Coming of Age in the Milky Way - Timothy Ferris (2003)
Part I. SPACE
Chapter 11. THE EXPANSION OF THE UNIVERSE
Nature lives in motion.
Eyesight should learn from reason.
When Einstein began to investigate the cosmological implications of the general theory of relativity, he found something strange and disturbing: The theory implied that the universe as a whole could not be static, but must be either expanding or contracting. This was a completely novel idea, and one for which there was, at the time, no observational evidence whatever: The astronomers he consulted informed Einstein that stars wander more or less randomly through space, but display no concerted motion of the sort that would suggest cosmic expansion or contraction. Faced with this disjunction between his theory and the empirical data, Einstein reluctantly concluded that there must be something wrong with the theory, and he modified its equations by adding a term that he called the cosmological constant. Symbolized by the Greek letter lambda, the new term was intended to make the radius of the universe hold steady with the passing of time.
Einstein never liked the cosmological constant. He called it “gravely detrimental to the formal beauty of the theory,” pointing out that it was nothing more than a mathematical fiction, without any real physical basis, one that had been introduced solely to being the theory into accord with the observational facts. As he wrote in 1917:
[W]e admittedly had to introduce an extension of the field equations of gravitation which is not justified by our actual knowledge of gravitation…. That term is necessary only for the purpose of making possible a quasi-static distribution of matter, as required by the fact of the small velocities of the stars.1
Moreover, as soon became apparent, the term did not even accomplish its avowed function of making the relativistic universe stand still. The Russian mathematician Aleksandr Friedmann found that Einstein in introducing the term had made an algebraic error, dividing by a quantity that could be zero. When Friedmann corrected the error, general relativity broke free of its fetters and the relativistic universe, to Einstein’s frustration, once again took on wings.
Connoisseurs of irony’s serrated edge will appreciate that it was in 1917, the very year that Einstein besmirched his general theory of relativity by introducing the cosmological term, that the American astronomer Vesto Slipher published a paper containing the first observational evidence that the universe is in fact expanding.
Slipher knew nothing of general relativity. He was a nose-to-the-grindstone staffer at Lowell Observatory, in Flagstaff, Arizona, an isolated and idiosyncratic private institution so remote from the theoretical physics community that it might as well have been on the far side of the moon. His employer was Percival Lowell, of the Boston Lowells, a loftily unconventional thinker remembered chiefly for having charted the (illusory) canals of Mars, which he took to be global waterways dug by a parched alien civilization desperately importing water from the polar ice caps. Like many astronomers of his day, Lowell thought that the spiral nebulae were Laplacian solar systems aborning. To test this thesis, he assigned Slipher to take spectra of a number of spirals, using a new and more efficient spectrograph, in order to search for the rotation velocities characteristic of Laplacian nebulae eddying their way into stars and planets. Slipher did, indeed, find evidence of rotation in the spirals—as Edwin Hubble would find, this was actually the motion of billions of stars orbiting in spiral galaxies—but he also found, superimposed on the rotation velocities, an enormous displacement in the spectral lines of most spirals toward the red end of the spectrum.
The only reasonable explanation for this astonishing finding was that Slipher was observing Doppler shifts. The name comes from the Austrian physicist Christian Johann Doppler, who noted in 1842 that light, sound, or other radiation coming from a moving source is received at a higher frequency if the source emanating it is approaching, and at a lower frequency if the source is receding. (It is owing to this “Doppler shift” that an automobile horn sounds higher in pitch if the car is approaching and lower if it is speeding away.) Astronomers had long been making use of Doppler shifts in spectra to measure the velocities of stars: The spectral lines of stars that were moving toward the sun would be displaced toward the blue, while those of stars moving away from the sun would be displaced toward the red, or lower frequency, end of the spectrum. Indeed, it was by virtue of just such measurements that astronomers had been able to inform Einstein that stellar motions in the Milky Way were generally random.
The velocities of the spiral nebulae implied by Slipher’s redshifts, however, were much more rapid than those of the stars. Two of the first fifteen spirals Slipher observed were moving at over two million miles per hour. Even more unexpectedly, their motions were concerted: Twenty-one of the twenty-five spirals for which Slipher had accumulated spectra by 1917 were displaced toward the red, indicating that they were flying away from each other and from the earth. (The exceptions, we understand today, were nearby galaxies that are gravitationally bound to the Milky Way in the Local Group of galaxies, and therefore do not participate in the cosmic expansion.)
But, though Slipher’s findings were provocative, they did not in themselves indicate that the universe is expanding. Slipher had no way of knowing the distances to the galaxies whose redshifts he obtained—nor, for that matter, of ascertaining that they were galaxies. That shoe was to be dropped, instead, by Edwin Hubble.
Hubble was thirty years old by the time he was discharged from service as a private in the American Expeditionary Force in France and began training the big telescopes at Mount Wilson on the spiral nebulae, but he worked fast: Only five years later he was able to write Harlow Shapley that he had found Cepheid variable stars in the spirals, establishing that they are galaxies and making it possible to estimate their distances. Five years after that, in 1929, he was able to plot the distances of twenty-five galaxies against his and Slipher’s measurements of their redshifts. The result was a straight line—a direct correlation between distance and velocity of recession.
Inscribed in the Hubble diagram was the signature of cosmic expansion. Imagine that the earth were expanding, and let the two-dimensional surface of the earth stand for three-dimensional cosmic space. In such a situation, every observer would find that every other city on Earth was receding from his city, each at a velocity directly proportional to its distance. If, for instance, the rate of expansion were such that the earth doubled in diameter every hour, then the distances between cities also would double each hour. Chicago and Memphis are five hundred miles apart; therefore observers in Chicago would find that Memphis was receding at a rate of five hundred miles per hour. Looking out to San Francisco, eighteen hundred miles away, the Chicagoans would find that its velocity of recession was fully eighteen hundred miles per hour. And that, a velocity-distance relation, is what Hubble found for the galaxies.
It was also, of course, just what the general theory of relativity had predicted, at least before being fettered by the lambda term. (Fumed Einstein, “If Hubble’s expansion had been discovered at the time of the creation of the general theory of relativity, the [cosmological constant] would never have been introduced.”)2 Yet Hubble, like Slipher, was isolated by the gulf that still separated the world of the American observational astronomers from that of Einstein and the other leading theoretical physicists in Europe. Hubble knew next to nothing of general relativity; neither did his boss, George Ellery Hale, who confessed that “the complications of the theory of relativity are altogether too much for my comprehension,” adding, “I fear it will always remain beyond my grasp.”3 Nor had either man heard of relativity’s prediction that the universe might be expanding. Lacking a theoretical explanation for what he had observed, Hubble, alert to the fact that “observations always involve theory,”4 was reluctant to draw conclusions about the meaning of his own discovery.* He spoke of the “redshift-distance relation,” and sometimes of “velocity-shifts,” but seldom of what his finding has been called ever since—the discovery of the expansion of the universe. Years later, he was still describing the notion of cosmic expansion as “rather startling.”5
The Hubble law, that galaxies are receding from one another at velocities directly proportional to the distances separating them, holds true across the known universe. This plot includes ten major clusters of galaxies. The boxed area at the lower left represents the galaxies observed by Hubble when he discovered the law.
As it happened, the man who put together Einstein’s relativity with the redshifts of the spirals was neither an eminent theorist nor a skilled observer, but an obscure Belgian priest and mathematician named Georges Lemaître. The son of a Louvain glassmaker and a brewer’s daughter, Lemaître had decided at the age of nine to become both a scientist and a clergyman: “There is,” he liked to say, “no conflict between science and religion.”6 Something of a joiner, Lemaître at Eddington’s suggestion made a tour of the United States, attending scientific conferences and passing out cards bearing his name and address. During the trip he learned of Slipher’s redshifts, and upon his return to Brussels wrote, in 1927, a prophetic paper that erected a mathematical superstructure connecting the observed redshifts with the expanding universe of general relativity.
Nobody noticed. Lemaître published the paper in an obscure journal—an admirably humble but professionally hobbling habit of which he was never to rid himself—and he did not in any event cut the sort of figure that suggests the stamp of genius. Plumply bourgeois in appearance, a homeboy in a priest’s collar, Lemaître was brushed off by the luminaries he approached when the Solvay Conference on Physics convened in Brussels that October. Even the customarily forbearing Einstein lost patience with the entreaties of this pillar of middle-class normalcy. “Vos calculs sont corrects, mais votre physique est abominable,” Einstein told Lemaître—“Your calculations are correct, but your physics is abominable.”7 (Einstein later reconsidered, and in Brussels in 1933 turned over a lecture to Lemaître, reassuring the nervous cleric as he stuttered through his talk by uttering sotto voce pronouncements of “tres joli, tres, tres joli”—“very, very beautiful.”)8
The situation became clearer to Lemaître, if not to his more established colleagues, with the publication of Hubble’s 1929 paper on the redshift-distance relation. In January 1930, Eddington, DeSitter, and the other recognized maestros of theoretical cosmology gathered at a meeting of the Royal Astronomical Society and there labored mightily, and in vain, to erect a mathematical bridge between DeSitter’s relativistic cosmology and Hubble’s discovery. Lemaître read of their efforts in the February issue of The Observatory and wrote to Eddington, reminding him that he had already solved the problem. Eddington sent a copy of Lemaître’s paper to DeSitter, and then, with the generosity and judgment that had informed his similar efforts on Einstein’s behalf years earlier, set about proclaiming to the world that a little-known Belgian mathematics professor had authored the first expanding-universe cosmology. It was thus that Hubble began to learn of the potential significance of his own findings.
Meanwhile, Lemaître had started thinking about the origin of the universe. An expanding universe clearly must once have been very different than it is at present. The galaxies today are millions of light-years apart; once they must have been closer together. Indeed, in the beginning, everything must have been close to everything else. The density of the young universe might have been very high indeed—as high, perhaps, as that of an atomic nucleus. Thinking along converging tracks stretching backward in time, Lemaître began forging the first links between cosmology, the science of the very large, and nuclear physics, the science of the very small.
This extrapolation did not sit well with Lemaître’s champion Eddington. “Since I cannot avoid introducing this question of a beginning,” he wrote, “it has seemed to me that the most satisfactory theory would be one which made the beginning not too unaesthetically abrupt” (Eddington’s italics).9 Eddington imagined that the universe had begun as a stable system, perhaps akin to a star cluster, that had fallen apart in such as way as to produce the cosmic expansion. Lemaître took a more radical tack. He proposed that the universe might have begun as an infinitely small pinpoint—a “singularity,” in mathematical terms—at time zero, “a day without a yesterday” when space was infinitely curved and all matter and all energy was concentrated into a single quantum of energy.10 Lemaître called this genesiac state the “primordial atom,” and its eruption the “big noise.”11 Later the astrophysicist Fred Hoyle, who disliked the whole idea even more than did Eddington, designated the creation event by an intentionally ugly name that stuck. Hoyle called it the big bang.
The gap between European theory and American observations began to close in the early 1930s, when Einstein and many more German Jews, intellectuals, and other undesirables read Hitler’s handwriting on the wall and began emigrating to the United States. In 1931, Einstein visited Mount Wilson, where Hubble, puffing his pipe in Churchillian disdain of observatory protocol, gave him a tour of the dome and showed him the spectrographic evidence of the cosmic expansion that the general theory had foreseen. Two years later, back in southern California, Einstein heard Lemaître describe his theory of the primordial atom at a lecture in the library of the Mount Wilson observatory office on Santa Barbara Street in Pasadena. “In the beginning of everything we had fireworks of unimaginable beauty,” Lemaître said, waxing rhapsodic. “Then there was the explosion followed by the filling of the heavens with smoke. We come too late to do more than visualize the splendor of creation’s birthday.” Einstein rose to his feet at the end of the talk and called Lemaître’s theory “the most beautiful and satisfying interpretation I have listened to.”12*
Insufficiently accomplished to be called a theory, Lemaître’s concept of genesis as a nuclear decay event might better be described as a working hypothesis. Lemaître understood this as well as anyone, and reminded readers of his book The Primeval Atom that “too much importance must not be attached to this description of the primeval atom, a description which will have to be modified, perhaps, when our knowledge of atomic nuclei is more perfect.”13 And yet, however tentatively, Lemaître’s approach anticipated the course of cosmology in the latter twentieth century, helped to set science on that course, and had the salutary immediate effect of inviting nuclear physicists into the cosmological arena. Some accepted the invitation, and the result was an infusion of fresh blood and brainpower into the field. Soon physicists of the caliber of Enrico Fermi, Carl Friedrich von Weizsäcker, and Edward Teller were applying their considerable talents to the question of what went on in the first moments of the big bang.
To the forefront of this effort ambled the deceptively easygoing, good-natured Russian émigré George Gamow. Witty, iconoclastic, and irreverent about the doings of humanity if not those of nature, Gamow like Einstein was one of those rare individuals who seem never to lose their childhood curiosity and sense of wonder. One of the things he wondered about the most was how the universe began.
