Dark Force Redux

Hubble has weighed in on the other major ingredient of the uni­verse: dark matter. The existence of dark matter was first indicated in an observation of the Coma Cluster of galaxies by the maver­ick Caltech astronomer Fritz Zwicky. Zwicky measured redshifts or radial velocities for galaxies in the cluster and saw velocities that were much higher than anticipated. The galaxies were buzzing around like angry bees at over 500 kilometers per second, over a million miles per hour.42 If the cluster had the mass indicated by the stars in all the galaxies, it wasn’t enough to keep those galaxies bound in one region of space. In fact, visible mass was insufficient by a factor of ten. The Coma Cluster should be flying apart. But it’s not, so Zwicky hypothesized an invisible form of matter to hold it together. It had to be a form of matter that exerted gravity but didn’t radiate light or even interact with radiation—dark matter.

This observation was so odd and so unexpected that most as­tronomers simply set it aside. (It didn’t help that Zwicky was bril­liant but extremely cantankerous, and he made a lot of enemies in the profession with his blunt and often rude comments.) But in the 1970s the rotation of spiral galaxies also indicated unseen forms of matter extending far beyond the visible stars. Astronomers re­visited Zwicky’s observation and found that it was correct, and that other clusters of galaxies showed the same effect. Dark matter was a ubiquitous feature of the universe, on galactic scales and beyond galaxies in the space between them.

The Hubble Space Telescope has cemented the measurement of dark matter in a very elegant way. Einstein’s theory of general rela­tivity says that mass bends light, and this prediction was confirmed in 1919 with starlight bending around the limb of the Sun during a solar eclipse. We’ve seen that both Cassini and Hipparcos had a hand in showing that relativity was correct. Zwicky realized that a galaxy could bend light by a detectable amount and he urged astronomers to search for the effect. Lensing was finally seen for the first time five years after Zwicky died, when the twin-i mage mirage of a single quasar was observed in 1979. In the mid-1980s, the phenomenon of lensed arcs was discovered with 4-meter tele­scopes on the ground. Each lensed arc is an image of a galaxy be­hind a rich cluster, magnified and distorted by the cluster. Cluster lensing has a very particular signature because the arcs are frag­ments of concentric circles centered on the cluster core. The beau­tiful part of the effect is that the gravitational deflection is sensitive to all mass, light or dark, so it’s a reliable way to weigh a cluster.

Hubble’s exquisite imaging has been used to study lensed arcs in dozens of clusters.43 Each background galaxy that gets imaged is a little experiment in gravitational optics (and a confirmation

of general relativity). In some clusters there are hundreds of little arcs, so the mass measurement of the cluster is very reliable— it’s like having an optics experiment with hundreds of light rays (figure 11.5). The observations clinch the fact that dark matter exceeds normal matter by a factor of six, or the ratio of the 27 percent to 4.5 percent contributions mentioned earlier. They also allow the dark matter to be mapped in the cluster, and the spatial distribution is critical information in helping decide the physical nature of this universal but mysterious substance.

Hubble has also played a pivotal role in locating a large type of dark object: massive black holes. Since the discovery of quasars in the 1960s, it has been clear that only a gravitational “engine” could generate so much energy from such a small volume. The energy source for quasars is thought to be a supermassive black hole, billions of times more massive than the Sun. As we saw in the last chapter, black holes aren’t always black. Nothing can escape from the event horizon, but a black hole will gather hot gas into a rotating disk around it. The disk siphons material into the black hole while the poles of the spinning black hole act as giant particle accelerators. The accretion disk emits huge amounts of ultraviolet and X-ray radiation while the jets emit radio waves. For a long time, astronomers thought that the galaxies surrounding the qua­sars were special because they housed such a black hole. Over the last fifteen years, Hubble has used its spectrographs to study stellar velocities near the centers of apparently normal galaxies near the Milky Way. Often, the data showed a sharp rise in star velocities near the nucleus of the galaxy.44 Calculating the density of mat­ter that would generate such high velocities in such a small vol­ume, the only possible explanation was an efficient gravitational engine—a black hole.

But these black holes were surprising in two ways. First, they weren’t as massive as the black holes that powered quasars. They ranged from 10 million to a few hundred million times the mass of the Sun. Second, the galaxies otherwise looked completely normal. Apparently, these black holes weren’t active even though there was plenty of “food,” or gaseous fuel in the center of these galaxies. As the data accumulated, it became clear that every galaxy has a black hole, with mass proportional to the mass of old stars in the galaxy, but those black holes must only be active a small fraction of the time and inert the rest of the time.45

This result has led to a paradigm shift in extragalactic astron­omy. There’s no division between “normal” and “active” galaxies— all galaxies are active at some level but not all the time. Black holes are a standard component of a galaxy. There are a few intermedi­ate mass black holes residing in globular clusters and in dwarf galaxies, filling in the mass range from a thousand to a million solar masses. Nature knows how to make black holes spanning a factor of a billion in mass! The low end of the range is collapsed stellar corpses the size of a city and the high end is behemoths in galaxies ten times larger than the Milky Way. Moreover, in their active phases black holes eject mass and quench star formation and generally alter the properties of the surrounding galaxy. The co-evolution of galaxies and black holes is now a major field of research; with its exquisite resolution and sensitivity Hubble is playing a big role. Some time in the first hundred million years or so after the big bang, the first stars and galaxies formed, and black holes began to grow at the same time.