Origins: Fourteen Billion Years of Cosmic Evolution (4 page)

It’s philosophically sensible, and in line with all the predictions of modern physics, to presume that the bulk properties of antimatter will prove to be identical to those of ordinary matter—normal gravity, normal collisions, normal light, and so forth. Unfortunately, this means that if an anti-galaxy were headed our way, on a collision course with the Milky Way, it would remain indistinguishable from an ordinary galaxy until it was too late to do anything about it. But this fearsome fate cannot be common in the universe today because if, for example, a single anti-star annihilated with a single ordinary star, the conversion of their matter and antimatter into gamma-ray energy would be swift, violent, and total. If two stars with masses similar to the Sun’s (each containing 10
57
particles) were to collide in our galaxy, their melding would produce an object so luminous that it would temporarily outproduce all the energy of all the stars of 100 million galaxies and fry us to an untimely end. We have no compelling evidence that such an event has ever occurred anywhere in the universe. So, best we can judge, the universe is dominated by ordinary matter, and has been since the first few minutes after the big bang. Thus total annihilation through matter-antimatter collisions need not rank among your chief safety concerns on your next intergalactic voyage.

Still, the universe now seems disturbingly imbalanced: we expect particles and antiparticles to be created in equal numbers, yet we find a cosmos dominated by ordinary particles, which seem to be perfectly happy without their antiparticles. Do hidden pockets of antimatter in the universe account for the imbalance? Was a law of physics violated (or was an unknown law of physics at work?) during the early universe, forever tipping the balance in favor of matter over antimatter? We may never know the answers to these questions, but for now, if an alien hovers over your front lawn and extends an appendage as a gesture of greeting, toss it your eight ball before you get too friendly. If the appendage and the ball explode, the alien probably consists of antimatter. (How it and its followers will react to this result, and what the explosion will do to you, need not detain us here.) And if nothing untoward occurs, you can proceed safely to take your new friend to your leader.

CHAPTER 3

Let There Be Light

A
t the time when the universe was just a fraction of a second old, a ferocious trillion degrees hot, and aglow with an unimaginable brilliance, its main agenda was expansion. With every passing moment the universe got bigger as more space came into existence from nothing (not easy to imagine, but here, evidence speaks louder than common sense). As the universe expanded, it grew cooler and dimmer. For hundreds of 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 done so. 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.

As the universe expanded, the energy carried by each photon decreased. Eventually, about the time that the young universe reached its 380,000th birthday, its temperature dropped below 3,000 degrees, with the result that protons and helium nuclei could permanently capture electrons, thus bringing atoms into the universe. In previous epochs, every photon had sufficient energy to break apart a newly formed atom, but now the photons had lost this ability, thanks to the cosmic expansion. With fewer unattached electrons to gum up the works, the photons could finally race through space 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 leftover light from a dazzling, sizzling early universe. It’s a ubiquitous bath of photons, acting as much like waves as they do like particles. Each photon’s wavelength equals the separation between one of its wiggly wave crests and the next—a distance you could measure with a ruler, if you could get your hands on a photon. All photons travel at the same speed in a vacuum, 186,000 miles per second (naturally called the speed of light), so photons with shorter wavelengths have a larger number of wave crests passing a particular point each second. Shorter-wavelength photons therefore pack more wiggles into a given interval of time, so will have higher frequencies—more wiggles per second. Each photon’s frequency provides a direct measure of its energy: the higher the photon frequency, the more energy that photon carries.

As the cosmos cooled, photons lost energy to the expanding universe. The photons born in the gamma-ray and X-ray parts of the spectrum morphed into ultraviolet, visible light, and infrared photons. As their wavelengths grew larger, they became cooler and less energetic, but they never stopped being photons. Today, 13.7 billion years after the beginning, the photons of the cosmic background have shifted down the spectrum to become microwaves. That’s why astrophysicists call it the “cosmic microwave background,” though a more enduring name is the “cosmic background radiation,” or CBR. One hundred billion years from now, when the universe has expanded and cooled some more, future astrophysicists will describe the CBR as the “cosmic radio-wave background.”

The temperature of the universe drops as the size of the universe grows. It’s a physical thing. As different parts of the universe move apart, the wavelengths of the photons in the CBR must increase: the cosmos stretches these waves within the spandex fabric of space and time. Because every photon’s energy varies in inverse proportion to its wavelength, all the free-traveling photons will lose half their original energy for every doubling in size of the cosmos.

