According to current cosmological theories, approximately 14 billion years ago the universe was created at a single point in space-time in the "Big Bang". As the universe expanded and cooled, numerous particle physics processes and particle-antiparticle annihilations occurred (e.g., Krane, 1996). After about t = 6 s, the universe consisted of some number N protons, N electrons, 0.16N neutrons, 109N photons, and 109N neutrinos, all at a temperature of approximately 1010 K.
As protons and neutrons collided with each other it was possible to form deuterium nuclei via the reaction n + p ^ 2H + 7. (2.1)
However, interactions with photons with energy greater than 2.22 MeV can break up these deuterium nuclei via the reverse reaction. Hence, the universe had to cool to about 9 x 108 K before significant numbers of neutrons could stably combine with protons in this way. At about the same time, the deuterons formed could engage in further reactions such as
with an energy of formation of 5.49 MeV, and
with an energy of formation of 6.26 MeV. Both these reactions have energies well above the threshold of deuteron formation and thus photons not energetic enough to break up deuterons would certainly not have been energetic enough to destroy these nuclei.
The final steps in initial nuclei formation were
There are no stable nuclei with molecular weight 5 and thus no further reactions involving single nucleons were possible. Further reactions involving other nuclei did occur, but their products made up only a very small fraction of the final number of nuclei produced, which was dominated by !H and 4He. By t = 250 s, the original 0.16V neutrons had decayed to about 0.12V, and these combined with 0.12V protons to form very nearly 0.06V 4He nuclei via the reactions listed above. In other words, the amount of 2H, 3H (which rapidly decays to 3He), 3He, and heavier nuclei left over was very small. Hence, after this time the universe contained 0.82V nuclei, of which 7.3% were 4He and 92.7% were protons. This translates to a helium mass fraction of about 24%. While 3He and 3H can be produced by fusion in stars, deuterium (2H or D) is not produced in significant quantities by any cosmic process that has occurred since the Big Bang, although it is destroyed via stellar fusion. Hence, the deuterium nuclei present in the universe now are truly primordial, and the primordial value of D/ H is estimated (from observing absorption lines in the spectra of very distant, firstgeneration stars; Burles and Tytler, 1988) to be (3.4±0.25) x 10~5.
As time progressed, the universe expanded and cooled until at about t = 700,000 yr the energy of photons had reduced to such a level that the nuclei could combine with electrons to form neutral atoms. At this point the universe became transparent to electromagnetic radiation and astronomy could begin! The residual photons at this time typically had ultraviolet (UV) wavelengths, but as the universe has since expanded these have become considerably redshifted to the microwave part of the electromagnetic spectrum. This residual radiation from the Big Bang is called "cosmic microwave background radiation" and was first observed in 1964 by Arno Penzias and Robert Wilson (Penzias and Wilson, 1965). Cosmic microwave background radiation is found to be almost entirely isotropic (i.e., has almost the same intensity in all directions) and is consistent with black body (or cavity radiation) with a temperature of 2.7 K. Although it was not identified until 1964, the microwave "hiss" of cosmic microwave background radiation is actually responsible for a small fraction (approximately 1%) of the familiar white noise seen on television screens between channels in the days before digital television.
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