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Size of universe?

  1. Oct 14, 2007 #1
    If the universe is 13.7 Billion years old, how could the univere have a radius any longer than 13.7 Billion light years?
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  3. Oct 14, 2007 #2


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    While SR prevents anything from travelling through space at or faster than c, it has nothing to say about how fast space itself can expand. And space is expanding, which means the galaxies on average, are getting father from each other every day the universe ages. The farthest are moving away from us at speeds faster than light. These galaxies are beyond the boundary of the observable universe, and are, in every useful way, cut off from us forever.
    Last edited: Oct 14, 2007
  4. Oct 14, 2007 #3
    Excellent explanation, thanks.

    So there are galaxies even farther away than those in the Hubble Ultra Deep Feild?
  5. Oct 14, 2007 #4


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  6. Oct 14, 2007 #5


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    That is not quite right, Dave.
    Today the Hubble radius is about 13.8 billion LY. The recession speed at that distance is c---this defines the Hubble radius.
    The recession speed at, say 14 or 15 billion LY is faster than c.

    However someone in a galaxy which is currently 15 billion LY distant could, today, flash a signal to us which we would eventually receive. This is according to the Lambda CDM model, which is standard.

    So SOME galaxies which are at this moment receding faster than c are, in fact, NOT "cut off from us forever."
    In that respect your (otherwise excellent) statement is wrong.


    It is important to understand that we are currently receiving light from galaxies which were receding from us at several times the speed of light WHEN THEY EMITTED THE LIGHT WE ARE RECEIVING. If you don't already know this or don't understand how it can have happened, it would probably be a good idea to ask questions about it. It's been discussed several times here at Cosmology forum and there should be someone around who can explain or give you a link.

    The analogous thing still applies to a limited extent today. A galaxy receding at greater than c can emit light which will eventually reach us, by the same mechanism. But only if it is not too far away---if I remember right the limit (the Cosmological Event Horizon) is estimated to be about 16 billion LY. That is why, in my example, I said 14 or 15.
    Last edited: Oct 14, 2007
  7. Oct 17, 2007 #6
    Correct me if I'm wrong but what you are dealing with is the boundaries of the universe populated with planets. Technically, wouldn't it be safe to say that the universe with both planets and not with planets would end up being an infinite number of light years?
  8. Oct 17, 2007 #7


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    Unfortunately we don't know if U is spatial infinite or spatial finite.

    It can be mathematically modeled very successfully either way, using two different versions of the standard model called LambdaCDM that cosmologists normally use.

    Either way the fit is good. Cosmologist Ned Wright, in a january 2007 paper, indicated that the fit was just a teeny bit better using the finite version, but that the difference was not significant.

    In the finite version, space looks like the mathematical object called "Ess-three" which is written S3 and is analogous to the 2D surface of a ball except that it is 3D instead of 2D.
    In the infinite version, space looks pretty much like ordinary 3D euclidean space you learn in school, often called "Arr-three" and written R3

    Both S3 and R3 look pretty much the same---hard for a local observer to tell the difference.

    Just like analogously the surface of a very large ball looks locally nearly indistinguishable from an infinite 2D plane.

    LambdaCDM means "lambda cold dark matter"---it is just what they call their standard model universe that they fit to the data (galaxy counts, supernovae, cosmic microwave background temperature maps, etc.)

    Either version of LambdaCDM, finite or infinite, we only get to see a finite patch of it anyway---not a lot of practical difference. Both versions continue expanding forever.
    If you want links to websites, ask. If you want more explanation here, keep asking.
  9. Oct 17, 2007 #8
    That's interesting... and a little over my head. I'm just now a high school freshman so I really haven't expaned this far.

    I do understand things like dark matter, finite, and infinite so I could get the jist of what you said. Could you post links to some sites?
  10. Oct 17, 2007 #9


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    I have to go for now, but somebody else will get some links or i will tomorrow.
    BTW I didnt get what you said about planets. In the standard model the stars and galaxies are spread ALL OVER SPACE more or less even.
    There is no one place where you get a lot of stars and planets and galaxies and stuff---with the rest of it being empty. That is not the picture.
    Whether space is infinite or finite it is roughly uniformly filled with stuff, in the normal picture that the professionals use.

    There is a professional cosmologist named SpaceTiger who hangs out here some, and there may be others too. You should be all right. It may just take a little time
  11. Oct 17, 2007 #10
    Pretty much what I meant about the planets is is that if you could look at the planets, stars, and galaxies that make up the farthest reaches of the universe, everything from those planets, stars, and galaxies in I considared the boundary of the universe with stuff in it. The other areas outside the farthest reaches would be infinite, empty space. That, however, is assuming that the universe is, in fact, infinite and not finite.
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