Narration: synthetic voice. Script drafted with AI assistance.

In nineteen twenty-nine, many astronomers pictured a universe that stood still. Then a six-page paper by Edwin Hubble found a pattern.

Its title: a relation between distance and radial velocity among extra-galactic nebulae. Three simple ideas hide in it.

First, nebulae: faint, fuzzy patches in a telescope. Hubble had shown that some lie far beyond our Milky Way. They are galaxies.

Second, speed. An ambulance siren racing away has its sound waves stretched, so its pitch drops.

Light from a receding galaxy is stretched the same way, toward the red. The dark lines in its spectrum shift, and the shift gives the speed.

Third, distance. Identical lamps grow fainter further away. Know a star's true brightness, see how faint it looks, and you know how far.

His table puts the two together: for twenty-four galaxies, a speed from the spectrum and a distance from the faintness of their stars.

Plot one against the other. A few near ones even approach. Beyond them, the further a galaxy, the faster it recedes, roughly in a straight line.

The slope, which he called K, was about five hundred kilometres a second per million parsecs. Today's value is nearer seventy: his distances were too small.

Hubble was careful. He called it a first approximation, and thought it premature to discuss what it meant.

Others offered a meaning. Picture raisins in rising dough: as it swells, every raisin moves away from every other, and the furthest move fastest.

Swap the raisins for galaxies, and you have an expanding universe: the reading that won out.

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