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

I Estimate the number of CMB photons in 0.25-litre

  1. Jan 5, 2017 #1
    i know that the total energy density of a blackbody radiation is

    E= 4 segma/C * T^4

    so how i can use this to find the number of the CMB photons in 0.25 litre?
  2. jcsd
  3. Jan 5, 2017 #2


    User Avatar
    Science Advisor

  4. Jan 5, 2017 #3
    I recommend a textbook by Barbara Ryden https://www.amazon.com/Introduction-Cosmology-Barbara-Ryden/dp/1107154839/. In section "2.5 Cosmic Microwave Background" she makes calculations you are asking about and comes to a conclusion that "...there are about 411 CMB photons per cubic centimeter of the universe at the present day."
    Last edited by a moderator: May 8, 2017
  5. Jan 5, 2017 #4
    many thanks but i dont have the book
    Last edited by a moderator: May 8, 2017
  6. Jan 5, 2017 #5
    Thank you, but i how i can get the frequency and then how i can find the number of the photons? just i am confused
  7. Jan 5, 2017 #6


    User Avatar
    Science Advisor

    I'm not sure what level the question is being asked. The photon gas wiki page has the number of photons per volume as a function of temperature directly. You could just use that formula and get the answer straight away.

    If instead you're supposed to find out how to derive the number density, you'd have to derive it by integrating over the distribution of photons in a black body using Planck's Law.
  8. Jan 19, 2017 #7
    Use E=mc^2 n another relation in which uses T of this relation~

    N with plank relation E=hv, v=1/T n try..
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted