The Great Debate was settled then and there. Spiral nebulae were other galaxies, and the universe was every bit as vast as Einstein had imagined it to be. Hubble had gambled that the universe would submit to scientific inquiry, and he had won. Still he pressed on. By the following February, he had uncovered nine novas and a possible second Cepheid in Andromeda. He also found Cepheid variables in Triangulum and possibly in three other nebulae as well. Now that there could be absolutely no doubt of the meaning of his finding, he wrote to Shapley to needle him with the news. “You will be interested to hear that I have found a Cepheid variable in the Andromeda Nebula,” the letter began. Shapley needed to read no farther to understand the significance of Hubble's words. In his reply, Shapley raised a number of skeptical and cautionary arguments, but privately he despaired. “Here is the letter that destroyed my universe,” he morosely told Cecilia Payne-Gaposchkin, now a doctoral candidate at Harvard, who was in his office when Hubble's missive arrived. By an odd coincidence, Payne-Gaposchkin had also been present when Eddington announced the results of the eclipse results that confirmed general relativity, making her witness to two pivotal cosmological advances in the space of four years—a sort of modern-day Miriam in the shadows of the sci/religious miracles.
Once again, the size of the measurable universe had tremendously increased. Until the middle of the nineteenth century, the most remote identified object was Uranus, roughly two billion miles away. When Friedrich Bessel nailed down the distance to a star for the first time, he placed 61 Cygni ten light-years away, thirty thousand times farther than Uranus. Now Hubble had determined the first galactic distance and moved the boundary of measured space another one hundred thousand times farther out into the depths of space. The scale of human life shrank each time, while the evident power of the human intellect grew. Hubble had penetrated the secret depths of space where Shapley had imagined that true galaxies hid, like blue whales lumbering at uncharted fathoms in the murk of the oceans. If the God of the old-time religions was out there, he had fewer and fewer places to hide. But if knowing the universe is the same as knowing God, as Einstein preached, then astronomers were closing in on the divine.
Despite his obvious excitement at the Andromeda findings, Hubble was reluctant to publish his results. For all his surface confidence, he was terribly concerned about making a grand pronouncement that would appear naive or foolish. Every time he walked down from the summit to attend the formal 5 P.M. dinners at the Monastery, Mount Wilson's living quarters, Hubble had to face his astronomer brethren. Not all of them accepted the existence of other galaxies. Adriaan van Maanen, a playful and well-liked Dutch astronomer, had in fact argued vigorously in the other direction. He was convinced that he had observed some of the spirals rotating, which was possible only if they were relatively small and nearby. Indeed, one of the reasons Shapley refused to believe the spiral nebulae could be galaxies was that he completely trusted these rotation measurements. Unlike Shapley, Hubble didn't consider van Maanen a good friend. Nevertheless, he found it worrisome to have a doubter in his own midst and held back until he was utterly sure of his results. (Van Maanen never figured out where he went wrong and refused to admit his errors long after Hubble had proven conclusively that spiral nebulae are galaxies. In a fit of pique, Hubble reexamined his photographic plates and declared that “the large rotations previously found arose from obscure systematic errors and did not indicate motion, either real or apparent, in the nebulae themselves.”)
Word of Hubble's discovery inevitably leaked out to the media. As a result, the first public announcement of his astronomical breakthrough was a small story that ran in The New York Times on November 23, 1924. Still Hubble balked at publication. The noted stellar astronomer Henry Norris Russell pressed him to present his findings to a Washington, D.C., meeting of the American Association for the Advancement of Science, which offered a $1,000 prize for best paper. When he didn't submit anything, Russell snorted, “Well, he is an ass. With a perfectly good thousand dollars available, he refuses to take it,” then turned to find that Hubble's paper had just arrived. Hubble remained in splendid isolation at Mount Wilson while Russell read his official announcement to an enthusiastic crowd on January 1, 1925. Hubble shared the best-paper prize.
