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Expanding Universe

  1. Feb 21, 2009 #1
    I am trying to understand that in order for the universe to be expanding that galaxies at further distances must be moving away from us at faster speeds than galaxies closer to us. It seems enough that all galaxies are moving away from us (or each other if observed from another galaxy). Does anyone have a good way of explaining this? Thanks in advance.
  2. jcsd
  3. Feb 22, 2009 #2


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    Think about all the galaxies in a particular direction. If the nearest ones have a certain recession velocity, then those beyond it must be receding at some velociry from them. This means they are receding from you at ( roughly) the sum of those velocities.

    YOU ----------- v1 ------>galaxy A ------------v2------------->galaxy B
    ------------------------ 'sum' of v1 and v2 ---------------->
  4. Feb 22, 2009 #3


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    The 'velocity proportional to distance' part of it is necessary for the expansion to be entirely uniform. Imagine a rubber sheet with a bunch of dots drawn on it that is being stretched. If you stretch the sheet by a factor of two, then all distances between dots will increase by a factor of two. So if the dots are 1m apart, and the expansion takes one second, then the dots will be moving at about 1m/s with respect to one another, to end up at 2m apart. If, however, we look at two dots 2m apart, then they will end up 4m apart after the one second, so they will have had to be moving at about 2m/s.

    Note that it doesn't matter where on the sheet I look: the separation between any two dots is increasing by the same amount proportional to distance.

    Does that help?

    P.S. One minor nitpick: all galaxies are not moving apart. The expansion of the universe is a large-scale phenomenon. Galaxies close enough to one another are as frequently moving towards one another as they are moving away. It's only when you get far away that the expansion becomes important.
  5. Feb 22, 2009 #4
    Ok, that makes sense. Good analogy by the way, makes it much eaiser to picture the expansion in my mind. Thanks a lot!!
  6. Feb 23, 2009 #5
    Thats a very good explanation and something I've wondered about too. Thanks !

    This raises two more questions though.

    Does this same logic apply to the 'balloon' model commonly used to visualize the
    expanding univers? I assume the answer is yes?

    Does this mean that the universe has a constant rate of expansion rather than the accelerating rate that I keep hearing about?

    afterall it seems in the rubber sheet analogy tht the sheet is expanding at a constant rate which gives the appearance that the galaxies attached to it are receeding at an accelerating rate away from us. I guess my confusion comes because it seems like there are two distinct expansions going on. One, the accelerating distances between the galaxies, and two, the constant rate of expansion of the rubber sheet. In our real universe there is no rubber sheet, only the increasing distances of the real galaxies. Thats what confuses me, it seems like in our real universe something is missing. Where is the rubber sheet ? What is it that is really expanding? and is it constant or is it accelerating?
    Last edited: Feb 23, 2009
  7. Feb 23, 2009 #6


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    This is a nitpick, but it's not a model. It's an analogy. And since it's an analogy used to visualize the exact same phenomenon, yes, it applies there too. The main problem with the balloon analogy is that the distances you have to measure are along the surface of the balloon, which is a curved surface, and we're just not used to doing that (unless you're a cartographer).

    No. The point is that the rate of expansion is nearly constant in space (for what is called an "equal time slicing" of the universe). But it varies dramatically with time. The way in which gravity responds to the contents of the universe determines how this expansion rate changes.

    Actually, I didn't go into the rate of expansion at all. I just gave the relative size of the sheet at two discrete points in time. The rate of expansion could have, in principle, been doing all sorts of weird things in between. To start to piece together how the rate of expansion changes with time, we'd have to have more information, such as the relative size of the rubber sheet at more points in time.
  8. Mar 8, 2009 #7
    I've read many different things, and learned alot about expansion , and the balloon expansion. Mostly from reading in Marcus' topic areas. Most of what is said here very well explained and, understandable. I do have some questions that I get the feeling no one likes to talk about. Mainly on what are we expanding into. I've done some other reading and find that it could be possible to that spacetime is expanding into" space "which has no time yet because it really is'nt part of the universe until spacetime has expanded into it? Please forgive me but could this be true? I keep hearing that the pennies glued to a balloon represent galaxies moveing away from each other that are at rest with a point on the surface of the balloon. Ok fine, but it "appears "when the balloon expands pennies are at rest on the surface of the balloon but, not at rest with the rest of the space it's expanding into. Does this make any sense? Sorry I am newb, but wish to learn any help thx.
  9. Mar 9, 2009 #8


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    In the balloon analogy, space (the surface) is embedded in a higherdimensional space. It expands into this additional dimension, and the observable effects of expansion are due to this motion into the additional dimension. So that view is mathematically consistent, if one bends the rules a little.
    But this is a model, and GR deals with our fourdimensional spacetime only, without need for additional dimensions. Expansion can be consistently described within spacetime itself; if this is hard to imagine, well, that's always the case with relativity.
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