About Time by Adam Frank

Author:Adam Frank
Language: eng
Format: epub, mobi
Publisher: Free Press

FIGURE 7.7. From telecommunications to cosmology. Wilson and Penzias stand in front of the horn-shaped receiver (designed for satellite communications) they used to discover microwave “echoes” of the Big Bang.

While the universe would only be hot and dense enough to forge elements for a few minutes, the end of the nuclear era did not end the universe’s particle alchemy. As space continued to expand, the temperature and density of the cosmic soup continued to drop. Matter, in the form of protons, electrons, helium nuclei and other particles, was well mixed with photons (quanta of light). The physics of this mix imprinted the photons with a fixed signature—a fossil imprint of history—that was discovered by Penzias and Wilson’s curious probing.

In the late 1800s, physicists learned that any hot, dense object emits light with a characteristic pattern called a blackbody spectrum. The red glow of an iron rod in a fireplace is a common example of blackbody light. Within the heated iron rod matter and photons are strongly coupled together exchanging energy in rapid-fire reactions. Physicists also learned that most of the light a blackbody emits comes at a wavelength strongly dependent on the blackbody’s temperature. Place the same iron rod in a blast furnace and it will glow white-hot as the peak blackbody emission shifts to include more visible wavelengths (colors). Astronomers routinely exploit this property of blackbody emission. Hot, dense objects (such as a star) can have their temperatures taken from millions of light-years away simply by recording their spectra, confirming its blackbody character and noting where the peak emission occurs.

As exotic as the early universe might have been, it was still a collection of hot, dense stuff. Thus, the entire early universe was a blackbody. Matter particles and photons jostled together. Matter absorbed the photons and then spat them out again in endless reactions. In this way, the young universe was saturated with blackbody light. Then, about three hundred thousand years after the birth of time, the blackbody photons were frozen out of the party.

The key event was the capture of electrons by protons to form the first hydrogen atoms. The newly formed hydrogen could not absorb the blackbody photons. The physics of hydrogen’s internal constitution made interactions with the blackbody photons all but impossible. A “decoupling” of matter (the now ubiquitous hydrogen atoms) and the bath of blackbody photons occurred relatively swiftly. The photons were left without dance partners. Just a few thousand years before, they could not travel more than a millimeter through the universe without being absorbed, but after decoupling they were left orphaned, free to wander the universe unimpeded. As time marched forward, the only change this fossil light would experience was a stretching in wavelength directly tied to the expansion of space itself.

Through their calculations Alpher and Gamow saw how the nuclear thermodynamics of a hot Big Bang implied a universe full of fossil blackbody radiation. They even knew its temperature. In one of the great acts of scientific prescience they predicted a cosmic

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