Setting the time table
The present uniform expansion of the Universe suggests that there was an epoch in the past when the Universe was in a singular state with infinite density. Since most of the known laws of physics become inapplicable to such a singular state, we cannot extrapolate to earlier times before this epoch of singularity. We therefore concern ourselves only with what happened after this epoch of singularity, which is called the Big Bang. In the solutions discussed in §10.6 and §10.7, the time t was measured from the Big Bang.
The spacetime dynamics discussed in Chapter 10 sets the stage of the Universe. Now we shall look at the dramatis personae who were involved in the grand drama which unfolded and is still unfolding against this background stage of spacetime. How the temperature of the early Universe varied with time is given by (10.67) and (10.69). At times earlier than 1 s after the Big Bang, typical photons had energies somewhat larger than 1MeV. Since such photons are known to produce electron-positron pairs, the Universe at these early times must have been full of electrons and positrons which would have been approximately as abundant as photons. When photons had energies larger than 2GeV at still earlier times, they would have given rise to proton-antiproton pairs and neutron-antineutron pairs along with pairs of many other particles listed in elementary particle physics textbooks and their antiparticles.