Chasing Hubble's Shadows is an account of the continuing efforts of astronomers to probe the outermost limits of the observable universe. The book derives its title from something the great American astronomer Edwin Hubble once wrote: "Eventually, we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial."
The quest for Hubble's "shadows"—those unimaginably distant, wispy traces of stars and galaxies that formed within the first few hundred million years after the Big Bang—takes us back, in effect, to the beginning of time as we are able to perceive it, when the first discrete stellar objects appeared out of what has lately come to be known as the "cosmic dark age." The information that is being gleaned from these dim sources—chiefly with the aid of Hubble's namesake, the Hubble Space Telescope—promises to yield clues to many cosmic puzzles, including the nature of the mysterious "dark energy" that is now believed to pervade all of space.
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About the Author
Jeff Kanipe is an independent science journalist, specializing in astronomy, cosmology, and planetary science. His most recent book was A Skywatcher's Year.
Jeff Kanipe is an independent science journalist, specializing in astronomy, cosmology, and planetary science. His most recent book was A Skywatcher’s Year.
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Chasing Hubble's Shadows
The Search for Galaxies at the Edge of Time
By Jeff Kanipe
Farrar, Straus and GirouxCopyright © 2006 Jeff Kanipe
All rights reserved.
HUNTING FOR SNARKS
In the midst of the word he was trying to say,
In the midst of his laughter and glee,
He had softly and suddenly vanished away—
For the Snark was a Boojum, you see.
—Lewis Carroll, "The Hunting of the Snark"
"The cup of the Big Dipper contains over four hundred galaxies," said the planetarium director, emphasizing his remark by rapidly circling the famous star grouping with his red pointer. "Just imagine," he continued, "if there are that many galaxies confined to just this tiny area, then there must be hundreds of millions of galaxies scattered across the entire sky, each containing tens of billions of stars." And with that, he turned a dial on his console, and the stars began to march majestically across the sky to the opening strains of Percy Faith's "Theme from A Summer Place."
Fittingly, it was late summer 1968 and the first time I had ever set foot in a planetarium. My youthful interest in astronomy was still rudimentary, but already I had learned and could recognize on sight most of the constellations. I watched the familiar patterns rise in the east, drift overhead, and disappear in the west as the impossibly high strings of "Summer Place" soared in the background. Perhaps I am easily amused, but I was wowed.
Thirty-six years later, with the public release of the Hubble Ultra Deep Field (HUDF)—the deepest, farthest look into the universe ever made—my first-time planetarium experience came back to me and I was wowed all over again. Astronomers at the Space Telescope Science Institute in Baltimore announced that a region of sky just three thousandths the size of the cup of the Big Dipper contains at least ten thousand galaxies! That's an area you could easily cover with a pencil eraser held at arm's length. Assuming that the distribution of galaxies is similar across the sky, the number of galaxies embraced by the Big Dipper's cup would be well over 190 million, counting all those down to the faintest ones seen in the HUDF. If you extrapolate these values to the number of potentially observable galaxies across the entire sky, you get a truly mind-boggling number: 127 billion! Of course, the true population, consisting of these galaxies plus those that cannot be, or have yet to be, detected with current telescopes, would be far greater.
We know how deep Earth's oceans are. We know the width and breadth of the solar system. The size of the Milky Way is no longer in question; nor is the expanse of space lying between the nearby galaxies. But just how deep is the universe? Such a question has the most profound implications, because in this case, deep implies more than size and distance on the grandest of scales—it implies age as well. Some of the galaxies in the Hubble Ultra Deep Field, for example, lie more than 12 billion light-years away. But that immensity also represents a span of time—lookback time—of more than 12 billion years in the past, because it has taken that long for the photons from these remotest known galaxies to reach Earth, despite zipping along at a velocity of 300,000 kilometers per second. Hence the universe is far richer than people thought it was in the 1960s or, for that matter, in the three decades that followed. And today the answer still eludes astronomers. Not even the acute vision of the Hubble Space Telescope, the instrument used to make the remarkable HUDF image, can survey such realms. Still, the Hubble telescope has afforded us the first glimpse of the objects that live near the farthest reaches of the universe, billions of newly formed and inchoate galaxies. Only that telescope, at this time in history, could have provided humankind with such a penetrating look at what creation hath wrought.
What a marvel and scientific triumph the Hubble telescope has been. No other contrivance save Galileo's humble two-inch telescope ("Old Discoverer," he called his fifth and favorite model) has done more to advance humanity's understanding of the universe in so brief a time. Even with a modest 2.4-meter mirror, the telescope's location above the distorting effects of Earth's atmosphere has extended our visual acuity more than a billion times. If fireflies flickered in the dark recesses of the moon, we could see them with the Hubble telescope. Its abundant light-gathering power has enabled astronomers to study stars, star clusters, nebulae, and galaxies in far greater detail and at significantly greater distances than anyone had thought possible before it was placed into orbit in 1990. All the more reason to mourn its probable loss due to a federal budget constraint. Stamping out wasteful government spending is certainly a good thing, but wasting an invaluable telescope that has revolutionized the way we look at the universe does not seem to be in the best interest of science or society. But that's another topic for another day.
For the science of cosmology—the study of the origin, evolution, and structure of the universe—the Hubble telescope has been both a pathfinder and a plumb line into the vast depths of our cosmic origins. This was aptly demonstrated in January 1996 with the public release of the Hubble Deep Field, the first baby portrait ever made of the universe. Had it not been for the Deep Field and its complement a year later, the Hubble Deep Field South, we would have never had an Ultra Deep Field. Thankfully, we now have all three.
