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Question about expanding universe VS gravity pull.

  1. Dec 27, 2009 #1
    Hello all. I am a newcomer on the forum and no major math or astronomy knower.

    But I have often thought about this issue I would like to ask about here.

    From what I understand the Big Bang happened from a singularity expanding outwards (or exploding). As the speed of light is considered a constant, when the singularity expanded it can only reach the speed of light.

    Now as the universe expands bodies are made, stars, planets and all these bodies have a gravity pull on the expanding universe.

    So as now the expansion speed is bigger than the gravity pull of these bodies, but the gravity pull must slow the expansion down.

    So should science not be able to calculate when the expanse will stop and the gravity pull will be bigger than the escape velocity ?

    I know this might sound stupid or like I am a real dummie, but I am trying to learn more so please have patience with me.


  2. jcsd
  3. Dec 27, 2009 #2


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    If the universe were expanding at greater than escape velocity now, it would always be doing so. This is what escape velocity is. As the universe expands the individual galaxies move further apart, meaning that their gravitational hold on each other weakens. The expansion would slow but never slow to a stop.

    If however, the it is expanding at less than escape velocity, eventually gravity will overcome the expansion and it will stop and then reverse to a "big crunch".

    In the 1990's a study was done to determine just which one of these scenarios was right. By comparing a galaxies distance from us to its speed, they could plot out the rate of expansion over time ( Remember, the further a galaxy is away from us, the longer it takes for the light from it to reach us. We are seeing the galaxy as it was when the light left, not as it is now. This allows us to peer into the past of the Universe.) In this way we could determine the rate at which the universe's expansion changes over time, which allows us to figure out whether or not the expansion is slowing fast enough to eventually stop or not.

    The odd thing was that they found that the expansion wasn't slowing at all, it was in fact, speeding up. It was as if something were acting against gravity over long distances, pushing the universe apart. This something was given the term "dark energy", and its exact nature in under current examination.

    The upshot is that, by our best current knowledge, the universe will continue to expand forever, and it will never come to a stop.
  4. Dec 27, 2009 #3
    Thanks Janus for the response.

    I have heard about the expanding faster issue. What I am thinking about that is at some point the gravitional pulls from foreign galaxis and bodies will be so far from each other there will be no pull what so ever. Does this not mean that the universe will not continue to "hang together" as ONE unit, but split up in mini universes with nothing in between them ?
  5. Dec 27, 2009 #4


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    Gravitational pull never ceases. But it might be too weak to stop the expansion.
    Distances get larger amd larger, that doesn't necessarily mean that portions of the universe get separated without any chance of contacting each other.
    It's different with accelerating expansion, however. After ~100 bn years (according to the standard model), the universe will be split up into infinitely many regions without contact to their neighbours. One of those regions will be our vicinity, the supercluster we're in.
    We'll see/measure an event horizon surrounding this region, beyond which we can't see.
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