Rediscovering the Scientist-Priest Who Radically Changed Our View of the Universe
Guido Tonelli on the Intuition of Georges Lemaître
In 1917 Albert Einstein, developing the consequences of his general theory of relativity, postulated a homogeneous, static, spatially curved universe. Mass and energy warp space-time, and would tend to make it collapse into a point—but if you add to the equation a positive term that compensates for this tendency towards contraction, the system remains in equilibrium. The beginning of modern cosmology is ushered in with this maneuver. To avoid the catastrophic ending of the universe—the inevitable result if only gravity were present—an arbitrary term was invented. Wanting to maintain the prejudice regarding the stability and persistence of the universe that had lasted for millennia and still evidently held Einstein captive, he forcefully introduced the “cosmological constant,” a kind of vacuum energy which is positive and tends to push everything outwards, thus contrasting with and counterbalancing the gravitational pull and guaranteeing the stability of the whole.
Today, now we know that the universe is made up of 100 billion galaxies, it is shocking to realize that scientists in the first two decades of the last century, among them some of the most brilliant minds of all time, were still convinced that it consisted solely of the Milky Way. It was the slow concentric movement of the bodies belonging to this galaxy that gave the idea of a universe that was like a stationary, harmonious and ordered system. Soon afterwards this was brought into question by new kinds of observation, but a radical break with the old conceptions was also anticipated by the brilliant intuition of a young Belgian scientist.
In 1927 Georges Lemaître was a 33-year-old Catholic priest with a degree in astronomy from the University of Cambridge, and in the process of completing his PhD at the Massachusetts Institute of Technology. He is among the first to grasp that Einstein’s equations can also describe a dynamic universe, a system of constant mass but one that is expanding—with a radius, that is, which gets bigger with the passage of time. When he presents this idea to his older and much more established colleague, Einstein’s response is shockingly negative: “Your calculations are correct, but your physics is abominable.” So deeply rooted is the prejudice which for millennia had conceived of the universe as a stationary system that even the most elastic and imaginative mind of the period rejects the idea that it can be expanding, and that as a consequence of this expansion it must have had a beginning.
It would take years of discussion and fierce argument before this extraordinarily novel idea was generally accepted by scientists, and a great deal more time would have to pass before it became public knowledge.Lemaître’s intuition, confirmed by Hubble’s measurements, provided the basis for nothing less than a new vision of the world.
The key to its success is suggested by Lemaître himself, in an article in which he proposed his new theory, backed up with measurements of the radial speed of extra-galactic nebulae.
At the time, the attention of astronomers was concentrated on those peculiar objects resembling clouds which they conceived of as being groups of stars aggregated together with agglomerations of dust or gas. Today we know that they are in fact galaxies, each containing thousands of stars, but the telescopes of the time were not sufficiently developed to show them in much detail.
In order to calculate the speed at which a star or any other luminous body moved, astronomers had long known how to use the Doppler effect. The same phenomenon that we notice with sound waves from an ambulance siren can be observed with light waves. When the source recedes, the frequency of the waves that we receive is reduced: the sound of the siren gets fainter the further away it is. In the same way, the color of visible light shifts towards red with distance. By analysing the spectrum of luminous frequencies emitted by various celestial bodies, we can measure for each one this shift towards red, precisely the so-called red shift, and work out from this the radial speed with which they are receding from us.
But it was not easy to measure how far away these formations were, or consequently to determine whether they were situated within our galaxy or not. The solution was discovered by Edwin Hubble, a young astronomer working at the Mount Wilson Observatory in California, equipped with what at the time was the world’s most powerful telescope.
The technique employed was based on the study of Cepheids, pulsating stars of variable luminosity or brightness. Hubble begins his work just a few years after the death of Henrietta Swan Leavitt, one of the first American astronomers, a young scientist who had contributed enormously to this field and received, as is often the case, no appropriate recognition. In fact, at the beginning of the twentieth century it was considered unthinkable that a woman could use a telescope, and the extremely rare young female scientists were often deployed in subordinate roles. Leavitt was entrusted with the role of human “computer,” a wholly secondary and badly paid job: her task, in fact, consisted of examining, one after another, thousands of photographic plates containing images taken through telescopes, and recording the characteristics of stars and other celestial objects. She was assigned, in particular, the task of measuring and cataloguing the apparent brightness of these stars.
The young astronomer focused her studies on the stars with variable luminosity belonging to the Small Magellanic Cloud, a nebula which at the time was thought to be part of our own galaxy. It was Leavitt’s incisive observation that the brightest stars were also those with the largest pulsation period. Once this correlation was established, an estimate of the absolute brightness of a star could be obtained, which in turn would allow us to measure its distance from us. The brightness of an object varies according to the inverse square of the distance from the observer, so by knowing its absolute brightness, one need only measure the apparent brightness to calculate the distance.
Leavitt measured the relation between luminosity and period in the Cepheid variables of the Small Magellanic Cloud, and by hypothesizing that the stars were largely at the same distance, she was able to construct a scale of intrinsic luminosity, starting from the visible ones recorded on the plates.
Thanks to the incredible intuition of this brilliant young astronomer, we have at our disposal standard candles, that is to say light sources of known intensity, through which it is possible to deduce an absolute measure of distance.
This is what Hubble did when he used the Cepheids of the Andromeda nebula to reach the conclusion that these celestial bodies are too far away to be part of our Milky Way.
Lemaître was familiar with the first measurements made by Hubble, which not only placed these nebulae beyond our galaxy but also endowed them with an impressive speed of recession. His theory of an expanding universe made it possible to explain these unprecedented observations, as long as it was accepted that an enormous system was involved, immensely bigger than anything previously supposed. A gigantic structure in which there are countless galaxies similar to our own, with everything inclined to move away from everything else.
After having placed the Earth at the centre of the universe for thousands of years, and having reluctantly accepted that it is just one of the many bodies that rotate around the Sun, a further, final illusion suddenly crumbles. The solar system and our beloved Milky Way have no special position. We are an insignificant component of an anonymous galaxy—just one among the myriad of others to be found throughout the universe. And as if this was not enough, the entire system changes over time. Like all material objects it had a point of origin, and it will in all probability also have an end.
Lemaître’s intuition, confirmed by Hubble’s measurements, provided the basis for nothing less than a new vision of the world. In his original article, written in French, the astronomer-priest had gone so far as to predict a relationship of strict proportionality between distance and the speed at which astronomical objects recede. If his idea about the expanding universe was right, the more distant galaxies would have to move away from us at higher speeds, and would consequently exhibit a greater red shift. And this is precisely the result that Hubble obtained as his catalogue of observations grew in complexity and richness. But for a long time, Lemaître’s intuition was ignored because the Belgian journal in which he’d published his article had such a limited circulation. For this reason, until very recently, the scientific world had always referred to this correlation as “Hubble’s law.”
Thanks to a careful work of reconstruction, the contribution of the Belgian scientist has finally been recognized. It took almost a century, but today the relation that made it possible to establish the essentially dynamic nature of the universe is called, appropriately, the “Hubble–Lemaître law.” In the early 1930s, faced with large amounts of experimental data, Einstein also ended up abandoning his initial scepticism. Legend would have us believe that when reluctantly admitting that the Belgian priest and the American astronomer were right, the eminent scientist regretted his previous failure to understand, remarking that the cosmological constant “had proved to be the biggest blunder I have made in my life.”
Starting from an initial state in rapid expansion, there was no need to introduce this ad hoc correction, and so the cosmological constant disappeared for many decades from the fundamental equation of cosmology. By an irony of sorts, however, there would be a further reversal in the second half of the twentieth century when the discovery of dark energy caused the term that had so tormented its inventor to be reintroduced.
It was Lemaître once more who was the first to speculate that the expansion of the universe could actually be accelerating—and who, not by chance, kept Einstein’s cosmological constant in the equation, albeit reduced to a much lower value. Lemaître described the birth of the universe as a process that had taken place some time between 10 and 20 billion years ago, starting from an elementary state which he called the “primeval atom.” His hypothesis drew together the most advanced scientific theories of the period and the numerous mythological narratives that made everything originate from a kind of cosmic egg. But before doing so, he established the connection between microcosm and macrocosm that would prove so very fruitful in the coming decades.
From the outset, the formulation of this groundbreaking theory produced a great deal of perplexity. In truth, world opinion was otherwise engaged: the Wall Street Crash of 1929, the emergence of Fascism and Nazism in Europe, the many indications that the entire planet was about to descend into another global war. But even in scientific contexts where there was interest, the skepticism directed at the new cosmological theory was extremely strong. A good number of the most eminent scientists of the age refused even to countenance the idea of a beginning to space-time, or a birth of the universe. The problem lay in the fact that it bore a terrible resemblance to the biblical Genesis, and to the creation theory advocated by many religions. And if this wasn’t bad enough, the first proponent of the theory happened to be a priest as well as a scientist, and a Roman Catholic one at that.
The idea of an eternal universe, of an uncreated and everlasting stationary state, had first been supported by Aristotle, and it still fascinated many scientists. One of the best known of these was Fred Hoyle, a British astronomer who simply considered the theory proposed by Lemaître to be utterly repugnant. Hoyle remained convinced by his own ideas right up until his death in 2001. In 1949, in a program made for BBC radio, it was Hoyle who coined the term “Big Bang”—a description he intended as derogatory. Ironically, the image of a great explosion that Hoyle had used with the intention of ridiculing the new cosmological theory ended up penetrating so deeply into the collective imagination that it contributed significantly to its success.It was experimental results, once again, that would determine the definitive success of Big Bang theory.
One of the bastions of the most tenacious opposition to the theory was provided by Soviet science. For decades, Soviet scientists stigmatized the Big Bang as a pseudoscientific and idealistic theory that hypothesized a form of creationism—far too similar to the religious kind not to be deeply suspect. It mattered little, for them, that Lemaître had scrupulously separated science from faith, to the extent of reacting with horror when in 1951 Pope Pius XII could not resist the temptation of referring to the Big Bang described by scientists as resembling the biblical moment of Creation. It was an attempt by the Pope to provide a sort of scientific basis for creationism, to reinforce the rational basis of faith—and Lemaître strongly objected to it.
It was experimental results, once again, that would determine the definitive success of Big Bang theory. Among the theoretical developments of the new cosmological theory there had been, in the 1950s, the prediction of a radiation diffused throughout the universe: fossil waves, the relics of a moment in which photons had irrevocably separated from matter and that continued to fluctuate around us. These were very weak electromagnetic waves, stretched for billions of years by the expansion of space-time, an attenuated energy that would have given to the interstellar void a typical temperature of a few degrees kelvin.
The stunning discovery that confirmed this was made almost by chance in 1964 by the American astronomers Arno Penzias and Robert Wilson. The pair had been working for weeks to mend an antenna they hoped to use for radioastronomical observations in the microwave region, but they had failed to eliminate an annoying signal that seemed to be coming from every direction at once. At first they had assumed that it was interference caused by a radio station transmitting in the vicinity of the laboratory; then they had thought it might be electromagnetic disturbance connected with various activities in nearby New York. After even checking that a pair of pigeons that had nested in the antenna—leaving a coating of whitish dielectric material, also known as pigeon poo—were not responsible, they stopped searching and published their results in a short letter. The discovery of cosmic microwave background (CMB) radiation emanating from all directions and the observation that the universe had a temperature of a few degrees kelvin, that is to say around –270 degrees Celsius, sealed the success of the already indisputable new theory. Penzias and Wilson had effectively recorded the echo of the Big Bang, the mother of all catastrophes, the primal event, the proof that everything had begun 13.8 billion years ago.
Excerpted from Genesis: The Story of How Everything Began by Guido Tonelli, translated by Erica Segre and Simon Carnell. Published by Farrar, Straus and Giroux in April, 2021. Copyright © Giangiacomo Feltrinelli Editore, Milano. Translation copyright © 2020 by Simon Carnell and Erica Segre. All rights reserved.