Using the Big Bang theory, it is possible to formulate an answer to this 'cosmological' question, but without Big Bang cosmology there is no possible answer. Big Bang cosmology states that the universe formed a finite number of years ago, and that its initial conditions were that of high density and temperature. Present observations are consistent with an age somewhere between, say, 10 and 20 billion years, and the very existence of the cosmic background radiation being a perfect black body attests to the pre-existence of a dense, high temperature state in cosmic evolution.
In the Big Bang model, once you specify the ratio of density of the universe to the amount of cosmic background radiation, often given as 'photons per quark', and the rate of expansion today, given by Hubble's Constant, the Big Bang model predicts a hot dense state within the first 3-5 minutes after the Big Bang when helium nuclei formed from the free protons and neutrons. The above figure shows just such a plot of the abundances of the light elements during the early evolution of the universe. After 10 - 15 minutes, any remaining free neutrons decayed into more protons and some electrons, and you then end up with a fixed ratio of stable hydrogen ( protons) to helium. For the observed expansion rate and density of the universe today, the abundances of hydrogen and helium follow from the Big Bang model. An absolutely fantastic prediction, relating two observable factors to the abundance of the lightest elements!
Copyright 1997 Dr. Sten Odenwald
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