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Trying to find out how exactly we find out how big the visible universe is.

  1. Dec 19, 2009 #1
    Okay, can someone please tell me how you find how far light has traveled since the big bang. I know the answer is 13.2 billion year, and I know you can get it by using

    c / H = answer in parsecs

    Then I convert parsecs to light years and I can get the time and distance from there. But, how do you get the speed of light divided by Hubbles constant. It doesn't make sense to me.
  2. jcsd
  3. Dec 19, 2009 #2


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    What do you mean, "how do you get"? Do you mean you want a derivation?
  4. Dec 19, 2009 #3


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    You can do the division okay, as long as units are consistent; but what you get is not strictly how far light has traveled.

    It gives you the proper distance now to galaxies with a recession velocity in proper distance co-ordinates of c, which is a different thing.

    c is 300,000 km/s and H is about 74 km/s/Mparsec. The division will give you a distance in MegaParsecs of 4051, which is indeed about 13.2 billion light years.

    That this number is so close to the age of the universe is something of a co-incidence, associated with the fact that in the current epoch we have similar order of magnitudes for matter density and dark energy density (following the ΛCDM model).

    Cheers -- sylas
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