by Brian Greene
Inflationary cosmology modifies the big bang theory by inserting an intense burst of enormously fast expansion during the universe’s earliest moments. This modification, as we will see, proves essential to explaining some otherwise perplexing features of the relic radiation. But more than that, inflationary cosmology is a key chapter in our story because scientists have gradually realized over the last few decades that the most convincing versions of the theory yield a vast collection of parallel universes, radically transforming the complexion of reality.
Relics of a Hot Beginning
George Gamow, a hulking six-foot-three Russian physicist known for important contributions to quantum and nuclear physics in the early twentieth century, was as quick-witted and fun-loving as he was hard-living (in 1932, he and his wife tried to defect from the Soviet Union by paddling across the Black Sea in a kayak stocked with a healthy assortment of chocolate and brandy; when bad weather sent the two scurrying back to shore, Gamow was able to fast-talk the authorities with a tale of the unfortunately failed scientific experiments he’d been undertaking at sea). In the 1940s, after having successfully slipped past the iron curtain (on dry land, with less chocolate) and settled in at Washington University in St. Louis, Gamow turned his attention to cosmology. With critical assistance from his phenomenally talented graduate student Ralph Alpher, Gamow’s research resulted in a far more detailed and vivid picture of the universe’s earliest moments than had been revealed by the earlier work of Friedmann (who had been Gamow’s teacher back in Leningrad) and Lemaître. With a little modern updating, Gamow and Alpher’s picture looks like this.
Just after its birth, the stupendously hot and dense universe experienced a frenzy of activity. Space rapidly expanded and cooled, allowing a particle stew to congeal from the primordial plasma. For the first three minutes, the rapidly falling temperature remained sufficiently high for the universe to act like a cosmic nuclear furnace, synthesizing the simplest atomic nuclei: hydrogen, helium, and trace amounts of lithium. But with the passing of just a few more minutes, the temperature dropped to about 108 Kelvin (K), roughly 10,000 times the surface temperature of the sun. Although immensely high by everyday standards, this temperature was too low to support further nuclear processes, and so from this time on the particle commotion largely abated. For eons that followed, not much happened except that space kept expanding and the particle bath kept cooling.
Then, some 370,000 years later, when the universe had cooled to about 3000 K, half the sun’s surface temperature, the cosmic monotony was interrupted by a pivotal turn of events. To that point, space had been filled with a plasma of particles carrying electric charge, mostly protons and electrons. Because electrically charged particles have the unique ability to jostle photons—particles of light—the primordial plasma would have appeared opaque; the photons, incessantly buffeted by electrons and protons, would have provided a diffuse glow similar to a car’s high beams cloaked by a dense fog. But when the temperature dropped below 3000 K, the rapidly moving electrons and nuclei slowed sufficiently to amalgamate into atoms; electrons were captured by the atomic nuclei and drawn into orbit. This was a key transformation. Because protons and electrons have equal but opposite charges, their atomic unions are electrically neutral. And since a plasma of electrically neutral composites allows photons to slip through like a hot knife through butter, the formation of atoms allowed the cosmic fog to clear and the luminous echo of the big bang to be released. The primordial photons have been streaming through space ever since.
Well, with one important caveat. Although no longer knocked to and fro by electrically charged particles, the photons have been subject to one other important influence. As space expands, things dilute and cool, including photons. But unlike particles of matter, photons don’t slow down when they cool; being particles of light, they always travel at light speed. Instead, when photons cool their vibrational frequencies decrease, which means they change color. Violet photons will shift to blue, then to green, to yellow, to red, and then into the infrared (like those visible with night goggles), the microwave (like those that heat food by bouncing around your microwave oven), and finally into the domain of radio frequencies.
As Gamow first realized and as Alpher and his collaborator Robert Herman worked out with greater fidelity, all this means that if the big bang theory is correct, then space everywhere should now be filled with remnant photons from the creation event, streaming every which way, whose vibrational frequencies are determined by how much the universe has expanded and cooled during the billions of years since they were released. Detailed mathematical calculations showed that the photons should have cooled close to absolute zero, placing their frequencies in the microwave part of the spectrum. For this reason, they are called the cosmic microwave background radiation.
I recently reread the papers of Gamow, Alpher, and Herman that in the late 1940s announced and explained these conclusions. They are marvels of theoretical physics. The technical analyses involved require hardly more than a grounding in undergraduate physics, and yet the results are profound. The authors concluded that we are all immersed in a bath of photons, a cosmic heirloom bequeathed to us by the universe’s fiery birth.
With that buildup, you may find it surprising that the papers were ignored. This was mostly because they were written during an era dominated by quantum and nuclear physics. Cosmology had yet to make its mark as a quantitative science, so the physics culture was less receptive to what seemed like fringe theoretical studies. To some degree, the papers also languished because of Gamow’s unusually playful style (he once modified the authorship of a paper he was writing with Alpher to include his friend the future Nobel laureate Hans Bethe, just to make the paper’s byline—Alpher, Bethe, Gamow—sound like the first three letters of the Greek alphabet), which resulted in some physicists taking him less seriously than he deserved. Try as they might, Gamow, Alpher, and Herman could not interest anyone in their results, let alone persuade astronomers to devote the significant effort required to attempt to detect the relic radiation they predicted. The papers were quickly forgotten.
In the early 1960s, unaware of the earlier work, the Princeton physicists Robert Dicke and Jim Peebles went down a similar path and also realized that the big bang’s legacy should be the presence of a ubiquitous background radiation filling space.1 Unlike the members of Gamow’s team, however, Dicke was a renowned experimentalist and so didn’t need to persuade anyone to seek the radiation observationally. He could do it himself. Together with his students David Wilkinson and Peter Roll, Dicke devised an experimental scheme to capture some of the big bang’s vestigial photons. But before the Princeton researchers could put their plan to the test, they received one of the most famous telephone calls in the history of science.
While Dicke and Peebles had been calculating, the physicists Arno Penzias and Robert Wilson at Bell Labs, less than thirty miles from Princeton, had been struggling with a radio communications antenna (coincidentally, it was based on a design Dicke had come up with in the 1940s). No matter what adjustments they made, the antenna hissed with a steady, unavoidable background noise. Penzias and Wilson were convinced that something was wrong with their equipment. But then came a serendipitous chain of conversations. It began with a talk Peebles gave in February 1965 at Johns Hopkins University, which was attended by the Carnegie Institution radio astronomer Kenneth Turner, who mentioned the results he heard Peebles present to his MIT colleague Bernard Burke, who happened to be in touch with Penzias at Bell Labs. Hearing of the Princeton research, the Bell Labs team realized that their antenna was hissing for good reason: it was picking up the cosmic microwave background radiation. Penzias and Wilson called Dicke, who quickly confirmed that they had unintentionally tapped into the reverberation of the big bang.
The two groups agreed to publish their papers simultaneously in the prestigious Astrophysical Journal. The Princeton group discussed their theory of the background radiation’s cosmological origin, while the
Bell Labs team reported, in the most conservative of language and with no mention of cosmology, the detection of uniform microwave radiation permeating space. Neither paper mentioned the earlier work of Gamow, Alpher, and Herman. For their discovery, Penzias and Wilson were awarded the 1978 Nobel Prize in physics.
Gamow, Alpher, and Herman were deeply dismayed, and in the years that followed struggled mightily to have their work recognized. Only gradually and belatedly has the physics community saluted their primary role in this monumental discovery.
The Uncanny Uniformity of Ancient Photons
During the decades since it was first observed, the cosmic microwave background radiation has become a crucial tool in cosmological investigations. The reason is clear. In a great many fields, researchers would give their eyeteeth to have an unfettered, direct glimpse of the past. Instead, they generally have to piece together a view of remote conditions on the basis of evidence from remnants—weathered fossils, decaying parchments, or mummified remains. Cosmology is the one field in which we can actually witness history. The pinpoints of starlight we can see with the naked eye are streams of photons that have been traveling toward us for a few years or a few thousand. The light from more distant objects, captured by powerful telescopes, has been traveling toward us far longer, sometimes for billions of years. When you look at such ancient light, you are seeing—literally—ancient times. Those primeval comings and goings transpired far away, but the apparent large-scale uniformity of the universe argues strongly that what was happening there was also, on average, happening here. In looking up, we are looking back.
The cosmic microwave photons allow us to make the most of this opportunity. No matter how technology may improve, the microwave photons are the oldest we can hope to see, because their elder brethren were trapped by the foggy conditions that prevailed during earlier epochs. When we examine the cosmic microwave background photons, we are glimpsing how things were nearly 14 billion years ago.
Calculations show that today there are about 400 million of these cosmic microwave photons racing through every cubic meter of space. Although our eyes can’t see them, an old-fashioned television set can. About 1 percent of the snow on a television that’s been disconnected from the cable signal and tuned to a station that’s ceased broadcasting is due to reception of the big bang’s photons. It’s a curious thought. The very same airwaves that carry reruns of All in the Family and The Honeymooners are infused with some of the universe’s oldest fossils, photons communicating a drama that played out when the cosmos was but a few hundred thousand years old.
The big bang model’s correct prediction that space would be filled with microwave background radiation was a triumph. During a mere three hundred years of scientific thought and technological progress, our species went from peering through rudimentary telescopes and dropping balls from leaning towers to grasping physical processes at work just after the universe was born. Nevertheless, further investigation of the data raised a pointed challenge. Ever more refined measurements of the radiation’s temperature, made not with television sets but with some of the most precise astronomical equipment ever built, showed that the radiation is thoroughly—uncannily—uniform across space. Regardless of where you point your detector, the temperature of the radiation is 2.725 degrees above absolute zero. The puzzle is to explain how such fantastic uniformity came to be.
Given the ideas presented in Chapter 2 (and my comment four paragraphs ago), I can imagine your saying, “Well, that’s just the cosmological principle at work: no location in the universe is special when compared with any other, so the temperature at each should be the same.” Fair enough. But remember that the cosmological principle was a simplifying assumption that physicists, including Einstein, invoked to make the mathematical analysis of the universe’s evolution tractable. Since the microwave background radiation is indeed uniform throughout space, it provides convincing observational evidence for the cosmological principle, and it strengthens our confidence in conclusions the principle helped reveal. But the radiation’s astounding uniformity shines a glaring spotlight on the cosmological principle itself. Reasonable though the cosmological principle may sound, what mechanism established the cosmos-wide uniformity that observations confirm?
Faster Than the Speed of Light
We’ve all had the mildly unsettling sensation of shaking someone’s hand and finding it steamy hot (not so bad) or clammy cold (definitely worse). But were you to hold on to that hand, you’d find that the modest temperature differential would quickly subside. When objects are in contact, heat migrates from the hotter to the colder, until their temperatures are equal. You experience this all the time. It’s why coffee left on your desk eventually comes to room temperature.
Similar reasoning would seem to explain the uniformity of the microwave background radiation. As with holding hands and standing coffee, the uniformity presumably reflects the familiar reversion of an environment to an overall common temperature. The sole novelty of the process is that the reversion is supposed to have taken place over cosmic distances.
In the big bang theory, however, the explanation fails.
For places or things to reach a common temperature, an essential condition is mutual contact. It may be direct, as with shaking hands, or, minimally, through an exchange of information so that conditions at distinct locations can become correlated. Only through such mutual influence can a shared, communal environment be achieved. A thermos is designed to prevent such interactions, thwarting the drive to uniformity and preserving temperature differences.
This simple observation highlights the problem with the naïve explanation of the cosmic temperature uniformity. Locations in space that are very far apart—say, one point way off to your right, so deep in the night sky that the first light it ever emitted has only just reached you, and a second, similar point way off to your left—have never interacted. Although you can see both, light from one still has an enormous distance to cover before it reaches the other. Thus, hypothetical observers situated at the distant left and right locations have yet to see each other, and since the speed of light sets the upper limit for how fast anything can travel, they’ve yet to interact in any way. To use the language of the previous chapter, they are beyond each other’s cosmic horizon.
This description makes the mystery manifest. You’d be floored if inhabitants of these distant locations spoke the same language and had libraries filled with the same books. With no contact, how could a common heritage have been established? You should be equally floored to learn that without any apparent contact, these widely separated regions share a common temperature, one that matches to an accuracy of better than four decimal places.
Years ago, when I first learned of this puzzle, I was floored. But on further thought, I became puzzled by the puzzle. How could two objects that were once close together—as we believe all things in the observable universe were at the time of the big bang—have separated so quickly that light emitted by one wouldn’t have time to reach the other? Light sets the cosmic speed limit, so how could the objects achieve a spatial separation greater than what light would have had time to traverse?
The answer highlights a point that’s often not adequately stressed. The speed limit set by light refers solely to the motion of objects through space. But galaxies recede from one another not because they are traveling through space—galaxies don’t have jet engines—but rather because space itself is swelling and the galaxies are being dragged along by the overall flow.2 And the thing is, relativity places no limit on how fast space can swell, so there is no limit on how fast galaxies that are being pushed apart by the swell recede from one another. The rate of recession between any two galaxies can exceed any speed, including the speed of light.
Indeed, the mathematics of general relativity shows that in the universe’s earliest moments, space would have swelled so fast that regions would have been propelled apart at greater than light speed. As a result, they would have been unable to exert an
y influence on one another. The difficulty then is to explain how nearly identical temperatures were established in independent cosmic domains, a puzzle cosmologists have named the horizon problem.
Broadening Horizons
In 1979, Alan Guth (then working at the Stanford Linear Accelerator Center) came up with an idea that, with subsequent critical refinements made by Andrei Linde (then carrying out research at the Lebedev Physical Institute in Moscow), and by Paul Steinhardt and Andreas Albrecht (a professor-student duo who were then working at the University of Pennsylvania), is widely believed to solve the horizon problem. The solution, inflationary cosmology, relies on some subtle features of Einstein’s general relativity that I’ll describe in a moment, but its broad outline can be readily summarized.
The horizon problem afflicts the standard big bang theory because regions of space separate too quickly for thermal equality to be established. The inflationary theory resolves the problem by slowing the speed with which the regions were separating very early on, providing them ample time to come to the same temperature. The theory then proposes that after the completion of these “cosmic handshakes” there came a brief burst of enormously fast and ever-quickening expansion—called inflationary expansion—which more than compensated for the sluggish start, rapidly driving the regions to vastly distant positions in the sky. The uniform conditions we observe no longer pose a mystery, since a common temperature was established before the regions were rapidly driven apart.3 In broad strokes, that’s the essence of the inflationary proposal.*
Bear in mind, however, that physicists don’t dictate how the universe expands. As far as we can tell from our most refined observations, Einstein’s equations of general relativity do. The viability of the inflationary scenario thus depends on whether its proposed modification to the standard big bang expansion can emerge from Einstein’s mathematics. At first glance, this is far from obvious.