Thread: What happens with photons after Big Bang

The most common theory says that during Big Bang both matter and antimatter were created. Due to a little asymmetry of their ratio matter survived the annihilation. At this process photons were formed, but are these photons still existing.

Where are the photons now? Are they still "flying" away from the point of creation?
Or does these photons vanished into thin air ?

The possibilities are numerous:

1) Pair production
If the energy of a photon is high enough, it can spontaneously produce a new matter antimatter pair (that eventually can be annihilated again).

2) Scattering
Photons can scatter at particles. During this process, they can transport energy to the particle increasing its kinetic energy and reducing the photon's energy = increasing the photon's wavelength (Compton scattering). As a result, the temperature of the matter is increased.

3) Absorption and re-emission
Photons can ionise or excite atoms and ions. They are then absorbed. New photons will be emitted after re-capturing an electron or during the cascade of electrons down the ladder of atomic energy levels. The newly emitted photons usually have a different energy distribution than the initial photon(s).

4) Observation: Cosmic Microwave Background
During the early hot (several thousand degrees) ionised phase of the universe, the photons only had a very short mean free path, i.e. they were scattered, absorbed, etc. very quickly after their emission or production. Especially the free electrons that are abundant in an ionised hot plasma, scatter photons very efficiently. As a result, the universe was intransparent, because like in a fog, the photons did not reach very far. Such a plasma also exists in stars, where we cannot look into their inner layers, either. Effectively, such a situation is a nearly perfect Black Body radiator, whose spectrum only depends on the temperature. This is exactly what we see in the Cosmic Microwave Background, however strongly redshifted due to cosmic expansion. So, at least the photons that were released as soon as the universe became transparent, i.e. when the temperature was low enough to allow for the formation of neutral atoms, are still around and are observed as the CMBR.