The Epoch of Recombination

After BBN, the universe is a hot soup of protons, electrons, helium nuclei, and photons. Initially, the energy density of radiation was greater than the energy density of matter and we were in the radiation era. During the radiation era, the energy from the radiation governs the expansion of the Universe But the energy density of radiation drops faster than that of matter:

Matter: energy density proportional to R^-3 (volume effect)
Radiation: energy density proportional R^-3 (volume effect) times another R^-1 (redshift effect), so R^-4.

After about 55,000 years, T ~ 9000 K and the energy density of radiation falls below that of matter: we enter the matter era.

Now, photons and particles are still closely coupled, due to all the free electrons running around. Electrons scatter photons through Thomson scattering, so that photons cannot free stream. The Universe is opaque, and matter and radiation behave as a single fluid.

Imagine a little piece of the Universe, which has an excess amount of matter. It will want to start collapsing under its own gravity. But it can't: the photons are coupled to the particles and keep them from collapsing. So structure (galaxies, stars, clusters, etc) cannot form!

When the temperature of the universe drops below 3000 K or so, when the Universe is ~ 200,000 years old, the electrons and nuclei combine to form atoms. No free electrons are running around, so photons can free stream and matter decouples from radiation. This is a fundamentally important time in the Universe's history: called the epoch of recombination. The Universe becomes transparent, we see it as the microwave background, and structure can start to form...