Atomic nuclei form in minutes, atoms form in hundreds of thousands of years, but the “dark ages” rule thereafter, until stars finally form.
When it comes to our cosmic history, it’s incredible to realize how impactful the earliest moments were in creating the conditions that would allow for our very existence many billions of years later. The earliest stages we can say anything meaningful about actually occurred even prior to the start of the hot Big Bang. Cosmic inflation took place and then ended, seeding the Universe with quantum fluctuations and then giving rise to the hot Big Bang. The Universe cooled and expanded from its hottest, densest stages to produce more matter than antimatter, then stable protons and neutrons, then atomic nuclei, and eventually even neutral atoms, all amidst a background sea of radiation and neutrinos.
You might think that once neutral atoms form, the next step would be driven by gravitation: the formation of stars. But the timescales required to form them are immense compared to everything that came before. By the time just half-a-million years have passed, the Universe is dominated by matter, the radiation sea is cool enough that atoms cannot become ionized, and gravitation gets to work in earnest. Even with those ingredients, it will still take somewhere between 50 and 100 million years for even the very first star in the Universe to form. For all the time in between, the Universe experiences the darkest part of the era known as the cosmic dark ages. Here’s what it was like back then.
The formation of neutral atoms isn’t simply important for setting into place the building blocks of all the complex chemical structures that can arise from molecules, ions, and any combination of atoms bound together. It’s also very important for “freeing” the photons, or particles of light, left over from the hot Big Bang. When neutral atoms first formed, that marks the time when photons stopped scattering off of free electrons, since free electrons are only present when your atoms are ionized in the form of a plasma. Once all…
