A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae
Galaxies flee faster the farther they lie — the first sign the whole universe is expanding.
Look far enough into space and almost every galaxy is racing away from us — and the more distant it is, the faster it flees.
The big idea
In 1929 Edwin Hubble compared how far away galaxies are with how fast they are moving, and found a startlingly simple rule: a galaxy twice as distant runs away twice as fast. A galaxy's speed is just its distance multiplied by a fixed number, now called the Hubble constant.
Nothing is special about our address — the same thing would be seen from any galaxy. The only picture that explains it is an expanding universe: space itself is stretching, carrying the galaxies apart like raisins in rising dough. Wind that expansion backwards and everything was once crowded together — the seed of the Big Bang idea.
How it came about
Hubble worked at Mount Wilson in California with the largest telescope in the world. To gauge distance he leaned on special pulsing stars, Cepheids, whose steady throb betrays how far away they are. The speeds were harder won; almost all of them had been painstakingly measured years before by Vesto Slipher, who had first noticed that the faint "spiral nebulae" were nearly all rushing away from us.
Hubble plotted the two against each other and saw the line. But he was not quite first: the Belgian priest and physicist Georges Lemaître had derived the same law from Einstein's gravity two years earlier, and even estimated the number — only to publish it in French in a journal almost no one read. For that reason the rule is now often called the Hubble–Lemaître law.
Why it mattered
Before Hubble, most scientists — Einstein included — assumed the universe was static and eternal. One faint diagram overturned that. A universe that expands is a universe with a history and a beginning, and that single shift opened the door to all of modern cosmology: the Big Bang, an age for the universe, and the question of how it all might end.
A way to picture it
Draw dots on a balloon and start blowing it up. Every dot moves away from every other dot, and a dot twice as far moves away twice as fast — yet no dot is the centre; the whole rubber surface is simply stretching. The galaxies are the dots, space is the rubber, and that is exactly the pattern Hubble found written across the sky.
Where it sits
Hubble's law rests on Einstein's general relativity (1915), whose equations Friedmann and Lemaître had already shown could describe an expanding cosmos. It runs forward to the discovery of the universe's faint background heat in 1965, to the 1998 finding that the expansion is speeding up, and to today's gravitational-wave observatories — like LIGO elsewhere in this Library — which now offer a fresh, independent way to weigh the same Hubble constant.
The results establish a roughly linear relation between velocities and distances among nebulae for which velocities have been previously published, and the relation appears to dominate the distribution of velocities.
The outstanding feature, however, is the possibility that the velocity-distance relation may represent the de Sitter effect, and hence that numerical data may be introduced into discussions of the general curvature of space.