photon decoupling

Electrons began to bind with nuclei to form atoms. The atomic nuclei slowed down enough that they 'recruited' the storm of free electrons flying around. The matter started to become neutral, greatly reducing its interaction with the background radiation.

The mean free path of photons became very large, in that photons could travel very long distances without interacting with matter. Suddenly, as if a dense fog had disappeared, the universe was filled with light. Since this time the wavelength of the light has been stretched by the expansion of the universe, but we can still detect it as it permeates all of space - it is the Cosmic Microwave Background Radiation. It gives us a representation of how matter was distributed at this time.

Cosmic Microwave Background Radiation (source)

This event was particularly brief, even in the context of the earliest moments of the universe, and took place approximately three minutes after the moment of creation. The universe was 1600 times smaller than its present size.

- ASh, Fx (ed)