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 | | Edwin Hubble |
Most scientists would be thrilled to make one great discovery in their lifetime. But for Edwin Hubble, one was not enough. He made two revolutionary discoveries, separated in time by less than a decade.
In 1923-24, Hubble proved unequivocally that the great "spiral nebula" in the constellation Andromeda (M31) was a separate "island universe"—a galaxy unto itself. Evidence was already mounting for the existence of external galaxies, but Hubble drove the final nail in the coffin to the idea that the entire universe was encompassed by our Milky Way Galaxy. Hubble’s proof of external galaxies extended the size and scale of our Universe by many orders of magnitude, a profound shift in the human conception of the cosmos.
Just six years later, Hubble, along with Milton Humason, published his famous Hubble Law: the farther a galaxy is located from Earth, the greater its light is shifted toward the red end of the spectrum. Many astronomers quickly came to understand the paradigm shift implied by Hubble's Law: the universe must be expanding. The redshift comes from expanding space, which causes the light of distant objects to be "stretched" to longer wavelengths, shifting it toward the red end of the spectrum. This discovery was a critical moment in cosmology, since it offered compelling observational support for the emerging Big Bang theory of our Universe’s origin. (The term "Big Bang" would not be coined for several more decades.)
 | | Velocity-Distance Relation among Extra-Galactic Nebulae. Radial velocities, corrected for solar motion, are plotted against distances·estimated from involved stars and mean luminosities of nebulae in a cluster. The black discs and full line represent the solution for solar motion using the nebulae individually; the circles and broken line represent the solution combining the nebulae into groups; the cross represents the mean velocity corresponding to the mean distance of 22 nebulae whose distances could not be estimated individually. |
Since Hubble's day, increasingly larger and more sophisticated telescopes have extended Hubble's Law to the far reaches of our Universe, offering irrefutable evidence for cosmic expansion. The space telescope named after Hubble has also shown that galaxies in the distant universe appear very different from those in the local universe, meaning the universe is evolving, as one would expect from an expanding universe. Surveys of how galaxies cluster on large scales, along with many other independent lines of evidence, all point to the same conclusion: we live in a universe that has been expanding for billions of years.
Ironically, Hubble himself never completed accepted cosmic expansion, and went to his death in 1953 without fully embracing the implication of his greatest discovery.
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