1. The problem statement, all variables and given/known data By extrapolating the Fricdmann equations back in time, they discovered that the energy density of this radiation field must have initially considerably exceeded that of matter (as defined by E = mc^2). Radiation density is the amount of energy in a given volume of space, and it can be expressed as the temperature of the black body emitting the same energy. The observed 75 25 per cent hydrogen/helium ratio* implied that the temperature of the radiation must have been 10^9 K. Electromagnetic radiation readily interacts with electrically charged protons and electrons, so while the fireball was dominated by the radiation field the mean distance travelled by a photon was minuscule. As the Universe expanded, the radiation field would have cooled*. After some 300,000 years, its temperature would have fallen to 10^3 K, cool enough for electrons to associate with protons to produce neutral hydrogen atoms. 2. Relevant equations 3. The attempt at a solution I would like to do this calculation. I don't even know where to begin since there are quite a lot of Friedmann equations out there. Which Friedman equation do I use and how do I use it?