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

Age vs. size of universe

  1. Nov 25, 2015 #1
    I'm trying to reconcile the age and size of the universe. The size of the universe is more than twice the distance light can travel in the time it has existed. Does this mean that the universe is expanding faster than the speed of light or that it did initially and has slowed down since? I have read that the universe is actually accelerating due to dark energy or some other unknown effect so I guess it isn't slowing down. Does this come down to relativity and the 14.7B years is in a certain frame of reference?
  2. jcsd
  3. Nov 25, 2015 #2


    User Avatar
    Gold Member

    The light that we see today from the edge of the observable universe was MUCH closer when it was emitted (I forget the numbers but it was 100,000s of millions of light years or less as I recall). After it was emitted, it was carried away by the expansion of the universe, making progress towards us but very slowly even though locally it always travels at c. Where the emitting objects are "now" is about 47 billion light years away and are receding from us at about 3c.

    I put "now" in quotes because it's not that simple.
  4. Nov 25, 2015 #3


    Staff: Mentor

    Not really; the expansion doesn't have a "speed", and the rate of increase of the scale factor, which is what pop science treatments usually mean when they talk about the "speed of expansion", is not a "speed" in the ordinary sense and is not limited to the speed of light. In the early universe, the rate of increase of the scale factor was in fact much faster than ##c##.

    You might want to check out this fairly recent thread on the various possible meanings of "rate of expansion":

  5. Nov 26, 2015 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  6. Nov 27, 2015 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Hi Myric, welcome.
    A light year is the distance light can travel in one year without the help of distance-expansion.

    The pattern of expansion shouldn't be confused with ordinary motion because nobody gets anywhere by it, no body approaches a goal, everybody just becomes farther apart by some percentage growth rate. And the expansion of any given distance is not limited by any particular speed, like c. It could be more it could be less--it's a percentage change like the interest rate in a bank account. the rate changes over time according to an equation that is basically Einstein's gen. rel. equation of 1915. GR is something to accept and get used to

    It's not so surprising that light could travel 47 billion light years in a period of time that is just 13.8 billion years, if you allow for expansion
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook