So what’s the correct temperature of the universe?
0° K............................
Excellent question!
Scientific answer: whatever the temperature is at the time that we measure it.
You CAN'T go back...you must go ahead with what you have for the PRESENT population.
It’s not about the universe having a correct temperature, it’s about the fact that the universe is expanding, while supposedly it’s a closed system with no energy input coming in from the outside.
In that situation, as the universe expands, the temperature should be steadily dropping, since the same fixed amount of energy is being spread thinner and thinner. Instead, they are observing the opposite happening. Either that means the universe has not been expanding this whole time, or there is some other source of energy input, or some other variable we are not accounting for.