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Astronomy - Distance with different scale factors

  1. Jul 8, 2011 #1
    The problem statement, all variables and given/known data
    Expansion of the universe is described by the scale factor R(t), where t is the time after the Big Bang. For a flat universe the scale factor is today
    [itex]R(t)=C_{1}\cdot t^{\frac{2}{3}}[/itex]

    When the Universe was radiation dominated, for t <200,000 years, the scale factor was
    [itex]R(t)=C_{2}\cdot \sqrt{t}[/itex]

    Today, about 12 billion years after the Big Bang, we measure the distance to another superhop to 1 billion light years. How big would the distance between these sites one year after the Big Bang, if we assume that we live in a flat universe.

    The attempt at a solution
    I express the distance in meters
    [itex]R(t)=9.461\cdot 10^{24}\, m[/itex]

    Then I express the time in seconds
    [itex]t=3.787\cdot 10^{17}\, s[/itex]

    I found that the constant is
    [itex]C_{1}=1.8075\cdot 10^{13}[/itex]

    I need to know the constant [itex]C_{2}[/itex], but I'm not sure how I'll be able to calculate it.

    I have tested just to put [itex]C_{1}=C_{2}[/itex], but the result for the distance was wrong. I then realized that the two constants have different units.

    Does anyone have a suggestion how I may proceed?
     
  2. jcsd
  3. Jul 10, 2011 #2

    Redbelly98

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    For t = 200,000 years, both expressions for R(t) should give the same result.

    To make your life easier, consider using units of years and light-years rather than seconds and meters.
     
  4. Jul 10, 2011 #3
    Thanks for your reply, now I was able to solve the problem.

    Thanks for your advice, I'll keep it in mind.
     
  5. Jul 10, 2011 #4

    Redbelly98

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    Glad it worked out.

    I know we get SI units drilled into our heads in intro physics classes, but they are not always the most convenient. If the problem statement uses alternate units, that's a hint for you to consider it too. :smile:
     
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