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If the universe is flat and finite?

  1. Aug 15, 2010 #1
    If the universe is flat and finite which is where the evidence points today (WMAP) then wouldn't there be an end to space (like an edge). I don't think the universe is curved back onto itself like a sphere because its flat. I mean what would happen if you were to reach the edge of space itself? What if you go past it?
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  3. Aug 15, 2010 #2
    Although the universe seems to be finite, it does not have an 'edge.' Even though its 'flat,' it can still be cyclical. This is almost certain because the universe appears isotropic in all directions.
  4. Aug 15, 2010 #3
    The universe in theory is isotropic in all directions, but we all know this is not the case. Also, the universe would have to be closed if it was cyclical which makes no sense. The universe must be open if its flat.
  5. Aug 16, 2010 #4
    The universe is not 'in theory' isotropic, it is observationally so. Homogeneous as well. Yes, the universe would have to be closed to be cyclical, but it can still be locally flat, even on the size scale of the observable universe. How does the universe being closed make no sense?
  6. Aug 16, 2010 #5
    The WMAP didn't show the universe was "locally flat". Its accepted that its all flat with 2% marginal error. Also, back to Isotropy. Look at this link and tell me if this is in every direction.
    So given that the entire universe is completely flat on average what would happen if you were to reach the ends of space?
  7. Aug 16, 2010 #6
    The 'cold spot' is perfectly consistent with statistical fluctuations. Its still only 'cold' by 5e-5; as apposed to the average fluctuation size of 1e-5. And its still relatively small, about 0.05% of the sky....
    Just because I only see the moon in one direction doesn't mean the universe is anisotropic.... clearly its not perfectly uniform, the idea is that 1) physical laws and processes are consistent, and 2) its relatively equilibrated.

    Good question, way out of my league, excited to hear what an expert says.
  8. Aug 16, 2010 #7


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    Back to the first post for a moment,
    I'm aware that observational data indicate that the universe, if not completely flat on large scales, is at least close to being flat, in-so-far as we can measure anyway.

    But I'm not aware of any data that indicates that it is finite. Sure there may be theories of closed curvatures and/or cyclical topologies (just like there are open and/or flat and infinite theories too). But I'm not aware of any observational evidence that the universe is finite.
  9. Aug 16, 2010 #8
    Well we can use logic to deduce the universe is finite in size if it is completely flat. We know that the universe used to have a finite amount of space right at inflation. The universe cannot expand infinitely fast, so it must be finite in size now. Its just like saying I have an infinite velocity, its not possible. Everything can be assigned a quantity.
    Last edited: Aug 16, 2010
  10. Aug 16, 2010 #9
    I think there is a misconception that the Universe is finite. What we know is finite is the observable Universe. Correct me if I'm wrong.
  11. Aug 16, 2010 #10


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    I don't think that's established. We know the universe must have been far, far, more dense than it is now just at and just after the big bang. But I don't think we can go so far to say with certainty that it was finite, as measured from within its own construct.

    The general consensus now is not that the universe expanded into some sort of preexisting space at the time of the big bang, but rather the 3 dimensions of space themselves came out of the big bang. My point is that the universe, even at and just after the big bang, is not like something you could measure from the outside, like measuring the diameter of an apple, being outside of the apple. Everything is inside -- even infinity itself. Not only is it not possible to look at it from the outside, it doesn't even make any sense.

    It kind of reminds me of an old riddle. "If a plane crashes exactly on the boarder of the USA and Canada, where are the survivors legally buried?" The answer to the riddle is that the survivors are not buried. They didn't die (being survivors), so it doesn't even make any sense to discuss where they are buried. When discussing what was outside the universe at or before the big bang, many scientists reply 'nothing'. But it is not the kind of 'nothing' as in empty-space nothing. Instead it's like the 'undefined' kind of nothing. The kind of nothing that doesn't even make logical sense to talk about.

    Even if one adopts the idea of the universe being a 0-dimensional point singularity at the moment of the big bang, that singularity wasn't sitting inside of some area of space. that singularity was still all of space itself and everything in it.

    Maybe the universe (entire universe) is finite. I don't know. I just don't think we've established that it is. That's my only point.
    Yes, that much we can say. The observable universe is and was finite.
  12. Aug 16, 2010 #11
    Very interesting point. You have given me a lot to think about and has only proved how much more we are ignorant of the universe. I don't know what to think now lol.
  13. Aug 17, 2010 #12
    Some physicists (Smolin, Rees, Susskind) have posited a "multiverse" (or "megaverse", Susskind's preferred term) in which infinite universes co-exist, in which case there is something "outside" of our universe. Obviously this is something that cannot be verified through observation, but presumably they have some theoretical physics-based reasons for thinking this may be so.

    Apparently Susskind and Smolin violently disagree about the details, but both seem to favour the notion of a multiverse or megaverse.
  14. Aug 20, 2010 #13
    There are ten distinct closed flat 3-manifolds. See, for example, Topology and the Cosmic Microwave Background http://arxiv.org/abs/gr-qc/0108043.

    Also, flat is very difficult to prove: 1.00000001 is spherical not flat and 0.9999999 is hyperbolic not flat.

  15. Oct 29, 2010 #14
    I agree from what I have read the observable universe is finite. (I cant observe this for myself but I will accept the observations calculations and theories of others). I also agree that the observable universe is shrinking but that doesnt mean that what we cannot observe is not there. Also some think that mass beyond the observable universe is responsible for dark flow near the edge of observable universe which again suggests that the universe continues beyond the observable horizon that we are aware off.

    So the question is how much bigger is the rest of the universe than the small part we can see? Penrose has mentioned a figure of 10 to power 30 which is BIG! Others talk about Quasi Infinite although I am not sure whether that means a googolplex or more!
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