Knocking on Heaven's Door - Lisa Randall [177]
At the time Zwicky made his measurements, he didn’t have the resolution to see individual stars. Much more solid evidence for dark matter came from Vera Rubin, an observational astronomer, who much later—in the late 1960s and early 1970s—made detailed quantitative measurements of stars rotating in galaxies. What first seemed to be a “boring” study of stars orbiting in a galaxy—a study Vera turned to since it provided less-well-trodden territory than other astronomical activities at the time—emerged as the first solid evidence of dark matter in the universe. Rubin’s observations with Kent Ford yielded incontrovertible evidence that Zwicky’s conclusion years earlier had been correct.
You might wonder how someone could look through a telescope and see something dark. The answer is that she could see its gravitational consequences. The properties of a galaxy, such as the rate at which its stars orbit around, are influenced by how much matter it contains. With only visible matter present, one would have expected those stars well beyond the galaxy to be rather insensitive to the galaxy’s gravitational influence. Yet stars ten times farther away than the luminous central matter rotated with the same velocity as stars closer to the galaxy’s center. This implied that the mass density did not fall off with distance, at least to distances as far from the galaxy’s center as ten times the distance of the luminous matter. Astronomers concluded that galaxies consisted primarily of unseen dark matter. The luminous matter we see is a significant fraction, but most of the galaxy is invisible, at least in the ordinary sense of the word.
We now have a good deal of other supplementary evidence for dark matter’s existence. Some of the most direct is from lensing, illustrated in Figure 75. Lensing is the phenomenon that occurs when light passes a massive object. Even if that object itself doesn’t emit light, it does exert a gravitational force. And that gravitational force can cause light emitted by a nondark object behind (as seen from our vantage point) to bend. Because the light bends in different directions according to the path it takes around the dark object and because we automatically project straight lines for light, this lensing can produce multiple images of the original bright object in the sky. These multiple images allow us to “see” the dark object—or at least infer its existence and properties by deducing the gravity needed to bend the observed light.
[ FIGURE 75 ] Light passing a massive object can bend, which from the perspective of the observer appears to create multiple images of the original object.
Perhaps the strongest evidence to date that dark matter, rather than a modified gravitational theory, explains such phenomena comes from the Bullet cluster, which involved two colliding clusters of galaxies. (See Figure 76.) Their collision demonstrated that the clusters contain stars, gas, and dark matter. The hot gas in the cluster interacts strongly—so strongly that the gas remains concentrated in the central collision region. Dark matter, on the other hand, doesn’t interact—at least not very much. So the dark matter just passed through. Lensing measurements showed that the dark matter was indeed separated from the hot gas in just the way implied by a model of very weakly interacting dark and strongly interacting ordinary matter.
[ FIGURE 76 ] The Bullet cluster indicates that clusters of galaxies contain dark matter, and that their dynamics are unlikely to be explained by modified gravitational laws. That’s because we can see a separation between the more