The 4-Percent Universe

by Richard Panek

  Einstein might have agreed. A sound theory needs to make at least one specific prediction. General relativity made two. One involved an infamous problem of Einstein's era. The orbit of Mercury seemed to be slightly wrong, at least according to Newton's laws. The observable differences between Newton's and Einstein's versions of gravity were negligible—except in circumstances involving the most extreme cases, such as a tiny planet traveling close to a gargantuan star. Newton's equations predicted one path for Mercury's orbit. Observations of Mercury revealed another path. And Einstein's equations accounted precisely for the difference.

  Another prediction involved the effect of gravity on light. A total eclipse of the Sun would allow astronomers to compare the apparent position of stars near the rim of the darkened Sun with their position if the Sun weren't there. According to general relativity, the background starlight should appear to "bend" by a certain amount as it skirted the great gravitational grip of the Sun. (Actually, in Einstein's theory it's space itself that bends, and light just goes along for the ride.) In 1919 the British astronomer Arthur Eddington organized two expeditions to observe the position of the stars during an eclipse on May 29, one expedition to Principe, an island off the west coast of Africa, the other to Sobral, a city in northeastern Brazil. The announcement in November 1919 that the results of the experiments seemed to validate the theory made both Einstein and general relativity international sensations.*

  Yet Einstein himself downplayed the theory's power to predict "tiny observable effects"—its influence on physics. Instead, he preferred to emphasize "the simplicity of its foundation and its consistency"—its mathematical beauty. Mathematicians tended to agree, as did physicists such as Dicke's professor at the University of Rochester. General relativity's known effects in the universe—an anomaly in the orbit of a planet, the deflection of starlight—were obscure in the extreme; its unknown effects on the his tory of the universe—cosmology—were speculative in the extreme. Even so, Einstein also acknowledged that if the theory made a prediction that observations contradicted, then, as would be the case with any theory under the standards of the scientific method, science should amend or abandon it.

  By the time Dicke joined the Princeton faculty in the 1940s, after the war, Einstein was as much a spectral presence in life as his theory was in experimental physics. Sometimes a seemingly homeless man would shuffle into a faculty party, and the younger folks in the crowd would need a moment to recognize the shock of hair and the basset-hound eyes. During the 1954–55 academic year, Dicke took a sabbatical leave at Harvard, and he found himself returning to thoughts of general relativity. As a scientist who was equally at ease designing equipment and constructing theories, Dicke realized he could do what previous generations couldn't have done with their existing technology. When he returned to Princeton, he resolved to put Einstein to the test.

  His experiments over the coming years would involve placing occulting disks in front of the Sun to determine its precise shape, which affects its gravitational influence on the objects in the solar system, including Mercury; bouncing lasers off the Moon and using the round-trip time to measure its distance from Earth, which would indicate if its orbit was varying from Einstein's math in the same way that Mercury's orbit varied from Newton's math; and using the chemical composition of stars to trace their age and evolution, which in turn would be important for tracing the age and evolution of the universe, which in turn would involve an attempt to detect the relic radiation from the primeval atom, cosmic fireball, Big Bang, or whatever you wanted to call it. Dicke wondered if a theory of the universe could avoid not only a Big Bang singularity but the Steady State's spontaneous creation of matter, and he proposed a compromise of sorts: an oscillating universe.

  Such a universe would bounce from expansion to contraction to expansion throughout eternity, without ever reaching absolute collapse or, between collapses, eternal diffusion. During the expansion phase of such a universe, galaxies would exhibit redshifts consistent with what astronomers were already observing. Eventually the expansion would slow down under the influence of gravity, then reverse itself. During the contraction phase, galaxies would exhibit blueshifts as they gravitated back together. Eventually the contraction would reach a state of such compression that it would explode outward again, before the laws of physics broke down. Dicke's oscillating universe would therefore neither emerge from nor return to the dreaded singularity, though the earliest period of its current expansion would resemble a Big Bang. During one particularly muggy meeting of the Gravity Group, Dicke ended a discussion of that theory by turning to two of his Dicke birds, Peter Roll and David Todd Wilkinson, and saying, "Why don't you look into making a measurement?" They could build a radio antenna to detect the radiation from the most recent Big Bang. Then he turned to a twenty-nine-year-old postdoc and said, "Why don't you go think about the theoretical consequences?"

