Throughout January 1913 Slipher focused on measuring his plates with utmost precision. The result astonished him. The Andromeda Nebula was rushing toward the solar system at the ridiculous speed of 300 kilometers (186 miles) per second (a total of 670,000 miles per hour). This was about ten times faster than the average motion of stars in the Milky Way. If the nebula was really a nearby star and planetary system in formation, it was wildly abnormal.
Instead of announcing this result in a major astronomical journal, Slipher chose to publish a brief account in the Lowell Observatory Bulletin. True to form, Slipher held off any grander statement until he had some confirmation. Yet even one spiral-nebula velocity was an exceptional accomplishment. Lowell was enormously pleased. “It looks as if you had made a great discovery,” he wrote Slipher. “Try some more spiral nebulae for confirmation.”
Working on Andromeda, though, was a holiday compared with gathering enough light from other white nebulae. Andromeda is the biggest spiral in the sky; the others only get smaller and dimmer, which made it even harder for Slipher to obtain their velocities. “Spectrograms of spiral nebulae are becoming more laborious now because the additional objects observed are increasingly more faint and require extremely long exposures that are often difficult to arrange and carry through owing to Moon, clouds and pressing demands on the instrument for other work,” he noted.
Slipher’s first target after Andromeda was M81. He then worked on a peculiar nebula in southern Virgo, NGC 4594, which he described as a “telescopic object of great beauty.” It’s now known as the Sombrero galaxy. Slipher eventually found that it was moving at a speed “no less than three times that of the great Andromeda Nebula.” This time, however, the nebula was not traveling toward us, but away—at some one thousand kilometers (620 miles) per second. Slipher was greatly relieved. Finding a nebula that was racing outward rather than approaching removed any lingering doubts that the velocities might not be real. “When I got the velocity of the Andr. N. I went slow for fear it might be some unheard-of physical phenomenon,” he wrote his former Indiana professor John Miller.
A Hubble Telescope view of NGC 4594, known as the Sombrero galaxy. In 1913, Vesto Slipher measured this object as moving away from the Milky Way at some 1,000 kilometers per second.
(NASA, ESA, Hubble Heritage Team [STSci/AURA])
In the succeeding months Slipher kept expanding his list. His accomplishment was all the more amazing considering the relative crudeness of his instrument. The 24-inch telescope had only manual controls, and they weren’t yet sophisticated enough for fine guiding. Yet Slipher had to hold the tiny image of each nebula on the slit of the spectrograph steadily for hours on end as the telescope tracked the turning sky. When asked years later how he was able to do this, Slipher replied dryly, “I leaned against it.”
By the summer of 1914 Slipher had the velocities of fourteen spiral nebulae in hand. And with this collection of data, an undeniable trend at last emerged: While a few nebulae, such as Andromeda, were approaching us, the majority were rapidly moving away.
Suddenly the older idea that the white nebulae were other galaxies—other “island universes” of stars at fantastically great distances (an idea dating from Immanuel Kant in 1755)—looked newly plausible. “It seems to me, that with this discovery the great question, if the spirals belong to the system of the milky way or not, is answered with great certainty to the end, that they do not,” Danish astronomer Ejnar Hertzsprung wrote Slipher. The speeds were too great for them even to stay within our home galaxy. But Slipher at this stage was still on the fence: “It is a question in my mind to what extent the spirals are distant galaxies,” he responded. But he was absolutely sure of his velocity measurements.
For most of his career Slipher published few detailed papers of his work outside of Lowell’s in-house bulletin. He published very little at all from 1933 until his retirement in 1954, having turned much of his attention to local business pursuits and community affairs. The great standout in his otherwise sparse research record was his work on spiral-nebula velocities. He was absolutely confident of what he was seeing—so confident that he for once overcame his homebound nature and traveled in August 1914 to Northwestern University in Evanston, Illinois, to present his results in person.
At Northwestern, sixty-six astronomers from around the United States gathered by Lake Michigan for their annual meeting. Slipher reported in his talk that the average speed of the spirals was now “about 25 times the average stellar velocity.” Of the fifteen spiral nebulae he had measured so far, three were approaching Earth and the rest were moving away. The velocities ranged from “small,” as it was recorded on his list, to an astounding 1,100 kilometers (680 miles) per second, the greatest speed of a celestial object ever measured up to then. When Slipher finished delivering this remarkable news, his fellow astronomers rose to their feet and gave him a resounding ovation. No one had ever seen such a spectacle at an astronomical meeting. And with good reason: Slipher alone had climbed to the top of the Mount Everest of spectroscopy. In the audience was a young, ambitious astronomer named Edwin P. Hubble, just starting his graduate degree, who would later seize on Slipher’s work and extend it.
After a few more years, the cautious Slipher at last came around to Hertzsprung’s view and began to envision the Milky Way as moving among other galaxies just like itself. He even speculated in 1917 that the spirals might be “scattering” in some way—a precocious intimation of cosmic expansion that took many more years to fully recognize. But acceptance of spiral nebulae as distant galaxies could not be fully achieved until astronomers could determine how far away Andromeda and its sister nebulae truly were.
That, of course, famously occurred in 1923–24 when Hubble, using the 100-inch telescope on California’s Mount Wilson, identified Cepheid variable stars within Andromeda and used their pulsation periods as cosmic yardsticks to establish that the nebula was indeed a separate island universe. Five years later, in 1929, working with Milton Humason, Hubble identified a mathematical trend in the flight of the galaxies. The velocity at which the galaxies were moving away from us steadily increased as he peered ever deeper into space. The greater the distance of the nebula, the higher its velocity. The numerical value describing this trend became known as the “Hubble constant.”
Hubble was quite possessive of this finding and kept close watch on it. When Dutch astronomer Willem de Sitter, in a 1930 review article, casually referred to several astronomers linking a galaxy’s velocity to its distance, Hubble picked up his pen and reminded de Sitter who should receive the lion’s share of the credit. “I consider the velocity-distance relation, its formulation, testing and confirmation, as a Mount Wilson contribution and I am deeply concerned in its recognition as such,” he wrote.
Hubble conveniently forgot to tell de Sitter that the galaxy velocities he first drew upon in his historic 1929 paper were actually Slipher’s old data, which Hubble used without acknowledgment, a serious breach of scientific protocol. Hubble partially made up for this nefarious deed much later, in 1953. As Hubble was preparing a talk, he wrote Slipher, asking for some slides of his first 1912 spectrum of the Andromeda Nebula, and in this letter he at last gave the Lowell Observatory astronomer due credit for his initial breakthrough. “I regard such first steps as by far the most important of all,” wrote Hubble. “Once the field is opened, others can follow.” In the lecture itself, Hubble professed that his discovery “emerged from a combination of radial velocities measured by Slipher at Flagstaff with distances derived at Mount Wilson.” Privately, Slipher was bitter that he didn’t receive more immediate public credit but was too humble to demand his share of the glory.
In some ways, Slipher’s accomplishment resembled that of Arno Penzias and Robert Wilson several decades later. In 1964 the two Bell Laboratories researchers were calibrating a horn-shaped antenna in New Jersey in preparation for some radio observations and found unexpected static wherever they pointed. Just as Slipher made a remarkable cosm
ological find that took others time to fully interpret, Penzias and Wilson needed fellow astronomers to tell them what they had found: the afterglow of the Big Bang. But whereas Penzias and Wilson received the Nobel Prize for their serendipitous discovery, Slipher, as the years passed, was nearly forgotten in the momentous saga of the fleeing galaxies. A namesake like the “Slipher Space Telescope” was never to be.
