Light Scattering: Free Electron Explanation

[GarryS]

Light is scattered many times by free electrons. The scattering goes on until the electrons freeze out, after 300,000 years. Can anybody explain this to me.

 

[Ken G]

The key is that free electrons have a much higher chance of scattering visible-light photons than do electrons bound in hydrogen atoms. The "freezing out" means that the temperature gets cold enough that the electrons in effect stick to protons and make neutral hydrogen. At that point, the "veil lifts", because the bound electrons do not interact strongly with the visible photons of the CMB at that time. This means the time it takes a photon to scatter off a bound electron in a hydrogen atom is longer than the age of the universe at that time (some 380,000 years), and if it doesn't scatter in that time, it probably won't later on either, because the density just keeps dropping with time.

 

[GarryS]

The key is that free electrons have a much higher chance of scattering visible-light photons than do electrons bound in hydrogen atoms. The "freezing out" means that the temperature gets cold enough that the electrons in effect stick to protons and make neutral hydrogen. At that point, the "veil lifts", because the bound electrons do not interact strongly with the visible photons of the CMB at that time. This means the time it takes a photon to scatter off a bound electron in a hydrogen atom is longer than the age of the universe at that time (some 380,000 years), and if it doesn't scatter in that time, it probably won't later on either, because the density just keeps dropping with time.

Does this mean that the scattering process takes away some energy of the electron each time?

 

[Ken G]

The cooling wasn't due to the scattering process, it was due to the expansion of the universe. That cooled the radiation and the electrons, and allowed the atoms to form.