  Jim Peebles had already forced himself to learn cosmology. As a graduate student at Princeton he had been required to pass the Physics Department's general examinations, and when he looked at previous years' exams, he saw that they reliably included questions on general relativity and cosmology. So he studied the standard texts of the day, Classical Theory of Fields, by Lev Landau and Evgeny Lifschitz, from 1951, and Relativity, Thermodynamics, and Cosmology, by Richard C. Tolman, from 1934. Both books came with a whiff of formaldehyde; they presented cosmology in the embalmed terms of long-settled truths. The more Peebles educated himself about cosmology, the less he trusted it. General relativity itself excited him; he was a loyal and enthusiastic member of the Gravity Group. What appalled him were the assumptions that theorists had forcibly yoked to general relativity in order to create their cosmologies.

  The trouble, Peebles saw, started with Einstein. In 1917, two years after arriving at the theory of general relativity, Einstein published a paper exploring its "cosmological considerations." What might general relativity say about the shape of the universe? In order to simplify the math, Einstein had made an assumption: The distribution of matter in the universe was homogeneous—that is, uniform on a large scale. It would look the same no matter where you were in it. In calculating the implications of Einstein's theory, Georges Lemaitre and, independently, the Russian mathematician Aleksandr Friedman had adopted the same assumption and added one more, that the universe is isotropic—uniform in every direction. It would look the same no matter which way you looked. Then the Steady State theory went the Big Bang one assumption better: The universe is homogeneous and isotropic not only throughout space but over time. It would look the same in every direction no matter where you were in it and no matter when.

  Peebles tried to be fair; he attended a lecture on the Steady State theory. But he came away thinking, "They just made this up!" To Peebles, a homogeneous universe, whether in space or time or both, was not a serious model. Tolman's book came right out and said as much: Theorists assumed homogeneity "primarily in order to secure a definite and relatively simple mathematical model, rather than to secure a correspondence to known reality." This approach reminded Peebles of those oversimplified problems on exams: Calculate the acceleration of a frictionless elephant on an inclined plane.

  "Boy, this is silly," Peebles thought. Why, he asked himself, would anyone imagine the universe to be, of all the things that a universe could be, simple? Yes, scientists preferred to follow the principle of Ockham's razor, dating back to the fourteenth-century Franciscan friar William of Ockham: Try the simplest assumptions first and add complications only as necessary. So Einstein's invocation of a homogeneous universe had a certain logic to it, a legacy behind it—but not enough to be the basis of a science that made predictions that led to observations.

  Yet when Dicke approached him about figuring out the temperature of the most recent Big Bang in an oscillating universe, Peebles immediately accepted the challenge. First, the request came from Bob Dicke, and you had to trust his hunches. Besides, Peebles shared not only his mentor's enthusiasm f
or exploring general relativity but Dicke's reservations about cosmology. Only a year earlier, in 1963, in an article on cosmology and relativity for the American Journal of Physics, Dicke had written: "Having its roots in philosophic speculations, cosmology evolved gradually into a physical science, but a science with so little observational basis that philosophical considerations still play a crucial if not dominant role."

  What appealed to Peebles was the chance to shore up that "observational basis"—the experimental implications. It was the possibility that his calculations might lead to an actual measurement, one that Roll and Wilkinson would make, using the radio antenna that Dicke had assigned them to build. They would be doing cosmology the scientific way: The appearances were going to have to accommodate Jim Peebles's math.

  The first hint that radio waves might offer a new way of seeing the universe dated to the 1930s—again, through an accidental detection at Bell Labs. In 1932 an engineer who had been trying to rid transatlantic radiotelephone transmissions of mystery static figured out that the noise was coming from the stars of the Milky Way. The news made the front page of the New York Times but then receded into obscurity. Even astronomers regarded the discovery as a novelty. Not until after World War II did the use of radio waves to study astronomy become widespread.

