Up on the mountain, Hubble encountered his greatest scientific rival, Harlow Shapley, who had already made his reputation by measuring the size of the Milky Way. Using bright stars called Cepheid variables as standardized light sources, he had gauged the galaxy as being an astounding 300,000 light-years across — 10 times as big as anyone had thought. Yet Shapley claimed that the Milky Way was the whole cosmic ball of wax. The luminous nebulae were, he insisted, just what they looked like: clouds of glowing gas that were relatively nearby.

Hubble wasn't so sure. And in 1924, three years after Shapley departed to take over the Harvard Observatory, Hubble found proof to the contrary. Spotting a Cepheid variable star in the Andromeda nebula, Hubble used Shapley's technique to show that the nebula was nearly a million light-years away, far beyond the bounds of the Milky Way. It's now known to be the full-fledged galaxy closest to our own in a universe that contains tens of billions of galaxies. "I do not know," Shapley wrote Hubble in a letter quoted by biographer Christianson, "whether I am sorry or glad to see this break in the nebular problem. Perhaps both." (Hubble was not entirely magnanimous in victory. To the end he insisted on using the term nebulae instead of Shapley's preferred galaxies.)

Hubble's scientific reputation was made almost overnight by his discovery that the universe is vast and the Milky Way insignificant. But he had already moved on to a new problem. For years, astronomers had noted that light from the nebulae was redder than it should be. The most likely cause of this so-called red shifting was motion away from the observer. (The same sort of thing happens with sound: a police car's siren seems to drop in pitch abruptly as the car races past a listener.)

Hubble and his assistant, Milton Humason, began measuring the distances to these receding nebulae and found what is now known as Hubble's Law: the farther away a galaxy is from Earth, the faster it's racing away. Could it be that the universe as a whole is rapidly expanding? That conclusion was extraordinary, almost mind-blowing, yet seemed inescapable.

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Feb. 9, 1948

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