Rick's Cosmology Tutorial: Chapter 7 Abstract

At times before about 1 second the universe contains virtually equal numbers of electrons and positrons, comparable with the number of black-body photons. It is shown that about half the positrons have annihilated by about 14 seconds. This is often quoted as the electron-positron annihilation time. However, this may be rather misleading since there are still of order 10^9 positrons for every proton at this time. It is shown that the number of positrons becomes comapable with the number of protons only by about 30 minutes.

From Chapter 6 we understand the potential importance of reactions being frozen-out by cosmic expansion. If the electron-positron annihilation reaction were to be frozen-out then there might be a residual cosmic positron background even today. However, it is shown that the annihilation reaction does not freeze-out. Consequently the positron abundance becomes negligible within a few hours.

Close-up of Jupiter's red spot showing turbulence of weather patterns. "Close up" is perhaps misleading since this picture is many times bigger than Earth