  Radio astronomy turned out to be part of a larger dawning of awareness among astronomers that the range of the electromagnetic spectrum beyond the narrow optical band might contain useful information. The wavelengths to which human eyes have evolved to be sensitive range from 1/700,000th of a centimeter (red) to 1/400,000th of a centimeter (violet). To either side of that narrow window of sight, the lengths of electromagnetic waves increase and decrease by a factor of about one quadrillion, or 1,000,000,000,000,000. The Princeton experiment would concentrate on some of the longest waves because they would have the lowest energy—the kind that radiation that had been cooling from very nearly the beginning of time would have reached by now.

  Peebles began by using the present constitution of the universe to work backward toward the primordial conditions. The present universe is about three-quarters hydrogen, the lightest element; its atomic number is 1, meaning that it has one proton. In order for such an abundance of hydrogen to have survived to the present day, the initial conditions must have contained an intense background of radiation, because only an extraordinarily hot environment could have fried atomic nuclei fast enough to keep all those single protons from fusing with other subatomic particles to form helium and heavier elements. As the universe expanded—as its volume grew—its temperature fell. Extrapolate from the current percentage of hydrogen how intense the initial radiation must have been, calculate how much the volume of the universe has expanded since then, and you have the temperature to which the initial radiation would by now have cooled.

  A radio antenna, however, doesn't measure temperature, at least not directly. The temperature of an object determines the motions of its electrons—the higher the temperature, the greater the motions. The motions of the electrons in turn are what produce radio noise—the greater the motions, the more intense the noise. The intensity of the noise therefore tells you how much the electrons are moving, which tells you the temperature of the object—or what engineers call the "equivalent temperature" of the radio noise. In a box with opaque walls, the only source of radio noise will be the motions of the electrons in the walls. If you place a radio receiver in a box that happens to be the universe, then the intensity of the static will tell you the equivalent temperature of the walls of the universe: the relic radiation.

  In 1964 Peebles got to work predicting the current temperature of the relic radiation—the equivalent temperature of the static that an antenna would need to detect. Meanwhile, his colleagues Roll and Wilkinson began work on the antenna—technically, a Dicke radiometer, invented by Dicke to refine radar sensitivity during the war, while he was working at the Radiation Laboratory at the Massachusetts Institute of Technology. In early 1965, Peebles received an invitation from Johns Hopkins University's Applied Physical Laboratory to give a talk, and he asked Wilkinson if he could mention the radiometer in public.

  "No problem," Wilkinson said. "No one could catch up with us now.

  What happened next happened fast. Peebles delivered his talk on February 19. In the audience was a good friend of Peebles's from graduate school (and a former Dicke bird), Kenneth Turner, a radio astronomer at the Carnegie Institution's Department of Terrestrial Magnetism (DTM), in Washington, D.C. The experiment made an impression on Turner, and a day or two later he mentioned the colloquium to another radio astronomer at DTM, Bernard Burke. Another day or two later, during a communal lunch, Burke got a phone call from a Bell Labs radio astronomer he'd met in December on a plane ride to an American Astronomical Society meeting in Montreal. Burke went into the kitchen's anteroom to take the call. After a brief discussion, Burke made small talk. "How is that crazy experiment of yours coming?" he said.

  On the flight to Montreal, Arno Penzias had described for Burke the work he and Bob Wilson were doing on Crawford Hill. He had told Burke that they hoped to study the radio waves from the stars not in the big bulge at the center of the Milky Way, where most astronomers had been looking, but in the other direction, at the fringe of the Milky Way halo. But now, he said to Burke on the phone, they'd run into a problem even before they could begin their observations.

  "We have something we don't understand," Penzias said. He explained that he and Wilson couldn't get rid of an excess noise corresponding to a temperature near, but not quite, absolute zero. When Penzias had finished describing their efforts and their frustration, Burke said, "You should probably call Bob Dicke at Princeton."

