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Thread: pre 1998 expectations

  1. #1
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    pre 1998 expectations

    In 1998, it came as a surprise that the expansion of the universe is accelerating. Apparently, until then it was expected that the total gravity from all matter would work against the expansion. I can't understand why there was such an expectation.

    Let's consider a silly/simplistic view of space and matter: space is infinite in all directions and has always been there. At Big Bang some matter pops up at a specific point and flies away in all directions from that point. In this scenario I can understand that gravity would slow down the expansion and try to move everything back to the point of origin.

    But we are tought that no point is special, that there is no center and an equal amount of matter in all directions from every point. Shouldn't the gravity of everything then pretty much cancel itself out and have no large scale net effect?

    So the question is: why did/do cosmologists expect gravity to work against the universe's expansion? (sorry for adding yet another expansion thread :-)

  2. #2
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    Quote Originally Posted by drhex View Post
    In 1998, it came as a surprise that the expansion of the universe is accelerating. Apparently, until then it was expected that the total gravity from all matter would work against the expansion. I can't understand why there was such an expectation.

    Let's consider a silly/simplistic view of space and matter: space is infinite in all directions and has always been there. At Big Bang some matter pops up at a specific point and flies away in all directions from that point. In this scenario I can understand that gravity would slow down the expansion and try to move everything back to the point of origin.

    But we are tought that no point is special, that there is no center and an equal amount of matter in all directions from every point. Shouldn't the gravity of everything then pretty much cancel itself out and have no large scale net effect.
    I guess if the critical mass density was great enough then based on the big crunch theory then matter would be slowed down and start receding back under the effect of gravity to a point where it all converges. This i suspect would be the definition of the center of the detectable universe i.e matter&energy. You will get better explanations off others on this site.

  3. #3
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    What if the big bang was actually a homogenous(in terms of matter-energy) and extensive region of space that expanded. Therefore, there's more stuff outside of our bubble than inside of it.

    Too many assumptions with too little time.

    I, as well, did not expect an accelerating expansion. I was also 13 when the news came out.

  4. #4
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    I started a thread two months ago asking exactly the same question:
    http://www.bautforum.com/questions-a...expansion.html
    Ken G provided arguments based on general relativity for slowing of the
    expansion, but I am not yet convinced. My impression is the same as
    yours: A finite and/or closed universe would decelerate, but an infinite
    and open universe should not, because (in my non-GR description) the
    net gravitational force on every particle is the same in all directions.

    -- Jeff, in Minneapolis

  5. #5
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    I administer an open source project at BigCrash.org that explores a big bang theory along the lines of (not, but similar to) Loop Quantum Gravity. Your welcome to contribute...

  6. #6
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    Quote Originally Posted by drhex View Post
    In 1998, it came as a surprise that the expansion of the universe is accelerating. Apparently, until then it was expected that the total gravity from all matter would work against the expansion. I can't understand why there was such an expectation.

    Let's consider a silly/simplistic view of space and matter: space is infinite in all directions and has always been there. At Big Bang some matter pops up at a specific point and flies away in all directions from that point. In this scenario I can understand that gravity would slow down the expansion and try to move everything back to the point of origin.

    But we are tought that no point is special, that there is no center and an equal amount of matter in all directions from every point. Shouldn't the gravity of everything then pretty much cancel itself out and have no large scale net effect?

    So the question is: why did/do cosmologists expect gravity to work against the universe's expansion? (sorry for adding yet another expansion thread :-)
    You're confusing expansion with movement.

    Movement is where our star is moving away from, say, Polaris. It wouldn't matter whether the universe is expanding or contracting - we'd still be moving away from Polaris through the fabric of space-time.

    Think of marbles shot across a thin sheet of rubber which is either static, being slowly stretched in all directs, or is unstretching, becoming smaller.

    Expansion is where the fabric of space-time itself is growing larger. Same marbles, only glue them in place (zero relative movement). Now slowly stretch the rubber (the rubber represents space-time itself). That's expansion, and the most distance marbles experience the most red-shift, while adjacent marbles experience almost none at all.

    Again, in the latter case, the marbles aren't moving through space-time (across the sheet of rubber) at all. It's space-time itself which is expanding.

  7. #7
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    Mugaliens,

    I'm afraid you didn't address the question at all.

    What you do say is mostly a standard description of expansion using the
    rubber sheet analogy, and irrelevant to the OP, but several aspects of the
    analogy are controversial.

    What glue fastens galaxies to the fabric of spacetime, which prevents
    the galaxies from slipping and sliding as spacetime expands?

    And it is disturbing that you choose a star within the Milky Way to
    illustrate an aspect of cosmic expansion, since there is no expansion
    observed within galaxies, as you must well know.

    -- Jeff, in Minneapolis

  8. #8
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    The universe is accelerating due to the effects of anti-gravity(dark energy). If matter has a force that can contract, then nothing can expand.

  9. #9
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    I'm afraid Ken G's posts went over my head in that other thread were Jeff Root asked the same thing. But there was an interesting idea: the notion I (and Jeff) has that all the gravity from other particles would cancel out at any point is more obvious if the force of gravity did not care about distance. But it does, and causes local clumping that could be inferred to cause more clumping at ever greater scales until everything is in one big clump and the universe has effectively shrunk. (ignoring dark energy that would stop the clumping when we get above super-cluster size).

  10. #10
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    drhex,

    Clumping doesn't change the gravitational force between clumps.
    If two clumps of matter are not gravitationally bound, and are moving
    away from each other, their mutual gravitation will never be enough
    to bring them together.

    A big question is: Is the Universe gravitationally bound? It appears
    to be right on the knife edge between being bound and not.

    -- Jeff, in Minneapolis

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