Here's another thing: I remember learning that all the nice usable energy we get from the sun is due to the fact that the big bang produced lots of hydrogen, bu

Here's another thing: I remember learning that all the nice usable energy we get from the sun is due to the fact that the big bang produced lots of hydrogen, but fusing four hydrogens into a helium yields a bunch of energy. (And you can still get some energy out by fusing further, all the way down to iron.) So if matter and radiation were at equilibrium at the big bang like Baez says, why wasn't more helium (and heavier elements down to iron) formed, leaving nothing for the stars to burn? I think the answer has to be that the temperature was incredibly high at the time of the big bang: At really high temperatures, it's more favourable for nucleons to be flying around separate from each other as protons. At lower temperatures, it starts being better for them to be things like helium and carbon, and at lower temperatures still, iron. So if the universe's temperature had lowered very very gradually, we would be stuck with nothing but iron at this point, leaving nothing for the stars to burn. So it seems we owe our sunlight, in some sense, to the fact that the universe cooled fairly quickly?