It is exactly at this point that l'abbé Georges Lemaître enters the cosmological scene. In 1927 he published a paper in a Belgian scientific journal, in which he, for the first time, compared observational predictions computed from models based on relativity with red shift measurements of distant galaxies indicating that the universe is actually expanding. This work marks the beginning of cosmology as an observational science. It was later translated into English by Sir Arthur Stanley Eddington and published in the Notices of the Royal Astronomical Society.
Expansion of the universe implies a beginning, a starting point with the infinite matter density. Although his original world model had no beginning, Lemaître soon realised that in a more realistic case it cannot be removed by any simple means. It was Einstein who suggested to him the sort of calculations that have to be made to get rid of the embarrassing "initial state". Lemaître did the calculations and demonstrated that with Einstein's assumptions, the beginning is even more persistent. More than 30 years later, Roger Penrose and Stephen Hawking proved the same conclusion in a fully general setting; their famous "singularity theorems" incorporate Lemaître's earlier result.
If the "beginning" cannot be avoided, it should be included in the realistic model of the universe. In 1931, Lemaître published a short paper in Nature presenting an outline of his Primaeval Atom hypothesis. Our sun and all stars, in every second, emit immense numbers of radiation quanta. If we go backwards in time, the number of quanta diminishes, and the quanta themselves become more and more powerful. At the limit, we are eventually confronted with one extremely concentrated quantum of energy - the Primaeval Atom.
Starting from this hypothesis, Lemaître elaborated a scenario of cosmic evolution - the first-ever attempt to reconstruct physical processes interacting with space-time dynamics. Subsequent disintegrations of the Primeval Atom, in the rapidly-expanding space, were supposed to fill in the universe with a variety of chemical elements. Then the expansion slowed down to an almost static rate, creating the possibility of condensations of matter to form galaxies and clusters of galaxies. We are now living in the third period of cosmic evolution, in which the recession of galaxies is again accelerating. Cosmic rays, constantly bombarding the Earth, are distant descendants of the Primaeval Atom, the last generation of its fragmentation.
