Where Did the Big Bang Come From?
The story of Georges Lemaître shows how cosmology moves in mysterious ways
I was once talking to a freshman studying political science. He just happened to have read a popular book on quantum theory and was shocked that in physics it is not the majority that decides which theories should be accepted, and that the universe does not alter when accepted views change – happily enough, because substantial changes in our scientific image of the world happen quite often.
In 1998, two teams of astrophysicists, investigating supernovae with the help of ultra-modern technology, announced an astonishing discovery: the overall recession of galaxies is not slowing down, as it should under the influence of gravity, but is actually accelerating. The consequences are tremendous. To account for this phenomenon, one has to admit that about 75 per cent of the material content of the universe is composed of some unknown anti-gravitating entity, dubbed “dark energy”. Scientists are not easily prone to accept ideas that overturn their established views. But observational evidence, from independent sources, started to accumulate, and cosmologists had to face the gigantic problem of the “dark” component of the universe.
Is this indeed an entirely new, unprecedented problem? The history of physics teaches us that “equations are wiser than those who wrote them down”. In 1917, Einstein applied his gravitational field equations to model the global structure of the universe, and was very upset when it turned out that the universe, as described by his model, is gravitationally unstable: it either collapses or expands, but is never static. To prevent this catastrophe, Einstein added to his equations a famous cosmological constant whose aim was to stabilise the universe. However, a few years later, a Russian mathematician, Alexander Friedman, discovered that Einstein’s equations with the cosmological constant admit a host of cosmological models with various “stability properties”, among them many expanding universes (with different rates of expansion). This embarras de richesses became the fundamental strategy of modern cosmology: physical theory presents a plethora of possible cosmological models (in principle, an infinite number of them), and it is up to observational data to select among them a model, or a class of models, that best corresponds to observational results.
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.
Lemaître was a Catholic priest, and many people regarded his cosmological model as an attempt to smuggle God into modern cosmology, although he himself carefully distinguished creation in the religious sense, which is outside the reach of science, and the “natural beginning”, which can be a subject-matter of scientific hypotheses. In 1948, Thomas Gold, Hermann Bondi and Fred Hoyle created the so-called steady state cosmology, doubtless with the aim of liberating cosmology from theological ingredients. In this model, the universe is expanding, but the receding galaxies are replaced by new ones formed by matter continuously created out of nothingness. This obviously contradicts the law of energy conservation, but the creation rate, indispensable to sustain the constant density of the universe, is so small that it cannot be experimentally detected. This hypothesis guarantees to the universe its “steady state” without any beginning.
It was Hoyle who coined the name Big Bang. It was intended to suggest that Lemaître’s model owed its popularity to big propaganda rather than to solid scientific arguments. The dispute between defendants of the Big Bang model and the steady-state scenario dominated cosmology over a decade. In the last years of his life, Lemaître, deeply disappointed, turned from cosmology to his old passion – numerical calculations and calculating machines.
The controversy was settled in 1965 by the discovery of microwave background radiation by Arno Penzias and Robert Wilson, soon interpreted as a remnant of the Big Bang (as had been predicted by George Gamow and his group in 1948). In 1966, a few days before his death, Lemaître was informed by Odon Godart, his former assistant, about this discovery. He was glad that his ideas were winning the battle, although it was not cosmic rays that contributed to this success, but rather a new type of radiation.
Almost 50 years later, a stream of new data is still coming in. Their recent analysis indicates that the oldest clusters of galaxies exhibit deceleration, but about six billion years ago the trend was reversed and the universe started to accelerate. It has been asked: “Was the Abbé Lemaître guided by Divine Providence, by scientific prescience, or was he just lucky?” It might also be that old masters were simply cleverer than some of us.