Let there be light: some 380,000 years after the big bang, the universal fog lifted and the cosmic background radiation was set free

Natural History, Oct, 2003 by Neil deGrasse Tyson

In the beginning of everything, when the universe was just a fraction of a second old, a ferocious trillion degrees hot, and glowing with an unimaginable brilliance, its main agenda was expansion. With every passing moment the universe got bigger. But it also got cooler and dimmer. And for millennia, matter and energy cohabited in a kind of thick soup, in which speedy electrons continually scattered photons of light to and fro.

Back then, if your mission had been to see across the universe, you couldn't have. Any photons entering your eye would, just nanoseconds or picoseconds earlier, have bounced off electrons right in front of your face. You would have seen only a glowing fog in all directions, and your entire surroundings--luminous, translucent, reddish white in color--would have been nearly as bright as the surface of the Sun.

Eventually, right around the time the young universe reached its 380,000th birthday, its temperature dropped below 3,000 degrees. Electrons began to slow down enough to be captured by protons, thus bringing atoms into the world. With fewer unattached electrons to gum up the works, the photons could finally race around without bumping into anything. That's when the universe became transparent, the fog lifted, and a cosmic background of visible light was set free.

That cosmic background persists to this day, the remnant of the light left over from a dazzling, sizzling early universe. It's a ubiquitous bath of photons--massless vehicles of energy, always moving at the speed of light, which act as much like waves as they do like particles. As the cosmos continued to cool, photons that had been born in the visible part of the spectrum lost energy to the expanding universe and eventually slid down the spectrum, morphing into infrared photons. As their wavelengths grew in size, they became cooler, that is, less energetic, but they never stopped being photons.

Today, some 13.7 billion years after the beginning, the photons that make up the cosmic background have cooled further still, shifting down the spectrum to become microwaves. That's how they got their modern moniker: "cosmic microwave background" or CMB for short. A hundred billion years from now, when the universe has expanded and cooled even more, astrophysicists will be writing about the cosmic radio wave background.

The temperature of the universe is directly related to the size of the universe. It's a physical thing. If the universe grows to twice its original size, all its free-traveling photons lose half their original energy. A growing universe forces a photon's wavelength to get longer, stretching along with the spandexlike fabric of space and time.

A photon's wavelength is simply the separation between one wave crest and the next--a distance you could measure if you had a small enough ruler. Because all photons move at the same speed, the shorter their wavelengths the more wave crests have to pass a given point in a given interval of time. Those are the higher-frequency photons. And a photon's frequency is a direct measure of its energy. That makes sense, too: the higher its frequency--that is, the faster it wiggles--the more energy it carries.

When an object glows from being heated, it emits radiation in all parts of the spectrum. But that radiation always peaks somewhere. The peak energy output of ordinary household lightbulbs lies in the infrared part of the spectrum, which people detect as warmth on the skin. But of course lightbulbs also emit plenty of visible light, or we wouldn't be buying them.

The peak output of the cosmic background has a wavelength of about a millimeter, which is smack-dab in the microwave part of the spectrum. The static you hear on a walkie-talkie comes from an ambient bath of microwaves, a few percent of which are from the CMB. (The rest of the noise comes from the Sun, cell phones, police radar guns, and so on.)

The existence of the CMB was predicted by the Ukrainian-born U.S. physicist George Gamow and his colleagues in the 1940s, culminating in a 1948 paper that extrapolated the known laws of physics into the early universe. The foundation of those ideas came from the 1927 work of Georges Edouard Lemaitre, a Belgian astronomer and Jesuit priest who is generally recognized as the father of big bang cosmology. But it was two U.S. physicists, Ralph A. Alpher and Robert C. Herman, both of whom had worked with Gamow, who estimated what the temperature of the cosmic background ought to be.

In hindsight, theirs is a relatively simple argument, one that I've already made. The fabric of space-time was smaller yesterday than it is today, and if it was smaller, basic physics requires it to have been hotter. So the physicists turned back the clock and imagined an epoch when the universe was so hot that all its atoms were completely ionized--when all atomic nuclei were laid bare and all electrons roamed free. Under those conditions, they hypothesized, photons would not have sped uninterrupted across the universe, as they do today. The photons' free ride today would have required that the cosmos get cooler--cool enough for the electrons to combine with atomic nuclei, forming atoms and allowing light to move without obstruction.