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Is the universe round or flat?

  1. Mar 11, 2012 #1

    I am a bit confused on what the current model of the universe is. some are saying it is round, while others are saying it is flat. which is true?

    If the universe is flat, then is it that only some part of it is flat or the whole universe is flat?

    how can we prove it theoratically and mathematically?

    Thanks in advance
  2. jcsd
  3. Mar 11, 2012 #2

    Ken G

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    Observations are consistent with a spatially flat universe. But observations also suggest that there is much more universe "beyond" what we can actually see, so whether or not it continues to be flat ad infinitum is unknown and may not be knowable.
  4. Mar 11, 2012 #3

    The COBE and WMAP satellites launched by NASA in 1992 and 2003 measured the Cosmic Microwave Background, and determined that the Universe must be either flat or extraordinarily close to it. If it was even slightly curved in the beginning, it would have become more and more curved over time, so it seems the universe it flat

    Also, remember what people mean when the say the Universe is round: they don't mean it is in the shape of a ball ,or anything like that. It is actually impossible to picture. To get an idea of what a round(or closed) universe would be likely, think of a 2 dimensional example: imagine a person who lives on the surface of a sphere - but that is all there is, no center of the sphere or outside. To him, the universe has no boundaries, but if he continuously travels in one direction, he will eventually cross through either the top or the bottom, and then eventually return to his position.

    For the 3 dimensional version of this, you would need to be able to picture a fourth dimension, which we can't do. But what you can do is know that in a round universe, you will never see a boundary, and that parallel lines will eventually cross over far enough distance.

    If the universe is flat, as the evidence suggests, then it can be infinite or finite. If it is infinite you can easily picture it, it just goes forever in every direction. If it flat and finite, we once again can not imagine the whole universe, but it would be like a 3 dimensional version of the world pacman lives in - finite, but no boundaries.
  5. Mar 11, 2012 #4
    There are probably many different ideas of what you could mean by a Universe that is flat, round, finite, infinite, etc. My personal preference is to look at the possibility of closed space like curves. If you travel long enough in a straight line in one direction (i.e. traveling along a space like geodesic), then you could end up where you started from; space would be compact.
  6. Mar 12, 2012 #5


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    But, you would never know when you reached your departure point in a finite, unbounded universe. It would be like walking around the earth on foot. It would take so long your point of origin would no longer be recognizable by the time you returned. The expansion of the universe further compounds the problem.
  7. Mar 12, 2012 #6
    I'm not sure if anyone has answered your question but I think the shape of the universe and if it is flat are two different things. You can have a round (spherical) universe that is flat. I think the standard model describes a universe that is spherical and expanding but it could contract or be flat. I think flatness has more to do with the density of the universe than it does with the shape. I think
  8. Mar 13, 2012 #7
    thanks to all of you for the reply

    recently, I did some research on google on the matter, and i found out that in the recent model proposed by physicists, they are now thinking that the universe has a shape similar to a flat trumpet like shape. In this model, they think the universe is narrow at one end and widens on going from one end to the other end.

    What do you people think of this !
  9. Mar 13, 2012 #8


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    welcome to pf!

    kaushik_s! welcome to pf!
    no, that's a size-time graph of the size of the universe, not a picture of space
  10. Mar 13, 2012 #9
    No one thinks this is actually true :) Sometimes popular magazines pick up weird publications to promote, that is all.

    And just for reference (in case you did not read the article completely), they are postulating a very weird topological structure for the universe just to explain the leftmost datapoint on this plot. Now, the blueish region on the plot corresponds to natural statistical uncertainty (poetically called "cosmic variance"), arising from the random nature of the initial density fluctuations. You can see that it almost manages to cover the data point, meaning that the data is already almost within the expected error margin.
  11. Mar 13, 2012 #10
    Re: welcome to pf!

    No, I'm pretty sure that's not true... The 3-space metric they consider is
    ds2 = (dx2+dy2+dz2)/z2
  12. Mar 13, 2012 #11


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    No, you can't. You cannot globally cover the sphere with the metric of flat space. In other words, a sphere has different geometry than the plane. Of course, it is possible that the sphere be sufficiently large that it locally looks flat, but there is still not an exact equivalence. The standard model of cosmology makes no a priori claims as to the global geometry of the universe. Recent CMB measurements coupled with those from the HST indicate that the *observable* universe is flat to within 1%. This measurement, however, tells us nothing of the global geometry of the universe.
  13. Mar 13, 2012 #12
    Asking if the universe is round or flat is like asking if a body of water is round or flat
  14. Mar 13, 2012 #13
    why is that?
  15. Mar 13, 2012 #14
    Because the nature of the universe is to equalize in much the same way water neutralizes
  16. Mar 13, 2012 #15
    What do you mean by equalize? I thought the universe was expanding.
  17. Mar 13, 2012 #16
    In other words its not relevant water is water and space is space and it doesn't mater what shape the container is
  18. Mar 13, 2012 #17
    I don't think the question is what shape the universe is contained within, it's what the curvature of the universe is on the large scale.
  19. Mar 13, 2012 #18


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    This discussion is becoming nonsensical. It is perfectly valid to question the global geometry of the universe. Anyone who says otherwise must be unaware that general relativity is a geometric theory of spacetime, and that the content of the universe determines its global geometry.

    It's not about "containers". The distinction is between flat and curved geometries: for example, is the earth round or flat? Is that a sensible question? If you think so (as I hope you do), then the question regarding the geometry of the universe is equally sensible -- in fact, it's simply a higher-dimensional analog.
  20. Mar 13, 2012 #19
    how are you observing the universe?
    and yes its is very sensible to be curious about the geometry of the universe
  21. Mar 13, 2012 #20


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    I don't mean to be rude, but please don't spread misinformation when you are not knowledgeable on a topic. Your responses in this thread have been cryptic, vague, and worst of all, completely incorrect. I think bapowell's nice concise answer is basically the end of the geometric discussion.

    However, there is an interesting point here that I'd like to make, which is that the curvature of the universe (positive, negative, or zero) is a separate question from the topology of our universe. For example, since we know the universe is flat (curvature is zero), the easiest solution to have is the simple plane. But there are other solutions, for example, the universe could be shaped like a torus! There are other, more exotic global topologies of the universe, and it's interesting to think about but often difficult to experimentally verify (since one can always make the radius of the doughnut, for example, very large such that there is no hope to observe its global structure on the timescales available to us).
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