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Most textbooks say Edwin Hubble discovered that the universe is expanding. The fuller story starts with a Belgian physicist, Georges Lemaitre, who wrote down the relationship between galaxy speed and distance in 1927 and explained it with general relativity. This post covers what he did, why it was overlooked, a worked example on the expansion rate, and what his way of working teaches.
Who was Georges Lemaitre?
Lemaitre was a mathematician and theoretical physicist who was also ordained a Catholic priest in 1923. That year he won a three-year Belgian government fellowship to study abroad, and by 1925 he was a student of the astronomer Arthur Eddington. These details come from the physicist Jean-Pierre Luminet's history of Lemaitre's work (2015), which draws on the Lemaitre archives at Louvain.
What did the 1927 paper show?
In 1927 Lemaitre published, in French in a Belgian journal, a paper whose title translates as "A homogeneous universe of constant mass and increasing radius accounting for the radial velocity of extra-galactic nebulae". He found solutions of Einstein's field equations in which the radius of the universe grows with time. Then he tested them against observations.
Luminet quotes the key passage. Using 42 galaxies from lists by Hubble and Strömberg, Lemaitre found an average distance of 0.95 million parsecs and an average speed of recession of 600 km/s, which gave 625 km/s at a distance of one million parsecs. He wrote this as an equation (his Eq. 24): the speed of recession is proportional to the distance. That is the relation later called Hubble's law.
The paper went almost unnoticed. Eddington, to whom he sent a copy, did not respond at first, and Luminet records that Einstein, when they met at the 1927 Solvay Conference, accepted the mathematics but rejected the physical idea of an expanding universe.
How was the result lost in translation?
Only when Lemaitre wrote to remind him did Eddington, somewhat embarrassed, take up the model. He then sponsored an English translation, published in 1931 as A Homogeneous Universe of Constant Mass and Increasing Radius in Monthly Notices of the Royal Astronomical Society. But the paragraph containing the data and the expansion rate was replaced by a single sentence giving the result without the calculation.
For years people suspected someone had cut it to protect Hubble's claim. In 2011 the astrophysicist Mario Livio reported in Nature that letters in the archives settled it. As Luminet summarises, Lemaitre translated the article himself and chose to delete several paragraphs and notes without any outside pressure. He was even encouraged to add comments, but preferred to put his new ideas in a separate paper.
What was the primeval atom?
In 1931 Lemaitre went further. In a short letter to Nature, The Beginning of the World from the Point of View of Quantum Theory, he argued from thermodynamics and quantum theory that "if we go back in the course of time we must find fewer and fewer quanta, until we find all the energy of the universe packed in a few or even in a unique quantum." He called this starting point the primeval atom. It is the ancestor of the big bang model.
Luminet notes that Lemaitre kept his science and his religion separate: for him the physical beginning of the world was a scientific question. Lemaitre died in 1966, a few weeks after learning that the cosmic microwave background had been detected.
Worked example: what does the expansion rate tell you?
The expansion rate H₀ is in km/s per megaparsec (Mpc). Take 1 Mpc = 3.086 × 10¹⁹ km.
Speed of a galaxy. With Lemaitre's value, H₀ = 625 km/s/Mpc, a galaxy 10 Mpc away recedes at v = H₀ d = 625 × 10 = 6250 km/s. With the modern value from the Planck 2018 results, H₀ = 67.4 km/s/Mpc, it is 674 km/s.
A rough age scale. If everything had always moved at today's speed, the time since all galaxies were together would be 1/H₀.
Convert H₀ to 1/s: 625 km/s per 3.086 × 10¹⁹ km = 2.03 × 10⁻¹⁷ s⁻¹.
Invert: 1/H₀ = 3.086 × 10¹⁹ / 625 ≈ 4.94 × 10¹⁶ s.
Convert to years using 1 billion years ≈ 3.156 × 10¹⁶ s: about 1.56 billion years.
With 67.4 km/s/Mpc the same steps give about 14.5 billion years. Lemaitre's rate was nearly ten times today's value, so his age scale came out nearly ten times too short.
Common mistakes:
Not converting Mpc to km. H₀ has two distance units that must cancel. Convert before inverting.
Treating 1/H₀ as the exact age. It is an estimate; the true age depends on how the expansion rate has changed.
Confusing the cause. Redshift here comes from space expanding, not only from galaxies moving through space like a car. The Doppler idea helps you start; our post on Christian Doppler explains it.
When an idea needs a picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
What can a student take from Lemaitre's way of working?
Test theory against data at once. He did not stop at the equations; he went to the galaxy lists.
Do not let authority settle a question. Einstein and Eddington were both unconvinced at first.
Keep your working visible. The dropped paragraph is a reminder of what is lost when a calculation is cut.
The observational side of the story is in our post on Edwin Hubble, and the next step, the hot early universe, is in our post on George Gamow.
Try it now: repeat the 1/H₀ calculation for H₀ = 73 km/s/Mpc and see how much the age scale changes.
