 The Big Bang Theory holds that there was a time when the universe was very small and very hot. The contents of the universe would have been in thermal equilibrium at that time. Particles in thermal equilibrium all have the same temperature. Roughly speaking, temperature is a measure of the kinetic energy of the particles. In this example, we have a small volume of protons and electrons in thermal equilibrium at 10,000 degrees Kelvin, the temperature at the surface of our sun. At 10,000 degrees, the electrons and protons are too hot to combine into hydrogen. If we add photons, they will scatter off the charged particles. Light cannot travel far through this space because it is constantly interacting with these free-moving charged particles. The plasma is opaque. As the universe expands, it cools. At some point, it cools enough for protons to capture electrons to form electrically-neutral hydrogen. This process is called recombination. Once the transformation to electrically-neutral particles is complete, light will travel through space without any further interactions. This is called decoupling. The radiation is said to have decoupled from the electrons and protons. The plasma becomes transparent. In addition to all the electrons and protons in the universe packed into this relatively small space, there were around 1.6 billion photons for every baryon in the universe. Baryons are the protons and neutrons in the plasma. Being in thermal equilibrium, these photons would be characterized by the black-body radiation curve. Knowing the energy that it takes to separate an electron from its proton in a hydrogen atom, we can calculate the temperature at which recombination and decoupling would occur. The current figure is around 3,000 degrees Kelvin, the surface temperature of a cool star like Proxima Centauri.