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We are told that

1) CMB provides about 10^9 photons/baryons.

2) For matter dominated era, assume flat university with H_o = 73km/s/Mpc

3) Stefan-Boltzman law for energy density: ρ_rad = αT^4 where α = 7.56x10^-15 ergs / cm^3 K^4

I read Marcus' explaination on

https://www.physicsforums.com/showthread.php?t=163941

but it is really difficult for me to understand as I am taking a 1st year AST course.

Any help will be appreciated, thx.