Science Forums: At what redshift does energy density of matter = energy density of radiation? - Science Forums

Jump to content

Welcome to ScienceForums.Net!

Welcome to ScienceForums.Net! We welcome science discussion at all levels — from beginners to researchers, covering topics from biology to computer science, and much more. Registration is fast and free, and allows you to post on the forums, so register now and join the discussions!
  
After you've registered, come in and introduce yourself, or visit the forum index. If you need any help  registering, posting, or if you just have some questions about our site, please feel free to contact us at staff at scienceforums dot net.

  • Start new topics and reply to others
  • Subscribe to topics and forums to get automatic updates
  • Create a ScienceForums.Net Blog!
Guest Message © 2012 DevFuse
Page 1 of 1
  • You cannot start a new topic
  • You cannot reply to this topic

At what redshift does energy density of matter = energy density of radiation? Rate Topic: -----

#1 15 March 2010 - 08:09 PM zeion 


Lepton
Hello. This is question for my course work, I was wondering if I could get some insight, here is the question:

Assume that the vast majority of the photons in the present Universe are cosmic microwave radiation photons that are a relic of the big bang. For simplicity, also assume that all the photons have the energy corresponding to the wavelength of the peak of a 2.73K black-body radiation curve. At Approximately what redshift will the energy density in radiation be equal to the energy density in matter?

(hint: work out the energy density in photons at the present time. Then work it out for baryons, assuming a proton for a typical baryon. Remember how the two quantities scale with redshift to work out when the energy density is the same.)

\rho_M \propto a^{-3}
\rho_\gamma \propto a^{-4}
T \propto a^{-1}
1 + z = \frac{v}{v_0} = \frac{\lambda_0}{\lambda} = \frac{a(t_0)}{a(t)}

How do I calculate the energy density of photons and protons at the present time? Do I use E = mc^2?
0

#2 15 March 2010 - 08:17 PM Radical Edward 


Primate
what are your variables there? It's been a while since I worked with that sort of stuff. lambda I guess is wavelength, v, velocity, t and to are time, rhos are density, but what about a and z?
The Ancient
0

#3 15 March 2010 - 08:36 PM zeion 


Lepton
a is the scale factor for the size of the universe where the a = 1 at the present time.. and 0.5 sometime in the past when it was half the size, etc.. and z is the redshift.
0

Share this topic:


Page 1 of 1
  • You cannot start a new topic
  • You cannot reply to this topic

1 User(s) are reading this topic
0 members, 1 guests, 0 anonymous users