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How does the expansion of the universe work?

  1. Dec 29, 2015 #1
    How does universe expansion work? I thought that the universe was infinite and the celestial corps were getting further distance from each other. If the universe is infinite, how does someone calculate something when infinity is getting bigger? From a reference point? Is the rate of expansion the same for every point in the space?
     
    Last edited by a moderator: Jan 12, 2016
  2. jcsd
  3. Dec 29, 2015 #2

    andrewkirk

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    It isn't getting bigger. Things are just getting further apart.

    Imagine the two-dimensional number plane with a star at every point with integer coordinates. Then imagine that, starting at time 0, the stars start moving so that the coordinates at time t of the star with coordinates (a,b) at time 0 will be ((t+1)a,(t+1)b). Then the stars are all getting further away from one another, even though the extent is infinite.
    By the way, that formula is not the one that describes the actual way that galaxies in our universe move apart. It is a much simpler, somewhat unrealistic, formula that is intended solely to help you visualise this kind of thing.
     
  4. Dec 29, 2015 #3

    Drakkith

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    When we talk about expansion we have to keep in mind that we are talking about how physical objects (and light) behave. What universal expansion means is that galaxies and galaxy clusters which are not bound strongly enough to each other through gravity will recede from each other over time. In other words, the distance between these unbound galaxy clusters will increase over time. This increase in distance follows a set of rules that can be naively described as an 'expansion' similar to how objects attached to a rubber band recede from each other as the rubber band is stretched (or a rubber sheet if you want to talk about expansion in 2 dimensions).

    I want to emphasize that expansion is about the increasing distance between objects, not about space literally stretching like a rubber sheet. The rubber sheet is simply an analogy.
     
  5. Dec 29, 2015 #4
    Thank You
     
  6. Dec 29, 2015 #5
    Thank You
     
  7. Dec 29, 2015 #6

    phinds

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    @EdColider If you think infinity can't get bigger, google "Hilbert's Hotel".
     
  8. Dec 29, 2015 #7
    Cool. I've never heard about it before.
     
  9. Dec 29, 2015 #8
    I understand that we can always add 1 more item to a list.
    What I can't understand is how does someone calculate the variation of something infinity.
     
  10. Dec 29, 2015 #9

    phinds

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    Your question is not clear. Do you mean "how much bigger does it get" ? If so, it is not a meaningful question, or put another way, it does not have a meaningful answer. Or, put another way, infinity plus 1 is exactly the same size as infinity so you can say it doesn't get any bigger at all.
     
  11. Dec 29, 2015 #10
    Thank You
     
  12. Dec 29, 2015 #11
    Sorry for the meaningful question. You really helped me out. :D
     
  13. Dec 29, 2015 #12

    Drakkith

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    Look back at my post. Cosmologists don't typically deal with infinity, they deal with finite numbers, such as the distance between galaxies.
     
  14. Dec 30, 2015 #13

    Orodruin

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    Note that in the case of the expanding universe, the stars are not actually moving. They are getting further apart due to the expansion of space. Compare with two ants holding on to a rubber band being stretched out.
     
  15. Dec 30, 2015 #14

    PeterDonis

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    "Moving" is relative. They are not moving relative to standard FRW coordinates, but they are moving relative to each other.

    We should probably be careful about how we use the term "expansion of space", since in other threads people are being told that the inferences they are drawing from that phrase are wrong. :wink:
     
  16. Dec 30, 2015 #15

    Orodruin

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    Obviously, but you know as well as I do that when no frame is specified in cosmology, we are usually referring to the comoving coordinates and I believe this is the standard assumption students will make if not told otherwise.

    So what would you use instead in this case? It is what it is in comoving coordinates with cosmological time as the simultaneity convention. I think starting to get into these issues in an I thread is pulling it a bit too far.
     
  17. Dec 30, 2015 #16

    PeterDonis

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    Yes, I agree. But I'm still not sure that saying distant stars/galaxies are "not moving", meaning not moving relative to these coordinates, will avoid confusion. See below.

    I would say that objects (or comoving objects if more precision is needed) are getting further apart (basically how andrewkirk started post #2). But that does imply that they are moving--saying they are "not moving", to me, implies that they are not getting further apart, which is why I think using the term "not moving" in this connection is likely to cause confusion.
     
  18. Dec 31, 2015 #17

    Orodruin

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    Thats funny, I have the exact opposite experience, ie, that calling things "moving" is a source of widespread confusion such as ascribing cosmological redshift to the Doppler effect. Also, I think moving seems to imply changing spatial coordinates with time more than increasing distance.

    Of course, it may be better to avoid using the word "moving" at all.
     
  19. Dec 31, 2015 #18

    Jorrie

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    I'm with you here. In co-moving coordinates, the only 'movement' will be peculiar movement. Cosmological redshift is then due to metric expansion, but the peculiar movement may have a Doppler effect that changes the observed redshift marginally.
     
  20. Dec 31, 2015 #19

    PeterDonis

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    That would be my preference, yes. Unless a spacetime is stationary, there is no way to construct a coordinate chart such that things are "moving" in one sense (nonzero coordinate velocity) iff they are moving in the other sense (nonzero observed redshift/blueshift of light signals between objects). And as soon as these two senses of "moving" are uncoupled, you have the potential for confusion, since our intuition says they should be coupled (more precisely, that we should always be able to choose coordinates so that they are coupled).

    The problem is that it's really hard to describe, say, the properties of the FRW cosmological models in ordinary language without using the words "moving" or "expansion". We can do it with math, of course, but then we have to explain the physical meaning of the math, and again, that's really hard to do without using those two words.
     
  21. Dec 31, 2015 #20

    marcus

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    FWIW (my two cents) in the past I have said things like:
    "Hubble law distance expansion is not like ordinary motion in the sense that nobody gets anywhere by it, everybody just becomes farther apart".

    Nobody approaches a goal or destination by it, relative positions don't change, all the distances just increase by a fixed percentage per unit time.

    It's like dots on an expanding sphere each staying fixed at the same latitude and longitude---not moving around in the ordinary sense but becoming farther apart.

    This is allowed by spacetime curvature, the unintuitive thing at the heart of GR. GR says you have no right to expect distances NOT to change between relatively stationary objects---objects each of which is not moving in the space around it.

    Language like that might or might not help newcomers.
     
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