** under construction **
We should have a graphic here with time on the top axis and scale factor on the bottom axis, and mass density on the y axis. Label epochs such as double Compton freeze-out, e-/e+ annihilation, quark-hadron phase transition, weak interaction freeze-out, BBN, matter-radiation equality, and recombination. Maybe we stop a little ways after recombination.
We could have three such graphics, one with speculative very early-universe stuff (starting with inflation and reheating), one from quark-hadron phase transition to recombination, and one from first stars and quasars to present day.