CHAPTER TWENTY-SIX
The Primeval Atom
A Belgian cleric laid the groundwork for both the
expanding universe and the Big Bang
THE idea that the universe is expanding was one of the most revolutionary and unsettling findings of modern astronomy. As seen in the previous chapter, the germ of the idea arose not solely with Edwin Hubble at the Mount Wilson Observatory in California in 1929, as so many textbooks suggest. In addition, we must look to the halls of MIT and Harvard a few years before Hubble even initiated his historic measurements of galaxy distances and motions. There the very theory of an expansion was hatched in the mind of a Jesuit priest, who was studying at MIT’s physics department.
A military hero, Georges Lemaître had received the Croix de Guerre for his service in the Belgian artillery after Germany invaded his homeland in World War I. He went on to earn a doctorate in mathematics at the Catholic University of Louvain; afterward, perhaps affected by the horrors he had observed from the trenches, he enrolled in a seminary. Although he was ordained in 1923, the Church permitted him to continue his scientific pursuits. Captivated by the beauty of Einstein’s new general theory of relativity, the abbé proceeded to the University of Cambridge to broaden his understanding of the theory’s equations under the guidance of the astrophysicist Arthur Eddington, who deemed his student “exceptionally brilliant.”
In 1924, after a year in England, Lemaître traveled to the United States to study at Harvard’s observatory and enroll in MIT’s Ph.D. program in physics. His dark hair combed straight back and his cherubic face adorned with round glasses, he could easily be spotted on the college campuses by his attire—a black suit or an ankle-length cassock, set off by a stiff white clerical collar. Some could find him just by following the sound of his full, loud laugh, which was readily aroused.
In pursuit of his second Ph.D., Lemaître became interested in applying general relativity to the universe at large, which many in the 1920s believed to consist entirely of our own galaxy. By then totally absorbed by astronomy, he made sure to attend the 1925 meeting of the American Astronomical Society in Washington, D.C., where a crucial discovery was announced: Edwin Hubble had proved that certain spiral nebulae, previously thought to be gaseous clouds within the Milky Way, were actually separate galaxies far beyond its borders.
While others in the room were focused on Hubble’s revelations about the true nature of these long-perplexing nebulae, Lemaître was two jumps ahead. Though new to astronomy, he quickly realized that the newfound galaxies could be used to test certain predictions that general relativity made about the universe’s behavior. Soon after the meeting, Lemaître began formulating his own cosmological model.
Georges Lemaître (left) and Albert Einstein in
1933 at the California Institute of Technology.
(Courtesy of the Archives, California Institute of
Technology)
Two models were already in circulation in the astrophysical community. According to the first, proposed by Einstein himself in 1917, the universe contained so much matter that space-time wrapped itself up into a hyperdimensional ball—a closed, stable, enduring system. The second, posited soon after by the Dutch astronomer Willem de Sitter, was very different: it assumed that cosmic densities were so low that the universe could be considered empty. The unique properties of space-time that arose in this model caused light waves to get longer the farther they traveled from their source. This aspect of the model was consistent with some recent astronomical news that de Sitter was well aware of, but Einstein wasn’t.
At the Lowell Observatory in Arizona, most of Vesto Slipher’s spiral nebulae spectra were shifted to the red, a shift that implied the nebulae were moving outward into space—indeed, at the greatest celestial velocities that had ever been observed (as noted in the previous chapter). But de Sitter posited that the nebulae might only appear to be moving; instead, he suggested, the light waves themselves were getting longer and longer as the light traveled toward Earth.
Lemaître was not comfortable with either model. De Sitter’s could explain the redshifted nebulae but required a universe that was empty (which he was sure it was not); Einstein’s accommodated a universe filled with matter but couldn’t account for the fleeing nebulae. Lemaître aimed, as he put it, to “combine the advantages of both.”
While studying at MIT, Lemaître visited Slipher at the Lowell Observatory and Hubble at Mount Wilson to learn the latest velocity and distance measurements for what were now known to be spiral galaxies. With this information in hand, he took a first stab at a new solution, but he had not fully developed it by the end of 1925, when he handed in his Ph.D. thesis and left MIT. His thesis contained a preliminary model, a modification of de Sitter’s view of the universe. On returning to Belgium, where he became a professor at the Catholic University of Louvain, he fleshed out that modification into an entirely new model, which he published in 1927. Nearly two full years before Hubble provided the definitive observational proof, Lemaître unveiled a cosmological model in which space-time continually stretches, and galaxies move outward on the wave. (The gravitational field of a galaxy, far stronger than the field outside it, keeps the galaxy intact during the expansion.) The galaxies’ retreat, he wrote in his paper, is “a cosmical effect of the expansion of the universe.” He even estimated a rate of expansion, a number close to the figure that Hubble eventually calculated and which came to be known as the “Hubble constant.”
This was a tremendous accomplishment and offered an astounding vision of how the universe operates. But no one noticed—no one at all. Lemaître’s paper was completely ignored, probably because he inexplicably published it in an obscure Belgian journal. A similar solution, conceived independently in 1922 by the Russian mathematician Aleksandr Friedmann, went unnoticed as well. At a 1927 meeting in Brussels, Lemaître cornered Einstein and tried to persuade him to accept this new vision of the universe. But the world-renowned physicist would have none of it. “Your calculations are correct, but your physical insight is abominable,” he replied. Einstein refused to imagine a universe in which space-time was stretching.
This impasse stood for a couple of years. But in 1929, Hubble verified that the galaxies were moving outward in a uniform way. And in 1931 Lemaître’s paper was finally noticed by Eddington and consequently reprinted in the more prominent Monthly Notices of the Royal Astronomical Society. Why Hubble saw the velocities of the galaxies steadily increase with distance was finally explained. Only then was the expanding universe truly recognized. Astronomers and theorists alike were thunderstruck by this radically new cosmic setup, breathtaking in its grandeur and terrifying in its implications.
Perhaps most consequential was the question that Lemaître first posed in his original 1927 paper: How did this expansion get started? “It remains to find the cause,” he wrote at the time. But within four years he boldly suggested in the journal Nature that all the mass-energy of the universe was once packed within a “unique quantum,” which he later called the primeval atom. From Lemaître’s poetic scenario arose the current vision of the Big Bang, a model that shapes the thought of cosmologists today as strongly as the idea of crystalline spheres, popularized by Ptolemy, influenced natural philosophers in the Middle Ages.
Unlike Galileo, who was condemned to house arrest for his defense of a Sun-centered universe, Lemaître was lauded by the Church for his cosmic breakthrough. Indeed, he ultimately rose to the rank of monsignor and was made a fellow and later president of the Pontifical Academy of Sciences. But he recoiled from any suggestion that his primeval atom had been inspir
ed by the biblical story of Genesis. Throughout his life, he insisted that his theory about the origin of space and time expanding outward from a quantum nugget sprang solely from the equations before him.
Lemaître made few notable contributions to cosmology after the 1930s, spending more time on celestial mechanics and pioneering the use of electronic computers for numerical calculations. But he continued to hope that the explosive origin of the universe would be validated by astronomical observations.
In June 1966, as Lemaître was fighting leukemia, Odon Godart, his successor at the Belgian university, visited him at the Hospital of Saint Peter with news of a report that had appeared in the Astrophysical Journal the previous year. That report, which would later win the Nobel Prize for Arno Penzias and Robert Wilson, had detailed the discovery of the cosmic microwave background; Godart brought confirmation that this was the remnant echo of the Big Bang. Lemaître died a few days later, on June 20, knowing that the universe was indeed launched from a compact bundle of energy, just as he had posited nearly four decades earlier.
CHAPTER TWENTY-SEVEN
Proving the Big Bang
Sometimes scientists don’t realize the answer is
hidden in plain sight
SOMETIMES a great scientific idea needs time to take root. Sometimes the world simply isn’t ready. Continental drift comes to mind as an example, as well as germ theory. Continents moving about? Microscopic bugs? Each of those propositions when first proposed seemed too bizarre to accept right off. In such situations, scientists have to be convinced that a new concept is worth looking into.
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