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I Deriving the second Friedman equation from the first.

  1. Aug 3, 2015 #1

    My professors asked me why we don't usually consider the second Friedman equation. He told me that the solutions for the first always give a solution for the second. He told me that the first equation along with it's time derivative can imply the second equation. I have been trying the algebra for a long time now, but I am not being able to get ahead.

    I wanted to know if the statement that the professor made is true in the first place.
  2. jcsd
  3. Aug 3, 2015 #2


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    You cannot just take the time derivative. You will also need to apply the continuity equation (i.e., the fact that the stress-energy tensor is divergence free).
  4. Mar 5, 2016 #3
    Thank you, that helped!
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