Astronomy coursework question

  • Thread starter zeion
  • Start date
  • #1
467
0

Homework Statement



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.)

Homework Equations




[tex] \rho_M \propto a^{-3}[/tex]



[tex] \rho_\gamma \propto a^{-4}[/tex]


[tex]
T \propto a^{-1}
[/tex]

[tex]
1 + z = \frac{v}{v_0} = \frac{\lambda_0}{\lambda} = \frac{a(t_0)}{a(t)}
[/tex]

The Attempt at a Solution



Not sure where to start.. how do I work out the energy density for photons and protons at the present time? Do I use E = mc^2?
 

Answers and Replies

Related Threads on Astronomy coursework question

  • Last Post
Replies
2
Views
2K
Replies
3
Views
4K
Replies
0
Views
1K
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
10
Views
3K
  • Last Post
Replies
0
Views
903
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
2
Views
7K
  • Last Post
Replies
1
Views
1K
Top