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Expanding universe into what?

  1. Aug 13, 2012 #1
    Dear all,

    I just read a great question on TED.com, however I think people over here could answer this question perhaps better.

    "If the universe is expanding - what is it expanding into?

    what is THAT space called
    and why isn't it part of the universe now?"
  2. jcsd
  3. Aug 13, 2012 #2
    For me the next analogy comes in handy:

    If a table could've been given the possibility of speech, upon asking what it's creator is made of it would answer with terms it is familiar with. Wood, oak, brown/black, painted, 4-footed, etc.

    When trying to explain something which consists of flesh and blood, it would fail to understand because of the lack of familiarity with these kind of substances.

    This analogy helps me in trying to (or rather lack) understand how the universe can expand within the maximum space assigned for it already.
  4. Aug 13, 2012 #3


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    There is no simple answer to this question. The universe is expanding, but, nothing is actually moving in any meaningful way. The empty space between the galaxies increases while the galaxies sit around doing nothing. If it helps, you may simply assume the universe is infinite, in which case the question becomes meaningless - even though we dont actually know if it is spatially infinite or just really big.
  5. Aug 13, 2012 #4


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    The problem with positing an edge to the universe (which would be needed if it were actually expanding INTO something) is that there is no known way to explain the utter weirdness that would happen to all our physical models at the edge. It just doesn't make sense. Try this:

  6. Aug 13, 2012 #5
    You're misunderstanding what it means for the universe to be expanding. Before I explain, I want to very strongly emphasize a few points.

    1. The universe has no 'edge'. This is because we model the universe as homogeneous and isotropic. That is, there are no special places. Also, the universe at large is described by general relativity, which describes spacetime as a smooth manifold, which excludes the possibility of an edge.

    So, one possibility is that the universe is infinite, it just extends forever. However, it may also be finite. If this is the case, then it's either simply or non-simply connected. That means it would be like the three-dimensional analogue of the surface of the earth - it's definitely finite, but it has no edge. So, in a finite universe, going far enough will bring you back to your original position.

    So, you can see from that why your question is flawed - there is no such thing as 'outside' of the universe because it has no edge.

    2. The universe has no center, and the big bang was *not* an explosion. This will make more sense in a little bit, but it's key to remember that the universe has no center - it expands from every point in intergalactic space. The big bang was the point in time when the entire universe was filled with a hot, dense, plasma and radiation. As space underwent expansion, the radiation redshifted and cooled, and the plasma cooled into hydrogen gas. This then accumulated into clumps, which became galaxies. The space in between these galaxies continues to expand.

    So, what does it mean that the universe is expanding? It means that the distance between bound objects (i.e. galaxies) is increasing. The galaxies themselves aren't moving (well, they have a very movement that has nothing to do with expansion, but this is besides the point), they just appear to be because the distance between them is getting larger. So, you can see that 'expanding' in this context is a lot different than how you think of it - the distance in between all galaxies getting larger.

    Now it makes sense why the universe doesn't have a center - since the space in between everything is growing, every galaxy sees every other galaxy moving away. So, everyone appears to be at the center, which is why there isn't an center (depending on whether you're a half glass full or half glass empty type of person, you may say the universe has no center or that every point is the center, both mean the same exact thing).

    I highly recommend reading this article, it should clear up a few misconceptions:

  7. Aug 13, 2012 #6
    Interesting replies, thank you all.

    The balloon-analogy helped deeply in getting a grasp of the thought how everything expands.

    Another thought has kept me awake for quite some time.
    Every planet has its own orbit, whereby every bundle of planets also have their own orbit (looking at the Milky Way). There has been, in my knowledge, no collision up to date. When we would make a model of these planets' orbit, it would just be a matter of time for them to collide.

    In the balloon-analogy this was stated:

    This of course explains that expansion takes place without the orbits of smaller systems being disrupted. However I do not think that solves the issue of the possibility of collision.

    Any thoughts about this?
  8. Aug 13, 2012 #7
    This question is asked once a week, to all the senior members, I applaud you for answering it in detail every single time.
  9. Aug 13, 2012 #8


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    The expansion of the universe can be thought of as having the distance between every object increased on average at large scales. All the galaxies move farther apart from each other, so the universe becomes more sparse. It's like zooming into a sheet of grid paper, except that the grid paper stretches on to infinity so there is no edge.
  10. Aug 13, 2012 #9


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    There are entire GALAXIES that collide, so I don't know where you get the idea that there are no collisions. I seem to remember reading an estimate, however, that if two galaxies of a billion stars each collide, as happens sometimes, the chances of two solar systems within them colliding is near zero and it's likely that if there are any such collisions, it's only a couple.
  11. Aug 13, 2012 #10
    Its to do with the properties of infinity.

    Infinity +100 = infinity

    You can't make it any bigger by adding to it, its already infinite.

    So if the universe is infinite in size and its expanding its still the same size (infinite), basically its just expanding into itself.
  12. Aug 13, 2012 #11
    No, this isn't right. Even if the universe is finite, it isn't expanding into anything. See my above post.
  13. Aug 13, 2012 #12
    I didn't say its expanding into anything other than itself, its falling outward.

    Its dimensionless, it has no definite size to begin with and therefore its perfectly happy to expand and not get any bigger. I think looking at the properties of an infinite size number and realising you cant make it bigger by adding to it is a good analogy for expanding space. It gets rid of the idea that its expanding into something else.

    No matter how much it expands its still the same size.
  14. Aug 13, 2012 #13
    The important property of expanding space is that by volume matter is getting scarcer.
  15. Aug 13, 2012 #14


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    I think you have a misunderstanding of what we mean by saying that the universe is infinite but expanding. The distance between ALL objects not gravitationally bound to each other gets larger over time. That is it. There is nothing that says that space is moving anywhere, or expanding into anything else, or anything of the sort. The universe isn't expanding into itself or anything else. Things are simply moving away from other things without getting closer to anything.
  16. Aug 13, 2012 #15

    I'm pretty sure I understand it.

    If you mean the distance then yes I know.

    I never said it did, saying that it expands into itself isn't the same as saying it expands into "something else"
  17. Aug 13, 2012 #16
    If I have an infinitely long elastic band and I stretch it some more does it get longer? Of course not.
    So if space is infinite in size and it expands does it get bigger? obviously not.
    Point is infinities are not additive and I think it makes perfect sense to say that space simply expands into itself.
  18. Aug 13, 2012 #17


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    I'd say that it most certainly gets longer. It's original length can be finite or infinite, but if you stretch it then it is longer. And again, space isn't expanding. Matter and energy within spacetime is getting further apart. To my knowledge spacetime is that which everything occupies, not something physical in itself.
  19. Aug 13, 2012 #18
    If I had an infinitely long elastic band and I stretched it some more by say 10 feet then by your reckoning its now infinitely long + 10 feet, that's just silly try measure it :) , its still infinitely long.

    Half of infinity is infinity, you cannot divide infinity into a measurable unit, nor can you add to it to make it bigger.

    If an infinite volume of space expands it doesn't get bigger, its still infinite.

    If you stretch something that has a finite length then it gets longer but if its infinitely long its length stays the same.
  20. Aug 13, 2012 #19
    You cannot think of an infinite distance as "measurable" its simply infinite even if it expands or contracts.
  21. Aug 14, 2012 #20


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    Take an infinitely long rubber band with marks every 1 inch and stretch it. the marks are all now further apart. The rubber band has stretched but it has not stretched or expanded into itself. I would certainly call that longer, but as I am not educated in mathematical infinity I won't try to argue it, I'll save that for one of the other guys here.
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