Page 7, Large Scale Structure and Cosmology

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Nuclear Fusion and Formation of the CBR

Cosmic Fusion
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!