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Age of universe

  1. Apr 3, 2007 #1
    Hi i am confused as to how to calculate the age of the universe with redshift
    say for example

    The age of the universe now is 13.4 billion years old (and a critical universe).
    How do i find the age of the universe if it was a redshift at say 0.6??

    Do i have to find the scale factor first?
    I am not very sure, please help!!
     
  2. jcsd
  3. Apr 3, 2007 #2

    Dick

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    You need to know the ratio of the scale factors. z=(a0/a)-1 where a is the scale factor then and a0 is the scale factor now.
     
  4. Apr 3, 2007 #3
    how do i get that? do i have to integrate first
     
  5. Apr 3, 2007 #4

    Dick

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    No. To a good approximation the universe has been matter dominated all of it's history. What does a(t) look like as a function of t in this case?
     
  6. Apr 3, 2007 #5
    so how would that work then?
     
  7. Apr 3, 2007 #6

    Dick

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    What does a(t) look like as a function of t in a matter dominated universe? You can look it up, it's a power law.
     
  8. Apr 3, 2007 #7
    to the power of 4
     
  9. Apr 3, 2007 #8

    Dick

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    Nope. Use the Friedmann equation. H^2 is proportional to rho. If a(t)=t^n. H is proportional to 1/t. rho is proportional to 1/a(t)^3. So 1/t^2 is proportional to 1/a(t)^3. So a(t) is proportional to t^(2/3).
     
  10. Apr 3, 2007 #9
    I'm sorry I'm not an expert on this I dont usually take science courses... but are u saying I'm gna take a(t)= k * t^(2/3) . and that gives me the answer?
     
  11. Apr 4, 2007 #10

    Dick

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    Yes. Since z=(a0/a)-1, this gives z=(t0/t)^(2/3)-1. (t0 is the age of the universe and t is time at redshift z. It's an approximation but it should do. You are asking some pretty hard questions for a non-science course person. What's the course?
     
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