5, 2011 #4


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    The force of gravity between two objects is unaffected by the rest of the universe. As long as they stay at the same distance with the same mass the force remains unchanged.

    It will not change it. Expansion does not take place with galaxies and galaxy groups as they are gravitationally bound to each other stronger than expansion can push them apart.

    Not from expansion. Over the next 4 billion years the orbit of the Earth will increase slightly as the Sun loses mass through radiation and solar wind. But that will not happen on anything close to a human timescale.

    The expansion is responsible for the decreasing density of the universe. The resulting decrease in matter/mass density is one possible cause for acceleration of that expansion.
  6. Dec 5, 2011 #5
    Within the realm of Newtonian physics you are correct. But, Einstein implied something else.

    So how is the universe expanding if the mass does not move apart?

    Less Sun mass less bending of space.

    Humans have been around for about 5 million years. before that were were chimps. Most species only last 10 million years so we will evolve into another species and they will recalled how they evolve from humans, ha, ha.

    But, if the density goes down the then there is more volume of space and this has to create greater distance between the mass/matter in space. However, above you stated that gravity does not allow separation of matter So the expansion of the universe only involves space?
  7. Dec 5, 2011 #6


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    Oh? What did he imply?

    If we look at our local group of galaxies we see that they are all gravitationally bound to each other well enough to avoid being pushed apart. This is referred to as a "local". Locally expansion doesn't happen. Once you get into the scale of galaxy superclusters, the force of gravity is not enough between these clusters to keep them bound to each other.

    This only happens for local objects. Our local galaxy cluster and a galaxy cluster 200 million light years away are NOT bound to each other and will move away from each other due to expansion.
  8. Dec 5, 2011 #7
    Thanks for the explanation, it should have been obvious to me.

    I guess Newtonian physics were just a tad off in relationship to Einstein.
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