The Light at the Edge of the Universe: Dispatches from the Front Lines of Cosmologyby Michael D. Lemonick
Will the universe expand forever? Or will it collapse in a Big Crunch within the next few billion years? If the Big Bang theory is correct in presenting the origins of the universe as a smooth fireball, how did the universe come to contain structures as large as the recently discovered "Great Wall" of galaxies, which stretches hundreds of millions of light years? Such are the compelling questions that face cosmologists today, and it is the excitement and wonder of their research that Michael Lemonick shares in this lively tour of the current state of astrophysics and cosmology.Here we visit observatories and universities where leading scientists describe how they envision the very early stages, the history, and the future of the universe. The discussions help us to make sense of many recent findings, including cosmic ripples, which supply evidence of the first billionth of a second of the universe; anomalous galactic structures such as the Great Wall, the Great Void, and the Great Attractor; and the mysterious presence of dark matter, massive but invisible. Lemonick assembles this information into a comprehensive, up-to-date picture of modern cosmology, and a portrait of its often contentious practitioners."Thoroughly recommended to those who want not only to learn about contemporary cosmology but also to obtain some impression of how cosmologists think and work."-Malcolm S. Longair, Sky & Telescope
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The Light at the Edge of the Universe
Dispatches From the Front Lines of Cosmology
By Michael D. Lemonick
PRINCETON UNIVERSITY PRESSCopyright © 1993 Michael D. Lemonick
All rights reserved.
Judging from the appearance of the nighttime sky, Mount Hopkins, Arizona, could be on a different planet from my home in Princeton, New Jersey. Back East, the stars are sparse and dim, mostly lost in haze, air pollution, and the reflected glare of city and suburban lights. Only a few constellations are visible even on the darkest nights. You only notice them if you make an effort. The bright planets—Venus, Jupiter, and sometimes Mars—are easily confused at a casual glance with the steady parade of airliners and small planes that cross overhead. If the ancient Greeks and Arabs, whose observations underlie most of Western astronomy, had tried to work under these conditions, they would probably have given up.
At Mount Hopkins, by contrast, you can't escape the stars. The heavens are carpeted with them, and bisected by the even brighter band of the Milky Way, broad and diffuse but unmistakable. The glow of Tucson, thirty-five miles to the north, of the twin cities of Nogales, Arizona, and Nogales, Mexico, twenty-five miles to the south, and of Interstate 19, which connects the two urban centers, does nothing to diminish them. The stars are so numerous and so bright that they have almost a physical presence. It would have been a little bit unnerving to be alone on the mountain during a visit I made in the middle of February 1991, with what seemed to be a heavy curtain of light pressing down from overhead.
Fortunately, I had a guide. His name is John Huchra, and he is one of the world's hardest-working and most talented observational astronomers. This is his own opinion and the opinion of most of his colleagues. At the time of my visit, Huchra was an administrator as well as a scientist, serving a term as associate director for optical and infrared astronomy at the Harvard-Smithsonian Center for Astrophysics—the CfA for short. Serving is the appropriate verb: Huchra's own abbreviated description of the job was "jail." Normally, Huchra spends about 130 nights a year at observatories around the world, but his new position had limited him to only ninety or so, still an enormous number for most astronomers, but a starvation diet for Huchra. He knew the administrative work was important, and that having someone with his drive and working knowledge of observational astronomy in the position would be good for the other observers at the Smithsonian. But like any veteran inmate, he always knew to the day, without bothering to consult a calendar, exactly how much time he had left on his sentence.
