First Light: The Search for the Edge of the Universe (33 page)

At the center of a youthful galaxy, a cloud flattens into a wheel of gas, dust, and (perhaps) planetesimals—an accretion disk. At the hub of the disk, a giant protostar undergoes nuclear ignition. The protostar feeds on inwardly falling gas until it grows too heavy to support itself. The poles of the star suddenly collapse, and the star turns into a spinning doughnut. The hole in the doughnut falls out of the universe—it implodes, collapses in space, collapses in time, acquires great entropy, and redshifts to infinity. It becomes a black pinpoint, a black hole. The spinning doughnut-star sheds its inner rim into its own black hole. The doughnut eats itself. The surrounding accretion disk—gas, dust, planetesimals, whatever—draws inward and swirls around the hole. The disk spins faster, heating up through friction. More and more gas arrives in the disk and tries to flow down the hole. This is a solar system that went down a rat hole in spacetime, sucking a cataract of matter after it.

The disk spins so rapidly that the material in the disk has trouble getting into the hole. The disk glows from turbulence and friction. Matter spills from the inner lip of the disk into the black hole, putting some torque on the hole. The hole spins up until it is rotating at a velocity known as the extreme Kerr solution—approaching the speed of light. The hole screws space and time around itself and tugs at the inner lip of the accretion disk. Magnetic fields whip through the disk. The disk fattens and begins to shine in unspeakable colors—the theorists toss around phrases such as “upscattered soft photons” and “synchrotron radiation”—but nobody really knows what the colors of a burning accretion disk might be. Gunn said, “The disk progresses slowly into the black hole, and friction inside the disk is what gives rise to the fireworks.” A hydrogen bomb converts about 0.7 percent of its core mass into radiant energy. A burning accretion disk can liberate up
to one third of its mass into out-streaming light as the remainder of the mass gargles down the drain.

A black hole is a very large amount of mass crammed into a very small place. If the earth, for example, were to be forced into gravitational collapse, it would make a black hole the size of a golf ball. If a black hole the size of a tomato were placed into a low orbit around the earth, its gravitational pull would drag oceanic tides over the continents. It would crack up the continental crust, triggering volcanic eruptions. The earth would go into a binary orbit with the tomato. The tomato would pull the earth into a teardrop. If the hole and the earth touched, the hole would go into orbit
inside
the earth. It would spiral toward the center of the earth, devouring matter. The earth would melt, vaporize, emit X rays, and churn down the black hole. After the tomato had eaten the earth, the tomato would be a little bit fatter, tending toward a Burpee’s Big Boy. A hole that has swallowed a hundred million suns would fill the orbit of Mars. The brightest quasars may contain a hole that has eaten several billion suns, which would fill the orbit of Pluto.

Around this hole the accretion disk would be burning brilliantly, but the disk might extend outward a great distance, perhaps as far as a light-year, dimming off gradually. At its outer limits the accretion disk might merge imperceptibly into the disk of the galaxy itself and sparkle with newly formed stars, which had condensed out of the disk as it spiraled toward the hole. “If you immerse one of these monsters in gas, as at the center of a galaxy,” Gunn said, “it is going to grow. As these things get bigger, they become brighter. And more voracious. There’s a common misperception, though, that black holes have to eat everything in a galaxy. Most of the matter in a spiral galaxy can’t get near the galaxy’s central black hole, because it is rotating around the center of the galaxy—as we are. There are natural limits to the growth of a black hole. Pretty soon the black hole runs out of food. It starves. The rise of quasars is the growth and saturation of a monster. The monster’s food supply gets cut off, and the decay of the quasars chronicles the monster’s starvation.” These starved objects would still sit in the centers of some galaxies. They are not feeding anymore, or they are feeding only sporadically. The core of the Milky Way may
contain a modest black hole that has swallowed the mass of a million suns (not enough mass ever to have caused the Milky Way to go quasar).

Astronomers are sure about only one thing regarding quasars: they are sure that they do not understand quasars. Bohdan Paczýnski, an astrophysicist who, like many, has spent a portion of his career contemplating the enigma of quasars, had this to say: “Our understanding of these accretion disks is comparable to astronomers’ understanding of stars before the discovery of nuclear fusion.” Jim Gunn expressed it this way: “We don’t actually know that quasars have anything to do with black holes at all.” The only bona fide accretion disk that astronomers have ever seen through a telescope is the rings around Saturn.

But the theorists enjoy trying to imagine what kind of an object makes the colors of a quasar. If a giant accretion disk—the core of a bright quasar—was located in our sky at the distance of the nearby star Alpha Centauri, the hot, central part of the disk would appear to be about the size of a penny seen from one hundred yards away. The light coming out of that penny would be two hundred times brighter than the sun. If you went outdoors and tried to look at it, your hair and clothes would burst into flames and your skin would char. You would absorb a lethal dose of gamma rays and X rays. If the quasar was emitting plenty of microwaves, as some quasars do, you would get a dose of radar that might cause you to boil internally. The experience would be not unlike standing close to a nuclear fireball at the moment of ignition. If you could somehow look at a quasar from nearby, through a glass darkly, you might not see the accretion disk. The disk might be enveloped in a shining globe, filling a great portion of the sky—a corona of hydrogen gas, perhaps one light-year across. The pressure of light coming out of the accretion disk would support the corona in a delicate stasis against collapse, preventing the corona from shrinking into the hole at the center of the quasar. The corona might be threaded with tendrils of fast-moving hydrogen, emitting Lyman alpha light.

Space around the quasar would be packed with stars, because the center of a galaxy is a well of stars. The stars would be orbiting the quasar, feeling the gravity of the black hole. Stars would pass
through the quasar’s corona and through the accretion disk inside the corona. Most stars would come out intact, except for the red giant stars. If a red giant passed through the hot part of the accretion disk, the giant would get a haircut and come out as a white dwarf. Quasars have been known to let off great pulses of light—implying that a cloud of gas hit the accretion disk and shirred into it, which would cause the disk to spike hundreds of times in brightness, which might make the quasar’s corona puff and horripilate. The brightest quasars—the ultraluminous quasars—may be eating a steady diet of gas, each year, equal to the mass of a hundred stars the size of the sun. If the earth was vaporized and injected into an ultraluminous quasar, its total mass would power the quasar for one second.

You might see an opposed pair of jets streaming out of the quasar in opposite directions—the fearful symmetry of the particle beams. These are fountains of electrons and other subatomic particles that are thought to squirt from the poles of some spinning black holes. They can mushroom into clouds of gas millions of light-years across, large enough to swallow Local Groups of galaxies. The jet that Maarten Schmidt saw drilling out of his first quasar—the quasar called 3C 273—is apparently a particle beam as long as three galaxies.

The tapestries in Maarten Schmidt’s house made Don Schneider feel strange. They had been woven by Maarten’s wife, Corrie. She had titled one of them “3C 273.” It was a swirling disk, five feet across, knotted with gobs of material, and at the center Corrie had mounted a glass photographic plate of the quasar and its jet, taken on the Hale Telescope. Don now sat in the Schmidts’ living room, telling Maarten that he had dug seventy-three good quasar candidates out of a strip of sky. Maarten and Don discussed the possibilities. Maarten hoped that some of these candidates would turn out to be quasars near the edge of the known universe. “You can be cheated by statistics,” Maarten allowed, “just the way people can win or lose at Las Vegas. I’m knocking on wood right now. I don’t want to knock wood too hard for fear that the weather or the statistics will turn against us.” Don and Corrie and Maarten roasted
shish kebab in the backyard, watching the sun go down. The Principal Investigator was a little nervous because he had been waiting for a year and a half to see what kind of crop of quasars these strips of sky would yield.

The next afternoon, Maarten and Don drove in Maarten’s brown Ford to the summit of Palomar Mountain. They found Jim Gunn in 4-shooter’s garage, surrounded by pieces of 4-shooter. They bolted into the Hale Telescope a sensitive CCD instrument, built by J. Beverley Oke, known as the Double Spectrograph.

Jim left off tinkering with 4-shooter long enough to look at some sheets of paper that Don had brought with him and which contained jagged lines—crude, almost illegible spectra of the candidates. Jim weeded out a handful of unusual spectra. These objects, he thought, might be deeply redshifted quasars. Maarten and Don both claimed to be skeptical. At the close of twilight they fed the coordinates of the first candidate into the telescope.

On the television screens in the data room, something that looked like a star came up. They took a spectrum of it. It was a star. A dud.

They slewed the telescope to the next candidate. They saw an anonymous galaxy with a hot nucleus. They put the slit of the spectrograph across the nucleus of the galaxy and decomposed its light, to see what kind of a creature this was. They waited for the computer to digest the light, and then a jagged line appeared on the screen, a line describing the intensities of color in this galaxy. They studied the line, reading the text of the light: it was a Seyfert galaxy—a spiral galaxy with a miniquasar burning in its core. This miniquasar was irradiating unto death, for all anybody knows, a few million solar systems near the center of the galaxy. Nothing fancy.

At eight o’clock in the evening they put the slit of the spectrograph over the first of Jim’s deep quasar suspects, a point of light resembling a star in the constellation Aquarius. They opened the camera and gathered light for fifteen minutes. A spectrum came up on the screen, displaying the object’s broken light. They saw glowing hydrogen and carbon. The thing was obviously a quasar.

The quasar was a monster. It showed a Lyman alpha emission line. Ordinarily this line would be ultraviolet in color and therefore
invisible, but here it had been redshifted down into green. This quasar was receding at close to the speed of light, carried along in the Hubble flow, the expansion of the universe. It was a deeply redshifted quasar.

The astronomers continued to break up the light of candidate objects. They found another Seyfert galaxy. They found an N galaxy—a galaxy with a blue, starlike center. They complained about the seeing—the air over Palomar was rippling tonight. At eleven o’clock they pointed the Hale Telescope at another of Jim’s deep suspects, a starlike object in the constellation Cetus.

Jim studied Don’s paper. He said, “I think the redshift on this thing will be three-point-eight.”

They opened the shutter and collected the object’s light for half an hour. They closed the shutter and called up the spectrum. A ragged, up-and-down line appeared on the screen. The line resembled the silhouette of a conifer forest.

Maarten Schmidt had never quite gotten over his surprise at what a change the computers had wrought. He could read the text of light on a screen. This quasar had a stunning Lyman alpha peak, a spike of color that ordinarily would be ultraviolet, but this one had been dragged down into the color yellow. Now
there
was a redshift. The peak was lacerated with fine absorption lines—razor cuts in the spectrum, betraying clouds of invisible hydrogen gas in front of the quasar, perhaps swirling around the quasar. He saw ionized silicon in the quasar. He saw oxygen. He saw nitrogen. He saw the same elements that human bodies and the earth are made of, except that he was looking into the early universe. The photons just gathered in the Hale mirror had been possibly older than the Milky Way.

Punching buttons on a calculator, Maarten estimated that this quasar had a redshift of 3.8, or 380 percent. Gunn had been right. Later the astronomers would give the quasar a name: PC 0131 + 0120. “I suppose we could have named it after one of Maarten’s daughters,” Don would remark, “but Maarten has discovered too many quasars, and he only has three daughters.”

By the end of the next night the astronomers had found five more quasars with deep redshifts. “A couple of years ago,” Don
remarked, “I would have gone nuts finding just one of these things. Familiarity breeds contempt.”

The following night Gunn rebuilt 4-shooter in the garage, working by flashlight. Juan Carrasco noticed a weather front coming in. Maarten spent much time on the catwalk, watching clouds gather. He saw lightning on the horizon, which bothered him. Lightning can contaminate sensors and make it difficult to read the colors of a quasar. As the weather deteriorated, they managed to get another remote quasar. A few minutes before dawn, they shot the last deep suspect, which turned out to be another quasar near the beginning of lookback time. The sun came up, and clouds enveloped the mountain. The team had now turned up nine high-redshift quasars, all of them at the limits of the optically known universe.