Oddly enough, it was Shapley, not Hubble, who suggested that astronomers should adapt their nomenclature to the new reality and call the external star systems “galaxies.” Like many revolutionaries in the new temple of sci/religion, Hubble still carried within him the conservative views of the world he overthrew. He was also naturally inclined to disagree with any idea that came from his rival, Shapley. Hubble, the man who proved that the Milky Way is but one of innumerable galaxies, forever called the objects by the archaic name “extra-galactic nebulae.”
As Hubble watched the cyclical flaring and dimming of the Cepheids in Andromeda, he did more than establish the distance scale of the universe. He also erased the lingering concern, aired by Curtis in the Great Debate, that stars lying at great distances from us might behave differently from those in our immediate celestial neighborhood. And now that scientists could determine the distances to other galaxies, they could establish the constancy of the universe over time as well. If the Andromeda galaxy is one million light-years away, that means the light we see now started on its way earthward a million years ago. That is, we are seeing the stars in Andromeda as they were a million years ago—yet they look identical to nearby stars. As Hubble and other astronomers looked out to ever greater distances, they added ever more evidence for the principle of temporal uniformity. This constancy of nature lent credibility to the search for a single set of overarching cosmic rules. Or, as Einstein might have put it, it showed that God does not change the house rules of the cosmos.
What Hubble did not settle was the lingering question of whether Einstein was correct when he assumed a static universe, held in place by Lambda. De Sitter had shown that Einstein's cosmology was not the only possible interpretation of general relativity. Friedmann had issued a challenge in suggesting that Einstein's own equations implied a dynamic universe. And by 1925, Slipher had collected spectral data on forty-one spiral nebulae, almost all of which showed a strong redshift. This perceived reddening of light implied that the sources were moving away from us at hundreds of miles per second; the record holder was racing 1,100 miles farther away every second. As long as the nebulae could be dismissed as bits of flotsam breaking free from the Milky Way, their breakneck speeds didn't seem such a vital matter. Now that Hubble had demonstrated that each smudge of light was another galaxy as mighty as our own, the redshifts took on a much greater significance.
If there was a clear pattern to the galactic redshifts, that would spell trouble for Einstein's cosmology. His static universe did not alter light from distant bodies and did not allow for any large-scale motions in the universe. The de Sitter universe, aka “solution B,” was in better shape—it predicted that light might grow redder as it passes through space, in which case galaxy redshifts should increase in proportion to their distances. But de Sitter didn't interpret his redshifts as velocities, and even he didn't consider his mass-free universe anything more than a mathematical idealization of the real world. That left a third possibility, that the redshifts really represented a true, systematic motion of galaxies away from us. The notion of a literally expanding universe was too weird for Friedmann, whose work was at the time still unknown to most of the astronomical world anyway. All the same, what creation had written in the book of nature seemed to be suggesting just that, even before Hubble got to work.
As early as 1921, astronomer Carl Wirtz in Germany thought he saw signs of a proportional relationship between distance and red-shift in Slipher's observations. Ludwik Silberstein, a Polish-born physicist then working in England, stirred up the pot in 1924, when he alleged to have proof of such a link. In making his claim, however, he selectively included studies of star clusters in our own galaxies. His colleagues, smelling a rat, quickly denounced t
he results, and many of them soured entirely on the provocative but endlessly ambiguous redshift studies. Undaunted, de Sitter continued to search for evidence of his cosmological reddening effect. Early in 1928, de Sitter had a chance to discuss the matter with Hubble, who was traveling through Europe. Hubble had read about de Sitter's research and was persuaded by the distinguished theorist's entreaties to unleash Mount Wilson's huge Hooker telescope and resolve the mystery of the redshifts.
