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Regarding the Big Crunch

  1. Jun 27, 2010 #1
    Greetings, PF community. I have a very simple question regarding the Big Crunch theory.

    As far as I know, the Big Bang was not an explosion in space, but expasion of space itself. But how about a possible Big Crunch? Does this theory deal with the contraction of space itself, or is it just galaxies attracting eachother due to gravitational forces?

    I may have a misconception, but I believe the Big Crunch could only possibly happen on a closed universe, and has to do with the overall density of the universe. But the point I'm still missing, is that I'm not sure if this theory implies a decrease in the expasion of the universe until a critical point in which space would begin to contract, or would it simply be an scenario where galaxies begin to attrack eachother and we end up with a very dense concentration of mass in an almost-empty universe.

    Perhaps I'm missing some fundamental concepts here, sorry if this is too basic.

  2. jcsd
  3. Jun 27, 2010 #2


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    This is correct.
  4. Jun 27, 2010 #3


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    Good point.

    The universe is expanding and space along with it. There are several theories as to what is causing this, but whatever it is, it is something more exotic than the 4 fundamental forces.

    So, if gravity slows and stops the expansion, and then begins to contract again, by what logic would space start to contract as well? Seems to me, the matter in the universe would start to coalesce but empty space would not; it would be "left behind".

    When the Big Crunch came, it would be an event in space, not of space.

    I think.

    Not sure if it is meaningful to talk about empty space "out there" if there's no matter in it.

    [EDIT]Upon reflection, I'm thinking expansion and compression must be symmetrical.
    Last edited: Jun 27, 2010
  5. Dec 8, 2010 #4
    so what's really happening then?

    does the dark energy cause the expansion of space directly or it acts on matter thus forcing space to create itself and overall seem like an expansion? (from my understanding, the former would be in consensus with the present theory, but i want to be sure..)

    and how is gravity seen in this picture? it is said that gravity curves space.. does that mean gravity, supposing it would mash together all matter, will bend space in such manner that it would intertwine with matter and contract along with it? or the curvature of space doesn't take place that way and it would still be possible for it to exist outside matter?

    (these may well be questions already asked and possibly answered on this forum, but if there is something to be blamed, that is my inability to have found those threads.. do not let this stop you from answering to my inexperienced concerns..)
  6. Dec 9, 2010 #5
    My question: how was calculated the singularity(zero volume, infinite density) of the Universe, black holes?
    Because I consider the Universe(black hole) can not have a volume less than the mass of the Universe(black hole) multiplied with the Planck density.
  7. Dec 11, 2010 #6
    i know my question may have been so elementary that it isn't worth answering, but still can anyone show me in the right direction? just trying to put pieces together regarding our universe like anybody else (even if it is at a lower level of understanding, since i have just begun seriously studying physics..)
  8. Dec 11, 2010 #7
    Dark energy causes the expansion of space itself (at least this is the current understanding, but we don't really know what dark matter/energy is!)

    If there will be a contraction, space itself will contract so I am not sure what you're really saying about matter, gravity and space. Try explaining it again :)

  9. Dec 11, 2010 #8
    thank you for confirming that..

    i was only curious about the nature of gravity and it's relation with space.. that's how i found this specific thread and the common curiosity of the OP..

    you said in case of contraction 'space itself will contract'.. does that mean gravity has the same property as dark energy, only that it affects space by contracting it? or it actually has an impact only on matter, but incidentally while all the matter in the universe would converge to a point, a singularity (or however would it be properly called) due to gravity, that very fact would also 'drag' space along with it?

    and the other question from my previous post was kind of the reverse of this second possibility: if gravity contracts matter, but not space, would it remain mostly empty in the end?

    (some of these questions were also mentioned in the other posts before mine.. but since they remained unanswered, i wanted to update my/our concerns..)

    on another note, i still don't understand why would there be a big crunch, since the universe is expanding and not only that, but also accelerating.. of course it can be said that dark energy, since we don't know its nature, might not always influence space as in this moment.. or perhaps the universe is expanding only to find out it will collide with itself at one moment.. but these are topics for other threads..

    in any case, big crunch or not, the questions are more related to the way space behaves and why..

    hopefully i made myself clearer..

  10. Dec 11, 2010 #9
    I just want to make clear that, as far as I understand, dark energy tends to increase the rate of expansion of the universe. It is not the primordial source of expansion.

