1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Recasting integration variable

  1. Jun 2, 2009 #1
    1. The problem statement, all variables and given/known data

    Recast the integration variable of the following integral to da

    [tex]\tau[/tex]=[tex]\int[/tex]k dr

    k is a positive constant

    2. Relevant equations

    dr= c*dt/a

    3. The attempt at a solution

    I have no idea, I'm sure it's something really simple, but I really just can't think.

    btw, this is part of a bigger problem, that I know how to solve, but I've forgotten how to do this.

    Last edited: Jun 2, 2009
  2. jcsd
  3. Jun 3, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi maddav! Welcome to PF! :wink:
    Do you mean dt rather than da? :confused:

    If not, can you tell us the bigger problem, so we can see what it all means? :smile:
  4. Jun 3, 2009 #3


    User Avatar
    Science Advisor

    If, as tiny tim suggested, you meant "recast to dt", then just replace the "dr" in the integral with "cdt/a" which you say is equal to it.
  5. Jun 3, 2009 #4
    Yeah, the problem was less mathematical than i'd assumed, it's a cosmology question, so it's fairly simple.

    I initially replaced it with dt (prior to it being suggested here), but of course, i'm told that universe is entirely matter and cosmological constant, so I can just rewrite dt as da using the friedmann equation.

    Thanks anyway guys!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook