The Day We Found the Universe - Marcia Bartusiak [125]
A selection from Milton Humason's measurements showing how the
spectral lines for calcium (marked as KH) move farther to the right (red
end of the spectrum) as both the distance for the galaxy and its velocity
increase (1 parsec = 3.26 light-years) (From Astrophysical Journal 83
[1936]: 10-22, Plate III, courtesy of the American Astronomical Society)
So important did the Mount Wilson administration consider this endeavor that Hubble and Humason got almost all the “dark time” on the 100-inch throughout the 1930s, to the dismay of other galaxy researchers. Only during those precious few nights each month when the Moon, with its disruptive light, remained hidden below the horizon could the two investigators carry out the measurements on their extremely faint targets. “The intense publicity that swirled around Mount Wilson's nebular department, with Hubble the bright star at its core,” noted Allan Sandage, “was anathema to the spectroscopists [at Mount Wilson].” Most astronomers at this time, in the United States and elsewhere, were focused on determining the life histories of stars. “Here they were,” continued Sandage, “toiling away on stellar astrophysics—the most exciting and exotic facet of contemporary astronomy, to their eyes—yet the public seemed to find them, well, boring.” Even though less than 5 percent of Mount Wilson's major publications in this era involved cosmology, the topic dominated the news stories coming out of the observatory. “Some spectroscopists began to feel resentful,” said Sandage. Even to this day, the legend persists that the Mount Wilson Observatory's sole focus at that time was galaxies, so great was the attention focused on Hubble and his accomplishments.
But what did it all mean? What was causing the galaxies to flee from the Milky Way in such a methodical way? Were these swift velocities even genuine? It was easy to equate the redshifts with velocity, as that was the simplest interpretation and the most straightforward way to talk about the phenomenon in scientific papers. Everyone used the terms interchangeably. But perhaps some new law of physics was at work and the galaxies weren't truly racing away after all. Maybe the retreat was entirely a chimera.
Hubble, the consummate observer, did not consider that question his main concern. He was reluctant to speculate. He wanted only to weigh the data that the universe provided him. Given that leaning, Hubble devoted most of his 1929 paper to establishing the link between a galaxy's distance and its redshift, its six pages filled with tables of numbers, a few equations, and a single graph. Only in the very last paragraph did he bring up a potential explanation. “The outstanding feature,” he wrote, “is the possibility that the velocity-distance relation may represent the de Sitter effect,” the most active model then in play. Maybe the light waves were lengthening as they traveled, setting up the illusion of movement; or maybe matter was truly scattering outward due to the weird nature of de Sitter space. More significant to Hubble was that bona fide data could now be offered in discussions of cosmological models. For centuries cosmology was a realm of speculation and imagination alone. Anybody's vision of the cosmos could be entertained—its origin, its behavior, its structure—simply because there was no way to refute it. But now actual cosmic measurements could be brought to the debate. Theory now had to meet the test of observation. It was one of the triumphs of twentieth-century astronomy, and Hubble initiated the endeavor. The galaxies became his “great beacons scattered through space,” luminous markers for mapping the topography of the universe.
For the most part, Hubble remained focused solely on his observations, leaving theory to others. “The interpretation,” he told de Sitter at one point, “should be left to you and the very few others who are