What Was It Like When The Universe Made Its First Elements?
“The Universe does form elements immediately after the Big Bang, but almost all of what it forms is either hydrogen or helium. There’s a tiny, tiny amount of lithium left over from the Big Bang, since beryllium-7 decays into lithium, but it’s less than 1-part-in-a-billion by mass. When the Universe cools down enough that electrons can bind to these nuclei, we’ll have our first elements: the ingredients that the very first generations of stars will be made out of.
But they won’t be made out of the elements we think of as essential to existence, including carbon, nitrogen, oxygen, silicon and more. Instead, it’s just hydrogen and helium, to the 99.9999999% level. It took less than four minutes to go from the start of the hot Big Bang to the first stable atomic nuclei, all amidst a bath of hot, dense, expanding-and-cooling radiation. The cosmic story that would lead to us has, in truth, finally begun.”
The first stars wouldn’t form until somewhere between 50 and 100 million years after the Big Bang, but the elements that made them up were created in just the first 3-to-4 minutes. When the Universe was a fraction of a second old, there was a 50/50 split between protons and neutrons; when it was 3 seconds old, it was more like 85/15. But all of those protons and neutrons couldn’t just fuse together to form deuterium, helium, and then the heavier elements like they do in stars, even though the Universe was energetic and dense enough to make that happen.