Hubble 1929, explained: the full-frame cut

Transcript

PNAS, 1929

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

The paper

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

Idea 1: nebulae

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

Idea 2: speed

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.

Speed from light

The dark lines in its spectrum shift, and the shift gives the speed. Third, distance.

Idea 3: distance

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

The data

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

The result

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 number

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.

What he claimed

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

One reading

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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