Nineteen-ninety-three was a pretty big year for astronomer Robin Ciardullo.
In February, the Penn State assistant professor traveled to Boston to tell the American Academy for the Advancement of Science that he and collaborator George Jacoby of the Kitt Peak Observatory had found what he calls the "holy grail" of intergalactic measurements: the distance to the Virgo cluster of galaxies. Doing so gave Ciardullo and Jacoby a new value for the rate at which the universe is expanding—a figure that suggests a cosmos some eight billion years younger than the one which theorists have supposed. "Obviously," says Ciardullo, "something is fundamentally wrong with our understanding of the universe if these numbers hold true."
Robin Ciardullo
Then, in June, it was off to Berkeley, California, and the American Astronomical Society, where Ciardullo and Jacoby tipped another sacred cow. New measurements they had taken, they announced, proved the absence, in at least one galaxy, of the mysterious substance known as dark matter—that unseen, unfelt, and in plain terms unknown stuff which is widely posited to make up over 90 percent of the universe. "For the past 15 years," Ciardullo says with a shrug, "anyone who looks for dark matter finds it. I didn't find it."
Since the fate of the universe—whether it is "open" or "closed," i.e., whether it will expand forever or one day roll back on itself like an overtaxed windowshade—depends heavily on just how much dark matter exists, this hint that there might be less than supposed ramified considerably.
Two big attention-getting discoveries, in two very different and equally fundamental areas of today's astronomy. To what does Ciardullo attribute his success?
"I have a short attention span," he says.
It's a joke, but half-serious; for Ciardullo's roaming interest has led him to concentrate not on any one point in the heavens, but on assembling a bag of observational tools that will allow him to explore as widely as possible. Both of his recent discoveries, it turns out, depend on one tool that has become especially useful to him: planetary nebulae, or dying stars.
Planetary nebulae pepper the Milky Way, up to 50,000 of them. They make exceedingly lovely objects, appearing as diffused rings of light. ("I said at a talk once that they're the prettiest objects in our galaxy," Ciardullo confides, "but somebody took exception.") Once thought to be planets (hence their name), they are the remnants of red giant stars that have kicked off their outer layers, leaving an intensely hot core, called a white dwarf, surrounded by a dense cloud of gas. Our sun will one day evolve into a planetary nebula.
Dying stars, of course, exist in other galaxies too. During the 1970s, astronomer Holland Ford, then at UCLA, began to get interested in these more distant objects. By the early 1980s, he was working with two graduate students—Ciardullo and Jacoby—on the possibility of using extragalactic planetary nebulae as a way to measure dark matter.
It goes this way: Stars revolving in a galaxy are held together by a balance between gravity and velocity, the same principle that keeps the Earth from flying off into space or being drawn into the sun's inferno. The faster the stars are moving, the more gravity has to be present to hold them in place. More gravity requires more mass.
In theory, then, if you can measure the velocities of a galaxy's stars, you can determine its mass. Subtract from this total mass the weight attributable to stars, gas, and luminous dust and you're left with the measurement of a galaxy's dark matter: what you can't see but the laws of physics say has to be there.
The problem is, it's impossible to see individual stars in far-off galaxies. Astronomers can estimate velocities from collective starlight, but these estimates tend to be skewed to the bright, inner regions of a galaxy. Outer velocities can be very different, throwing the total way off.
Planetary nebulae, however, are far brighter than ordinary stars—easy to measure, once they're located. Furthermore, they lie spread throughout a galaxy, so that by measuring their velocities you can get a good overall sampling.
First, however, you have to find them. This means picking them out from all of the other light sources, near and far, that confuse the picture. To do so, Ford and his students learned to take advantage of the peculiar quality of a dying star's light.
A planetary nebula's outer cloud, Ciardullo explains, reprocesses the light still given off by its core. As a result, instead of emitting light in a continuum, as an "ordinary" star does, a planetary nebula emits at only a few specific colors, or wavelengths, the brightest being green in the optical spectrum. By making photographs that filter out everything but this wavelength, Ciardullo and his colleagues can easily isolate a galaxy's planetary nebula population. Then they measure the velocities of these objects using the Doppler shift of their light.
Commonly observed in sound waves—those, say, of an onrushing train whose velocity shifts the frequency of its roar, so that standing on the platform, we hear a drop in pitch as the engine hurtles by—the Doppler effect works also for light. When the source of light is moving toward us, the light waves we receive are shifted slightly to the blue end of the spectrum; when it moves away, the shift is toward red. Measuring the extent of the shift can yield an object's traveling speed.
While working at measuring velocities, Ciardullo couldn't help but notice a distinct uniformity in the appearance of planetary nebulae; it was something Ford too had seen, in a few nearby galaxies, back in the 1970s. Specifically, the brightest nebula in a given galaxy for some reason invariably had the same brightness as its counterpart in any other galaxy.
Ciardullo worked to expand this observation; he and Jacoby searched and found that the uniform luminosity held for all sorts of planetary nebulae, no matter what types of stars they came from, or where. What it meant, Ciardullo realized, was that planetary nebulae, in addition to being clear, well-placed beacons for measuring intra-galactic velocities, would also make a fine way of gauging the distances between galaxies—measures that are a crucial variable in determining the age of the universe.
Back in the 1930s, when Edwin Hubble at Caltech first showed that the universe is expanding, two corollaries quickly became clear. First, this expansion had to have started somewhere—at some finite point in the past. Hence, the "Big Bang" theory of cosmogony. Second, if we could determine the rate of this expansion, we could pinpoint when the Bang occurred: the moment the universe began. Hubble expressed his idea as an equation: the rate of universal expansion equals the velocity at which a galaxy is receding from a given observer divided by that galaxy's distance from that observer. The equation's solution became known as the Hubble constant.
Well and good, except that a satisfactory solution to Hubble's formula has remained elusive. Over the 60 years since its formation, scores of astronomers have come up with values for the constant—numbers so varied they give an age for the universe of anywhere from 10 to 20 billion years. To narrow that range has become one of cosmology's burning quests.
Measuring a galaxy's recessional velocity has not been the problem; again, that can be determined by the red shift of the Doppler effect. The sticking point has been fixing distance.
Consider it: You're standing on a beach on a very dark night looking out over the water. You hear the waves gently crashing, and see a light out in front of you—nothing but a single point of light. How far away is it?
Tough to tell. Apparent brightness is no good measure here: A powerful searchlight on a ship miles out near the horizon might seem no more luminous than a lantern aboard a rowboat sitting just beyond the first line of waves, or a firefly six feet in front of your face. But suppose you had a marker out there, an object you knew to be in the vicinity of the unknown source, and whose true brightness you had already fixed. A lighted buoy, maybe. From what you know, you could determine its distance, and then use that measurement as a yardstick to find the distance of the other, nearby object. In astronomy, these buoys are known as standard candles. And planetary nebulae, Ciardullo and Jacoby sensed, would make good ones.
Over the years, lots of different candles have been applied to the problem of extragalactic distances. Many other types of yardsticks have been applied as well. Each method has its fierce partisans, but no one way has predominated. "There are wars over this," Ciardullo says. "It's been going on so long that it's sort of burned into the astronomer's psyche—no one really knows the distance scale."
An especially important battleground has been the Virgo cluster, a group of galaxies far enough away that its outward velocity can be measured without the confusion of local random motion, yet close enough to pick out pretty well with existing telescopes. Virgo's distance has been sought—and fought over—for decades. So once Ciardullo and Jacoby had worked out their distance formula, they set their sights on Virgo.
They took preliminary pictures of the cluster using the four-meter telescope at Kitt Peak. Soon after this information was published, they were granted time to perform the necessary observations, again using the Kitt Peak instrument and also the Canada-France-Hawaii telescope, a three-meter instrument in Hawaii managed by an international consortium of scientists. Over a couple of nights of reasonably clear weather, "We took our measurements," Ciardullo remembers. "Everything seemed right. We were excited, thinking to ourselves we had nailed the distance to Virgo. Then we went back home to analyze the data."
At first, the computer kept insisting there were large errors in the measurements. For several days, "George and I were bouncing e-mail back and forth, racking our brains," Ciardullo says. What had gone wrong?
Then Ciardullo figured it out. In order to get a firm handle on the brightness of the planetary nebulae in Virgo, he had compared them against foreground stars, stars in our own galaxy. Standard candles for the standard candles, as it were. As it turned out, however, one of the standards he happened to pick was an RR Lyra star, a type whose brightness varies. "It took us a while to track it down."
When they did, they found a star that was "way, way out there, out where no star in our galaxy belongs," Ciardullo says. Our solar system, for comparison, is eight kiloparsecs (about 24,000 light years) from the Milky Way's core, out in the boondocks of a spiral arm. This thing was 50 kiloparsecs above the spiral arm. It was, as Ciardullo carefully puts it, "the farthest star in our galaxy with a well-known distance yet published," its discovery a remarkable bit of serendipity. What struck him, he remembers, was that the new star sat "right in front of a very popular Virgo galaxy—but no one had noticed it!"
Somewhat to Ciardullo's chagrin, the announcement of the Lyra star made considerable waves. "I was coming back from doing some observing in South America," he remembers, "thirty-six hours on a plane, and when I got to Tucson, there was this media circus! 'Furthest star discovered!' We, of course, had been looking for the Hubble Constant."
Once the mystery was cleared up, they did get what he calls an "unambiguous" set of distance measurements. And these measurements yielded a Hubble Constant of 80 kilometers per second per million parsecs, considerably higher than the classical estimates of 50 kilometers per second. In terms of turning back the clock, this figure translates to an "almost uncomfortably young" age, for the universe, of 8 to 12 billion years. Uncomfortable because currently accepted estimates of the ages of the oldest stars, derived not by distance measurements but by the physics of nuclear combustion, are 15 billion years. How can a star be older than the universe it is a part of?
The dilemma has not yet been resolved, but indications are that the quest is moving forward. Recently, Ciardullo and his partner assembled a group of their young distance-scale colleagues to compare notes.
"George and I simply got tired of people writing review articles saying this problem can't be solved. So we got several experts together, all with their different methods of measuring, and we said let's get to the bottom of this. The idea was just to talk honestly about how we measure distances, what are the strengths and weaknesses of each method. Let's include all the provisos on measurements that usually get left out."
The result, he jokes, was "probably the longest paper in the history of the subject," sixty some pages of explanations, appearing in the Publications of the Astronomical Society of the Pacific. But direct comparisons of their several methods, the authors conclude, "show surprisingly good agreement."
From the distance work, Ciardullo shifted his focus back to dark matter, where years of effort seemed finally ready to bear fruit. "In order to measure the motions of planetary nebulae in galaxies, things have to break well," he explains, and for a while they hadn't. "We had lots of bad luck—snowstorms, rain, instrument failure, trouble getting telescope time. Optical astronomers are at the mercy of these things."
Again, they had developed a new technique, based on the use of planetary nebulae. And again, to test that technique, Ciardullo and Jacoby decided to focus on a well-known object, this time the elliptical galaxy M105, which lies 33 million light years distant, in the constellation of Leo. It took two years to arrange the necessary observing time. "Telescopes like Kitt Peak are oversubscribed by a factor of about three to four times," Ciardullo notes. Finally, however, permission came through, and they settled into the work.
What they found, after a close scrutiny of the movements of the planetary nebulae within M105, was—nothing. That is: no dark matter.
Ciardullo explains. "Near the center of M105, where there is a lot of visible mass and the gravitational field is strong, the planetary nebulae move fast. On the outside of the galaxy, where the attraction due to the galaxy's visible matter is small, the nebulae move slowly. Its overall behavior is exactly what you would expect from a galaxy made of ordinary stars—no extra invisible matter is needed to explain the observations."
Curiously, when he first reported these results, in 1992, at a planetary nebulae meeting in Innsbruck, Austria, the silence was deafening. "Mine was one of the most ignored papers there," he remembers. "This audience was not interested in dark matter." His approach and reception at the American Astronomical Society meeting the following June could not have been more different. "I went to Berkeley kind of to relax a little bit, see some friends. I had this paper I could give, so I figured okay. . . . All of a sudden the Society's press officer is asking me if I'll sit in on a press conference on dark matter."
At the press conference, with five other researchers, he and Jacoby were odd-men-out: "It was like 'Dark matter, dark matter, dark matter, no dark matter,'" he laughs. Stories on M105 ran in newspapers around the world.
Dark matter theorist Jane Charlton, whose office is just down the hall from Ciardullo's, has told him: "I occasionally wake up with nightmares about your galaxy.'"
One could put Charlton's concern another way. If the M105 finding holds up, if, that is, it can be extended to include other galaxies, if there really is a lot less dark matter around than has been supposed, there's going to be an awfully big hole left in astronomical understanding, if not in the universe itself.
"There is pretty solid evidence of dark matter around spiral galaxies," Charlton notes. "And also around clusters. They couldn't hold themselves together at the velocities observed without enough dark matter.
"So there is dark matter around. But we do have to think seriously about the possibility of not having as much as some theorists would like. If that turns out to be the case, a lot of things go back on the list of stuff to be explained."
Really, though, it's way too early to tell. "I don't know what it means," Ciardullo admits. "Already somebody has speculated that maybe there are two classes of galaxies—some that have dark matter, some that don't. I argue only that my result is not controversial, that I'm not missing something."
His result is strengthened by the fact that the galaxy he chose for a first look, M105, is so positively normal, the standard elliptical galaxy described in textbooks. But still, it's only one galaxy.
"Let me look at another," Ciardullo says. "Let me get a class of two. Then, look at a galaxy of a different type. Then at a galaxy where other people believe they have measured dark matter. Then, maybe we'll be able to say something a bit more conclusive."
And by then, maybe it will be time to train the power of planetary nebulae on some other distant object.
Robin Ciardullo, Ph.D., is assistant professor of astronomy and astrophysics, 519 Davey Laboratory, University Park PA 16802; 814-865-6601. George H. Jacoby, Ph.D., is astronomer at Kitt Peak National Observatory in Tucson, Arizona. Their research is supported by the National Science Foundation.