    I think the question was more directed to a lack of understanding of how the Big Crunch would really work. When I tried to picture this event, I came up with an scenario in which gravitational force would overcome universal expansion, just like galaxies close enough to eachother attrack eachother in the galaxy clusters scenario. My first guess was that empty space would be simply left behind, but upon thinkin I think that Dave is right when he states that contraction and expansion must be symmetrical.
    Last edited: Dec 11, 2010
  11. Dec 11, 2010 #10
    where does that 'must' come from? (since i'm sure you're not talking about the aesthetics of symmetry..)
  12. Dec 11, 2010 #11
    Sorry if I did not make myself clear. The must in my previous statement was a mere representation of my lack of understanding of the subject on a more deep context, according to my current general definition of the Big Crunch.

    Perhaps I should rewrite my sentence as, 'As far as I understand, the Big Crunch proposes an scenario where the metric expansion of space eventually reverses. If that would be the case, there would be no point in talking of metric expansion reversing if the space is simply left behind. Therefore contraction should be symmetrical to expansion'.

    Of course this is merely intuitive, and I'm pretty sure that intuiton does not help much when we talk about these kinds of topics. Perhaps someone more expert in the subject can correct me, since its very probable that I'm missing some concepts here.
  13. Dec 11, 2010 #12
    Ok, let's talk a bit about the background of the big crunch theory because there are some confused ideas here.

    If one studies Friedmann's equation: [tex]\dot{R}[/tex]2 = [tex]\frac{8}{3}[/tex][tex]\pi[/tex]R2 - k + 1/3[tex]\Lambda[/tex]R2, where R is the scale factor of the universe and depends on time, k is the curvature of the Universe and [tex]\Lambda[/tex] is a constant to represent the presence of dark matter.
    From this equation, some models of the Universe can be extrapolated by letting one term (in the RHS) dominate over the others.
    To obtain the Big Crunch, we need to consider large values of R. That is pretty much now. So not small values of R when the Universe was still "small" because of the Big Bang.

    This would make the Lambda term the dominant one, and we would therefore obtain that R = e(1/3[tex]\Lambda[/tex])^1/2.
    If we let [tex]\Lambda[/tex] < 0, we obtain a negative acceleration.

    Therefore when the Universe reaches a large value of R, say now, the term due to dark matter will firstly slow down the Universe expansion and then reverse it to a deceleration. This would culminate into a singularity, the Big Crunch. Therefore the whole Universe (or space) would contract due to gravitational interactions between matter (dark and non). Gravity doesn't really come into play.

    As a final note, this model is *incorrect* as it has been observed that the Universe is undergoing an acceleration due to dark matter.

    But it was a fine theory.
    Last edited: Dec 11, 2010
  14. Dec 11, 2010 #13
    Thanks for the reply, but when you refer to dark matter, do you really mean it, or should I replace for 'dark energy', in all of the sentences? Or perhaps only in a few of them?
  15. Dec 11, 2010 #14
    Good point, it should be dark energy.
    But the difference is subtle, so it doesn't really matter.

  16. Dec 11, 2010 #15
    thank you for replying.. my question lied just in what you said right here (although you might have meant something else in the second phrase, since, from what you first said, gravity does come into play..):

    so space is contracted indirectly by wrapping itself around matter, 'shrinking' along with it..
  17. Dec 11, 2010 #16
    Yes, i would say so.

  18. Dec 11, 2010 #17
    all right.. now i should ask how come space would completely curl around matter making it impossible to exist on its own and also be filled with 'nothing' (only with the possibility to 'shelter' matter).. but that one i guess is a question i should put on hold until i'll gather more understanding of physics and its underlying math..
  19. Dec 11, 2010 #18
    Well, the thing is that "nothingness" can't exist. There is just no such thing.

    Try with this interesting thought: Imagine a room. Nothing can be outside of it. The room is your Universe.
    The room is full of objects. Maybe you can't see them, because they could just be particles such as electrons, protons, neutrons and photons.
    Now take everything away, every single particle, even light.
    Can you imagine the room filled by emptiness? I can't. :)
  20. Dec 11, 2010 #19
    why not? given your analogy, by removing every trace of matter from that room, wouldn't make the room disappear.. it has the property of harboring matter, but it happens to be empty.. thus by filling it again with matter (let's say through the crack in the wall which suddenly emerged because some god bashed his head on the other side) it would be a process which takes place differently, governed by different math, than the first time when the room had to be created piece by piece..

    little edit: just to be sure you know what i mean although i think you got it: that 'nothingness' i'm talking about is not related to that in which some people thought the universe is expanding into..
    Last edited: Dec 11, 2010
  21. Dec 11, 2010 #20
    you are assuming that space can exist without the presence of matter.
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