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Calculating universe age vs. size of universe

  1. Jul 28, 2010 #1

    I was pondering the age of the universe and reading about how it was calculated, but then I began questioning something.

    Two scenarios, BIG IF. (I know, it's a big IF. I'm sure I'll get a lot of comments about how it's not possible, or improbable. I get it. but...) IF:

    -We determine the exact size of the universe in both of these scenarios-

    1. The universe is "almost exactly" the size of the observable universe. (yes I know that's qualitative not quantitative, but you know what I mean)

    2. The universe is "approximately" 100 times the size of the observable universe. (see scenario 1 comment)

    Ok, so this begs the question: How do these two scenarios differ in the universe's age FROM 13.73 billion years?

    Would this alter the hubble constant possibly?

  2. jcsd
  3. Jul 28, 2010 #2


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    They don't.
    No. There's a theorem (Birkhoff) that sperical shells of matter (the universe outside the observable part) have no gravitational influence whatsoever on what's inside.
    So the evolution is the same whether the universe is as big as a cherry stone or infinite.
  4. Jul 28, 2010 #3
    Thanks for the response. It was really bothering me how they could determine the age without knowing the at least approximate true size.

    Can I ask while we're on the topic, what assumptions are being made in order to determine the age?
  5. Jul 28, 2010 #4


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    There has been some recent observations which suggest that there may be a large source of gravitational attraction outside the observable universe. It appears that a large group of galactic clusters are traveling in some particular direction, over and above the expected expansion.
  6. Jul 29, 2010 #5


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    The assumptions are:
    - General Relativity is an accurate enough description of gravity, space and time
    - The (observable) universe is homogeneous
    - The universe as we see it is isotropic

    The last assumption is more or less experimental fact (the alleged dark flow would be a perturbation, if it exists).
    The second assumption has a reasonable basis in observations, and is quite natural given the observed isotropy.
    The first assumption seems also safe, given the succes of GR in all tests done so far.

    All of these assumptions are being questioned all the time. However, as of now, there is no unambiguous evidence against them, and there have been no really viable alternatives proposed.

    With the assumption, you're constrained to the Friedman models of the universe. The current LCDM model is the one best fitting the observations.
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