Photon Epoch -
10 seconds to 70,000 Years
After all these annihilation processes, a huge number of photons remain; approximately one billion photons for every nucleon. The temperature is still high enough to allow photons, which are neutral particles, to interact with the charged particles, but is still too high to allow the formation of atomic nuclei. Several minutes later, however, it has dropped to the point where protons, which are essentially hydrogen nuclei, are able to combine with neutrons.
The nucleosynthesis epoch lasted from about 3 minutes to about 20 minutes after after the birth. It produced a small range of atomic nuclei other than those of Hydrogen, or more specifically H-1 the normal isotope of hydrogen, comprising a single proton. Essentially, these were: deuterium (a heavier hydrogen isotope with 1 proton and 1 neutron), the helium isotopes He-3 (2 protons, 1 neutron) and He-4 (2 protons , 2 neutrons), and the lithium isotopes Li-6 (3 protons, 3 neutrons) and Li-7 (3 protons, 4 neutrons). A few unstable, or radioactive, isotopes were also produced including tritium (H-3 with 1 proton, 2 neutrons); beryllium-7 (Be-7 with 4 protons, 3 neutrons), and beryllium-8 (Be-8 with 4 proton, 4 neutrons). These unstable isotopes either decayed or fused with other nuclei to make one of the stable isotopes.
Theoretical considerations result in expected vales for the amounts of the different elements, by mass, to be about 76% of Hydrogen-1, about 24% helium-4, about 0.01% of deuterium, and trace amounts of lithium and beryllium. This is exactly what has been seen in the Universe today providing strong evidence for the Big Bang theory. Note that the Big Bang did not create any heavier elements; the evolution of stars and SupernovŠ explosions have produced them and spread them through the Universe, but that is another story I discuss in the section on Stars.
Over time, photons and matter became less densely packed as the universe expanded. Most of the energy density of the universe was in the form of photons. As the Universe expanded, the density of photons and matter reduced. The stretching of space in the expansion increases the wavelength of the photons, reducing their energy. This does not happen to particles of matter. So, while both matter and photons (energy) were becoming less densely packed, the photons were also loosing energy. The energy density of the Universe was becoming biased towards the matter content rather than the photon content. By somewhere around 70,000 years, the energy density of the Universe was dominated by matter,
Astronomy & Cosmology -
The Early Universe
(or "What Banged in the Big Bang?")