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Homework Help: The radius of the universe

  1. Sep 24, 2004 #1
    It is believed that the universe began with a big bang about 19 billion years ago. Neglecting the expansion of the universe, we can reason that the most distant objects obersrvable are no further than the distant light could have traveled in 1.9 x 10^10 yr.

    If the speed of light is 2.99792 x 10^8 m/s, what is the maximum possible radius of the observable universe? Answer in units of m.

    Do I just need to multiply these two numbers??
  2. jcsd
  3. Sep 24, 2004 #2
    If you just multiply those two numbers will your units work out?
  4. Sep 25, 2004 #3
    I'm not really sure... does anyone know how to do this problem???
  5. Sep 25, 2004 #4


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    You know how to do the problem. Speed times time is distance but you aren't just multiplying numbers. If you simply multiply the speed and time as given your answer will be in units of meter-years per second. While technically correct it isn't particularly meaningful. Faust was properly suggesting that you use like units in your calculation so that you end up with simply meters or kilometers.
  6. Sep 25, 2004 #5
    I realize the units won't work if i just mulitply them together, but I'm not sure how to find the maximum possible radius of this question
  7. Sep 25, 2004 #6


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    That would give you the largest possible radius because nothing can travel faster than the speed of light - those photons that left early in the formation of the Universe will be further out than any matter.
  8. Sep 25, 2004 #7
    1 year = 365 1/4 days

    1 day = 24 x 60 x 60 = 86400 seconds

    1 year = 31557600 seconds.

    Use this to convert m/s to m/year
  9. Sep 25, 2004 #8
    you may want to chose a second more conservative age for the universe to go along with the 19 billion figure because it is still a debate. the oldest I have heard is 15 billion.
  10. Sep 25, 2004 #9
    you need to convert the seconds into years or the years into seconds.
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