Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Age and size of the universe

  1. Mar 14, 2009 #1
    Hi

    New on here and wanted to show my stupidity by asking a question that is perplexing me and hoped you could help

    Maybe someone could point out that some of my facts are wrong but here goes

    The universe is supposed to be 76 billion light years across

    The universe is supposed to be 13.7 billion years old

    Nothing can travel faster than light and a light year is the distance that light travels in one year

    The universe is expanding (well actually accelarating)

    What of those numbers or my understanding because is wrong because even if the universe expanded at the speed of light (or close to) it could only be 27.4 billion light years across. Thats 13.7 billion light years away from the point of the big bang at all points?

    dont know any astrophysicists so thought i would ask here

    Thanks
     
  2. jcsd
  3. Mar 14, 2009 #2
    Hey darkred

    No one knows what the size of the universe is. Some say it's infinite, some say it has boundaries, some say it's shaped like a dounut, some say it's an endless loop,some have no clue.

    Yes, the universe is estimated to be about 13.7 billion years old.

    Yes, a lightyear is the distance light travels in one year. Exactly 9,460,730,472,580.8 kilometers. Nothing yet known to man can travel faster than light.

    Yes, the universe is expanding and getting faster, due to dark energy.
     
  4. Mar 14, 2009 #3
    In general relativity, the expansion of space between two well-separated points isn't limited by the speed of light. Also, I think you're confusing "the size of the universe" with "the size of the visible-universe as extrapolated to now".

    The standard analogy is to think of ants crawling along the surface of a balloon, as the balloon is being inflated. By the time an ant reaches the observer, the space between the observer and the ant's starting point has stretched so much that, even if the expansion were to freeze where it is now, it would still take a much longer time for the ant to travel back again. (Similarly we are just now observing light that was emitted 13 billion years ago by something that by now must be 70 billion light years apart from us. Also, the reason we can't see anything that would be further than that by now, say the actual edge of the universe, is that the universe was opaque to light from its beginning until about 13 billion years ago.)
     
  5. Mar 19, 2009 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    ant-matter universe

    To continue cesiumfrog's :smile: analogy …

    when one ant is going past another (at touching distance), it cannot do so at more than the speed of light, but two "stationary" ants a long way away from each other may appear to be going faster than light simply beacuse the balloon is expanding :wink:
     
  6. Oct 30, 2009 #5
    I too am confused by this, and I don't think the answers provided here have really helped. Ok, so the universe is expanding at an accelerating rate, which is approximately now 74.2 km/sec. If it is indeed accelerating, then we know that previously, it was expanding more slowly. But let's assume that it has been expanding at nearly this rate for a long time. Now if you take its current rate of expansion and compare that to the distance of a light year in km, 9.4*10^12, it seems that this expansion is trivial in comparison. (If you extrapolate that back 13 billion years, then it looks like the universe has grown scarcely a trillion km in radius. That's practically nothing compared to the distance light travels in a single year.) So how does the expanding (accelerating) universe get to 76 billion light years across in only 13 billion years, when it is expanding at what amounts to an insignificant fraction of the speed of light?
     
  7. Oct 30, 2009 #6

    Ich

    User Avatar
    Science Advisor

    Maybe you want to look up this number again, and then ponder this "/MPc" thing. Then Hubble's Law and so on. Really, it would help you asking the right questions and help us giving the right answers.
     
  8. Oct 30, 2009 #7
    The misunderstanding is thinking the Hubble constant means for the WHOLE universe's rate of expansion. Not so. It's the rate at which the local region 1 megaparsec in size is seen to be expanding and increases by that 74.2 km/s for every megaparsec away from you, the observer, that an observed region of the Universe lies. In not too many megaparsecs the Universe is expanding faster than light - Special Relativity's speed-limit applies everywhere locally, but on cosmic scales can be surpassed because space-time itself is expanding.

    That also means that when light has travelled for billions of years the underlying expansion of the Universe has to be taken into account when working out how far away something lies presently. Thus the observable edge of the Universe is further away than the speed of light travel time implies.
     
  9. Oct 30, 2009 #8
    Now that is a great explanation we lay people can understand! (Or at least begin to fathom.) Thank you qraal!

    BTW - Ich. Please don't take offense to the tone of my question. We lay folk understand that we're asking questions way beyond the limits of our day to day sensibilities. I truly appreciate that you have even taken the time to try to point me in the right direction.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook