The Day We Found the Universe (38 page)

Hubble had been appointed acting chairman of the IAU Nebulae Commission, and in and around its July session, he took the opportunity to sit down with Willem de Sitter to discuss relativity and its application to cosmology. Hubble was undoubtedly familiar with (though hardly an expert on) the Einstein and de Sitter solutions to the universe's structure. At the very end of his magisterial 1926 paper “ExtraGalactic Nebulae,” Hubble had included a brief section titled “The Finite Universe of General Relativity,” in which he mentions both of them. Moreover, the following year at Hubble's direction, Milton Humason had remeasured the redshifts of two nearby galaxies. In his brief report, likely ghostwritten by Hubble, Humason specially noted that the galaxy speeds computed from those redshifts were unusually low, “consistent with the marked tendency already observed” for the closest galaxies to have the smaller pace.

Hubble was thus certainly aware of the general trend of galaxy velocities outward, but at Leiden he seemed to have finally grasped the tremendous hubbub the galaxy redshifts were generating among cosmologists and received some further lessons from one of the world's few experts on general relativity. Eager to have his model of the universe put to the test, de Sitter encouraged Hubble at this time to extend the redshift measurements of the spiral nebulae begun by Slipher at the Lowell Observatory. With only a puny 24-inch refractor with which to work, Slipher had essentially come to the end of his search. He had been able to acquire the redshifts of the brightest spiral nebulae, over forty of them, but trying to obtain a reading from ever fainter and smaller galaxies was impossible. Slipher had exhausted the power of his telescope and could simply not gather enough photons. “The Flagstaff assault on these objects stopped just short of some great excitement,” Shapley later pointed out. Most figured that to reliably establish whether a galaxy's redshift was related to its distance in a predictable way would require a far bigger telescope, like the 100-inch reflector available to Hubble at Mount Wilson. It was the perfect match of problem to instrument. De Sitter knew this, and Hubble was obviously convinced as well.

Upon returning to California, Hubble immediately made this pursuit his top observational priority. Having conquered the mystery of the spiral nebulae, he was now commencing his next great challenge—to see if there truly was a definitive trend to the redshifts of the galaxies as they rushed headlong into distant space. It was at this time that Hubble forged his industrious partnership with Humason, each taking on a specific task to get the overall job done. While Hubble searched for Cepheid variables to determine the distances to a sample of galaxies, his colleague focused on getting the redshift data to figure out the galaxies' velocities (if that indeed was how the redshifts were to be interpreted). Hubble's plan was to put these two pieces of information together and determine if there was a law—a specific formula—that linked a galaxy's distance to its measured redshift.

Humason was not too happy at first upon hearing of his new assignment. Hubble had come home from Leiden quite excited and quickly suggested to Humason that he try to obtain a galaxy redshift that was not yet known. But the prism on the 100-inch spectrograph had started to yellow, and the photographic plates then on hand were considerably slow-acting for such work. Humason knew it would take several nights to get a decent spectrum. “I didn't feel much enthusiasm about these long exposures,” he later recalled. “But [Hubble] kept at me and encouraged me.” Hubble was after fainter and fainter objects, the ones too distant for Slipher to have studied with his smaller telescope, and some were low on the southern sky. “To get these,” said Humason, “you had to climb onto the 100 inch and sit on the iron frame during the long winter nights, which was extremely cold and uncomfortable.” For hours on end, through the freezing night, he would have to keep his guide star on the center of the cross wires, to make sure his image remained sure and steady. “The eye-strain, the monotony, the constant awareness—it was a test of endurance,” he said. But Humason's unusual entrée into astronomy offered him superb preparation for this arduous undertaking. He was accustomed to hard work.

Born in Minnesota in 1891, Humason as a boy moved to the West Coast with his family and one summer as a teenager enjoyed a camping holiday on Mount Wilson. He fell in love with the mountain and soon dropped out of grammar school to work as a bellboy and handyman at the newly opened Mount Wilson hotel, a popular resort spot for local residents. He washed dishes, corralled the horses, and shingled the cottages. Once the 60-inch telescope was under construction, he drove the mule trains that took the equipment, piece by piece, up the rugged path to the top of the peak. When a mountain lion was found feasting on a prized goat in the area, Humason tracked the animal down and shot him between the eyes with a .22-caliber rifle. Several years after Humason married the daughter of the observatory's chief engineer, his father-in-law arranged for him to work as the observatory's janitor. Gradually he was allowed to help the astronomers as a night assistant and over time won their respect and trust in making observations on his own, despite his eighth-grade education and lack of formal training in astronomy. Seth Nicholson took the young man under his wing and taught him some mathematics; Shapley mentored him as well. With his round face and round eyeglasses, the quiet and self-effacing Humason came to look like an academic. In 1920 he was promoted to a staff position in the photography department and two years later moved up to assistant astronomer. Known for his patience and conscientious attention to detail, he became especially skilled at taking long photographic or spectroscopic exposures of the most faint celestial objects. A likable fellow and an inveterate gambler, he relieved the pent-up tension from this grinding work by playing poker with the other night assistants and shop workers. If his schedule allowed, he'd catch the late-afternoon horse races at the nearby Santa Anita racetrack, taking any astronomer who wanted to go with him.

Over time, Humason was even put in charge of arranging telescope time. Sharing Hubble's strong loyalty for the Republican Party, Humason tried to get as many Democrat observers as possible on the mountain, away from the polls, on election days. Solar astronomer Nicholson, a staunch Democrat, evened the score by making sure only Republicans were scheduled on the solar telescopes at the same time. Hubble, whose status could never be threatened by Humason's humbler origins, got along fine with his devoted junior partner.

Milton Humason at Mount Wilson
(Courtesy of AIP Emilio Segrè Visual Archives)

• • •

By 1929 Hubble had determined the distances to twenty-four galaxies (including the Small and Large Magellanic Clouds), the most remote then judged to reside some 6 million lightyears away. He accomplished this feat by establishing a ladder of measurements, one rung leading to the next. First he used Cepheid variables, his most reliable yardstick, to directly obtain the distances to six relatively nearby galaxies; then he judged the magnitude of the brightest stars in those galaxies. Figuring such stars were similarly bright in other, more distant galaxies, he proceeded to use them as standard candles. He sought out these radiant stars in more far-off galaxies—fourteen in all—and estimated each galaxy's distance based on the stars' apparent luminosities. Then, taking all twenty of these galaxies into account (the first six and the subsequent fourteen), he estimated the brightness for an average galaxy and used that value for judging the distance to four more remote galaxies. Hubble's moving outward like this, rung by rung, was similar to Shapley's strategy for his globular cluster distance measurements, but here Hubble was making an even braver leap into distant space.

Hubble then paired each galaxy's distance with its measured velocity to see if there was a connection, some sort of organized flow in which the galaxies flew outward into the depths of space. Humason by then had redone a number of the redshifts, but when Hubble prepared his first paper on the findings (“A Relation Between Distance and Radial Velocity Among ExtraGalactic Nebulae”), he primarily used Slipher's original measurements.

Hubble was more vigilant than usual in preparing this landmark 1929 publication, chiefly because of the checkered history of the subject. An earlier and rather clumsy attempt by the Polish-American mathematical physicist Ludwik Silberstein to find a relationship between a galaxy's distance and its redshift had been met with derision, especially by noted astronomers Knut Lundmark and Gustaf Ström-berg, who was Hubble's colleague at Mount Wilson. Silberstein had lumped globular clusters in with spiral nebulae, which led to a meaningless result. He was ridiculed for both his inept analysis and leaving out data that went against his prediction, which tainted everyone's outlook on the problem. To make sure this didn't happen to him, Hubble shrewdly sought out the advice of Silberstein's two harshest critics and specifically highlighted their contributions in his paper. “Mr. Strömberg has very kindly checked the general order of these values… Solutions of this sort have been published by Lundmark,” he wrote fawningly. Hubble knew he was dealing with a controversial finding, so he was taking every precaution. He was wooing potential enemies to his side. He didn't even like Lundmark, having earlier accused the man of plagiarizing his system for classifying the galaxies, and the point he had Strömberg verify was so simple it scarcely needed checking. As it was, Hubble held up publication of his data to make sure he had nailed down every argument, as well as gathered data on even fainter galaxies so he and Humason could quickly publish a follow-up and prevent others from jumping into what Hubble considered
his
field. He was being both careful and cunning; he was not just introducing an idea but selling it hard. Hubble knew he had to make an air tight case in order to convince his more skeptical colleagues. “There is more to the advance of science than new observations and new theories,” historian Norriss Hetherington has noted. “Ultimately, people must be persuaded.”

According to Hetherington, Hubble presented his first data on the problem as if he were standing before a judge and jury—again, not surprising given his legal training. Hubble even had witnesses. With Hubble citing their assistance, Strömberg and Lundmark were brought forward to serve as objective bystanders to verify his competence. What Hubble saw was a definite pattern to the galaxies' retreat, a rule that was simple and yet so elegant. The velocity of the galaxies was found to steadily increase—rise in a linear fashion, as scientists say—as astronomers peered ever deeper into space. At double the distance, a galaxy's speed doubles as well. A galaxy 10 million lightyears away travels twice as fast as a galaxy 5 million lightyears distant. Hubble also calculated the rate of that increase. This number has since been amended (as better and better measurements were made over the years), but at first Hubble found that for every million parsecs outward (around 3 million lightyears), the velocity of a galaxy increased by 500 kilometers per second. He referred to this factor as
K
, the same term introduced by others in earlier analyses. By the late 1930s, though, astronomers were regularly referring to it as
H
, “Hubble's constant,” later shortened to
the
Hubble constant.

Hubble did not really “discover” this relation but rather demonstrated an effect already suspected, with data that at last convinced his fellow astronomers. In previous attempts the plotted measurements looked like scattershot across the page. But on Hubble's graph, even though there was still some scatter, the galaxies lined up far more tightly. The sure straight line he was able to draw through his points, a diagram that is now an honored icon in cosmology textbooks, gave everyone confidence in the results.

Hubble actually carried out two separate computations. In one, he calculated his rate of recession using all twenty-four of his galaxies. In the second approach, he figured out a rate of recession when he combined the galaxies into nine groups, according to their distance and direction on the sky. Both methods led to similar outcomes. “For such scanty material, so poorly distributed, the results are fairly definite,” concluded Hubble, almost with surprise.

In a clever move, Hubble didn't include on his historic graph the strongest bit of evidence then on hand. He left that for Humason to convey in a separate paper opportunely placed right before his in the
Proceedings of the National Academy of Sciences
. Humason's result was the attention grabber, setting up Hubble's paper, published under his name alone, for the fait accompli. After cutting his teeth on a few of Slipher's galaxies, Humason had gone after a fainter, previously unmeasured target, as Hubble directed. It was the galaxy NGC 7619 in the Pegasus constellation. “I agreed to try one exposure,” recalled Humason, who wanted to see whether it was even possible to venture farther out than Slipher. That exposure extended over a few nights, the sparse photons from the dim galaxy hitting the plate for a total of thirty-three hours. It was a lonely enterprise. Humason usually worked within the 100-inch dome with only the light of a tiny red bulb as company. For hours he would keep two crossed hairs—the guiding wires—smack-dab on his galaxy, a smudge of light barely visible through the barrel of the telescope. All this despite the fact, as one observer put it, “that the mountain itself is rolling eastward with the earth at ten times an express train's speed.” For assurance, Humason went back and did the measurement again, this time for forty-five hours.