On the first night of my visit, the stars began to pop out of the darkening sky almost as soon as the sun went down, and as full darkness came on they flooded the sky. Jupiter glowed like an electric arc to the east; Venus, even brighter, had set an hour earlier. Mars glittered overhead, and to the south was Sirius, the brightest star in the sky, but a poor second to Jupiter. We went inside the observatory building for a minute: Huchra's observing run would not start until two nights later, but the next evening he would be helping a postdoctoral fellow from the center use the telescope for a thesis project, so he had to spend some time this night on the computer, reading his electronic mail from Cambridge. "It used to be," he said, "that when I came observing, I could escape the real world. Now the eighty-seven people who are after me can still get to me. This is the wonder of E-mail." He called up one message after another, sighing every once in a while before tapping out a reply. Then, after a long silence, "Oh, no." It sounded as though something was terribly wrong. "My bowling team has slipped to third place."
Reading the messages would take him a while, so Huchra set me up with a pair of binoculars and went out to show me some of the celestial sights. I asked him to find me the great nebula in Andromeda, and, after a few minutes of triangulating from various guide stars, he pointed to a spot to the left of Cassiopeia.
"Right there," he said, and for the first time (other than in photographs) I saw the only full-fledged galaxy besides our own that is visible to the naked eye. Although it contains one hundred billion stars, it looked in the Arizona sky like nothing more than a fuzzy patch of light, much fainter than the Milky Way, a long and narrow oval stretching about four times the width of the full Moon. Until the early 1920s, astronomers did not even know for sure that Andromeda was a galaxy; many believed it was a cloud of glowing gas within the boundaries of the Milky Way. But it is now known to be a star system in its own right, an island of suns lying about two million light-years away.
A light-year is a measure of distance, and not, as one might guess, of time. It is the distance light travels, going at more than 186,000 miles per second, in one year. It equals nearly 6 trillion miles. The term is rarely used by astronomers. They prefer parsecs, shorthand for "parallax second," which is the distance away a star has to be to shift its apparent position on the sky by a second of an arc—a thirty-six hundredth of a degree—as Earth moves from one side of the sun to the opposite side over six months. Parsec is preferred because parallax is the way distances to nearby stars are measured.
Although the Andromeda galaxy is tens of thousands of times farther away than the individual stars of the Milky Way, there was no obvious way for me to tell, even in the clear skies of Arizona. The sky has no depth clues, and the most experienced astronomer sees it just as the ancient Greeks or Egyptians or Chinese did, and just as a child does as well: like a black bowl inverted overhead, its inner surface dotted with light. It would be possible to see the third dimension if stars came in one brightness only, like hundred-watt light bulbs do. If that were true, then a star's apparent brightness in the sky would indicate its distance; the closest ones would look brightest, and farther stars would seem dimmer. But in the real universe, a bright star might be average and nearby or brilliant and far away; a dim star may be intrinsically feeble or extraordinarily distant.
Yet for astronomers trying to understand how the universe is put together, and how it has evolved, it is essential to understand how it looks in three dimensions. Imagine a demographer trying to chart the spread of population across the United States over the past century. The most elementary piece of information, the starting point, for such a study would be the distribution of population today—how many people there are and where they live. If you don't know where they are today, you can't very well figure out how they got there.
Astronomers are faced with the same kind of problem. Until they trained telescopes on the Milky Way in the 1600s, they had no idea that its diffuse glow came from millions of stars, and until they found ways to measure the distances to the stars in the 1800s, they had no idea that the Sun and every other star in the sky is part of a single unit called the Milky Way galaxy. It was only by extending that third dimension further, measuring the distance all the way to Andromeda and other, fainter galaxies that Edwin Hubble proved, early in the present century, that the Milky Way is only one of many galaxies in the universe. Charting the directions and distances to other galaxies, Hubble became the first cosmic cartographer, the first to tackle the problem of constructing a modern map of the universe. Making careful measurements with the hundred-inch Hooker telescope on Mount Wilson, on what was then the outskirts of Los Angeles, he measured the distances to about twenty nearby galaxies. His work—he was also the one who discovered that the universe is expanding—made possible the science of cosmology, the study of the universe as a whole.
That was about seventy years ago. Since then, cosmologists have been refining Hubble's measurements and adding many of their own, making photographic surveys of the deep sky and assembling them into atlases that show millions of galaxies. The atlases, like the sky you see with your naked eye, are two-dimensional, though; they show where on the sky the galaxies lie, but not how far away they are. Their distance, the third dimension necessary to making a realistic map of the universe, is missing. The reason is that it is relatively easy to take a picture of the sky and capture a million galaxies at once, but very tedious and difficult to measure the distances to galaxies one by one. "In the early 1970s, when 1 started thinking about this problem," Huchra had told me when we met in his office in Cambridge a few months earlier, "there was essentially zilch data on galaxy distances. The most complete sample had 273 galaxies in it—not an overwhelming number."
By the end of the 1970s, thanks largely to Huchra and another astronomer named Marc Davis, the sample had expanded tenfold, and there were hints that the universe was more complicated than anyone had expected. Astronomers had always assumed that matter was, by and large, distributed evenly through space. On small scales, of course, that is not true. Matter in the Solar System comes in lumps—the planets and the Sun—with mostly empty space in between. The stars of the Milky Way are widely spaced lumps of matter, too, and the galaxies in turn are lumps in intergalactic space. There are clusters of galaxies as well, where galaxies are crowded together to form even larger lumps. But on really large scales, in collections of thousands or millions of galaxies, it was presumed that everything would even out, just as individual grains of sand merge into a smooth stretch of beach when they are viewed from a distance.
What Huchra and Davis started to see in the late seventies were hints that even on enormous scales the universe was somewhat uneven, with large areas that had more galaxies than average and large areas that had fewer. Other astronomers were seeing similar effects. Most notably, the husband-wife team of Riccardo Giovanelli and Martha Haynes had used radio telescopes to map an enormous chain of galaxies, which they called the Perseus-Pisces Supercluster, that stretched across the southern sky. Two thousand galaxies is a bigger sample than two hundred, but it is still not large enough for astronomers to say anything meaningful about the large-scale structure of the universe. So in the mid-1980s, Huchra started in on a much bigger survey, now working with a Harvard astrophysicist named Margaret Geller. And by 1986, with ten thousand galaxies in their catalog, Geller and Huchra and a graduate student, Valerie de Lapparent, were ready to present the first detailed map of a significant chunk of the universe. In January, Geller showed up at the winter meeting of the American Astronomical Society in Houston and gave a talk titled "A Slice of the Universe."
It made headlines—and not only in professional science journals. It appeared on the front page of The New York Times and in the pages of Time magazine, and Geller was whisked off to New York to appear on the Today show. The universe revealed in the map was not the one astronomers had expected. The slice of the universe charted by Huchra, Geller, and de Lapparent, tens of millions of light-years deep, was the biggest chunk of cosmos ever mapped in detail, and it was more structured than anyone expected—even though they had been prepared by the earlier surveys. The galaxies in the map were huddled together to form narrow filaments. In between the filaments, there was hardly anything at all—just enormous voids of empty space. The voids were more or less round, as though the astronomers' slice had cut across a froth of soap bubbles. And the walls of these bubbles were sharply defined. No theory had predicted the existence of structures this big, and no theorist could suggest any mechanism to explain how they had come about.
Huchra, Geller, and de Lapparent's bubbly slice of the universe was not the only jarring news that astronomers had to deal with that year. Sandra Faber, an astronomer from the University of California, Santa Cruz, gave a talk at a later conference in Hawaii, reporting on the observations of a team of seven astronomers (including herself) that subsequently became known as the "Seven Samurai." They, too, had been surveying galaxies, but they were interested more in motions than in distances; they wanted to see how galaxies in the neighborhood of the Milky Way are moving through space. Overall, the universe is expanding, and any two galaxies you choose will, on average, be moving apart as it does. Locally, though, gravitational attraction between galaxies can overcome the general expansion, creating "peculiar motions" that are superimposed on the expanding universe. Peculiar motions can make galaxies separate more slowly than they otherwise would, or even overcome the expansion altogether—the Milky Way and the Andromeda galaxy, for example, will have a close encounter, or even collide, in ten billion years or so. Clusters of galaxies, more massive than single galaxies, exert stronger gravity and create larger peculiar motions. The Milky Way and Andromeda, besides approaching each other, are together being pulled in the direction of the great cluster of galaxies in the constellation Virgo.
