- As soon as cosmologists understood that the Universe is expanding, they were left with a difficult question: What happened at the beginning of time?
- The surprising first answer, the precursor of the Big Bang model, was the "primeval atom." It was proposed by the Catholic priest and cosmologist Georges Lemaître.
- From the start, it was very hard to disentangle the scientific and biblical stories of creation.
In 1929, Edwin Hubble confirmed that the Universe is expanding. With that question settled, a far older one came back to haunt scientists: Did the Universe have a beginning? If so, what was going on before? Was there space, and was there time?
The quest for an answer has a fascinating history, and the search is still very much part of the conversation in cosmology. Maybe there are a few lessons to be learned from the wisdom of our predecessors.
A journey written in the stars
One of the first voices to address the issue of a beginning was the Belgian priest and cosmologist Georges Lemaître. Despite his love for physics, Lemaître followed his father’s advice (read: pressure). After a degree in civil engineering in 1913, he started to train as a mining engineer.
Sometimes a single factor can change someone’s course in life. In my case, it was inorganic chemistry labs that convinced me to change my studies from chemical engineering to physics (against my father’s advice as well.) In Lemaître’s case, it was years of exposure to the horrors of World War I. When the war was over, Lemaître knew it was time to pursue his dream. By 1920, he had joined both a graduate program in mathematical physics and the Maison Saint Rombaut, an extension of the seminary of the Archdiocese of Malines. There, he would be trained for priesthood.
In September 1923, Lemaître was ordained a priest. In October, he joined Arthur Eddington and his prestigious research group at Cambridge as a graduate student. After a year in England, Lemaître left for Harvard. He developed a solid foundation in theoretical physics and astronomy, a combination that would anchor his constant efforts to link the theoretical and observational aspects of cosmology.
Creative and independent, in 1927 Lemaître wrote a paper in which he basically rediscovered Alexander Friedmann’s cosmological solutions predicting an expanding Universe. In the same paper, he showed that these solutions, as well as Willem de Sitter’s, also led to a linear velocity-distance relation for receding galaxies.
Lemaître’s paper was published in an obscure journal and remained largely unnoticed. He did try to talk to Einstein about his results, but Einstein showed no interest. “Vos calculs sont corrects, mais votre physique est abominable,” Einstein told him — “Your calculations are correct, but your physics is abominable.” But Lemaître’s fate was about to change dramatically, and within a few years, Einstein himself would be applauding his ideas.
Lemaître’s abominable physics
When Hubble made his observations public, many cosmologists, including de Sitter and Eddington, scrambled to find a semi-realistic model of the Universe that could accommodate both matter and the expansion. When Lemaître heard of their efforts, he reminded his former adviser that he had solved the problem in 1927. Eddington finally read Lemaître’s paper and managed to get a translation published in Monthly Notices of the Royal Astronomical Society.
His prescient ideas finally vindicated, Lemaître pressed on with a more ambitious plan: to develop a complete, even if qualitative, history of the Universe, including its mysterious origin. “The purpose of any cosmogonic theory is to seek out ideally simple conditions which could have initiated the world and from which, by play of recognized physical forces, that world, in all its complexity, may have resulted,” he wrote.
In 1931, Lemaître published a paper in Nature. In it, he proposed the primeval atom and described the initial evolution of the Universe in terms of the decay of an unstable radioactive nucleus. He thus combined the new science of nuclear physics with the second law of thermodynamics, which says that over time, order tends to give way to disorder. He made no effort to explain where this original nucleus came from. This is how Lemaître unleashed his vision of cosmic birth:
“This atom is conceived as having existed for an instant only, in fact, it was unstable and, as soon as it came into being, it was broken into pieces which were again broken, in their turn; among these pieces electrons, protons, alpha particles, etc., rushed out. An increase in volume resulted, the disintegration of the atom was thus accompanied by a rapid increase in the radius of space which the fragments of the primeval atom filled, always uniformly.”
He then described how, from this prototypical matter, gaseous clouds would eventually form and condense into clusters of nebulae — what we call galaxies. With amazing intuition, he even proposed that the debris of these cosmic fireworks is detectable today as fossil rays, which he associated with cosmic rays. Little did he know that such rays indeed permeate the Universe. They are what we now call the cosmic background radiation, but they are not related to cosmic rays.
Finding the First Cause
Lemaître was clear that this model was only a rough approximation: “Naturally, too much importance must not be attached to this description of the primeval atom, a description which will have to be modified, perhaps, when our knowledge of atomic nuclei is more perfect.” Again, he was right. His cosmogonic vision, in a sense a cross between a creation myth and a scientific model, was to become the precursor of the modern Big Bang model of cosmology.
Despite obvious similarities with the “let there be light” biblical account, Lemaître insisted that his primeval atom hypothesis was a scientific model, and was not inspired by religious views on creation. He felt very uncomfortable when, in 1951, Pope Pius XII compared the initial state of the Universe as described scientifically with the Catholic interpretation of Genesis. In 1958, due to the pressure of several colleagues, Lemaître felt it was time to justify his position:
“As far as I can see, such a theory remains entirely outside any metaphysical or religious question. It leaves the materialist free to deny any transcendental Being… For the believer, it removes any attempt to familiarity with God… It is consonant with Isaiah speaking of the Hidden God, hidden in the beginning.”
Lemaître never ruled out the possibility that even the coming into being of the primeval atom could someday be explained scientifically, proposing that the answer may come from applying quantum mechanics to the Universe as a whole. This idea will re-emerge decades later, as Edward Tryon, Stephen Hawking, and others propose a quantum origin for the Big Bang. He left it to the future to determine whether any scientific theory can actually deal with the problem of the First Cause. (Where did the primeval atom come from?) As we will see, the problem remains unsolved.