When free electrons recombine with hydrogen nuclei, the electrons cascade down the energy levels, emitting photons as they go. In order for stable, neutral atoms to form in the early Universe, they have to reach the ground state without producing a potentially ionizing, ultraviolet photon. (BRIGHTERORANGE & ENOCH LAU/WIKIMDIA COMMONS)

What Was It Like When The Universe First Made Atoms?

It took hundreds of thousands of years to make atoms for the first time. If things were just a little different, it could have taken an eternity.

When it comes to our world, our Solar System, and everything we can see in our Universe, it’s all made up of the same ingredients: atoms. Electrons and atomic nuclei interact and link up to form not just individual atoms, but simple and complex molecules, some of which have given rise to macroscopic structures and even life. It’s one of the most impressive facts about the Universe: that it exists in such a way to admit the complex structure we find within it today.

But for hundreds of thousands of years, dating from the instant of the hot Big Bang, it was impossible to form even a single atom. It took a huge amount of cosmic evolution, and a number of important steps, in order to create them. Here’s the story of how we got here.

The density fluctuations in the cosmic microwave background (CMB) provide the seeds for modern cosmic structure to form, including stars, galaxies, clusters of galaxies, filaments, and large-scale cosmic voids. But the CMB itself cannot be seen until the Universe forms neutral atoms out of its ions and electrons, which takes hundreds of thousands of years. (CHRIS BLAKE AND SAM MOORFIELD)

By time the Universe is four minutes old, it’s already done fusing all the atomic nuclei it can fuse in this hot, dense, early state. There are no more free neutrons…