What the Seven Samurai did was to look at the peculiar motions of galaxies in an enormous volume of space, hundreds of millions of light-years across. They found that hundreds of galaxies, including the Milky Way, are all falling through space toward a point marked on the sky by the constellations Hydra and Centaurus. Astronomers already knew that the Milky Way was moving in that direction, another peculiar motion superimposed on the general expansion, and they thought they knew the reason: there is a large cluster of galaxies in Hydra-Centaurus, and its gravity was undoubtedly pulling the Milky Way toward it. But the Samurai discovered that Hydra-Centaurus was moving, too, and in the same direction. Something was pulling on it—something farther away, and much bigger. Alan Dressier, another one of the Samurai called this enormous chunk of matter, presumably a giant knot of galaxies, the Great Attractor. Like the bubbles and voids in the Harvard-Smithsonian survey, such an enormous structure was unexpected by most astronomers. It came as a shock.
Either one of these discoveries by itself might have been dismissed as a quirk, a kind of joke being played by Nature. If all astronomers had to go on was the Harvard-Smithsonian survey, they could have argued (and some did at first) that the universe really is smooth on large scales, and that Huchra, Geller, and de Lapparent had just happened on an area that was uncharacteristically full of structure. If all they had was the Great Attractor, they could say the same. With both phenomena, though, the evidence was compelling that the universe is structured on scales larger than anyone had previously imagined.
If astronomers' presumption of large-scale cosmic smoothness had been based purely on imagination and prejudice, they could simply have adjusted their worldview, just as an earlier generation of astronomers had done when they were forced to accept the Milky Way as just an average galaxy among many, rather than the only one in the universe. But there already existed direct evidence that the universe really is smooth on very large scales. That evidence is known as the cosmic microwave background radiation, and it is the echo of the Big Bang itself, still reverberating through the universe. It is the oldest light in the universe—for microwaves, like any form of electromagnetic radiation, including X-rays, infrared light, ultraviolet light, and ordinary visible light, are all basically the same stuff at different energy levels. For the first few hundred thousand years of its life, the universe was filled with both electromagnetic radiation and a hot, thick soup of elementary particles and atomic nuclei. The radiation couldn't penetrate the soup. But as the universe expanded and temperatures dropped to about five thousand degrees Kelvin (coincidentally, about the same temperature as the surface of the Sun), the soup thinned out, and the radiation burst free. With nothing appreciable to stop it over the billions of years between then and now, this light is still with us, weakened and cooled, but still detectable.
The discovery in 1965 of that feeble but pervasive radiation, in the form of microwaves (the same kind emitted by microwave ovens and radar transmitters), streaming in from all directions, turned the Big Bang at a stroke from a plausible theory into a widely accepted model for how the cosmos began. According to the model, the universe started in a condition of infinite heat and density (a condition beyond human imagination, and also beyond the descriptive boundaries of physics) and proceeded to expand and cool, the temperature and pressure dropping all the while. After perhaps .0000000000000000000000000000000000000000001 second that science can't yet deal with, the known laws of physics began to apply. They predict, among other things, that the microwave background should exist and that the universe should be expanding—both of which are observed, and neither of which is convincingly predicted by any other model. It's a "model" or a "theory" rather than the absolute truth, because no one can say for sure whether another idea won't work better, predicting the same observed phenomena that the Big Bang model does, but explaining other still-mysterious facts about the universe as well. Astronomers are currently proceeding on the assumption that the Big Bang is basically right, but most of them acknowledge that it could be modified or, conceivably, even overthrown someday.
Excerpted from The Light at the Edge of the Universe by Michael D. Lemonick. Copyright © 1993 Michael D. Lemonick. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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