3.1 Gamow's Hypothesis

From the observation of redshifts of stellar spectral lines in galaxies, Hubble ^[26] deduced that the motion of galaxies is regular: (i) galaxies generally move away from one another, and (ii) their separation velocity v is proportional to their separation distance d ( v = Hd, with the Hubble constant H). These observations of the expanding universe are consistent with the assumption of the cosmological principle, i.e., the universe is the same at all places and in all directions (isotropy and homogeneity). Reversing the direction of time, and following today's expansion of the universe backward leads to the conclusion that our observable universe was in a state of very high density and temperature at early times; this is the Big Bang hypothesis. Based on this hypothesis, Gamow and his collaborators (see, e.g., Refs. 27 and 28) pointed out that the early universe could have played the role of a gigantic fusion reactor, in which numerous nuclear reactions (neutron captures and beta-decays) synthesized all atomic nuclei from an initial equilibrium state of protons and neutrons. This idea of primordial nucleosynthesis would have supplied a simple explanation for the observations that the elemental abundances are similar everywhere in the observable universe, and that they decrease rapidly with increasing mass number (see Fig. 1.1).

Today we know, however, that this original idea fails as there exist no nuclei with mass numbers 5 and 8 that are stable...