After 3 minutes, the density and temperature of the universe was similar to that inside stars. The protons and neutrons FUSE. One quarter of the Universes protons fused to form Helium! Trace amounts of Deuterium (heavy Hydrogen), Tritium (heavier Hydrogen) and a wee, wee bit of Lithium were created. Almost as quickly as it starts, fusion processes end, leaving us with the "cosmic abundances" of Hydrogen (71%), Helium (27%) and a few trace amounts of the other elements mentioned. It would be a long time before any other atoms are fused.
Slowing Down
After the first 30 minutes, when the
last atomic nuclei (Deuterium, He, Li) was formed, the next
million years was an utter bore! During
this time, the universe, always expanding, was made of hot atomic
nuclei and electrons, bathed in a dense radiation field.
The photons and electrons are completely
coupled.
Parting Company (Recombination)
After 1 million years, the temperature of the universe
dropped to 3,000 K. Electrons began combining with
protons and the photons no longer had the energy to break them
apart. This is when the Cosmic Background Radiation
was created. Nothing can change the signature of this
radiation, which we now see at a temperature of 2.7 K degrees
(cooled due 15 billion years of expansion). Now MATTER dominates!
For an animated and highly descriptive tour of the birth and early evolution of our universe, click HERE.