In the early 1920s the bounds of the universe were still contested. Were the faint "nebulae" in the sky gas clouds within the Milky Way, or separate galaxies far beyond it? Most astronomers took the universe to be a single galaxy — the Milky Way — and static. Even Einstein had, in 1917, added a "cosmological constant" to his equations to hold the universe still.

At the Mount Wilson Observatory in California, using the largest telescope of the age (the 100-inch Hooker), Edwin Hubble (1889–1953) found Cepheid stars within the Andromeda "nebula" whose brightness varied in a regular way. These stars had a known property: their period of variation betrayed their true brightness, so their distance could be calculated. The result was decisive: Andromeda lay far beyond the Milky Way — it was a separate galaxy. The universe was made not of a single island but of billions of galaxies.

In 1929 Hubble went further. Studying the light of the galaxies, he saw that the spectrum of almost all of them was shifted toward red — that is, they were receding from us. Moreover, the farther a galaxy, the faster it fled; there was a straight proportion between distance and speed. This relation is known today as the Hubble–Lemaître law. The only reasonable interpretation was staggering: the galaxies were not moving through space; space itself was expanding.

An expanding universe, wound backward, pointed to a beginning — a moment when everything set out from a single hot, dense state. The Belgian priest-astronomer Georges Lemaître had already foreseen this idea; Hubble's measurement gave it observational ground. Einstein withdrew his cosmological constant as "the greatest blunder of my life" (tellingly, dark energy would bring it back in 1998). The idea that the universe has a history — this very timeline — was born in Hubble's redshifted galaxies.