All objects with temperatures above absolute zero will radiate photons throughout all parts of the spectrum. But that radiation always has a peak somewhere. The peak energy output of an ordinary household light bulb lies in the infrared part of the spectrum, which you can detect as warmth on your skin. Of course light bulbs also emit plenty of visible light, or we wouldn’t buy them. So you can feel a lamp’s radiation as well as see it.

The peak output of the cosmic background radiation occurs at a wavelength of about 1 millimeter, smack dab in the microwave part of the spectrum. The static that you hear on a walkie-talkie comes from an ambient bath of microwaves, a few percent of which are from the CBR. The rest of the “noise” comes from the Sun, cell phones, police radar guns, and so on. Besides peaking in the microwave region, the CBR also contains some radio waves (which allow it to contaminate Earth-based radio signals) and a vanishingly small number of photons with energies higher than those of microwaves.

The Ukrainian-born American physicist George Gamow and his colleagues predicted the existence of the CBR during the 1940s, consolidating their efforts in a 1948 paper that applied the then-known laws of physics to the strange conditions of the early universe. The foundation for their ideas came from the 1927 paper by Georges Edouard Lemaître, a Belgian astronomer and Jesuit priest, now generally recognized as the “father” of big bang cosmology. But two U.S. physicists, Ralph Alpher and Robert Herman, who had previously collaborated with Gamow, first estimated what the temperature of the cosmic background ought to be.

In hindsight, Alpher, Gamow, and Herman had what today seems a relatively simple argument, one which we have already made: the fabric of space-time was smaller yesterday than it is today, and since it was smaller, basic physics requires that it was hotter. So the physicists turned back the clock to imagine the epoch we have described, the time when the universe was so hot that all its atomic nuclei were laid bare because photon collisions knocked all electrons loose to roam freely through space. Under those conditions, Alpher and Herman hypothesized, photons could not have sped uninterrupted across the universe, as they do today. The photons’ current free ride requires that the cosmos grew sufficiently cool for the electrons to settle into orbits around the atomic nuclei. This formed complete atoms and allowed light to travel without obstruction.

Although Gamow had the crucial insight that the early universe must have been much hotter than our universe today, Alpher and Herman were the first to calculate what its temperature would be today: 5 degrees Kelvin. Yes, they got the number wrong—the CBR actually has a temperature of 2.73 degrees Kelvin. But these three guys nevertheless performed a successful extrapolation back into the depths of long-vanished cosmic epochs—as great a feat as any other in the history of science. To take some basic atomic physics from a slab in the lab, and to deduce from it the largest-scale phenomenon ever measured—the temperature history of our universe—ranks as nothing short of mind-blowing. Assessing this accomplishment, J. Richard Gott III, an astrophysicist at Princeton University, wrote in
Time Travel in Einstein’s Universe
: “Predicting that the radiation existed and then getting its temperature correct to within a factor of 2 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.”

When Gamow, Alpher, and Herman made their predictions, physicists were still undecided about the story of how the universe began. In 1948, the same year that Alpher and Herman’s paper appeared, a rival “steady state” theory of the universe appeared in two papers published in England, one coauthored by the mathematician Hermann Bondi and the astrophysicist Thomas Gold, the other by the cosmologist Fred Hoyle. The steady state theory requires that the universe, though expanding, has always looked the same—a hypothesis with a deeply attractive simplicity. But because the universe is expanding, and because a steady state universe would not have been any hotter or denser yesterday than today, the Bondi-Gold-Hoyle scenario maintained that matter continuously pops into our universe at just the right rate to maintain a constant average density in the expanding cosmos. In contrast, the big bang theory (given its name in scorn by Fred Hoyle) requires that all matter come into existence at one instant, which some find more emotionally satisfying. Notice that the steady state theory takes the issue of the origin of the universe and throws it backward an infinite distance in time—highly convenient for those who would rather not examine this thorny problem.

The prediction of the cosmic background radiation amounted to a shot across the bow of the steady state theorists. The CBR’s existence would clearly demonstrate that the universe was once far different—much smaller and hotter—from the way we find it today. The first direct observations of the CBR therefore put the first nails in the coffin of the steady state theory (though Fred Hoyle never fully accepted the CBR as disproving his elegant theory, going to his grave attempting to explain the radiation as arising from other causes). In 1964, the CBR was inadvertently and serendipitously discovered by Arno Penzias and Robert Wilson at the Bell Telephone Laboratories (Bell Labs, for short) in Murray Hill, New Jersey. Little more than a decade later, Penzias and Wilson received the Nobel Prize for their good luck and hard work.

