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I Size of the Universe?

  1. May 9, 2016 #1
    Hi all,
    I've just been having an interesting pub conversation with (what I would call) an amateur cosmologist and I posed the following question "the observable universe is stated as being c 50 billion light years wide but (even putting aside issues such as expansion / contraction) if nothing can travel faster than the speed of light and the age of the universe is c 13.7bn years old how is this possible"? Between us we were stumped but I'd really appreciate any explanations of this.

    Cheers, Pete.
  2. jcsd
  3. May 9, 2016 #2


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    The speed of light limit is only a local limit. That is, nothing can outrun a light ray.

    There is no well-defined notion of velocity for far-away objects at all, so there can't really be any speed limit.

    With regard to the fact that we can see matter that is currently nearly 50 billion light years away, that matter was much closer when the light we see was emitted. For example, the furthest matter that we can see is the matter that emitted the CMB. The CMB was just a few hundred thousand years after the "Big Bang", and the part of the universe that emitted the CMB we observe today is about 46 billion light years away, but it was only about 42 million light years away when it emitted that light. It's taken 13 billion years for the light to cross that distance because the expansion was so fast back then.
  4. May 9, 2016 #3
    Thanks Chalnoth, that's really helpful. A bit of a follow up though. If I had a (really long) ruler I could measure the observable universe as being 46 bn light years wide. As I understand it, and also from your response, we are measuring matter from the CMB here. I could understand if the observable universe were 13.7 bn light years wide. I could also understand if it were a "bit" more due to expansion in the early moments after the Big Bang but the differential between 46 bn ly and 13 bn ly is quite extreme. Is that all down to early expansion?

    Cheers, Pete.
  5. May 9, 2016 #4


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    Not really all that extreme. The matter that emitted the light was only about 42 million light years away when it emitted the light. It took over 13 billion years to cross a distance that was originally only about 42 million light years away because the expansion at the time was some 8,000 times as fast as the expansion rate today.
  6. May 9, 2016 #5
    Fabulous. Thanks very much for your helpful explanation.
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