  The Big Bang was a creation myth, but by 1965 it was a creation myth with a difference: It came with a prediction. By the time Penzias placed his call to Dicke, Peebles had arrived at a temperature of approximately 10° Celsius above absolute zero, which is more commonly referred to as 10 Kelvin.* Penzias and Wilson had found a measurement of 3.5 K (plus or minus 1 K) in their antenna. Because Peebles's calculations were rudimentary and Penzias and Wilson's detection was serendipitous, the approximation of theory and observation was hardly definitive. Yet it was also too close to dismiss as coincidence.

  At the very least it was worth recording for posterity. After the Crawford Hill meeting and a reciprocal meeting at Princeton, the two sets of collaborators agreed that they would each write a paper, to appear side by side in the Astrophysical Journal. The Princeton foursome would go first, discussing the possible cosmological implications of the detection. Then the Bell Labs duo would confine their discussion to the detection itself, so as not to align their measurement too closely with a wild interpretation that, as Wilson said, it "might outlive."

  On May 21, 1965, even before their papers appeared, the New York Times broke the story: "Signals Imply a 'Big Bang' Universe." (The reporter had been in contact with the Astrophysical Journal about another upcoming paper when he heard about these two papers.) The prominence of the coverage—placement on the front page; an accompanying photograph of the Bell Labs telescope—impressed some of the scientists in the two collaborations with the possible impact of their (possible) discovery. Peebles, though, didn't need the news media to tell him they were onto something big. All he had to do was look at Dicke. Dicke could be humorous and lighthearted, but not about physics. In recent weeks, though, he had clearly been enjoying himself in a different way. After talking to Dicke, one longtime Princeton astronomer reported back to his peers that Bob Dicke was "bubbling with excitement."

  A subsequent search of the literature turned up other predictions and at least one previous detection. In 1948, the physicist George Gamow had written a Nature paper that predicted the existence of "the most ancient archeological document pertaining to the history of the universe." He was wrong on the details but right on the general principle: The early universe had to be extremely hot to avoid combining all the hydrogen into heavier elements.
That same year, the physicists (and sometime collaborators of Gamow's) Ralph Alpher and Robert Herman published their calculation that "the temperature in the universe" should now be around 5 K, but astronomers at the time assured them that such a detection would be impossible with current technology. (In retrospect Wilson felt that they probably could have performed it with World War II-era technology, as long as they had properly connected the antenna to the cold load.) In a 1961 article in the Bell System Technical Journal, a Crawford Hill engineer wrote that the Echo antenna was picking up an excess of 3 K; but that reading fell within the margin of error, and the discrepancy wasn't going to make a difference for his purposes anyway, so he ignored it. In 1964, Steady State champion Hoyle, working with fellow British astronomer Roger J. Tayler, investigated the oscillating-universe scenario and performed calculations similar to those of Alpher and Herman. Also in 1964, even as Penzias and Wilson were directing their antenna to every point on the horizon in a futile effort to find the source of their excess noise, two Russian scientists published a paper pointing out that a detection of the cosmic background radiation was currently possible—and that the ideal instrument was a certain horn antenna on a hilltop in Holmdel Township, New Jersey.

  Jim Peebles had a high metabolism; he could eat whatever he wanted and not worry about gaining weight. This inherent restlessness extended to his intellectual life. He loved identifying the next big problem, solving it, seeing where it led, identifying that big problem, solving it, seeing where it led: a bend-in-the-knees, wind-in-the-face rush into the future. (He was an expert downhill skier.) Even the description of his intellectual restlessness that he once gave to a journalist was restless: "a random walk, no, an undirected walk, or rather a locally directed walk: as you take each step you decide where the next one is going to go." The library part of the scholarly process, however, the burrowing into the stacks, the boning up on the literature—maybe it didn't bore him, exactly, but it didn't engage him either. In any case, he hadn't done his homework.