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Calculating the size of the universe?

  1. Aug 12, 2011 #1
    Is my understaning correct that we don't know the size of the universe?

    Am I correct that it is not 26'ish billion light years across - it is larger than this?

    Can you show how to calculate the size of the universe?

  2. jcsd
  3. Aug 12, 2011 #2
    I have this same question. I think the map of the Universe told all of that (forgot the name of it), but I hadnt looked into it
  4. Aug 12, 2011 #3
    The amount of time it has taken for light to travel from the farthest distance we can see (the cosmic microwave background (CMB)) is ~13.7 billion years. If we assume the universe is of constant density in all directions (as is observed in the observable part of the universe for distance scales larger than a few hundred million light-years), and consider the CMB to have all been emitted at (approximately) the same time, the objects that formed from the areas of the CMB we see are currently ~48.8 billion light-years away. At the time the light was emitted from these objects they were ~44.7 million light-years away (ref. http://icosmos.co.uk/index.html" [Broken].
    Last edited by a moderator: May 5, 2017
  5. Aug 12, 2011 #4


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    Sparky, Isometric has answered your question for the OBSERVABLE universe. Is that what your question was about? You really need to be specific. If your question is about the universe beyond the observable universe, we don't know; by definition we can't observe it. Infinite is one of the more reasonable conjectures.
  6. Aug 15, 2011 #5
    Phinds –

    I understand we can see / measure back 13.7 billion light years.
    I am asking about is it possible to calculate how big the “entire universe” is – how big it is today –
    I was thinking one could use the expansion rate and say it has been expanding for 13 billion years and so on?

    Bottom line – I’m asking about the seen +unseen volume or radius.

  7. Aug 15, 2011 #6


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    IsometricPion has already answered your question as regards the observable universe (something like a 95 billion light year diameter) and beyond that, there is no answer. The unseen universe is beyond our observation and is subject only to various hypotheses. There certainly is no measure of its size and it's not at all clear that there ever will be.
  8. Aug 15, 2011 #7


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    There was a NASA report a few years back that gave a lower bound ASSUMING the entire thing was finite and analogous to the surface of a sphere (but 3D).

    An earlier poster pointed out that infinite volume is a reasonable assumption, and you can assume that and base your calculations on that. But then you don't get any finite answer.

    So what they did was present the data that they had collected in two different ways. Assuming infinite, and also assuming that spatially it would resemble the 2D surface of a sphere but would be 3D----it would be a hypersphere with a certain curvature and a certain circumference.

    And then they calculated in effect a LOWER BOUND on the size with 95% confidence. They found that with 95% confidence the circumference of this thing would have to be AT LEAST 650 billion light years.

    That is, if you could freeze the expansion process today (so the circumf would not change while you were measuring) and send a flash of light off in one direction, it would take at least 650 billion years for it to get back to you from the other direction, having made a tour.

    Something like 650. Maybe 630, I don't remember exactly.

    Keep in mind that this figure is derived from the data on observed curvature and it is ASSUMING that she is spatially finite----like the surface of a 2D balloon, where the inside and the outside of the balloon do not exist, all existence is on the 2D surface---and then think of the 3D analog of that.

    Basically creatures that live on a 3D hypersurface have a hard time telling if it is an infinite "flat" 3D or if it is just very very big and very slightly curved hyperspherical hypersurface.
    The measurement/estimation of largescale curvature is delicate and difficult. One methode is by counting galaxies at each distance and seeing how the number changes with distance.

    If there is slight positive curvature then the number you find at distance R will not grow quite as fast as R2. But if she is infinite it will grow like R2. the difference could be very slight.

    If you want a link to the technical report, just say. The paper was by Komatsu et al (many big names Spergel, Wright, ...) and this info was embedded in a table with a lot of other results of analyzing the 5-year data from the WMAP mission mapping the microwave background.
  9. Aug 15, 2011 #8


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    Google is great!
    I put in "Komatsu five year WMAP" and the first hit was:

    Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
  10. Aug 15, 2011 #9


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    We have a FAQ entry that calculates what you're asking for: https://www.physicsforums.com/showthread.php?t=506987 [Broken]
    Last edited by a moderator: May 5, 2017
  11. Aug 16, 2011 #10
    Yes the WMAP thank you

    ^ Ben, I will try and read that later when I have the time, thank you
  12. Aug 18, 2011 #11


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    Here's another paper that sets a lower bound on the size of the universe if it is closed-
    They say it's 21 particle horizons in volume, at least, according to WMAP data.

    Say 400 nonillion(4e32) cubic light-years, give or take.

    Considering how big a cubic light-year is, all I can say is... wow.
    Last edited: Aug 18, 2011
  13. Aug 18, 2011 #12
    Been away for a while traveling, but glad to be back.

    Would most Cosmologists prefer to believe that the universe wraps around so that light eventually returns to the place where it was emitted? The only alternative to this that I can think of is an edge of some kind or discontinuity or infinite space. Are there others?

    Does the finite age of the universe give us any clues as to the size of the whole universe? eg. It is hard to imagine infinite space when the time from the BB seems relatively finite.
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