I'm reasonably convinced that physicists know what happened in the universe at certain temperatures: just find out what happens when you reach those temperatures in a particle accelerator. I still have yet to come across the equation that measures the time before the CMB and what the universe's temperature was at that time. Most physicists agree that the CMB occurred 379,000 years AB (after the BB). I'm assuming they know this because of an equation. Well, what is it?