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Dr. Edwin Hubble: Study of Extragalactic Nebulae, 1939

Constant Expansion

Literally overnight, Hubble had discovered the cosmos. He went on observing nebulae, and set up a classification system for all known galaxies according to their distance, shape, contents, size, and brightness. Hubble made another incredible find in 1929 by observing "redshifts" in the light wavelengths emitted by galaxies: all galaxies appear to recede from us, and there is a relationship between a galaxy's distance from us and its velocity through space. In other words, the farther away a galaxy is from Earth, the faster it is racing away from us. This became known as Hubble's Law, and is interpreted as evidence that the universe is constantly expanding. Hubble's Law became a central idea behind the big bang theory.

More than ten years prior to Hubble's find, Albert Einstein had already set out his general relativity theory, and produced a model of space based on it, which claimed that space was curved by gravity and therefore must be capable of expansion and contraction. But Einstein had caved to the observational wisdom of the day and changed his original equations. With the discovery of Hubble's Law, Edwin had proved Albert right. Einstein paid a special visit to Hubble at Mount Wilson in 1931 to thank him for his work, and said that second-guessing his original findings was "the greatest blunder of my life."

Between 1922 and 1936, Hubble had solved several of the central problems concerning the nature of the universe and laid the foundations of cosmology with his work.