Density 4 x 10⁹

Universe is mostly light (photons ) "...it was light that then formed the dominant constituent of the universe, and ordinary matter played only the role of a negligible contaminant." Reminiscent of "Let there be light...".

Electrons and positrons created from light (pair -production ) and destroyed by annihilation at about equal rates. The pair-production threshold is 1 MeV, so the thermal energy kT=8.6 MeV was well above that.

Protons and neutrons being changed back and forth, so about equal numbers. The energy difference between neutron and proton is 1.29 MeV, so protons can be freely changed to neutrons at this temperature. Only about one baryon for 10⁹ photons, as inferred from the 3K background and density estimates. Since the conservation of baryon number is a strong conservation principle, it is inferred that the ratio of photons to baryons is constant throughout the process of expansion.

Density 30,000,000

Free neutrons decaying into protons, so there begins to be an excess of protons over neutrons. 62% protons, 38% neutrons

???There is still plenty of energy to convert protons to neutrons (1.29 MeV), so I don't see how the judgement of relative rate is made ???

Density 400,000

Primeval fireball becomes transparent to neutrinos, so they are released. It is presumed that the universe is filled with a background of those neutrinos now in addition to the 3K microwave background of electromagnetic radiation. Since they were released earlier, the calculated temperature is lower, about 2K. The expanding matter is still opaque to light and electromagnetic radiation of all wavelengths, so they are contained.

Electron-positron annilhilation now proceeding faster than pair-production. ?? A lot faster, I would think, since 1MeV is necessary for pair-production, so that is pretty far down the population tail for photons. ??? Probably using 3kT/2 for the thermal energy. 76% protons, 24% neutrons

Time:13.82 s, Temperature: 3x10⁹K

Below pair-production threshold, numbers of electrons and positrons rapidly decreasing. Nuclei such as ⁴He could form, but don't because of "deuterium bottleneck" - deuterium is not stable at this temperature. 83% protons, 17% neutrons

Time:3 min 2 s, Temperature: 10⁹ K

Electrons and positrons nearly all gone.

Photons and neutrinos are main constituents of the universe. Neutron decay leaves 86% protons, 14% neutrons but these represent a small fraction of the energy of the universe. The hydrogen/helium abundance of the present universe is a reflection of the equilibrium of particle populations established at this early time.

Time: 3min46s Temperature: 0.9x10⁹ K

The deuteron is now stable, so all the neutrons quickly combine to form deuterium and then helium nuclei (the highly stable alpha particles). There is no more neutron decay since they are stable in nuclei. Helium about 26% by mass in the universe from this early time. Nothing heavier is formed since there is no stable product with mass 5.

Note that if the expansion process had proceeded more slowly, almost all of the neutrons would have decayed and the universe would not have been able to form atoms as we know them.

Time:34min40s, Temperature: 3x10⁸ K Density 10

Nuclear processes are stopped, expansion and cooling continues. About 1 in 10⁹ electrons left because of slight excess of electrons over positrons in primeval fireball. The reason for the excess of matter over antimatter is a continuing investigation. Energy density is about 69% photons, 31% neutrinos.

Cool enough for hydrogen and helium nuclei to collect electrons and become stable atoms. Absence of ionized gas makes universe transparent to light for first time. At 3000 K, kT=0.26 eV so above this temperature atom formation is hindered.

Trefil (p41-42) has a good discussion of the transparency point and radiation pressure. He makes the analogy to the air in a tire - the pressure exists because the molecules bounce back from the tire "the tire remains inflated because the rubber walls are very efficient at scattering air molecules." Before the 700,000 year point the ions and electrons of the plasma were efficient scatterers of light, but after they condense into atoms, they are very inefficient scatterers of light - you can easily see 100 miles through air on a clear day.

From this point on, the radiation was decoupled from the particles and continued to cool as it was red-shifted by the cosmological expansion. With the discovery of the 3 K background radiation , we now observe the remnant of the radiation which was released at this transparency point. It is in fact the measurement of the characteristics of this cosmic background radiation which provided one of the key parts of the big bang model.