Again, Hubble's timing was perfect. Slipher, with his limited resources, had run into impenetrable theoretical and practical barriers by 1926. Analyzing the light of spiral nebulae was slow business, especially using the Lowell Observatory's modest twenty-four-inch refracting telescope. He would expose the same photographic plate over several nights, gradually building up a total exposure time of twenty hours or more. It was tedious work, and when he was done he could still only guess whether faraway objects were moving more quickly than nearby ones, because he had no way of measuring the distances to these nebulae. Hubble or Shapley could look at Cepheid variables because they had access to a one-hundred-inch mirror; such work was way out of the Lowell Observatory's league. Poor Slipher was reduced to looking at the sizes and brightnesses of the nebulae in order to guess their relative proximity. In his heart he believed these objects must be other galaxies. But he had pushed his equipment to the limit, and still he could not obtain the final answer.
Hubble confidently picked up where Slipher left off. He approached the challenge armed with two potent weapons: Mount Wilson's mighty one-hundred-inch telescope, which collected seventeen times as much light as Slipher's instrument; and Milton Humason, the observatory's crackerjack photographer. In his teens, Humason had worked at Mount Wilson as a mule driver, helping to deliver construction materials for the Monastery and other buildings around the observatory. Bewitched by the temple of astronomy taking shape high up in the San Gabriel Mountains, he returned there in 1917 as a janitor but proved himself curious and capable of much greater things. He started helping out one of the students, then began assisting Shapley, and finally he graduated into one of the finest photographers at the observatory. Hubble picked out the images and spectra he needed, and time and again Humason delivered, guiding the telescope and developing the photographic plates to perfection.
To get any kind of meaningful perspective on the spiral nebulae, now properly known as spiral galaxies, Hubble knew he had to obtain accurate distances and redshifts for a large sample of them. That meant going deep, well beyond the handful of nearby bright spirals. Starting in 1928, Hubble and Humason trained Mount Wilson's unblinking one-hundred-inch eye on two dozen of Slipher's galaxies, confirming and extending the dominance of redshifts that Slipher had recorded. Then Hubble did what Slipher could not do—he calculated the distances to those galaxies. For the nearest ones, he monitored Cepheid stars like the ones he saw in the Andromeda galaxy. Probing deeper into space, he looked for nova explosions or the most brilliant giant stars in each galaxy, which are all roughly equal in luminosity. Farthest out, Hubble noted that the brightest galaxies in large clusters all seemed quite similar, so he used them as crude “standard candles” for gauging distance. This clever stepping-stone approach allowed him to locate galaxies a hundred times more distant than Andromeda. By 1929 he had collected redshifts for forty-six galaxies and could tell the distances, more or less, to two dozen of them.
Finally, Hubble made a graph showing how the velocities—as indicated by how strongly the light is shifted toward the red end of the spectrum—relate to the distances of the galaxies. The graph showed a straight line: the farther away the galaxy, the more quickly it recedes. This linear relationship, now called Hubble's law, is the signature of an expanding universe. Princeton cosmologist James Gunn summed up this work: “Hubble's gigantic realm had been endowed with motion, motion which implied physical process, evolution, and origin.” Einstein delivered the prophecy of a dynamic, all-encompassing cosmic theory. Now Hubble witnessed the miracle that had been foretold and testified about it before the world.
Because the galaxies seem to move outward from us in all directions, it might seem as if we are in some unique, central location. But in an expanding universe, that is what every observer sees; it is precisely for that reason, in fact, that scientists quickly interpreted Hubble's law as proof of such cosmic expansion. Consider again the analogy in which Einstein's curved space is represented by the surface of a rubber balloon. Einstein had assumed that the balloon remained perfectly still. But suppose instead the balloon was expanding so that over a period of time its size doubled. What would an observer—our dear old hypothetical, two-dimensional friend Trevor—see from his tiny perch on the balloon's surface? A spot one inch away would, after the period of doubling, be two inches away. A spot two inches away would end up four inches away. A spot three inches away would end up six inches away, and so on. In other words, Trevor would observe each point on the surface of the balloon moving away at a rate exactly proportional to its distance. That is what happens when every part of the balloon (representing every part of space in the real world) expands at an equal rate. If the real universe were expanding, the same kind of thing would happen. Light from distant galaxies would be stretched and reddened, and the intensity of the effect would be in direct relation to how far away each galaxy is.
