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

Calculating Energy density in matter & radiation

  1. Mar 15, 2008 #1
    Hi guys, I'm in a pickle here with an assignment, hope someone can help...greatly appreciated in advance.

    I'm required to find the redshift at which the energy density in radiation equals the energy density in matter.
    The following are the givens we were...well, given:
    1) assume 1 baryon = 10^9 photon
    2) assume all photons have energy to the wavelength of the peak of a 2.73K black-body radiation curve
    3) scale factor (a) of matter: Pm = a^-3
    4) scale factor of radiation: Pr = a^-4
    5) we are to work out the energy density of a photon & baryon in present time and then apply the scale factor

    Apparently this isn't using complicated formulas, just simple E=mc^2 & E=h*nu...but being an economic student, you can say I'm pretty lost. I'm sure it's something simple that I'm just not seeing.

    Please help, any would be good.

    Thanks in advance
  2. jcsd
  3. Mar 15, 2008 #2
    oops, posted at the wrong section...please remove, i'll repost.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook