What Was It Like When The Universe First Made Atoms?
“The rest is history. Sure, it takes more than 100,000 years for the process to complete, but this is how the Universe does it. This two-photon transition, rare though it is, is the process by which neutral atoms first form. It takes us from a hot, plasma-filled Universe to an almost-equally-hot Universe filled with 100% neutral atoms. Although we say that the Universe formed these atoms 380,000 years after the Big Bang, this was actually a slow, gradual process that took about 100,000 years on either side of that figure to complete. Once the atoms are neutral, there is nothing left for the Big Bang’s light to scatter off of. This is the origin of the CMB: the Cosmic Microwave Background.”
The Universe got off to a start in quite a hurry. By time just four minutes had passed since the Big Bang, we had already created more matter than antimatter, gotten rid of all the unstable, fundamental particles, given mass to the Universe, and fused the first elements of the periodic table. The next step would be to make neutral atoms: the building blocks of all the complex structure we know of in the cosmos. Yet it didn’t take minutes, hours, days, or even months. It took hundreds of thousands of years to get there; an eternity compared to all that came before.