How large was the universe at the start of BBN? How large was it after?

Question

I'm trying to get a handle on the basics of BBN. I've read several authors quote the figure of 17 minutes. So BBN started at 3 minutes and ended at 20 minutes after the start of time. So how large was the when the process started and how large was it when it ended? How did they arrive at this value of 17 minutes?

Answer 1

According to Wikipedia’s “Chronology of the universe”, the radius of what has become our observable universe, at the end of the Big Bang nucleosynthesis era, was about 300 light years. At that time the universe was radiation-dominated and therefore the radius was expanding as $t^{1/2}$ so the radius would have been smaller by a factor of $\sqrt{3/20}=0.39$ at 3 minutes compared with 20 minutes.