Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The Center of the Universe

  1. Jul 12, 2006 #1
    I was reading about the center of our Universe, Here is a FAQ.

    "[URL [Broken] Where is the center of the Universe?


    Often when people are told that galaxies are receding from us, they
    assume that means we are at the center of the Universe. However,
    remember that the Universe is homogeneous and isotropic. No matter
    where one is, it looks the same in all directions. Thus, all galaxies
    see all other galaxies receding from them. Hubble's relationship is
    compatible with a Copernican view of the Universe: Our position is not
    a special one.

    So where is the center? *There isn't one*. Although apparently
    nonsensical, consider the same question about the *surface* of a
    sphere (note the *surface*). Where's the center of a sphere's
    surface? Of course, there isn't one. One cannot point to any point
    on a sphere's surface and say that, here is the center. Similarly,
    because the Universe is homogeneous and isotropic, all we can say is
    that, in the past, galaxies were closer together. We cannot say that
    galaxies started expanding from any particular point.[/URL]

    ------------------------- Question ---------------------------
    If every Galaxy acts as the center of the Universe would this be strong evidence for the infinite Universe Theory, If the Universe was infinite wouldn't all Galaxies appear to be the center relative to the observer?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Jul 12, 2006 #2


    User Avatar
    Science Advisor
    Gold Member

    "Evidence" means that we have some observation that directly supports it. IMO there is no evidence that "every Galaxy acts as the center of the Universe". All we have is that it appears homogeneous and isotropic on large scales, for as far (distance wise) as we can observe.

    Further, observation indicates that the universe is open, so we make an assumption that it is infinite, but we cannot be certain. If the universe is open but finite, then, in a way, there must be a center.

    What we can however say for sure is that we are very near the CENTER of OUR observable universe!
  4. Jul 12, 2006 #3
    Well doesn't it seem that the question as to what is the center we have to take into account that your space is not an euclidian space?
    It is instead a 4-dimensional space that is curved and not euclidian. That it is not euclidian means space and time axes are distinctly different.

    So what would "center" even mean in a 4-dimensional Minkowski manifold?
    Last edited: Jul 12, 2006
  5. Jul 12, 2006 #4


    User Avatar
    Science Advisor
    Gold Member

    I don't think we live in "a 4-dimensional space" - did you mean "a 4-dimensional space-time"? "Center" as I used it means a "spatial center" and not a space-time center!
  6. Jul 12, 2006 #5
    But isn't that the problem?

    For instance, if I understand it correctly, in space-time the light cones can be bent by the distribution of mass, hence when one is to take a time slice of space-time at the local frame of reference one is not at all guaranteed to have a view of space only.

    Isn't it true that one cannot take an absolute slice of time in space-time since that simply does not work in a Minkowski space. Each local frame of reference determines time. And in GR it only gets worse since the cones are turned by the curvature of space-time.

    So are you not trying to ask or answer a question about space as if it were something different as what it really is?
    Last edited: Jul 12, 2006
  7. Jul 12, 2006 #6


    User Avatar
    Science Advisor
    Gold Member

    I think you are moving into GR on local scales, where much of what you say is correct, except that in GR one should rather think Schwarzschild spacetime and not Minkowski!

    However, cosmology (and the original question of a center of the universe) works only on cosmological scales, where matter is taken to be spread homogeneously.
  8. Jul 12, 2006 #7
    Well you seem to appear much more knowledgable about all this than me, but my understanding is that our spacetime is definitly a 4-dimensional Minkowski manifold. I never heared of a Schwarzschild spacetime, Schwarzschild was simply the guy who provided a solution to Einstein's field equations for a spherical non-rotating mass. As far as I know he did not invent any different kind of space-time.

    Yes and how does that invalidate the ideas of GR?

    Are you saying that the question as to where is the center of the universe has any meaning?
    So all the GR concepts of space-time are no longer valid because it is "cosmological"?
  9. Jul 12, 2006 #8

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What do you mean by

    and by

  10. Jul 12, 2006 #9

    Ok, I'm probably ignorant, but anyway...: If the universe is infinite, how come it started expanding from one point about 13.7 billion years ago? In other words, if it is expanding, then how can it be defined as "infinite"? Can something "infinite" have a beginning and be expanding? Or does that really mean that the universe is expanding towards infinity?

    Sorry for being off-topic, but I have wondered about this for a while now. :redface: :smile:
  11. Jul 12, 2006 #10
    Isn't saying the Universe has an edge just as bad as saying it is infinite if they haven't the optical power to prove it?

    I think we need to increase our light amplification Quadrillions fold in telescope technology, If our optical power runs out by square laws then the
    Universe should be called infinite because no edge was found strictly by evidence of no edge. I think an edge should only be called if an edge is found and no other systems exist outside the edge.
  12. Jul 12, 2006 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Regardless of our observational powers, we cannot see objects that are so distant their light has yet to reach us. The "edge" of the observable universe is not a physical barrier; it's just the limit of what objects are close enough to us to be visible.

    - Warren
  13. Jul 12, 2006 #12
    no theory can ever be "proven without a doubt".

    We do actually see to the edge of the observable (in terms of photons) universe when the cosmic microwave background is observed. Beyond this, the universe is opaque to photons, however we should be able to see further back with gravitational wave detectors (once they become sensitive enough) and neutrino detectors.
  14. Jul 12, 2006 #13
    warning: (these things migt have been said before)

    If we view the universe from a view of string theory then the universe has to be finite.
    This is because, according to string theory or whatever they call it now, the universe is like 10 demensions (or 11 i haven't read on it for a long time), f our universe was infinite, then we would assume all of the dimensions to be infinte. So, we should be able to go to every single one if we wanted to, but we can't, because 6 are rapped in a little, little, curl.

    So, why should 4 be infinite and six not?

    The Big Bang theory also dissallows the infinite universe idea. Since there was a big bang, time has started, meaning, 3-infinite demensions, one-expanding, and 6 curled up? No, 4 expanding and 6 curled up is more realisitc.

    That finishes the infinite part.

    Now, galaxies don't help it. Lets make it from 3 space, to 2 space. Now, lets say we have a bunch on dots in a circles. The space between them begins to expand. The circle that holds them is expanding also because we find, the circle is actually a 3-d dohnut. There for, our dots are around the entire hyperdonut. The ones at the top, push the ones at the bottom away. The ones at the left push the ones at the right away. This causes an illusion that the universe is infinite.
    Because its a hyper-torus (dohnut). And a circles is seemingly infinite.
  15. Jul 13, 2006 #14


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    I'm no expert on this but I believe physicists actually do have some suggestions about why this should be so. In any case the problem is not why the four dimensions should be "extended" but why the others should be "compact". And since the four ARE extended (we see them to be so), they might be infinite without straining the theory at all.
  16. Jul 13, 2006 #15


    User Avatar
    Gold Member

    Let's go back to the sphere analogy posed in the answer and see what we get:

    "If every point on the sphere acts as the center of the sphere, would this be strong evidence for the sphere being infinite in size?"

    It now becomes obvious that symmetry-of-viewpoint provides no evidence at all about the size of the object in question.
  17. Jul 13, 2006 #16


    User Avatar
    Science Advisor
    Gold Member

    Infinite universe, or not?

    It seems this (not so off-topic) question is still unanswered! Let me give it a try:

    The Big Bang does not necessarily start from a single point - it is thought to have started from an infinitely dense state, which may, or may not, have been infinite in size already! Say it was infinite in size - can it still expand? Yes, for sure! Twice infinity is still infinity, or is my engineering mind missing something? :smile:
  18. Jul 13, 2006 #17
    Why so you say we cannot be certain?

    Seems to me that an open universe if by definition infinite and a close universe is finite.
  19. Jul 13, 2006 #18


    User Avatar
    Staff Emeritus
    Gold Member

  20. Jul 13, 2006 #19
    I thought the singularity was infinitely small at t=0? :confused:

    Well, no. If it expands, it expands from a certain size, and if it has a certain size, it's not infinite... If my engineering mind isn't missing something, of course. :biggrin:
  21. Jul 13, 2006 #20


    User Avatar
    Science Advisor

    Jorrie is right. Have a look at the link loseyourname provided where I tried to explain this by an analogy...(post #9)
    Last edited: Jul 13, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook