In 1927 a Belgian priest and physicist published a paper in a Brussels journal showing that a universe governed by general relativity must be either expanding or contracting, deriving the relationship between a galaxy’s distance and its recession speed, and calculating the rate from the observational data then available.
Two years later, Edwin Hubble published the same relationship from his own measurements at Mount Wilson. The result became known as Hubble’s law, and it stayed that way until 2018.
The story of who found the expansion of the universe involves at least five people, and most popular accounts mention one.
The Static Universe
When Einstein applied general relativity to the universe as a whole in 1917, he ran into a problem. His equations would not permit a static solution. Gravity pulls matter together, and a universe filled with matter must either collapse or fly apart.
Since everyone knew the universe was static, Einstein added a term to the equations, the cosmological constant, tuned precisely to hold everything in balance. It is a repulsive effect that exactly cancels gravity at large scales.
The fix is unstable, as was pointed out later: nudge such a universe slightly and it runs away. But in 1917 there was no observational reason to think the universe did anything at all.
In 1922 the Russian mathematician Alexander Friedmann found the dynamic solutions to Einstein’s equations, describing universes that expand or contract. Einstein published a note claiming Friedmann had made an arithmetic error, then withdrew it and conceded the mathematics was correct, while adding that the solutions had no physical significance. Friedmann died of typhoid in 1925, aged thirty seven.
Lemaitre’s Paper
Georges Lemaître was a Belgian Catholic priest who had studied at Cambridge with Arthur Eddington and at Harvard, and had a doctorate from MIT. He knew both the relativity theory and the American observational work.
His 1927 paper, published in the Annales de la Société Scientifique de Bruxelles, did three things. It derived an expanding solution to Einstein’s equations independently of Friedmann. It showed that expansion implies a linear relationship between the distance of a galaxy and its recession velocity. And it estimated the constant of proportionality from existing data, obtaining a value of around 600 kilometres per second per megaparsec.
The data he used were not his own. Vesto Slipher at the Lowell Observatory had spent from 1912 measuring the spectra of spiral nebulae and had found that almost all of them are redshifted, indicating motion away from us. Hubble had published distances for a number of the same objects. Lemaître combined the two sets and extracted the slope.
The paper was in French, in a journal with limited circulation, and almost nobody read it. Lemaître sent it to Eddington, who appears not to have registered its significance until several years later.
He did discuss it with Einstein at the Solvay Conference in 1927. Einstein’s reply, which Lemaître recorded, was that the calculations were correct but the physics was abominable.
Hubble’s Measurements
Edwin Hubble had the largest telescope in the world, the hundred inch reflector on Mount Wilson, and he had already used it to settle the biggest question in astronomy.
Until the mid 1920s it was disputed whether the spiral nebulae were nearby gas clouds within the Milky Way or separate galaxies at enormous distances. In 1924 Hubble identified Cepheid variable stars in the Andromeda nebula and used Henrietta Leavitt’s period luminosity relation to calculate its distance. It was far outside our galaxy. The universe was suddenly enormous and full of other galaxies.
That result depends entirely on Leavitt’s work at Harvard, where she had catalogued variable stars in the Magellanic Clouds and discovered that a Cepheid’s pulsation period fixes its intrinsic brightness. Comparing intrinsic with apparent brightness gives distance. It is the rung of the cosmic distance ladder that made everything above it possible, and Leavitt died in 1921 without living to see what was built on it.
In 1929 Hubble published a two page paper plotting the recession velocities of forty six galaxies against their distances. Forty one of the velocities came from Slipher’s spectra. The distances were Hubble’s, obtained with the help of Milton Humason. The points showed a clear linear trend, with a slope of about 500 kilometres per second per megaparsec.
Hubble was cautious about interpretation. He described the finding as a velocity distance relation and avoided asserting that the universe is expanding, preferring for years to speak of an apparent redshift whose cause might be something else.
The Missing Paragraphs
In 1931, at Eddington’s urging, Lemaître’s 1927 paper was translated into English for the Monthly Notices of the Royal Astronomical Society.
The translation is missing the passages in which Lemaître derives the rate of expansion from the data.
This was noticed, and for decades it was suspected that someone had suppressed the priority claim. In 2011 the astrophysicist Mario Livio found Lemaître’s correspondence with the journal’s editor. Lemaître had removed the paragraphs himself. His reason, stated plainly in the letter, was that by 1931 Hubble’s observations were much better than the fragmentary data he had used in 1927, and he saw no point in reprinting a superseded estimate.
It is an unusually clean resolution to a historical suspicion, and it says something about Lemaître, who seems to have been almost entirely uninterested in claiming credit.
In 2018 the International Astronomical Union voted to recommend that the relation be called the Hubble Lemaître law.
The Primeval Atom
Lemaître took the next step, which Hubble never did. If the universe is expanding, then running the film backwards brings everything closer together, and at some finite time in the past it was all in one place.
In 1931 he proposed that the universe began as what he called a primeval atom or a unique quantum, and that its subsequent expansion and fragmentation produced everything we observe. This is the first statement of what became Big Bang cosmology.
The name came from an opponent. Fred Hoyle, who favoured a steady state universe in which matter is continuously created, referred to the rival model in a 1949 BBC radio broadcast as this big bang idea. The label stuck, and Hoyle maintained afterwards that he had intended it descriptively rather than as mockery.
Lemaître was consistently careful about the theological reading of his own theory, and he had more reason than most to be. When Pope Pius XII gave an address in 1951 suggesting that Big Bang cosmology confirmed the doctrine of creation, Lemaître worked through the Pontifical Academy to discourage the Vatican from repeating the argument. His position was that a physical theory about initial conditions says nothing about creation in the theological sense, and that mixing the two damages both.
What Settled It
The expanding universe was not universally accepted until the 1960s. The steady state model, developed by Hoyle, Bondi and Gold, was a serious competitor and made testable predictions of its own.
Two observations decided the question. In 1965 Arno Penzias and Robert Wilson detected a uniform microwave background radiation at about three kelvin, coming from every direction, which the hot Big Bang model predicts as the cooled remnant of the early dense phase and which the steady state model has no way to produce. And counts of radio galaxies showed that the universe looks different at great distances, meaning it changes over time, which a steady state universe cannot do.
Big Bang nucleosynthesis provided a third line of evidence, predicting the observed cosmic abundance of helium and deuterium, which cannot be accounted for by stars alone. That prediction connects directly to the element first identified in the spectrum of the Sun: roughly a quarter of ordinary matter by mass is helium, and most of it was made in the first few minutes of the universe.
The Number Nobody Agrees On
The Hubble constant sets the rate of expansion and, with it, the age of the universe. Hubble’s original value of 500 implied a universe younger than the Earth, which was a serious problem until the distance scale was corrected in the 1950s.
The modern value is around 70 kilometres per second per megaparsec, giving an age of about 13.8 billion years. The measurements do not quite agree, and the disagreement has not gone away. Methods based on the cosmic microwave background give about 67. Methods based on Cepheids and supernovae in nearby galaxies give about 73. Both have small enough error bars that the gap is significant, and it is one of the sharpest open problems in cosmology.
The techniques involved are direct descendants of what Leavitt, Slipher and Hubble were doing: measure a standard brightness, compare it with what you see, and read the redshift off a spectrum. The successors to those instruments produced the deep field images that show galaxies as they were when the universe was a fraction of its present age.
The line of work that made all of it possible, from the first attempts to measure the sky systematically onwards, is collected in our six book pack on the history of astronomy.
Who Discovered It
Slipher measured the redshifts. Leavitt provided the method for measuring the distances. Friedmann found the mathematics. Lemaître combined the mathematics with the data, derived the law, and drew the physical conclusion. Hubble made the definitive measurements with a far better telescope and published the plot that convinced everyone.
The discovery is what happens when five people in four countries, mostly not in communication, each supply one component of an argument that none of them could have completed alone.
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