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Blueshift and Redshift

  1. Mar 25, 2010 #1
    I have a question.. So far, from what I've browsed on the forums, I haven't found an answer to it, and I figured this would be the best place for it.

    I'm aware of the balloon analogy, but I have a question regarding blueshifts and redshifts. If you take the balloon analogy, everything in the universe is expanding away from everything else, because as you blow up a balloon, you're creating more space between the dots on the balloon, so it's impossible for them to be getting closer. My question is, how can their possibly be any galaxies that are blueshifting if the balloon analogy is correct?
  2. jcsd
  3. Mar 25, 2010 #2


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    In addition to the expansion, galaxies are moving relative to each other. The redshift expansion dominates for galaxies that are far apart. However, nearby galaxies can get closer together. Our nearest galaxy neighbor (Andromeda) is expected to eventually collide with ours.
  4. Mar 25, 2010 #3


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    Remember that the balloon analogy is just a device to help you understand the consequences, not the cause, of the expansion of the Universe. The Universe expands because everything is moving away from everything else, the initial motion having been kicked off by the Big Bang. The expansion slows over time because of the effects of gravity (although in the last few billions years has sped up because of dark energy, but that is just a complication).

    On large enough scales the Universe is pretty smooth, so the expansion rate is altered by gravity roughly evenly, such that the Balloon analogy gives you a pretty good idea about how things work. On smaller scales, the lumpiness of the Universe becomes more apparent, so the effects of gravity are uneven. For instance galaxies have formed because of very slightly more overdense regions of the early universe have expanded more slowly than on average, due to the extra gravitational effects from more matter, and eventually actually 'turned-around' from expansion to contraction and collapsed to a dense object. If you wanted to extend the balloon analogy to understand this (which I don't think I would advise but none the less....) you could imagine that if you zoomed into the balloon's surface, you'd see that different bits of it were inflating at different rates, and some parts would actually be contracting. This means that for things very close to each other on the surface these local effects are important, but for things far away from each other, the overall expansion dominates.

    On larger scales than galaxies, we get all kinds of motions due to the gravitational interactions between galaxies and galaxy clusters, but as you go to larger and larger scales these become small compared to the overall expansion.
  5. Mar 25, 2010 #4
    Okay, that makes a lot of sense. Thanks a lot for the explaination! I appreciate it :)
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