In 1915, Albert Einstein’s own math proved that empty space can expand. But he found the concept so profoundly absurd that he altered his equations to hide the discovery.
To force the universe to remain static, Einstein added a mathematical fudge factor to his theory of general relativity.
The first to mathematically prove that empty space could expand was Russian physicist Alexander Friedmann in 1922, followed independently by Georges Lemaître, a Belgian Catholic priest, in 1927. While Friedmann derived the expanding universe from Einstein's equations, Lemaître took the crucial next step. He proposed that the stretching of space itself would cause distant galaxies to appear to be moving away from Earth, and he calculated the rate at which this expansion was happening.
Lemaître’s mathematics provided the theory, but the empirical proof came in 1929 from American astronomer Edwin Hubble. Using the massive 100-inch Hooker telescope at the Mount Wilson Observatory in California, Hubble observed the light from distant galaxies. Building on earlier spectral measurements by astronomer Vesto Slipher, Hubble discovered a clear correlation: the farther a galaxy was from Earth, the faster it appeared to be receding. Its light was "redshifted"—meaning the light waves were physically stretched to longer wavelengths as they traveled across the expanding fabric of space.
The mechanism of this expansion is entirely counterintuitive. It is not an explosion of matter moving outward into a pre-existing empty void. The void itself is growing. The classic analogy is a loaf of raisin bread rising in an oven. As the dough expands, every raisin moves farther away from every other raisin, even though none of them are moving relative to the dough immediately surrounding them.
In the cosmos, galaxies are the raisins. They are largely stationary within their local regions of space, held together by their own gravity. It is the empty space between them that is stretching. Because this metric expansion happens everywhere evenly, a galaxy that is twice as far away has twice as much expanding space between it and the observer, causing it to recede twice as fast.