Hubble, ever the cautious researcher, dared not come right out and say that the universe is expanding. He merely set out his findings in a paper, soberly titled “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae,” that appeared in the March 15, 1929, issue of Proceedings of the National Academy of Sciences. It was just six pages long, terse yet confident. His actual data points were scattered all over the page, more like potshots than a scientific bull's-eye. There were large random errors in his distance measurements, as well as large random galactic motions that distorted the pattern. He drew a hopeful line through the data points, depicting the linear relationship between distance and red-shift that he sensed was there. “For such scanty material, so poorly distributed, the results are fairly definite,” he wrote unapologetically. Through a mix of inspired genius and dumb good luck, he saw the correct pattern in the clutter.
Given what it spawned, Hubble's paper was surprisingly stingy on big concepts. “This discovery finally brought the question of the beginning of the universe into the realm of science,” writes Stephen Hawking. But Hubble didn't talk about beginnings. He didn't talk about the expanding universe. He didn't even talk about galaxies and motion. The physical implications of his redshifts lay outside his empirical conception of science. Eight years later, after most of his colleagues had firmly converted to sci/religion and rewired their brains to accept the idea of a cosmos that grows, Hubble still balked. “Well, perhaps the nebulae are all receding in this peculiar manner. But the notion is rather startling,” he said. Hubble was not a theorist. He understood little about general relativity, and he knew nothing of Friedmann's dynamic cosmological models. Here again, Hubble comes across as the near opposite of Einstein, although Hubble's unflinching adherence to observation echoes the philosophy of Einstein's onetime hero Ernst Mach. Mach was so allergic to speculation that in 1906 he was still writing about “the artificial hypothetical atoms and molecules of physics and chemistry.” Although less extreme in his views, Hubble was generally content to attempt to comprehend the universe through data alone. He stuck to his austere approach, reading nature's holy scripture without constructing his own arguments.
Nevertheless, Hubble sensed that his role as the grand explainer of the universe required him to comment on theoretical interpretations, no matter how alien they seemed. Hubble made one awkward attempt at the end of his 1929 paper to connect his observations with what little he knew of cosmological theory. “The outstanding feature, however, is the possibility that the velocity-distance relation may represent the de Sitter effect, and hence that numerical data may be introduced into discussions of the general curvature of space,
” he wrote. Quite likely he viewed this endorsement as a kind of repayment to de Sitter for having shared his theoretical ideas and encouraging Hubble to pursue the redshift measurements. The de Sitter universe also appealed to the conservative in Hubble, because it explained the redshifts yet in its abstruse way maintained a classical calm. Too bad for Hubble, he was rather late to this party. Nobody else was sold on this confusing and unrealistic formulation, and de Sitter abandoned it not long after Hubble presented his findings.
Such flubs only deepened Hubble's aversion to revealing the theoretical or philosophical goals underlying his research. Many current observers still follow in this tradition. They aspire to a kind of mastery of the universe, itching to know the most distant, obscure details of the construction of the cosmos. Yet if pressed, they swear they don't have a mystical bone in their bodies. Hints of Hubble's yearnings break through his wall of self-censorship. Nearing the end of The Realm of the Nebulae, his book summarizing his telescopic explorations, he addressed the limits of his explorations: “With increasing distance, our knowledge fades, and fades rapidly. Eventually we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial.” Here Hubble expressed his dark spiritual fear, that his telescopes would never be able to penetrate the kingdom of God. He never went to church and professed not to have any personal faith, but Hubble's worries were those of a believer. Empirical observation was his Lord, and the limits of his telescopes represented the grim end points in his search for truth.
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