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The size of the universe

  1. Apr 17, 2008 #1
    I was wondering how can the redshift tell you the size of the universe when there is a cosmological horizon where scientist can't see since light hasn't travelled here yet.
  2. jcsd
  3. Apr 17, 2008 #2
    By "universe", most of the time we actually mean the "observable universe".
  4. Apr 17, 2008 #3
    oh ok thanx
  5. Apr 17, 2008 #4


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    Who is the "we" that you are talking about. It is not scientists, in my experience. The astronomers I know are very careful to say observable universe when that is what they mean.

    And when they say universe, they do NOT mean only the observable part.

    Yenchin, I suspect you do not know what you are talking about. So I would ask you to find a scientific paper in some professional journal where the author agrees with you and says universe when what he actually means is the observable universe.
  6. Apr 17, 2008 #5


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    Cosmologists can only ESTIMATE the size of the universe. What you point out is perfectly right. They don't try to say what the size is with 100 percent certainty.
    There were a couple of papers by David Spergel and Niel Cornish that said it must be AT LEAST so-and-so big, based on WMAP measurements of the Cosmic Microwave Background. And that allows for the possibility that it is infinite in size.

    I'd be glad if you would reformulate your question, Hobo. What is it you want to know?
    There are a few professional estimates of the size, and they involve a range of uncertainty and allow for it being infinite. Do you want to know how those estimates are derived?

    Or do you want to know how they come up with the common figure of 46 or 47 billion lightyears, for the particle horizon (the farthest away things are that we can see, in the sense of getting light from them today.) Do you want to know how they get the figure of 46 or 47?

    That is the current radius of the part of universe that we are currently observing. The whole is naturally bigger---and the way they estimate its size is essentially by measuring the curvature of the part we can see. Assuming uniform average positive curvature, the flatter the larger-----and the curveder the smaller.

    So what do you want to know about? The size of the part we can see, or the estimates of the size of the (possibly infinite) whole?
  7. Apr 18, 2008 #6


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    That is an interesting point, marcus. I was under the impression cosmologists were referring to the observable universe unless otherwise stated.
  8. Apr 18, 2008 #7
    Marcus: By "we" I simply mean in the popular treatment of the subject where often the use of universe and observable universe is not explicitly spelled out and requires the readers to judge which is which. I believe the OP was confused by such a vague statement from the popular treatment, and thus the "we" was referring to the sloppy usage that he or she may have encountered somewhere. I certainly am not implying cosmologists are sloppy!

    I have also made the implicit assumption that the OP was asking about the size of the current observable radius, and not the whole universe (which might well be infinite though need not be), as most popular accounts and expository articles for lay readers concern with the former rather than the later.

    Should have been more careful and explicit, but I was rushing off for lecture this morning when I reply.
  9. Apr 18, 2008 #8


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    Yenchin, I totally agree... about the popular treatment. I think you are right about the OP, who may well have been confused by loose talk in the popular media.

    Here at PF Cosmology forum this confusion generated by vagueness in the media comes up repeatedly. If we ever have a FAQ, or a sticky thread for basic facts for newcomers, this distinction between the whole and the observed part should probably be fact number one.

    And then we'd need to define the various horizons/distance scales, like

    Hubble distance about 14 Gly
    Event horizon radius about 16 Gly
    Distance to CMB origin 45 Gly
    Particle horizon 46 Gly

    And list various redshifts, and the estimated distances they correspond to in the standard model.

    Or maybe that is too much to put in a "basic facts" thread, but at least the distinction between whole and observed part would be made explicit.
    Yenchin, you are welcome to try spelling these things out. I think we eventually will get a sticky thread and it will just be a question of who has the time to write the content and edit for brevity.
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