Dot Physics

I am going to try to answer another Ask a ScienceBlogger question. This one is from George P. Burdell. He asks:

“If the universe is expanding from the big bang, has anyone tried to reverse plot the galaxies to figure out the exact point where the big bang occurred?”

Actually, he asked two questions, but I am just answering the first. Second, let me note that I am not really a cosmologist – so I am just making up answer.

In short, the answer is that everywhere is the center of the universe. Think of it this way. Imagine that there are only two dimensions and these two dimension are the surface of a balloon. Also, this balloon is expanding. Here is the balloon at two different times with two locations.

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If I let this go back in time so that the balloon gets smaller, points A and B get closer together. If the balloon could super-shrink, they would both be at the center. The problem is that with this balloon example, everyone wants to think of the whole balloon as the universe. However, just the surface of the balloon is like the universe in this case.

That is my answer. I tried to keep it simple.

Comments

  1. #1 Matt Leifer
    May 20, 2010

    Good answer, but the balloon analogy is only really accurate if the universe is closed. If the universe is flat or open then it has always been infinite in extent, so it is not like the balloon surface which has finite area. Despite being infinite, it was still in some sense “smaller” in the past, in the sense of having higher energy density. To understand this properly, one would need to talk about spacetime and how the dimensions of space and time get mixed together under extreme gravitational conditions. To a physicist you can simply say that the big bang singularity is spacelike, so it is more like a “time” than a “place”, i.e. it still doesn’t have a center. However, I don’t know a good analogy for this.

  2. #2 Jesse
    May 20, 2010

    Works for me. Everywhere is expanding from everywhere else. The whole idea of the big ban is that if you traced it back, everything collapses to the same point. Thus, if you trace it back, everything is the center of the universe, as you said. I must admit, when I was first taking physics in high school and we talked a little bit about cosmology, this was a question I asked.

  3. #3 Jesse
    May 20, 2010

    That should read: “the big bang”

  4. #4 Chris
    May 21, 2010

    A very clever physics teacher once demonstrated how he showed his kids this concept. He took a transparency of some hubble view of lots of stars, and he overlayed it with another that he had slightly scaled down to show “some earlier time”. When you place them over each other you see one point from which all seem to be emanating. Try it!

    Of course, then he slid the transparencies so that a different star overlapped and that one suddenly became the center of the universe and location of the big bang, demonstrating the “everywhere is the center” concept quite well.

  5. #5 Jack
    May 26, 2010

    Hrmm, I have never been super satisfied with this answer to that question. The surface is the universe but not the whole balloon?

    In that case talking about a 2 spatial-dimensional surface expanding 3 dimensionally (ie. the balloon getting bigger). Your exapanding something in three dimensional space, yet only observing the effect in 2D.

    With a 3 dimensional universe curved around a 4th dimension this analogy can hold, and there is no, but I can’t get a hang of it in a flat universe.

    Someone help me out here – (to quote) Im not a cosmologist…

  6. #6 SeanH
    May 27, 2010

    Jack,

    I’m not a scientist or much to brag on at math, but I’ll give it a try.

    Your exapanding something in three dimensional space, yet only observing the effect in 2D.

    The trick is that you have to manage to shut that external third dimension off in your mind and picture yourself as a dot on the balloon. The entire universe is the skin of the balloon so that third spatial dimension it’s expanding into, being outside the balloon-skin universe, doesn’t exist.

    The balloon space isn’t closed because it’s spherical. You’re a dot on the 2-D skin of the balloon so that third dimension needed to see the universe as spherical isn’t available to you. You can only tell the balloon space is closed by observing that parallel lines drawn in the “real” 2-D universe will eventually meet. Or that the angles of a really big triangle add up to more than 180 degrees.

    but I can’t get a hang of it in a flat universe.

    Keep ignoring that third dimension, but instead of a balloon the universe is a torus and you’re a dot on the surface. Any parallel lines will now never meet and any triangles in the universe will add up to 180 degrees exactly so from your perspective in the “real” 2-D universe it’s spatially flat.

    I don’t know if that helps or if I just muddied things up worse, but that’s how I have to picture it. I constantly have to remind myself that there’s no universe but the balloon surface so seeing a third dimension or taking any perspective from outside the 2-D surface is invalid in the analogy.