From the Big Bang to the stars: The genesis of matter

After the second World War, it was believed that the whole chemical elements of the Universe were produced in the heart of the stars by successive nuclear fusion reactions. However, the relative abundance of some elements, such as deuterium, helium, lithium, beryllium or bore, are more abundant than expected by this simple theory. In 1970, the spallation mechanism was proposed to explain the formation of anomalous abundances of lithium, beryllium and bore in the interstellar medium by the splitting of heavy nuclei under the action of cosmic rays. However, this process cannot explain the observed quantities of deuterium, helium 3 and 4, and lithium 7. The explanation must be found in the first steps of the Universe creation. This paper describes the first minutes of the evolution of the primordial Universe and the successive chain reactions between neutrons, protons, and neutrinos, and then between neutrons and protons to produce deuterium and progressively heavier nuclei. This standard model of the Big Bang does not show any major contradiction between the observed relative abundance of elements and a common value for the baryonic number. However, the stellar activity has continuously modified the primordial abundance of the elements, in particular deuterium could have been ten times more abundant in the primitive matter than today. The observation and analysis of stellar matter, meteorites, stars and young galaxies indicate that the baryonic number cannot be greater than 5 10-10 and that today the Universe's baryonic density is lower than 3 10-31 g/cm3. This value is 15 to 30 times lower than the critical density on which our Universe evolution depends. Our Universe seems to be in a perpetual expansion as far as the existence of a huge quantity of non-baryonic matter (the famous missing mass) has not been demonstrated. (J.S.). 12 refs, 3 figs, 1 photo