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Inflation model of the universe

  1. Jun 18, 2012 #1
    I understand that (from what we can tell) the universe is expanding at an accelerating rate. I'm just curious if it's possible that the universe is expanding in some areas, and receding in others. despite this going against the cosmological principle, is it possible that beyond our bubble of expansion, there is a bubble of collapse?

    I'm just wondering if the universe is acting like entropy, in the sense that it could be expanding in some places, yet collapsing in others.
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  3. Jun 19, 2012 #2


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    In a way, this sorta kinda happens. Local regions which have a large overdensity of matter undergo collapse still. The accelerated expansion is only seen on very large scales (hundreds of millions of light years and greater).
  4. Jun 19, 2012 #3


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    Yes, there are still regions of the universe that are locally collapsing, but, as Chalnoth correctly points out, expansion is only obvious on very large scales.
  5. Jun 19, 2012 #4
    Also you may get odd things happening outside of the cosmic horizon.

    One limitation on the cosmological principle is the speed of light. Two pieces of the universe that aren't in communication can't exchange information to behave in the same way.
  6. Jun 19, 2012 #5


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    This does not seem incorrect, but it does seem like a somewhat misleading statement. The matter in some areas is coming closer together but that does not inhibit the expansion of space all around that region of matter, so the overall expansion is not affected.

    Here is what I base that on, and I'm open to correction:

    If you have galaxy A and galaxy B separated by 2 billion light years, and half-way between them there is a local cluster that is collapsing, the speed with which A and B are receding from each other is not affected by the existence of that cluster. Is this not correct?

    If it IS correct, then I think you can see why I say your statement is somewhat misleading. If it's NOT correct, then of course ... (and I do see how it might not be correct)
  7. Jun 19, 2012 #6
    I'd say it is NOT correct, but it's not an egregious error either. I don't like it on theoretical grounds:
    Cosmological expansion is based on homogeneous and isotropic uniformity, the FLRW model. It simply does not apply to lumpy portions of the cosmos. If you want to disregard the collapsing interim galaxy, then you can approximate the model conditions if you consider that 'ok for my purposes' [accurate enough].

    Thread here with some insightful discussion and details:

    Cosmological expansion of earth orbit
  8. Jun 19, 2012 #7
    Possible but not likely according the mainstream cosmology. I'm talking large scale 'collapse' here, like horizon to horizon type size.

    Eternal inflation models, bubble universes, and multiverses [all generally synonomous terms] are alternative models from the FLRW model most cosmologists believe best respresents our universe. The FLRW model has lots of verifying, although often indirect, observational evidence; insofar as I know, the other models have none, but that does not entirely rule them out.

    Summary information here:
    Eternal inflation
  9. Jun 19, 2012 #8
    FLRW which assumes a perfectly smooth universe is like assuming that the earth is spherical. The earth isn't spherical, but it's close enough so that for a lot of things.

    LCDM is like assume the earth is ellipsoid. LCDM contains the effects of linear perturbations. The earth isn't ellipsoid, but it's an even close approximation.
  10. Jun 20, 2012 #9
    You need to remember that GR is not a linear theory. That means, if you have a background solution which is expanding, and a local collapsing solution, the exact overall solution is not the combination of the two. I think it's very misleading to think that gravity and the expansion of the universe are somehow separate things, when in fact they are exactly the same thing (curving of the metric).
  11. Jun 20, 2012 #10


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  12. Jun 21, 2012 #11
    what is the difference between these two? wiki has them both listed as the standard model of big bang cosmology :/
  13. Jun 21, 2012 #12


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    FLRW is a description of how a smooth universe that is the same in all directions expands (or contracts) over time depending upon the contents of said universe. FLRW makes no statements, one way or another, about those contents (except that they are smooth and the same in all directions). It merely presents a relationship between expansion and contents.

    LCDM is explicitly about the contents of the universe: it is a universe where, besides normal matter, we have a cosmological constant (Lambda) and cold dark matter. This universe is approximately FLRW, but not exactly: matter in the universe tends to become rather clumpy with time. On large scales, on average, it is FLRW. But on smaller scales we have things like galaxy clusters, galaxies, stars, planets, and other objects that are much, much more dense than their surroundings.
  14. Jun 21, 2012 #13
    yes, ours are included: LCDM is fine tuning where we inlcude in FLRW some observational evidence that more closely describes our universe. For example we derive some information from type 1A supernova [a standard brightness] observations.

    Detailed parameters described here:
  15. Jun 21, 2012 #14
    ahh okay thank you. I understood that the cosmological principle was only applicable on large scales, and that helps clarify it
  16. Jun 21, 2012 #15
    sort of on topic, what are your thoughts on the possibility of a cyclic model for the universe? for example, the recent Baum–Frampton model
  17. Jun 21, 2012 #16


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    Personally, I think they're pretty highly unlikely, for various reasons. Most of the models I think are highly unlikely for entropy reasons: they require entropy to increase and then decrease periodically. This is just absurd.

    The Baum-Frampton model is kinda neat in its own way, but is very absurd for a different reason: it requires an equation of state for dark energy that is generally expected to be fundamentally impossible.

    My personal favorite model is this one:

    But I wouldn't even suggest that it's more likely than not. Just that I think the model is beautiful.
  18. Jun 21, 2012 #17
    I'll save this and read it after I start my astrophysics classes, so it make more sense to me haha
  19. Jun 26, 2012 #18

    cp = two principles of spatial invariance. The first invariance is isomorphism under translation = homogeneity (uniformity would be independent of the location one chooses to make the observations)
    invariance as isomorphism under rotation = isotropy (direction, such as North or South, can not be distinguished).
  20. Jun 27, 2012 #19


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    Although the two principles are not independent: global isotropy implies homogeneity.
  21. Jul 8, 2012 #20
    Heck, what I wanna know is this:
    If during the first second of the "Big Bang" space grew larger than light could travel, how big could the universe be?

    We have "figured out" the age of the universe, yet we have not "figured out" the expanse or breadth of it.
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