Gamow’s chief concern, as we will see, had to do with the formation of elements early in the history of the universe. He reasoned that the stuff of the young universe might have been hot and dense enough for atomic nuclei to have been fused into various combinations, creating the elements as we know them. This line of research was to have mixed results in Gamow’s hands (theoretical physics was still insufficiently mature to handle many of the calculations involved) but its portrayal of the early universe as a hot, dense, rapidly evolving plasma gave rise to one of the most potent predictions in the history of science—that of the cosmic background radiation, a ubiquitous, simmering energy left over from the big bang.
Gamow’s hot big-bang concept implied that, if the universe began hot and has been expanding and cooling ever since, its temperature today, though cold, would not be absolutely cold. There should be some residual heat remaining from the big bang. This energy would have been stretched out and thus lowered in frequency by the cosmic expansion: The photons carrying the energy of the big bang, having originated in the wavelengths of light, ought to have been redshifted by the subsequent expansion of the universe into the lower frequencies of electromagnetic energy that we call microwave radio radiation. Gamow’s colleagues Ralph Alpher and Robert Herman, while correcting arithmetic errors in one of Gamow’s big-bang papers, proposed that the universe today is permeated by an ocean of photons with an ambient temperature of “about five degrees” Kelvin.14
At the time, little attention was paid to the prediction of Alpher and Herman that relic radiation should be left over from the big bang. It seemed arcane and, in any case, impossible to verify; radio astronomy was in its infancy, and there was as yet no such thing on Earth as a microwave radiotelescope. A decade later, when radio astronomy had become a reality, Robert Dicke at Princeton University independently hit on the same idea, and set about building a microwave receiver to listen for the cosmic background radiation. He was still at work on it when he learned that two researchers at Bell Laboratories, Arno Penzias and Robert Wilson, were having trouble accounting for a persistent hiss in a microwave horn that Bell had built for satellite communications experiments. The temperature of this unwanted noise was 2.7 degrees. Though none of the three remembered the work of Gamow, Alpher, and Herman, this was just the value that they had predicted (once their calculations were updated to correct for subsequent improvements in the Hubble scale of the age of the universe). Penzias and Wilson won a Nobel Prize in physics for their discovery, and Lemaître, then seventy-two years old, learned of their finding in one of the last conversations of his life.
Today the assertion that we live in an expanding universe rests upon three fundamental lines of research. The first is the Hubble law: The relation between the distances of galaxies and the redshift of their light appears to pertain to the limits of present-day observation—out to hundreds of millions of light-years—and the only known consistent explanation for such a state of affairs is that the redshifts are produced by the recession velocity of galaxies in an expanding universe. The second piece of evidence is the cosmic background radiation: It traces out the “black-body” curve that would characterize the spectrum of photons released in the big bang, and it is received at equal strength from all directions, except for a small an isotropy (or “hot spot”) introduced by the absolute motion of the earth within the overall cosmic framework. The third piece of data is chronological: The age of the universe inferred from the expansion velocity, some ten to twenty billion years, fits with the ages of the oldest known stars, some twelve to sixteen billion years, and with the temperature of the cosmic background radiation itself.
Whatever its other implications for human thought—and there are many—the expansion of the universe had the tremendous advantage of investing cosmology with a dimension of cosmic history. The structure of the universe, from that of atomic nuclei to the vast superclusters of galaxies that stretch across hundreds of millions of light-years of space, may now be seen to have evolved from prior structures; to explain their present disposition clearly requires that we gain a better understanding of their history. Even natural laws themselves may prove to have a mutable past. These considerations will be discussed in Part III of this book. But first we need examine how our species came to comprehend the depths of terrestrial and cosmic history. It’s time for time.
*The one theory Hubble did know about that predicted the redshift-distance relation was that of the Dutch astronomer Willem de Sitter, who had published a model in which redshifts were generated, not by velocity in an expanding universe, but by the “De Sitter effect,” a bit of mathematical arcana with no known physical basis. The chilly reception afforded Hubble’s efforts to link his observations to the De Sitter effect did little to encourage Hubble’s already hesitant ventures into the theoretical side of cosmology.
*Einstein was referring to Lemaître’s contention that cosmic rays, high-energy subatomic particles from space, had been generated in the primordial fireworks. This did not hold up in its specifics, but it anticipated aspects of George Gamow’s subsequent prediction that the universe might be suffused by a cosmic background radiation composed of ancient photons released by the big bang.