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Age of universe vs size

  1. Dec 6, 2014 #1
    Please help me understand this.

    The age of the universe is given as being about 13.8 billion years. The size of the universe is estimated to be 93 billion light years
    ( https://en.m.wikipedia.org/wiki/Universe)

    Since the oldest light to reach us comes from 13.8 light years away, why do we estimate that the universe has a radius of 46+ billion light years instead of 13.8 ?

    I know that the universe expanded faster than the speed of light but why do we think that and why 93 billion light years?

    Thank you.
  2. jcsd
  3. Dec 6, 2014 #2

    Doug Huffman

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  4. Dec 6, 2014 #3
    First of all, it is important to emphasize this is the size of the observable universe, the part of the universe we can receive light from.

    Second, the radius is larger than 13.8 light years since the universe expands during the journey the photons make to reach us. So, the point where a photon that reaches us today started from can be 46+ billion light years away since it was much closer to us when the journey started.
  5. Dec 6, 2014 #4


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    If the universe were static, that is not expanding, then you could indeed do the simple calculation of the speed of light times the age of the universe to find out where the farthest objects you can see were at the time of emission (and still are).
    However, since the space in-between the source and the observer has been expanding, such calculations won't give you the right answer. The light has to travel the extra bits of space, so it takes it more time to reach us than it would in a static universe.

    In the expanding universe the light coming from faraway objects that you see was emitted closer than c*t, and the objects are at this very moment farther than c*t. The 46 billion ly figure is the distance to the farthest objects we see now, if you could stop the expansion and measure the distance as it is NOW. The distance to those objects at the time of emission was much closer - about 42 million ly.

    @Doug Huffman : I think inflation doesn't need to be invoked here, the confusion stems purely from not taking into account the regular expansion.
  6. Dec 6, 2014 #5
    Thank you all.
    I forgot that the universe is expanding further during the 13.8 billion years it takes for the oldest light to reach us.
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