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Shape of the Universe

  1. Jun 8, 2012 #1
    If the universe began from the Big Bang, then why is the universe not a spherical shape? I mean, if it expanded from a singularity, then why is it not spherical?
     
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  3. Jun 8, 2012 #2

    Bobbywhy

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    Last time I checked outside the universe is spherical. What makes you say it is not?
     
  4. Jun 8, 2012 #3
    This assumes that the universe expanded "into" something at uniform speed in all directions, which, of course, it did not.

    "Checking outside" allowed you to determine the topology of the Universe?
     
  5. Jun 8, 2012 #4
    The issue of what is the shape of the Universe is a consequence of Poincaré's conjecture, and it generalization. This problem has been solved a few years ago by Grigori Perelman.
     
  6. Jun 8, 2012 #5

    Bobbywhy

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    My apology for responding in such a flippant, terse, and cryptic manner. I assume our universe is homogeneous and isotropic. Therefore the local geometry or the “observable” universe must be either a. Flat and Euclidean, or b. Spherical with a small curvature, or c. Hyperbolic. Since most observations recently report that space is “flat” that would mean a. But if there is a slight positive curvature then b. becomes the best choice.

    As for the global geometry, I don’t know if the observable part encompasses the entire universe or not, so that seems impossible to decide.
     
  7. Jun 8, 2012 #6

    marcus

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    The word "singularity" fools a lot of people. It does not mean "point". So the standard theory does not say that the universe "expanded from a point".

    In math, singularity means where a man-made theory breaks down or blows up. Other physics theories have had singularities and they have been fixed by improving the theory so that it does not break down.
    The singularity (i.e breakdown) region does not have to be a single point, it can along a line or circle or over a whole surface. The word fools people into imagining an explosion outwards from a single point.

    Cosmologists do not assume a singularity actually occurred in nature, just in the classic GR theory. Efforts are under way to resolve the singularity by improving the theory.

    It will be a job to test the various replacement models in order to choose between them.

    I generally agree with what Bobbywhy said except that recent observations do NOT report that space is flat. The conclusion is nearly flat. Observations are compatible both with zero curvature and with slight curvature. For what it's worth (not very much :biggrin:) the center of the 95% confidence interval is slightly on the positive side. Its due to be narrowed down by new data within a year or so.

    Bobbywhy is quite right in a general way. If you go by the upper end of the current 95% confidence interval on curvature, you get a circumference of at least 750 billion lightyears. (spherical but almost flat)
    Have to go, no time to edit.

    Here's an essay about this:
    http://www.einstein-online.info/spotlights/big_bangs
    on the public outreach website of a pretty-good research institute outside Berlin called the AEI (Albert Einstein Institute)
     
    Last edited: Jun 9, 2012
  8. Jun 8, 2012 #7

    phinds

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    I think you need to read up on cosmology. No respected physicists claim to know what the shape of the universe is.
     
  9. Jun 8, 2012 #8

    phinds

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    I didn't (begin at a POINT, as you seem to thing it did). This is a very common misconception. "Singularity" in this context does not mean "point" it means "the place where our theories and math models break down and we have no idea WHAT was going on".

    See www.phinds.com/balloonanalogy
     
  10. Jun 8, 2012 #9
    You completely misunderstood my post. Read it again. It's the Poincaré conjecture that has been solved, not the shape of Universe. What Poincaré conjecture allows to deduce is the shape that is associated to every assumed topological structure. This shift the problem to whether a chosen topology for the Universe is better than another. It is more interesting to investigate the question in terms of topology than shapes, as topology allows connection with fields such as quantum field theory and string theory.

    Having said that, there was no need to be contemptuous in your reply. I am a theoretical physicist, and I know my stuff.
     
    Last edited: Jun 8, 2012
  11. Jun 8, 2012 #10
    No it doesn't. The Poincaré conjecture (the portion of it that was solved by Perelman) only shows that specific kinds of spaces (closed and simply-connected 3-manifolds) are homeomorphic to the 3-sphere.
     
  12. Jun 8, 2012 #11

    phinds

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    I have reread your post and I cannot see how my interpretation was at all unreasonable. It did not SOUND as though you know your stuff. Still, I regret the misunderstanding, even though I feel that it was the fault of your post more than of my interpretation.
     
  13. Jun 9, 2012 #12

    Chalnoth

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    There isn't any reason to believe that it started with a spherical shape, nor that it is a spherical shape today.
     
  14. Jun 9, 2012 #13
    apologies accepted. To be honest, I admit my own post was ambiguous, so no worries about that. :biggrin:
     
  15. Jun 9, 2012 #14
    The universe does expand spherically. this is just not in 3 dimensional space. if you watch my youtube videos on dimensions and the big bang, I explain a model: "The Pebble and the Pond", this is that the big bang was a pebble being thrown in a pond, this creates a circular shockwave, the circular shockwave is 3 dimensional space, and the movement over water is the passing of time. to us, in 3 dimensional space, it looks like the universe is infinite in every direction, because if you walk around a circle, you could do so infinitely, only, in 4 dimensional space the universe actually is a spherical shape (or actually, a hyper-spherical shape). this also explains the exponential expansion of the universe, because as time (the radius of the circular shockwave) moves constantly, 3 dimensional space (the circumference of the shockwave) expands exponentially.

    if you want to see my YouTube videos, my channnel is called "Fathom the Universe"
     
  16. Jun 9, 2012 #15

    phinds

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    Is this just some theory you made up or is it supported by any solid science? I was not aware that there was any KNOWN shape to the universe.
     
  17. Jun 9, 2012 #16
    No it is not my personal theory. Maybe my explanation was a bit clumsy, the model I talked about is an analogy used to explain the current big bang theory..I find it easier to scale down hyper-dimensional space to two or three dimensional models for clarification. Also on the question of shape.. there is not really a shape per se, as outside the universe has no meaning in this context. The internal dimensions expand in 4D from the moment of the big bang.
     
  18. Jun 10, 2012 #17

    phinds

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    In this statement you say "there is not really a shape per se" which is in direct contradition to the statement I was commenting on.

    My understanding, and I am NOT an expert on this, is that the concept of "4D" space (as opposed to 4D spacetime), it NOT accepted by modern physics as realistic. There have been threads (or posts rather) to that effect here on Physics Forum (which of couse does not in and of itself validate my point of view but it is supportive of it).
     
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