Does the Universe have a centre?

  1. As I understand it, I could be wrong of course, the Universe started from a singularity
    (Hawkins?).

    Does this mean it had some sort of centre?

    And if it did would it be possible to detect where it started from, and if so how?

    Is this the right place to ask this?
     
  2. jcsd
  3. Wallace

    Wallace 1,253
    Science Advisor

    This is the right place to ask if you aren't sure. This is actually a cosmology question but if you're not sure, and it's something to do with space, then general astronomy is fine.

    On to the question, the Universe, at least according to current theory, has no centre. The important thing to realise is that a 'singularity' is not a physical thing, i.e. you can't say 'oh look at that singularity over there'. In fact a singularity is merely a situation where our theories cease to be meaningful. The Big Bang model says that the Universe was hotter and denser in the past, but current theory doesn't actually let us wind the clock back to the very beginning. The equations themselves stop being meaningful at some point.

    This means that when we say 'the Universe is 14 Billion years old' what we really mean is that it is 14 Billions years from the oldest time our theories can make an accurate prediction. At some point as we wind the clock back and the Universe becomes more and more dense and hot we reach a point where we don't know how stuff behaves at those densities and temperatures. Our theories stop working, and we describe this as a 'singularity' but that is a description of an equation, not a physical 'thing'.

    Hopefully that answers part of your question. In terms of their being a 'centre', the Big Bang theory says that the Universe is roughly the same everywhere, so there are no special points. A centre would be such a special point. Big Bang theory is compatible with the Universe being infinite. So at the earliest time we can describe it with current theories, the Universe was probably infinite in extent and everywhere very hot and dense. From this point, everything in the Universe began moving away from everything else. This expansion is uniform, in the sense that at all points it appears to an observer at that point that they are at the centre, since the rate of expansion in all directions is the same. There is no universal centre.

    The other thing to realise is that the Big Bang was not something that occurred at some particular place, sending material outwards into previously empty space.
     
  4. Garth

    Garth 3,443
    Science Advisor
    Gold Member

    Boomerang, another way of illustrating the same thing as Wallace described is to ask the question: "Where did the Big Bang happen?"

    According to standard cosmology the answer is: "Everywhere!"

    Every point in the present universe was once in the Big Bang and every point has been expanding away from every other point ever since.

    Garth
     
  5. Boomerang, I'll reply more to your preceding thread, which was apparently locked for fear of creationist agenda.

    We are familiar with how gravity works locally (see Cavendish lab experiment), and we have navigated probes all 'round the solar system, confirming that the Earth and planets revolve around the sun.

    After observing red-shifts of many stars, we can determine the orbit of our solar system about the milky way galaxy. By extending this for distant galaxies, in addition to observing the average Hubble expansion, we measure that our galaxy itself is not quite at rest with respect to the expanding universe. (Deep sky surveys show that galaxies are also arranged into larger structures, and that the pattern has no single "special" centre.)

    So there is hard scientific evidence that the Earth is not the centre of the universe, let alone the sun or the galaxy even (though you might pedantically argue that the the Hubble Space Telescope satellite is at the centre of the observable universe). To put this another way, in light of modern cosmology: Can you tell me which city (or country) is at the centre of the globe?
     
    Last edited: May 1, 2008
  6. Why should there be a city at the centre of a globe?
    What evidence do we have to indicate there is a city at the centre of the globe?
     
    Last edited: May 1, 2008

  7. I was merely asking it there were any descripancies between short distance and long
    didtance observations really.
    So...if oue galaxy is not qute at rest have they made measurements for our
    solar system similarly?
     
  8. russ_watters

    Staff: Mentor

    That's exactly right and is a good illustration of why there is no center to the universe.
     
  9. I am sorry I realy do not see that at all, I don't see why the Earth should be some sort
    of mini Universe, that a purely groundless suggestion as far as I am concerned.
    I do not expect the Earth to be a mini Universe, you are not comparing llike with
    like so the comparison is pointless in the first place.
     
  10. please explain why our universe is not a sphere


    as that should be it's shape, and

    it has a center like any other sphere does
    it is just a very very BIG sphere
    BUT we have no idea where the center is
    rather then admit that odd fact we get the no center BS

    give them a few thousand years and they may get a better idea
     
    Last edited: May 1, 2008
  11. I didn't say the Universe is not a sphere, I just said it is not solid, like the Earth.
    Most of it is empty.
     
  12. russ_watters

    Staff: Mentor

    It's an analogy, boomerang/ray b. A 2d analogy to a 3d phenomena and except for the number of degrees, they work exactly the same. The universe has no center for the same reason that the surface of the earth has no center.

    Yes, this implies that if you flew in space in one direction for a very long time, you'd eventually arrive back where you started (there are unrelated reasons why this probably can't happen, though).
     
  13. To extend this analogy a bit (or whatever), the earth is 3D and as far as I am aware the
    universe is 3D. How, I asked about the centre, not whether there is a city at the centre,
    I am not sure where that idea came from. We have a pretty good idea where the
    centre of the earth is. I don't really see the validity of comparing the surface of the earth to
    to the universe in it's entirity, That's not comparing like with like, if you were to compare
    the 'surface' of the universe to the surface of the earth then I could accept that
    as showing the *surface* of the universe had no centre. However as I am not talking about the surface of the universe it is not an appropiate comparision.
    I mean we know where the centre of the observable universe is don't we? It's really not
    that difficult to work out.
     
  14. cristo

    cristo 8,411
    Staff Emeritus
    Science Advisor

    This isn't extending the analogy; this is completely changing it! The reason someone said "which city is at the centre of the earth's surface" was to try and put the analogy in layman's terms, and something you may be able to relate to. However, this does not seem to have been the case.

    Ok, so if you want a more realistic analogy, then why not imagine the three dimensional surface of a 4 dimensional sphere? The point is that one cannot, and thus it is easier to lose a spatial dimension and relate to the analogy as if we are sitting on the surface of the earth.

    So, again, imagine that we are sat on the surface of the earth. There is no centre; in fact the centre of such a surface is not defined. The same holds for the universe.

    Well, yes, I am the centre of my own observable universe-- but this is not the question you asked!
     
  15. The centre of the observable universe is YOU (i.e. the observer!). I'd guess it had a spherical shape in that up to a certain radius, things have no effect on you whatsoever because photons/gravitons, etc, can't reach you from beyond said horizon.

    Of course, the observable universe is always expanding.
     
  16. cristo

    cristo 8,411
    Staff Emeritus
    Science Advisor

    Since the OP is no longer with us, I'm locking this.
     
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