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Small, Flat Universe?

  1. Jun 26, 2009 #1
    So, I now understand based on things that I have read, and things that people on this forum have told me, that the universe if flat, must be infinite. My question is, when we look into the farthest reaches of space, we see a small, finite baby universe. How is this possible? The universe can be traced back to a point when it was 10^-33 cm small. Anything infinite begins infinite, from what I have always understood. What's going on, eh?
  2. jcsd
  3. Jun 26, 2009 #2


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    We do?
  4. Jun 26, 2009 #3
    The WMAP.

    (poorly stated by me)
  5. Jun 26, 2009 #4


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    OK, the WMAP shows that the universe was small and hot at one point. This it the prevailing theory. The prevailing theory holds that the universe is about 78G ly across.

    There are theories that the universe is infinite, but they are not prevailing theories, partly because, well, the evidence seems to poke a hole in them.
  6. Jun 26, 2009 #5
    The most recent evidence on the shape of the universe tells us that it is perfectly flat (to within experimental and measurable error). So, what you're saying is that the prevailing theory contradicts the newest intel?
  7. Jun 26, 2009 #6


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    I think that the starting premise: "if the universe is flat, it must be infinite" is flawed, or at least misunderstood.

    Flatness simply refers to the balance between expansion and contraction; it does not imply infinite extent.
  8. Jun 26, 2009 #7
    I have read and heard this. I'll look for links tomorrow.
  9. Jun 27, 2009 #8
    Last edited by a moderator: May 4, 2017
  10. Jun 27, 2009 #9
    It seems to me that you're confusing the idea of the entire universe with that of the observable universe. Since the universe has finite age and light travels at a finite speed, we can only observe structures that are close enough to us for light to have propagated to us within the time that the universe has existed. We generally expect that the universe is larger than just what we can see. I believe that the 78 Gly is the best number for the current size of the observable universe. It is unknown how much larger the whole universe is.
  11. Jun 27, 2009 #10
    So what you're sayin gis that the observable universe was once very, very small, but the entire universe's size is unknown, even at the big bang?
  12. Jun 27, 2009 #11
    Quite right.
  13. Jun 27, 2009 #12
    Ahh, now it makes sense.
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