What led Penzias and Wilson to their Nobel Prize? During the early 1960s, physicists all knew about microwaves, but almost no one had created the capability of detecting weak signals in the microwave portion of the spectrum. Back then, most wireless communication (e.g., receivers, detectors, and transmitters) rode on radio waves, which have longer wavelengths than microwaves. For these, scientists needed a shorter-wavelength detector and a sensitive antenna to capture them. Bell Labs had one, a king-size, horn-shaped antenna that could focus and detect microwaves as well as any apparatus on Earth.

If you’re going to send or receive a signal of any kind, you don’t want other signals to contaminate it. Penzias and Wilson were trying to open up a new channel of communication for Bell Labs—so they wanted to pin down how much contaminating “background” interference these signals would experience—from the Sun, from the center of the galaxy, from terrestrial sources, from whatever. They therefore embarked on a standard, important, and entirely innocent measurement, aimed at establishing how easily they could detect microwave signals. Though Penzias and Wilson had some astronomy background, they were not cosmologists but technophysicists studying microwaves, unaware of the predictions made by Gamow, Alpher, and Herman. What they were decidedly
not
looking for was the cosmic microwave background.

So they ran their experiment, and corrected their data for all known sources of interference. But they found background noise in the signal that didn’t go away, and they couldn’t figure out how to get rid of it. The noise seemed to come from every direction above the horizon, and it didn’t change with time. Finally they looked inside their giant horn. Pigeons were nesting there, leaving a white dielectric substance (pigeon poop) everywhere nearby. Things must have been getting desperate for Penzias and Wilson: could the droppings, they wondered, be responsible for the background noise? They cleaned it up, and sure enough, the noise dropped a bit. But it still wouldn’t go away. The paper they published in 1965 in
The
Astrophysical Journal
refers to the persistent puzzle of an inexplicable “excess antenna temperature,” rather than the astronomical discovery of the century.

While Penzias and Wilson were scrubbing bird droppings from their antenna, a team of physicists at Princeton University led by Robert H. Dicke was building a detector specifically designed to find the CBR that Gamow, Alpher, and Herman had predicted. The professors, however, lacked the resources of Bell Labs, so their work proceeded more slowly. The moment that Dicke and his colleagues heard about Penzias and Wilson’s results, they knew that they’d been scooped. The Princeton team knew exactly what the “excess antenna temperature” was. Everything fit the theory: the temperature, the fact that the signal came from all directions in equal amounts, and that it wasn’t linked in time with Earth’s rotation or Earth’s position in orbit around the Sun.

But why should
anybody accept the interpretation? For good reason. Photons take time to reach us from distant parts of the cosmos, so we inevitably look back in time whenever we look outward into space. This means that if the intelligent inhabitants of a galaxy far, far away measured the temperature of the cosmic background radiation for themselves, long before we managed to so do, they should have found its temperature to be greater than 2.73 degrees Kelvin, because they would have inhabited the universe when it was younger, smaller, and hotter than it is today.

Can such an audacious assertion be tested? Yup. Turns out that the compound of carbon and nitrogen called cyanogen—best known to convicted murderers as the active ingredient of the gas administered by their executioners—will become excited by exposure to microwaves. If the microwaves are warmer than the ones in our CBR, they will excite the molecule a little more effectively than our microwaves do. The cyanogen compounds thus act as a cosmic thermometer. When we observe them in distant, and thus younger, galaxies, they should find themselves bathed in a warmer cosmic background than the cyanogen in our Milky Way galaxy. In other words, those galaxies ought to live more excited lives than we do. And they do. The spectrum of cyanogen in distant galaxies shows the microwaves to have just the temperature we expect at these earlier cosmic times.

You can’t make this stuff up.

The CBR does far more for astrophysicists than to provide direct evidence for a hot early universe, and thus for the big bang model. It turns out that the details of the photons that comprise the CBR reach us laden with information about the cosmos both before and after the universe became transparent. We have noted that until that time, about 380,000 years after the big bang, the universe was opaque, so you couldn’t have witnessed matter making shapes even if you’d been sitting front-row center. You couldn’t have seen where galaxy clusters were starting to form. Before anybody, anywhere, could see anything worth seeing, photons had to acquire the ability to travel, unimpeded, across the universe. When the time was right, each photon began its cross-cosmos journey at the point where it smacked into the last electron that would ever stand in its way. As more and more photons escaped without being deflected by electrons (thanks to electrons joining nuclei to form atoms) they created an expanding shell of photons that astrophysicists call “the surface of last scatter.” That shell, which formed during a period of about a hundred thousand years, marks the epoch when almost all the atoms in the cosmos were born.