Big Bang Nucleosynthesis
After Inflation, the universe slowed down to the normal "Hubble Rate" expansion and was filled with radiation and elementary particles, sometimes called "quark soup". As the universe continued to cool, new particles were formed out of pre-existing ones. This early formation phase is called the Big Bang Nucleosynthesis (BBN). With the temperature falling below 10 billion Kelvin, BBN took place from about 10^-3 seconds to about three minutes.
The diagram at the left illustrates two of the common nuclear reactions which occurred during the BBN. It shows single proton ions and neutron ions combining to form deuterium nuclei, D, (containing one proton and one neutron) plus the emission of high energy photons, γ. Subsequently it shows two deuterium nuclei fusing to produce one nucleus of helium-3 (with two protons and one neutron) and one free neutron. Note that "atoms" were not yet forming, the above reaction shows just the "nuclei" of future atoms forming from ions. Experiments can be done in labs today demonstrating these early BBN nuclei reactions.
The BBN theoretical calculations result in a nuclei abundance of about 75% hydrogen (1 proton nucleus), about 25% helium (2 protons and 2 neutrons in the nucleus), and about 0.01% of deuterium (1 proton and 1 neutron nucleus). Without this abundance of hydrogen nuclei, there would be no water and therefore no life as we now know it. That the observed hydrogen and helium abundances in early distant galaxies are very consistent with the above theoretical calculations is considered "strong evidence" for the Big Bang Theory. Big Bang Nucleosynthesis is one of the three pillars of support for the Big Bang Theory. Measurements from the WMAP satellite over nine years of collecting data (2001 to 2010) has confirmed the above ratios with much greater precision.
One nice feature of the BBN is that the physical laws that govern the behavior of matter at these energy levels are very well understood. Hence the BBN lacks some of the speculative uncertainties that characterize earlier periods in the "theoretical" life of the universe. During all this time, electrons and photons interacted with each other so intensely (instantaneous collisions) that the universe was dark (opaque), no light (photons) could escape the darkness.
