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The Big Bang, and where it occured in space.

  1. Apr 8, 2008 #1
    I am fully aware that there is no point at which the big bang 'happened'. You cannot put your finger on it and say that is where it happened. My question is why? I have heard people explaining it as a rubber balloon, where the rubber represents the universe, as it expands you cannot pinpoint the centre, but then what does the air inside represent?

    These questions are coming from a newbie to this field and if this is a common question and people are tired of answering please redirect me to a thread that may help answer my question. As always I appreciate any help given.

  2. jcsd
  3. Apr 8, 2008 #2


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    Staff: Mentor

    The balloon analogy is a 2d analogy of a 3d event. The air inside is not part of the analogy. It doesn't represent anything. You can do a similar 1d analogy with a rubber band.
  4. Apr 9, 2008 #3
    Thanks Russ,

    Would there be a 3D analogy? If so would that 3D analogy explain why there is no centre, and that the big bang didn't happen in a set place.
  5. Apr 9, 2008 #4
    Maybe start from another 2D analogy: imagine a square rubber sheet with grid lines traced on it for reference, which is somehow stretched from all four sides at the same time, so that the grid on it expands evenly.

    To add another detail - while a square rubber sheet would have an obvious edge, you might also imagine it rather as an expansive plane of rubber sheeting extending in every direction as far as your eye can see. We don't know whether or not the universe has an edge; for all we know it may extend infinitely in every direction at this point in time, which I believe in conventional cosmological models would mean that all the way back to the Big Bang it would have extended infinitely in every direction, containing an infinite field of dense stuff.

    But the grid lines you would imagine on a rubber sheet don't represent the stuff within the universe, they represent space itself... it's sort of like the coordinate system of the universe is what's expanding, is the way I interpret it.

    For a 3D analogy, I don't know if you could come up with an analogy simple enough that the details wouldn't get in the way of the analogy, just because there aren't straightforward materials that expand or stretch that way. Maybe something like bread dough rising, if you imagined an infinite space full of bread dough that expanded evenly without constraint? But the problem is that dough is stuff whereas what we're trying to represent is the coordinate system, as I said. I was also thinking you might try something with a complex jungle-gym-like matrix of telescoping rods that would represent the equivalent of the grid lines.
  6. Apr 9, 2008 #5
    I'd like to add 2 points.

    First, one way to describe the Big Bang as simply the moment where the equilibrium state of the universe changed to something much cooler and "less dense" then it was previously. Bang as in Explosion becomes relevent only in the most general of terms, rapid expansion.

    Second, I love Captain Quasar's name. This has no relevence to the topic but I love your handle Cap.
  7. Apr 9, 2008 #6


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    Staff: Mentor

    There is no 3d analogy. Space is the 3d 'thing' and is what we are trying to find an analogy for.

    1d: line
    2d: surface
    3d: space
    Last edited: Apr 9, 2008
  8. Apr 9, 2008 #7
    Hi Mayday,

    The most direct answer to your question is that the Big Bang occurred EVERYWHERE in space. However, all of space was extremely tiny at the time it happened -- tinier than one elementary particle today. So there is NO specific place where it happened.
  9. Apr 9, 2008 #8


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    One technicality. Whether the big bang occurs at a point or everywhere depends a little bit on what the topology of space looks like. For instance if you had a closed universe and the topology was S^3*R, you can continously deform the space to a point.

    Now, imagine the universe was a flat topological plane. There is no such priviledged point, but from the point of view of the metric, the distance between every point goes to zero, even for points that are arbitrarily far apart some epsilon away from the big bang.
  10. Apr 10, 2008 #9

    For full disclosure I must also point out that there's a guy with the name CaptainQuaser. We each thought of this name independently. He showed up on PF a couple of years before me but he spelled it wrong so I didn't know about him until about a month ago.

    One nitpick - it's only the observable universe, the bazillions-of-lightyears-across region that we can currently see with deep space telescopes, that was extremely tiny at the point of the Big Bang, which is not necessarily the same thing as all of space. We have no way of knowing whether or not it's the case that the universe ends just beyond where our telescopes can see (which would be the corollary to stating that the "observable universe" and "all of space" are the same thing). For all we know all of space was an infinite expanse in all three dimensions at the point of the Big Bang.
  11. Apr 10, 2008 #10
    It is not size of space, it is the average density of space that would provide the conditions for space expansion. Although it is right to say it occurred everywhere, but I don't think you can say tinier then one elementary particle. If we say space is infinite in size, then it can not be smaller then any finite size (aka your example of a particle), but the average density of space. (if you were to sample any finite region of space and compare it to any other(s) finite region(s) of space, and you would get the same density)(also not straight density because you would have an infinite variable in your formulas, hence average density)

    I think the density for expansion to occur would be 87% planks density.
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