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Is the universe infinite?

  1. Dec 19, 2010 #1
    Ok so me and a few of my physics (& Maths) friends were arguing this.

    I argued that it must be finite in size, since the universe contains a finite amount of matter and since no space is truly empty, how could the universe be finite.

    My friend who's a mathematician said that in her geometry subject this question was actually brought up. She said that the universe may be a 3-manifold (3D surface?) and it depends on the curvature (negative or positive) as to whether the universe is finite or not.

    I just want to know what the consensus is.
  2. jcsd
  3. Dec 20, 2010 #2


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    There is no consensus. Basically, our vision is limited in time and space, so we can't exactly say anything about whether or not distances are infinite. All we can say for sure is that our universe is many times larger than our observable region. But how much larger is currently up in the air.
  4. Dec 20, 2010 #3
    What is the/a universe?? Scientists suggest that there many universes emanating from their own "big bang". Space is infinite but a universe is bounded.
    Last edited: Dec 20, 2010
  5. Dec 20, 2010 #4
    The size of the Universe: the speed of light multiplied with 13.7 billion years...
  6. Dec 20, 2010 #5
    lol no
  7. Dec 20, 2010 #6
    if the matter that originally emitted the oldest CMBR photons has a present distance of 46 billion light years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years away...
  8. Dec 20, 2010 #7
    Assumption: mass creates space, right?

    If space were finite, then just by standing at the edge of space will create more space?
  9. Dec 20, 2010 #8


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    What evidence do you have of this?

    What evidence do you have of this?

  10. Dec 21, 2010 #9
    space is infinite, energy/matter has a maximum value
  11. Dec 21, 2010 #10
    Checking my assumptions:
    1. Matter create space?
    2. Matter expanse space?
    3. Matter needs space to exists?
  12. Dec 21, 2010 #11
    Space can create particles i.e. matter and energy seemingly from nothing? And the reverse?
  13. Dec 22, 2010 #12
    Answers to your assumptions:

    1) No, matter does not create space. Matter exists in space.
    2) No, matter does not expand space. At the earliest epoch of the Universe, there was no matter, per se, but rather pure radiation. The expansion of space is an intrinsic property, set by the initial conditions (Big Bang). Technically, the expansion of space would still occur even if there was absolutely no matter.
    3) As physical matter has spatial extension, then I guess yes, matter "needs" space within which to exist.
  14. Dec 22, 2010 #13
    Ah, thank you Deuterium2H
  15. Dec 24, 2010 #14
    How can we say that for sure? What in our observable universe would be different if the universe were 1.1 times larger then the observable universe versus 1,000,000,000,000,000 times larger?
  16. Dec 24, 2010 #15


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    Well, there are a number of possibilities that have to be considered. But let's just consider one where the universe wraps back on itself. If the universe wraps back on itself based upon its spatial curvature, then current measurements place the spatial curvature to be within 1% of zero, which makes the radius of curvature at least 10 times the Hubble radius, which is a factor of a few larger than the observable universe.

    If the universe is flat but still wraps back on itself, then this induces anisotropies, which we should be able to see in the CMB. But we don't. Thus if it does wrap back on itself, it would have to do that very, very far away (again, a factor of a few times the observable universe).
  17. Dec 28, 2010 #16
  18. Dec 28, 2010 #17
    You got that wrong, the paradox you mention was more to do with a static universe.
    Since our universe is all that there is (that is after all the definition of universe), it must then extend forever - yet it is still expanding!
  19. Dec 28, 2010 #18
    Last edited by a moderator: Apr 25, 2017
  20. Dec 28, 2010 #19


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    Olbers' Paradox takes two assumptions, and shows they cannot both be true:
    1. The universe is infinite (in time and space).
    2. The universe is static (no expansion).

    The discovery of the expansion of the universe demonstrates that the second assumption fails, which means that Olbers' Paradox cannot provide any additional information about the truth or falsity of the first.
  21. Dec 28, 2010 #20


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    A clarification appears to be in order. Olber hypothesized the universe cannot be both spatially and temporally infinite. We are fairly certain it is not temporally infinite. The jury is still out on the spatially infinite part. A temporally finite universe could be spatially infinite. My guess is it is not. My reasoning is an infinitely spatial universe would have observationally irregular 'edges'. I think this would be fairly obvious from WMAP data - and it is not. The alleged 'axis of evil' thing has been discredited due to selection effects - unsurprisingly.
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