Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Scalefactor calculations for special cases using the Friedmann Equation

  1. Mar 8, 2012 #1
    (i) Obtain the scale factor a(t) and redshift z when the energy density of matter and radiation were equal.
    (ii) Next use the a(t) relation for a matter-only universe to estimate the time of matter-radiation equality.
    (iii) Repeat (ii) but using the a(t) relation for a radiation-only universe. Which approximation, (ii) or (iii), is closer to the correct answer?

    Can someone help me?

  2. jcsd
  3. Mar 9, 2012 #2
    How far have you gotten? You should start by figuring out how dust and radiation energy densities depend on the scale factor. You can find this out using the conservation of energy-momentum, and the fact that radiation energy-momentum tensor is traceless.

    After that you just equate the expressions you got, and integrate Friedmann equations
  4. Mar 9, 2012 #3
    Ive understood the concept of how I'm supposed to do it. But, I am confused on how to start the radiation equation. As in what formula do I use to find the scale factor for a radiation dominated universe?
  5. Mar 9, 2012 #4
    The way I understand the question is that you should use the scaling laws to find the value of scale factor at matter-radiation equality as a function of their densities today.
  6. Mar 9, 2012 #5


    User Avatar
    Gold Member

    Just remember that the average energy/photon is proportional to 1/a. I'm guessing that you should assume zero curvature, that the density is always at critical, and that the kinetic energy of the matter can be neglected. You'll need to assume some value for the amount of matter per photon (about 10-35kg/photon).
  7. Mar 9, 2012 #6
    Oh that clears up part i for me. Thanks guys!! My answer for part i comes to a=2.8*10^-4
    Is that right?

    I am confused about the method we have been asked to use for part ii.
    Do i just plug in the value of a into the (da)^2 = Ho^2/a^2 equation and solve for 't' ?
    Am I missing something?

    Thanks Again.
  8. Mar 9, 2012 #7


    User Avatar
    Gold Member

    Also remember that the average energy per blackbody photon is ~2.7kT, which is a little higher than you might guess.
    Just to give a few clues, with radiation only, you'll find that a [itex]\propto[/itex] t1/2 whereas with matter only, a [itex]\propto[/itex] t2/3. Then [itex]\rho[/itex] [itex]\propto[/itex] k/tn where n is a certain positive integer that I won't disclose (same integer for radiation and matter), and k is a constant, although kradiation is slightly larger than kmatter.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook