Some 380,000 years after the big bang, the universal fog lifted and the cosmic background radiation was set free.
Back at the beginning of everything, when the universe was 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 a little bit bigger. But it also got a little bit cooler and a little bit dimmer. Meanwhile, matter and energy cohabited in a kind of opaque soup, in which free electrons continually scattered light to and fro. For 380,000 years things went on that way, until the temperature dropped below 3,000°. Right about then, electrons slowed down enough to be captured by passing protons, thus bringing atoms into the world, and a cosmic background of visible light was set free.
The cosmic background is the incarnation of the leftover light from a dazzling, sizzling early universe. It’s a ubiquitous bath of photons—massless vehicles of light energy that are as much waves as they are particles. As the cosmos continued to cool, the 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. Although the visible light photons had became weaker and weaker, they never stopped being photons.
What’s next on the spectrum? Today, more than 14-billion years after the beginning, the photons of the cosmic background have cooled to microwaves, giving us the modern moniker “cosmic microwave background,” or CMB for short. Continue to expand and cool the universe, and astrophysicists some 50 billion years from now will be writing about the cosmic radio wave background.
In the 3,000 degree youthful universe, photons didn’t travel far before scattering every which way off an electron. Back then, if your mission had been to see across the universe, you couldn’t have. Any photons you saw would, just nano- and pico-seconds earlier, have bounced off an electron right in front of your face, and so you would have seen only a glowing fog everywhere you looked. The entire luminous, translucent, reddish-white sky would have been nearly as bright as the surface of the Sun.
Every photon travels at the speed of light until it slams into an electron. Then it bounces off and travels in another direction at the speed of light, slams into another electron, bounces off, and so on. Only when photons are no longer slamming into electrons can they freely traverse the universe. And electrons didn’t cease their obstructionism until they made some more or less permanent commitments to atomic nuclei. That’s when the the universe became transparent, and that’s when the cosmic background was formed.
The temperature of the universe is directly related to the size of the universe. It’s a physical thing. If a photon lost half its original energy, it’s because the universe grew to twice its original size. As the universe grows, the photon’s wavelength gets longer, stretching with the expanding fabric of space and time.
A photon’s wavelength is the simple separation between one crest and the next, a distance you can measure with a ruler. A photon’s energy is measured by its frequency, or the number of wiggles it makes during a given interval of time. Photons of lower frequencies carry less energy than photons of higher frequencies.
When an object glows from being heated, it emits radiation (light) in all parts of the spectrum, but will always peak somewhere. For household lamps, the light bulbs all peak in the infrared, a part of the spectrum we detect only in the form of warmth on our skin. But bulbs also, of course, emit plenty of visible light. That’s why when you put your hand near your bedside lamp, you feel the light as well as see it. Currently the peak wavelength of the cosmic background is about 1 millimeter, which is smack dab in the microwave part of the spectrum. If you open a channel on your walkie-talkie where there isn’t a strong signal, you hear static. That static is ambient background microwaves, a few percent of which are the CMB; the rest comes from the Sun, cell phones, police radar guns, leaky microwave ovens, and so on.
Being the remnant of something that was once brilliantly aglow, the CMB has the wavelength “profile” we expect of a radiant but cooling object: it peaks in one part of the spectrum but radiates in other parts of the spectrum as well. In this case, besides peaking in microwaves, the CMB also gives off some radio waves and a vanishingly small number of photons of higher energy.
The existence of the CMB was predicted by the Russian-born U.S. physicist George Gamow and colleagues during the 1940s, culminating in a 1948 paper that laid out the physics of the early universe. The foundation of these ideas came from the 1927 work of the Belgian physicist and Jesuit priest Georges Lemaître, who is the generally recognized “father” of big bang cosmology. But it was US physicists Ralph Alpher and Robert Herman, in 1948, who estimated what the temperature of the cosmic background ought to be. They based their calculations on Einstein’s 1916 general theory of relativity, Edwin Hubble’s 1929 observations that the universe is expanding, and the particle and nuclear physics developed before, and during the Manhattan Project of WWII.
With hindsight, the argument is relatively simple. The fabric of space-time would have been smaller yesterday than it is today, and if it was smaller basic physics requires that it must have been hotter. So they turned the clock backward, and imagined a time when the universe was so hot that all the atoms would have been completely ionized, laying bare with all their electrons roaming free. And if that was the case, light would not always have arced, uninterrupted, across the universe. The photons’ free ride couldn’t have begun before a certain stage in the cooling of the cosmos. Not before the electrons would have combined with atoms and stopped interfering with the light. This transition occurs at a few thousand degrees.
While it was Gamow who suggested that the universe was once hotter, and that you could know the physics of the early universe, it was Herman and Alpher who proposed a temperature of 5 Kelvin. Yes, they got the wrong temperature—the CMB is 2.7 Kelvin—but together, those three physicists made an extrapolation unlike any other in the history of science. To take some basic atomic physics from a slab in the lab, and then deduce the largest-scale phenomenon ever measured, was nothing short of extraordinary. Writing about this feat, the US physicist J. Richard Gott says,
Predicting that the radiation existed and then getting its temperature correct to within a factor of two was a remarkable accomplishment—rather like predicting that a flying saucer 50 feet in width would land on the White House lawn and then watching one 27 feet in width actually show up.
At the time Gamow, Herman, and Alpher were coming up with their prediction, physicists were still undecided about the beginning of the universe. The year 1948 also saw the publication of a rival “steady state” theory of the universe by the English physicist Hermann Bondi and the American Astronomer Thomas Gold. The CMB proved to be the turning point. It indicated clearly that the universe had once been different—smaller, and hotter—whereas the rival, “steady state” model implied that the universe, while expanding, had always looked the same. The universe could not be hotter yesterday. And they provided for matter to pop into our universe from someplace else, at just the right rate, to leave the expanding universe with the same average density. The first direct observation of the CMB were the nails in the coffin of the steady-state theory, and that observation was made inadvertently in 1964 by Arno Penzias and Robert Wilson of Bell Laboratories. In 1978, they won the Nobel Prize for it.
In the 1960s everyone knew about microwaves, but almost no one had the technology to detect them. Back then most wireless communication was with radio waves, which are longer, and so the existing receivers and detectors and transmitters weren’t useful. You needed a shorter-wavelength detector and a suitable antenna to capture them. Bell Labs, pioneer in the communications industry, had a beefy horn-shaped antenna that could focus the microwaves down to the detectors.
If you’re going to send or receive a signal, you don’t want too many things contaminating it. Penzias and Wilson wanted to get a measure of the background interference—from the Sun, from the center of the galaxy, from terrestrial sources, from whatever. So they made an innocent measurement. They weren’t cosmologists; they were physicist-technologists looking for microwaves, unaware of the Gamow, Herman, and Alpher predictions. What they were decidedly not looking for was the cosmic microwave background; they were just trying to open up a new channel of communication for Bell Laboratories, and before doing that, they needed to characterize all the things that would contaminate a signal.
So Penzias and Wilson run their experiment, and corrected their data for all the sources of interference they know about, but there’s one part of the signal that doesn’t go away, and they just can’t figure out how to eliminate it. Finally they look inside the dish. They see pigeons nesting inside. And so they’re worried that a white dielectric substance (pigeon poop) might be responsible for the signal, because the signal comes from every direction, and it doesn’t change, and the only thing that’s all over their fancy horn-shaped antenna and doesn’t change is the pigeon poop. So they clean it up, the signal drops a little bit, but there’s still something left. The paper they publish in 1965 in the Astrophysical Journal talks about the unaccountable
excess antenna temperature.
But at the same time Penzias and Wilson were scrubbing bird droppings off their fancy antenna, a team of physicists at Princeton, led by Robert Dicke, were building a detector specifically to find the CMB. They, however, didn’t have the resources of Bell Labs, so their work went a little slower. At the moment Dicke and his colleagues heard about Penzias and Wilson’s work, they knew they’d been scooped. The Princeton team knew exactly what the observed excess antenna temperature was. Everything fit: the temperature, the fact that the signal came from every direction, and that it wasn’t linked in time with Earth’s rotation or position around the Sun.
Because light takes long stretches of time to reach us from distant places in the universe, if we look out in space we are actually looking back in time. So if the intelligent inhabitants of a galaxy far, far away were measuring the temperature of the cosmic background radiation at the moment captured by our gaze, they would get a temperature higher than 2.7 Kelvin, because they are living in a younger, smaller, hotter universe than we are.
Can such an assertion be tested? It turns out that the molecule cyanogen (CN)—best known to convicted murderers as the active component of the gas administered by their executioners—gets excited by exposure to microwaves; if the microwaves are warmer than the ones in our CMB, they excite molecule a little more. The cyanogen in distant, and thus younger, galaxies is exposed to a warmer cosmic background than the cyanogen in our galaxy. Indeed, their cyanogens lives at a higher exited state than ours. Although CN is a powerful cosmic marker, its behavior was all learned in the lab.
You can’t make this stuff up.
But why should any of this be interesting? The universe was opaque until 380,000 years after the big bang, so you could not have witnessed matter taking shape even if you’d been sitting front row center. You couldn’t have seen where the galaxy clusters and voids were starting to form. Before anybody could have seen anything worth seeing, photons had to travel, unimpeded, across the universe.
The spot where each photon began its cross-cosmos journey is where it smacked into the last electron that would ever stand in its way—the point of last scatter. As more and more photons escape unsmacked, they create a three-dimensional, expanding “surface” of last scatter, some 120,000 years deep. That surface is where all the atoms in the universe were born: an electron joins an atomic nucleus, and a little pulse of energy in the form of a photon soars away into the wild red yonder.
By then, some regions of the universe had already begun to coalesce by the gravitational attraction of its parts. Photons that last scattered off electrons in these regions developed a different, slightly cooler profile than those scattering off the less sociable electrons sitting in the middle of nowhere. Where matter accumulated, the strength of gravity grew, enabling more and more matter to gather. These regions seeded the formation of galaxy superclusters while other regions were left relatively empty.
When you map the CMB in detail, you find that it’s not completely smooth. It does have spots that are slightly hotter and slightly cooler spots than average. And by studying these temperature variations in the CMB—that is to say, by studying the structure of the surface of last scatter—we can infer what the structure and content of the matter was in the early universe. We know what the structure of matter is today, because we see galaxies and clusters and superclusters. To figure out how those systems arose, we use our best probe, the cosmic microwave background. The CMB is a remarkable time capsule and a way to reconstruct history in reverse. Studying its patterns is like performing some sort of cosmic phrenology. We’re looking at the skull bumps of the grown-up universe and inferring its behavior as an infant.
When constrained by other observations of the contemporary and distant universe, the CMB enables you to find out all sorts of fundamental cosmic properties. Compare the distribution of sizes and temperatures of the warm and cool areas and you can infer how strong the force of gravity was at the time and how quickly matter accumulated, allowing you to then deduce how much ordinary matter (4%), dark matter (23%), and dark energy (73%) there is in the universe. From here, it’s then easy to tell whether or not the universe will expand forever.
Ordinary matter is what we are all made of. It has gravity and interacts with light. Dark matter is a mysterious substance that has gravity but does not interact with light in any known way. Dark energy is a mysterious pressure that acts in the opposite direction of gravity, forcing the universe to expand faster than it otherwise would. What our phrenological exam says is that we understand how the universe behaved, but that most of the universe is made of stuff for which we have no clue what it is.
Our profound areas of ignorance notwithstanding, today, as never before, cosmology has an anchor, because the CMB reveals the portal through which we all walked: the surface of last scatter. It’s a point where interesting physics happened, and where we learned about the universe before and after its light was set free.
The simple discovery of the cosmic microwave background turned cosmology into something more than mythology. But it was the accurate and detailed map of the cosmic microwave background that turned cosmology into an experimental science. Cosmologists have plenty of ego—how can a person not be ego-driven when it’s your job to deduce what brought the universe into existence? But without data, their explanations were just tall tales. In this modern era of cosmology, each new observation, each morsel of data wields a two-edged sword: it enables cosmology to thrive on the kind of foundation that so much of the rest of science enjoys, but it also constrains theories that people thought up when there wasn’t enough data to say whether they were wrong or not. No science achieves maturity without it.
Let there be cosmology.