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Universe radius 1,000 years ago?

  1. Jan 2, 2012 #1
    Hi everyone!

    Nowadays, our Universe has a 47 Billions light-year long radius,
    but the tricky thing is that its expansion rate is increasing, so
    could you tell me how its radius was a couple of thousand years ago?

    Thanks and happy new year!
  2. jcsd
  3. Jan 2, 2012 #2


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    On a billion lightyear scale it wouldn't make much difference.

    The current rate of expansion is about 1/140 of one percent per million years.

    So going back in time by 1000 years would make distances to most things shorter by roughly
    1/140000 of one percent. So it's "in the noise" or "in the roundoff". The socalled particle horizon which you mention as 47 billion LY and sometimes people say is 46 billion LY is not determined very precisely.

    You've probably thought about it and realize there are two effects. One is expansion of distance, which we were just considering, and the other is that the horizon would be 1000 lightyears closer back then even ignoring the effect of expansion. Even without expansion there would not have been as much time for light to reach us, back then.

    So there are two miniscule negligible effects (A) the light travel time would be 1000 years less and (B) the distances would be less by 1/140000 of one percent. They would both have made our particle horizon (radius of observable) slightly less 1000 years ago than it is today, but too little to worry about.
    Last edited: Jan 2, 2012
  4. Jan 14, 2012 #3
    We also have to keep in mind that such calculations are based on observations of the detectable universe. The unseen undetectable part has been hypothesized to be like the earth is to an atom in comparison to the detectable one on which we are basing calculations as if they apply to the whole. So for the sake of accuracy, it's better to use the modifier "detectable" when making these calculations or at least mention a horizon as the previous poster did.
    Last edited: Jan 14, 2012
  5. Jan 14, 2012 #4


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    You are right! I didn't emphasize that enough in my post---only got around to it towards the end:

    The 46 billion LY that I assumed Termina was talking about is only the radius of the portion that is currently observable. There is presumably much much more to the universe :biggrin: One doesn't know how much more.
    This radius of the observable universe is called the "particle horizon".

    The modifier "detectable" which you suggest would be a good one to use, I think. If anybody wants to read up about the various horizons and the basics of expansion cosmology, the CMB etc, I can't think of anything better than an old (2003) easy-math article by Charley Lineweaver
    You can get it simply by googling "lineweaver inflation"
    because inflation is the first word in the title. Radrook if you have an introductory online source you like, please give the link, especially if more recent than this Lineweaver "Inflation and the CMB".
    Lineweaver is a worldclass cosmologist who just happens to be able to write clearly at a basic level.
    Last edited: Jan 14, 2012
  6. Jan 15, 2012 #5
    Thanks for your replies!:biggrin:
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