How the Expanding Universe Was Discovered

That the universe is expanding — that galaxies are rushing apart and the cosmos had a beginning — ranks among the most profound discoveries ever made. The story behind it involves a Belgian priest whose breakthrough was nearly lost because he published in the wrong language, and an American astronomer who got most of the credit.

A universe assumed to be still

For most of history, the cosmos was assumed to be static and eternal — unchanging on the largest scales. Even Einstein, when his new theory of general relativity in 1915 suggested the universe might not be stable, added a term to his equations specifically to hold it still. He later called this his greatest blunder. The idea of a changing, evolving universe was simply too radical.

The clue in the colours

The first hints came from light. In the 1910s, the astronomer Vesto Slipher measured the spectra of distant "spiral nebulae" — objects we now know are entire galaxies. He found that the light from most of them was shifted toward the red end of the spectrum. This redshift, by analogy with the familiar drop in pitch of a receding siren, suggested the galaxies were moving away from us. But no one yet grasped the full meaning.

The priest who saw it first

In 1927, a Belgian priest and trained astronomer named Georges Lemaître put the pieces together. Combining the redshift data with estimates of galaxy distances, he derived a remarkable relationship: the farther away a galaxy is, the faster it recedes. He recognised what this meant — the universe itself is expanding, and his theory followed directly from Einstein's own equations. He even estimated the rate of expansion.

Hubble's confirmation

Two years later, in 1929, the American astronomer Edwin Hubble published the observational proof. Using the powerful telescope at Mount Wilson and careful measurements of galaxy distances, he demonstrated the same proportional relationship between distance and recession velocity, now plotted convincingly from his own data. The evidence was undeniable: the universe is expanding. The relationship became known as Hubble's law, and the rate as the Hubble constant.

Setting the record straight

For decades the discovery was credited almost entirely to Hubble, while Lemaître's earlier theoretical work was overshadowed. Historians now see it more fairly: Lemaître provided the interpretation and the first estimate, while Hubble provided the decisive observational confirmation. In 2018, the International Astronomical Union voted to rename the relationship the Hubble–Lemaître law, restoring credit to the quiet priest who had seen it first.

The birth of the Big Bang

An expanding universe carries a stunning implication. If everything is flying apart now, then long ago everything must have been packed together. Run the expansion backwards and the cosmos shrinks to an unimaginably hot, dense beginning. Lemaître himself proposed this idea, calling it the "primeval atom." It grew into the Big Bang theory — the foundation of modern cosmology, later confirmed by the discovery of the cosmic microwave background.

Key takeaways

• The universe was long assumed static; even Einstein adjusted his equations to keep it so.
• Redshifts in galaxy light revealed they are moving away from us.
• Lemaître derived cosmic expansion in 1927; Hubble confirmed it observationally in 1929.
• The discovery led to the Big Bang theory and, in 2018, to the renamed Hubble–Lemaître law.