Edwin Hubble, an American astronomer working at Mount Wilson Observatory in California, provided the first observational proof that the universe is expanding. His landmark 1929 paper, “A relation between distance and radial velocity among extra-galactic nebulae,” showed that galaxies move away from us in all directions, and that more distant galaxies recede faster in direct proportion to their distance. But the full story involves several scientists whose work made that discovery possible, and one who actually figured it out theoretically two years before Hubble proved it with data.
What Hubble Found in 1929
By photographing dozens of distant galaxies and recording their spectra, Hubble identified a simple, striking pattern: the farther away a galaxy was, the faster it was moving away from Earth. The light from these galaxies was shifted toward the red end of the spectrum, a sign that they were receding. This redshift grew in direct proportion to distance, producing a clean linear relationship now expressed as velocity = H₀ × distance. That equation became known as Hubble’s Law, and the proportionality constant H₀ became the Hubble constant, one of the most important numbers in all of cosmology.
Hubble did this work using the 100-inch Hooker Telescope at Mount Wilson, which was by far the most powerful telescope in existence at the time. Its mirror had nearly three times the light-gathering ability of the next largest instrument, making it the only telescope capable of resolving individual stars in distant galaxies. Without it, the measurements that proved expansion would not have been possible for years.
Lemaître Got There First, on Paper
Two years before Hubble published his observational evidence, Belgian astrophysicist Georges Lemaître had already worked out mathematically that the universe must be expanding. His 1927 paper described a model in which galaxies move apart from one another, with more distant galaxies separating faster, precisely the velocity-distance relationship Hubble would later measure. Lemaître even estimated a proportionality constant from the limited data available to him.
The problem was visibility. Lemaître published in the Annals of the Brussels Scientific Society, a journal almost nobody outside Belgium read. His work went largely unnoticed until 1930, when he wrote to Arthur Eddington, his former teacher, to remind him of the paper. By then, Hubble’s observational proof had already made headlines. In 1931, Willem de Sitter publicly praised Lemaître’s “brilliant discovery” of the expanding universe, and Lemaître went further, proposing that the universe began as a primordial explosion, describing the present cosmos as “the ashes and smoke of bright but very rapid fireworks.” This was an early version of what we now call the Big Bang theory.
In 2018, the International Astronomical Union voted to officially rename Hubble’s Law as the Hubble–Lemaître Law, acknowledging that Lemaître had independently derived the expansion relationship before Hubble confirmed it with telescope data.
The Scientists Who Made It Possible
Hubble’s proof rested on two things: knowing how fast galaxies were moving and knowing how far away they were. Neither measurement was his alone.
The velocity data came largely from the earlier work of Vesto Slipher, an astronomer at Lowell Observatory. Starting in 1912, Slipher painstakingly measured the redshifts of spiral nebulae (what we now call galaxies), beginning with the Andromeda Galaxy. By the time Hubble began his analysis, Slipher had already catalogued the velocities of dozens of galaxies. Hubble used these measurements directly.
The distance measurements depended on a discovery by Henrietta Swan Leavitt, a Harvard astronomer who in 1912 identified a relationship between the brightness and the pulsation period of a type of star called a Cepheid variable. Brighter Cepheids pulse more slowly. Because you can measure the pulsation period from Earth, you can calculate the star’s true brightness, then compare that to how bright it appears to determine its distance. These became the first reliable “standard candles” for measuring distances across the cosmos, and they remain the gold standard today. Without Leavitt’s discovery, Hubble would have had no way to determine how far away those galaxies were.
Milton Humason, Hubble’s colleague at Mount Wilson, also played a key role. Humason was a remarkably skilled observer who photographed the spectra of distant galaxies, allowing the team to determine velocities by measuring wavelength shifts and interpreting them through the Doppler effect.
Why Einstein Called It His “Greatest Blunder”
Albert Einstein’s general theory of relativity, published in 1917, originally predicted that the universe should be either expanding or contracting. Einstein found this uncomfortable. The scientific consensus at the time held that the universe was static and eternal, so he added a term to his equations called the cosmological constant, a kind of anti-gravity force that held everything in place.
When Hubble’s observations proved that the universe was in fact expanding, Einstein realized his original equations had been right all along. There was no need for the cosmological constant. He reportedly called it his “greatest blunder,” a rare admission from the most celebrated physicist of the twentieth century. (In an ironic twist, modern cosmology has since revived the cosmological constant in a different form to account for dark energy, the mysterious force accelerating the universe’s expansion.)
The Expansion Rate Today
Hubble’s original estimate of the expansion rate was dramatically too high, implying a universe younger than Earth itself. Modern measurements have refined the number enormously, but a puzzle remains. Telescopes that measure the expansion using nearby galaxies and Cepheid variables get a Hubble constant of roughly 70 to 76 kilometers per second per megaparsec. Measurements based on the cosmic microwave background, the faint afterglow of the Big Bang, yield a lower value of about 67 to 68. Scientists call this gap the Hubble Tension, and it remains one of the biggest open questions in cosmology. Whether it reflects a measurement error or points to new physics is still unresolved.