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How can EVERY point in the universe be the center?

  1. Aug 1, 2012 #1
    It has been stated time and time again that the fact that galaxies are moving away from us makes us appear to be the center of the universe. Of course, someone making this observation from a different galaxy would come to the same conclusion. The implication is that any given observer in any given galaxy will perceive the same notion of being in the center due to the universe expanding.

    I can accept this to an extent, but according to Wikipedia, "The diameter of the observable universe is estimated to be about 28 billion parsecs (93 billion light-years), putting the edge of the observable universe at about 46–47 billion light-years away." Now, let's say I could somehow make an instant leap near this perceived edge; let's say I leap to 45 billion light years from here, and I observe the expansion of the universe. Would I really still be no more in the center than any other point, when I'm only a billion light years from the perceived edge? Or is it that I'm still as much in the center as any other position because the "actual" universe is apparently much larger, or even infinite?
  2. jcsd
  3. Aug 1, 2012 #2
    Edge of the observable universe does not mean edge of the universe period. The observable universe is simply the region of space from which light could've reached us by now. There may be regions of space beyond which aren't in our observable volume but may be in some other point's observable volume.
  4. Aug 1, 2012 #3
    So if the universe is finite, there are positions other than "ambiguously in the center"?
  5. Aug 1, 2012 #4
    Remember, the observable universe is simply a consequence of the fact that a) light has a finite speed and b) the universe has a finite age. So, we can only see so far.

    If you travelled to this 'edge', you would not notice anything peculiar. You would have your own observable universe, since you can also only see so far.

    Another important concept to note is that the universe does NOT have an edge. As you say, it could be infinite. However, a finite universe also has no edge. It would be like the surface of a sphere, or a torus. You will eventually wrap around to your original position after traveling far enough.
    Last edited: Aug 1, 2012
  6. Aug 1, 2012 #5


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    Absolutely not. If the universe IS finite, it is almost certainly not bounded. That is, think of it as something like a 3D version of the 2D surface of a sphere. Just because it isn't infinite doesn't mean there is a center. You would theoretically be able to see the back of your head if you had a good enough telescope (that's really a comment on the topography, not on real physics, because the speed of light gets involved)
  7. Aug 1, 2012 #6
    So it's kind of like how, if I were to examine a basketball, every point would be the center because the uniformity of the shape (a sphere) would prevent me from having any frame of reference for a perceived edge or boundary? That makes sense, although I'm having trouble imagining this 2D surface of the sphere being extrapolated into 3D.
  8. Aug 1, 2012 #7
    Yeah, it's best not to try to imagine the boundary of a 4d ball--that's basically what you're trying to do, and human beings have no realistic way of visualizing that without projections or the like. Instead, just imagine that, like being trapped on the surface of a ball, you can set out in one direction, go on for a while, and eventually end up where you started. In that sense, such a universe would be like a cheap 1990's video game, where if you go off the edge of the screen, you come back on the other side. Imagining a cube with the same sort of wraparound boundary may be the simplest thing.
  9. Aug 1, 2012 #8
    But if you have this expansive space where you can go in any direction--up, down, left, right, forward, backward--at which point are you being forced along a spherical surface?
  10. Aug 1, 2012 #9
    You aren't forced along a spherical surface. To you, the universe seems infinite - you can continue in whatever direction, for however long you want. Of course, you'll eventually start to notice that you're seeing the same galaxies, and that the universe wrapped around on itself.

    You can only visualize this from the inside, because the universe doesn't have an outside. It's self-contained, unlike the surface if the basketball. We say the surface of the basketball, a two dimensional sphere (a sphere is not to be confused with a ball, the three-dimensional inside of the basketball), is embedded in three-dimensional space. This is one type of geometry, in which you embed manifolds into other manifolds. This is the only type of geometry that the human brain can visualize, since everything around us is embedded in three dimensional space.

    Naturally, the universe must be a three dimensional manifold. Visualizing it would require you to image a four dimensional space to embed it into. As mentioned above, you can't do this.

    Now, you may say, does this mean that the universe is sitting in a four dimensional space? No, this is where the other type of geometry kicks in, intristic geometry. This describes the topology of a manifold without embedding it in something else. This would be the case with the universe.
  11. Aug 1, 2012 #10
    Now, just to be clear, when you say 4D space, you're not talking about the dimension of time, which is often referred to as the fourth dimension? You're talking about the the next dimension formed from drawing a right angle at the third dimension?
  12. Aug 1, 2012 #11


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  13. Aug 1, 2012 #12
    Yes, correct. Of course, our universe only has 3 dimensions of space, the fourth I've been mentioning is a mathematical abstraction. As I said, we can use intristic geometry to describe the manifold that represents the universe.
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