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17.9 Discovery of the Microwave Background Radiation

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George Gamow. Click here for original source URL

In the 1940s, Russian physicist George Gamow was exploring the implications of the expansion of the universe. He realized that if the expansion were traced backward, it would point to a time when the universe was hot and dense; perhaps even hotter and denser than a star. Gamow knew that the hot early universe must have been filled with high-energy radiation. Somehow, this environment of deadly radiation and high-speed particles must have evolved into the large and empty universe we see today. The big bang concept had been introduced in 1927 by another physicist, the Belgian priest George Lemaître, who called the early universe the primeval atoms and talked about "a day without a yesterday."

 

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Measured blackbody curve for the cosmic background radiation. Click here for original source URL.

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Temperature map of the universe, as measured by WMAP. Click here for original source URL.

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Measured blackbody curve for the cosmic background radiation. Click here for original source URL.

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Fluctuations in cosmic microwave background radiation as seen by COBE. Click here for original source URL.

In thinking about the evolution of the universe, Gamow realized that the early hot universe would expand and cool, much like any gas, but it would never become completely cold. His rough estimate put the current temperature in the range 5-10 Kelvin, incredibly cold. Like any blackbody, the universe as a whole emitted light with a characteristic color distribution that reflected the gas's temperature. For the first roughly 400,000 years of our universe's history, this thermal light didn't get to escape to be observed — individual photons couldn't travel a meaningful distance in the too-dense early universe, and were constantly absorbed by atomic nuclei and electrons that were all in constant collision. (You can think of the early universe as opaque.) Once the universe cooled enough that electrons and nuclei could bond, the photons could fly free.

Today, we see these photons, which originated as short-wavelength, high-energy photons, as the Cosmic Microwave Background. While they started out on the blue end of the electromagnetic spectrum when they formed in the early universe, cosmic expansion has caused them to be stretched out as the universe expanded. In effect, they have been red shifted to a much longer wavelength and lower energy. Gamow predicted that the universe should be bathed in this radiation that is a relic of the big bang, but is now red shifted to microwave wavelengths and a temperature only a few degrees above absolute zero. Unfortunately, Gamow's prediction could not be verified at the time it was made; radio astronomy was in its infancy, and no telescopes sensitive to microwave radiation existed. Gamow's prediction thus fell into obscurity for a decade.

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Robert Wilson, left, and Arno Penzias stand in front of the Bell Labs horn radio antenna in Crawford Hill, N.J., where they discovered cosmic background radiation confirming the Big Bang. Click here for original source URL.

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Arno Penzias. Click here for original source URL.

In 1964, Arno Penzias and Robert Wilson, two engineers at Bell Telephone Laboratories, were testing a sensitive, horn-shaped microwave antenna designed to relay telephone calls to communication satellites orbiting the Earth. As part of their tests, they were mapping the faint microwave radiation emitted by the Milky Way. An unknown source of noise affected their measurements even after they had carefully checked their equipment. The excess noise did not appear to change intensity with direction in the sky, time of day, or season, and it was not associated with any known astronomical source. At one point, they noticed that pigeons were roosting inside the antenna. Concerned that a residue left by the birds might be affecting their measurements (Penzias and Wilson euphemistically called it a "thin, white, dielectric film"), they cleaned the horn and started over. The noise persisted.

Through a colleague, Penzias and Wilson learned that a group of physicists at nearby Princeton University led by Robert Dicke was building a receiver to look for Gamow's proposed cosmic background radiation. Penzias and Wilson had been unaware of Gamow's work, but through luck and technical skill they found the microwave signal first. The extra noise they detected had exactly the temperature predicted by Gamow. The two Bell Lab engineers were awarded the Nobel Prize in 1978, ten years after Gamow's death. Their discovery was dramatic confirmation that the universe had once been hot and dense — they had seen the echo of the big bang.

Gamow was a brilliant scientist with an iconoclast's touch. Like Einstein, he never lost his sense of youthful curiosity and wonder at the workings of the universe. He wrote a series of books, illustrated by his own cartoons, which made abstract physics real by showing what the world would be like if you were shrunk to the size of a sub-atomic particle. While he didn't live long enough to get the Nobel Prize, he did get to see confirmation of his theories, which is a prize in itself.