Dark Matter Seen Through the Lens of Gravity

Revitalizing a method that is very old, Tyson and his colleagues have made use of the phenomenon of gravitational lensing, based on the theory of general relativity conceived by Einstein. Quantum mechanics considers light not only as a wave with a characteristic length, but alternatively as a particle called a photon. Einstein's special theory of relativity allows one to consider an effective but tiny mass for the photon of m = E/c2, where E is its energy and c is the speed of light. If a photon of light from some far galaxy enters the gravitational field of a closer object (not unlike a rocket or satellite sailing past a planet), the gravitational force between the object and the photon will pull the photon off course, deflecting it at an angle inward, where it will continue on a new, straight-line trajectory after it has escaped the gravitational field. If that subsequent straight line brings it within the purview of a telescope, scientists observing it can apply the formulas proposed by Einstein, as well as the simple geometry of Euclid. Knowing the angle of arrival of the photon relative to the location of the light-emitting source, as well as the distance to the mass, one can calculate the size of the mass responsible for altering the photon's path.

In 1915 Einstein had predicted just such a deflection of light by the Sun's gravitational field. It was twice the deflection predicted by classical Newtonian analysis. When England's Sir Arthur Eddington confirmed Einstein's prediction during a solar eclipse in 1919, general relativity won wide acceptance. Einstein refined the analysis years later, applying it to stars and predicting the possibility of what came to be called an Einstein ring, a gravitational lensing effect that would occur when a far-off star happened to line up right behind a star closer in. But Einstein dismissed such an alignment as being too improbable to be of practical interest. Zwicky, exhibiting his well-known prescience (he predicted the existence of neutron stars), went on to speculate that nearby galaxies, with masses 100 billion times greater than a star, would also act as gravitational lenses, splitting the light of more distant objects into multiple images.

Tyson picked up the story: "The first lensed object was discovered in 1979. It was a quasar seen twice—two images, nearby, of the same quasar—a sort of cosmic mirage." But the confirmation of Einstein, as Zwicky had predicted, was to vindicate their potential value as a sort of telescope of distant objects, a further source of information about the average large-scale properties of the universe, as well as an indicator of the presence of inhomogeneities in the universe, particularly those arising from dark matter. Tyson said that one of the reasons they seemed to offer a glimpse into the dark matter question was expressed in a seminal paper by Harvard University's William Press and James Gunn of Princeton that ruled out a universe closed by dark matter purely in the form of black holes. The technique was still considered limited, however, to the chance conjunction of a foreground gravitational lens with a distant quasar. In 1988, Edwin Turner of Princeton wrote that "only quasars are typically far enough away to have a significant chance of being aligned with a foreground object. Even among quasars, lens systems are rare; roughly 2,000 quasars had been catalogued before the first chance discovery of a lensed one in 1979" (Turner, 1988, p. 54). True enough, but if chance favors the prepared mind, Tyson's 10 years and more of deep-space imaging experience was perhaps the context for a new idea.

Discovering the Blue "Curtain"

Using the 4-meter telescopes situated on Kitt Peak in Arizona and at the Cerro Tololo Observatory in Chile, Tyson was trying to increase the depth of the surveys he was making into the sky. It was the distance of quasars that made them such likely candidates for lensing: the greater the intervening space, the greater the likelihood of fairly rare, near-syzygial alignment. Tyson was not looking exclusively for quasars, but was only trying to push back the edge of the telescopic horizon. Typically, existing earthbound telescopes using state-of-the-art photographic plates in the early 1970s were capturing useful galactic images out to about 2 billion to 3 billion light-years.

Tyson explained the brightness scale to his fellow scientists, alluding to the basic physics that intensity decreases as the square of the distance. "Magnitude is the astronomer's negative logarithmic measure of intensity. With very, very good eyesight, on a clear night, you can see sixth magnitude with your eye." As the magnitude decreases in number, the intensity of the image increases logarithmically—the brightest image we see is the Sun, at a magnitude of around-26. Though faint, the images Tyson was producing photographically registered objects of magnitude 24, at a time when the faintest objects generally thought to be of any use were much brighter, at about magnitude 20. Djorgovski supplied an analogy: "Twenty-fourth magnitude is almost exactly what a 100-watt light bulb would look like from a million kilometers away or an outhouse light bulb on the moon."

Although the images Tyson and others have produced photographically at this magnitude were extremely faint, they still revealed 10,000 dim galaxies on each plate covering half a degree on the sky. Astronomers for the last five decades have been straining their eyes on such photographs, and Tyson was doing the same when he began to realize that "my data were useless." In 1977, he had been counting these faint smudges for a while, and noticed that "as the week went on, I was counting more and more galaxies on each plate. I realized that I was simply getting more skillful at picking them out." Tyson fully realized the problem—the human element—and began to look for a more objective method of counting galaxies.

What he found reflects the impact of technology on modern astrophysics. He sought out John Jarvis, a computer expert in image processing, and together they and Frank Valdes developed a software program called FOCAS (faint object classification and analysis system). With it, the computer discriminates and classifies these faint smudges, producing a catalog from their size and shape that separates stars from galaxies, and catalogs a brightness, shape, size, and orientation for each galaxy. Now that he had established a scientifically reliable method for analyzing the data, he needed to image more distant galaxies (in order to use them as a tool in dark matter tests by gravitational lensing). In this pursuit he became one of the first astronomers to capitalize on a new technology that has revolutionized the field of faint imaging—charge-coupled devices, or CCDs, which act like electronic photographic plates. Even though many telescopes by now have been equipped with them, the usefulness of CCDs for statistical astronomy has been limited by their small size. Tyson and his collaborators are building a large-area CCD mosaic camera, which he showed to the scientists at the symposium, that will increase the efficiency. CCDs have dramatically increased the horizon for faint imaging because "they have about 100 times the quantum efficiency of photographic plates and are also extremely stable. Both of these features, it turns out," said Tyson, "are necessary for doing ultradeep imaging."

What happened next, in 1984, was reminiscent of Galileo's experience in the early 17th century. At that time Galileo had remarked that with his rudimentary telescope he could see "stars, which escape the unaided sight, so numerous as to be beyond belief" (Kristian and Blouke, 1982). Said Tyson, "Pat Seitzer and I chose a random region of the sky to do some deep imaging in. Based on what we knew at the time about galaxy evolution, and by going several magnitudes fainter with the aid of the CCDs, we expected to uncover perhaps 30 galaxies in this field." Previous photographic technology had shown none in such a field. Their region was ''a couple of arc minutes across, about 1 percent of the area of the moon," explained Tyson, for comparison. But instead of the expected 30, their CCD exposure showed 300 faint blue objects, which FOCAS and other computer treatments helped to confirm were galaxies. They continued to conduct surveys of other random regions of about the same size and came up with about 10,000 faint galaxy images. The bottom line, based on reasonable extrapolation, was the discovery of 20 billion new galaxies over the entire celestial sky. In effect, these pictures showed a new "curtain" of galaxies, largely blue because of their early stage of evolution. There still was insufficient light to measure their redshift, but they clearly formed a background curtain far beyond the galaxies that could be measured.