The Day We Found the Universe - Marcia Bartusiak [120]
Hubble began to focus his full attention on the cosmic exodus in 1928. That summer the International Astronomical Union was holding its triennial general assembly in the picturesque city of Leiden, set along the Old Rhine in southern Holland. With fine weather to entice them, more than three hundred delegates attended the gathering, where they were entertained with boat excursions down the city's noted canals, gliding past scenery painted by Rembrandt three centuries earlier. It was the height of the Roaring Twenties, and Europe was overflowing with tourists. “Most of the Americans appear to be over here this summer, always on the run scooping up culture with both hands, buying walking sticks and spats, post cards,” noted Lowell astronomer Carl Lampland, who attended the meeting.
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 “Extra-Galactic 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 red-shift 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.