The Formation of Compound Particles

The first particles formed following the big bang were quickly dispersed over a very wide space. As with any explosion, the force of expansion was very great indeed. Particles rushed outward, away from each other, with blinding speed. 

A deuteron is the nucleus of an isotope of hydrogen, deuterium. The nuclear symbol for a deuteron (D) is jH. In a nuclear symbol of this kind, the superscript (2, in this case) represents the mass number of the particle (the total number of protons and neutrons), while the subscript (1, in this case) represents the atomic number, or the total number of protons. 

Deuterons were relatively unstable during the early moments after the big bang because, when struck by gamma rays, they break apart into their constituent parts  

as long as high-energy gamma rays were widely available, the chances for the survival of deuterons was relatively slight. 

The key transition point in this story occurred when the temperature of the young universe fell to less than 109 K. At that point, gamma rays with sufficient energy to break apart deuterons (about 0.1 MeY) became much less abundant, and deuterons had a much greater chance of survival. It was then, about 100 seconds after the big bang, that deuterons began to accumulate in space. 

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