The original Deep Field was a bold proving ground for later explorations of the distant universe. For ten consecutive days, between December 18 and 28, 1995, astronomers focused the Hubble telescope on a single spot of sky in the constellation Ursa Major (Big Bear). Except for a few very dim stars that were used for guiding purposes, the region initially appeared devoid of galaxies. A total of 342 exposures were made, each ranging between 15 and 40 minutes, covering ultraviolet, optical, and infrared wavelengths. Ostensibly, the purpose of the multiple exposures was to see how many galaxies might be strung out toward the edge of the visible universe and whether any structural features could be discerned in them. Perhaps the image would reveal a few galaxies, or perhaps a few hundred. Maybe the galaxies would appear faint, fuzzy, and featureless or at best show some modest structural detail.
The result was a stunning still life of more than two thousand galaxies, a flurry of budding, tumultuous light whipped up in the shadowy primordial vacuum. This is how the universe looked when it was less than 2 billion years old: very strange; very chaotic. Upon seeing these galaxies for the first time, Robert Williams, then director of the Space Telescope Science Institute and leader of the Deep Field research team, said, "We have not been able to keep from wondering if we might somehow be seeing our own origins in all of this."
When I first sat down with my eight-by-ten copy of the Deep Field, I scrutinized it with a magnifying lens and then I just gawked, trying to fathom what I was seeing. Of course, it's the bright things the eye naturally fixates upon, but in this case my saccadic vision bounced around the photo like a pinball in play. Just left of center was a bright star—a foreground object in our galaxy. It was positioned next to a cream-colored spiral galaxy that probably lay a billion or more light-years beyond. Scattered around the field were nearly two dozen prominent galaxies, most of them spirals or partial spirals with bright, broad centers. Mixed in were a dozen or so ruddy spheroids that stood out like smoldering embers against innumerable smaller shards dappling the background.
Going back to my magnifying glass, I inspected the spaces between the fainter but still obvious "blobjects." Here were flecked wraiths of light that barely surfaced above the darkness—hints, perhaps, of other galaxies. Hubble's shadows, I thought to myself. He, more than anyone, would not have been surprised to see these features. Indeed, he would have been surprised not to have seen them. In autumn 1935 Edwin Hubble, who until his death in 1953 was the world's preeminent observational astronomer, delivered a series of lectures at Yale University in which he described his principal observations of the universe and their possible implications. The lectures were compiled the following year into a book called The Realm of the Nebulae, which is still a must-read for all astronomers because it was largely responsible for presenting the first modern view of the cosmos. At the conclusion Hubble reflected upon the challenges astronomers face in detecting objects at the edge of the observable universe: "Eventually, we reach the dim boundary—the utmost limits of our telescopes. There we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial."
Hubble's murky meditation is usually interpreted as an expression of the challenges astronomers face in observing so vast a universe with limited telescope technology, but for me his words took on new meaning as I looked at the Hubble Deep Field photograph that day in 1996. How overwhelmingly limitless the universe appears, even from the perspective of a powerful telescope unhindered by Earth's palpitating atmosphere. The shadows with which Hubble had to contend were indeed ghostly but not like those cast in the image appropriately bearing his name. Although the field encompasses a mere pinprick in the celestial vault, it is a profound perforation. The largest and brightest galaxies in the image are 7 to 8 billion light-years away, and a few would prove to shine from a universe that existed at least 12 billion years ago, when the universe was about 10 percent its present age. In all, there are at least twenty-five hundred galaxies assorted throughout billions of light-years along the line of sight of this extremely narrow visual tube. Just how many angels can flit through a soda straw?
Observational cosmology allows us to use telescopes as keyholes, through which we spy frozen moments of cosmic lookback time when objects shouted out their existence with heat and light. For nearby galaxies the lookback times range from a few million to hundreds of millions of years, by no means an inconsequential amount. But the lookback times of galaxies in the Hubble Deep Field require that we invert the way we typically denote benchmarks of spatial reference and age in the cosmos. The galaxies in the Deep Field flickered to life so long ago that they are immensely old relative to our present universe. On the other hand, because they are so old, they are young relative to the instant of the big bang. Hence we must now reorient ourselves from looking back in time, toward the beginning of the universe, to looking forward from that beginning until now.
The quest to find the most distant objects in the universe goes back many decades and has proved to be one of science's most challenging and arduous observational undertakings. Researchers had to ask themselves some very tough questions. How did the first stars and galaxies form, and when? What did they look like? How massive and how bright are (or rather, were) they? In the twilight years of the twentieth century, there were no ready answers to such questions. All assumptions as to the nature of primordial stars and galaxies were based on the known stars and galaxies in our immediate region of the universe. But trying to link the modern universe to the primordial universe was, and still is, a risky business, presenting astronomers with a kind of cosmological catch-22. Giant galaxies such as the Milky Way, it was assumed, had evolved since the early universe, but the effects of that evolution had to be understood before sound assumptions could be made about the first generation of galaxies. Astronomers knew, of course, that stars like the Sun were products of previous generations of stars that had cooked up atoms of hydrogen and helium in their cores to create the heavier elements—"metals," the astrophysicists call them—upon which life is so dependent. But the origin and nature of the primordial stars and their host galaxies were complete unknowns.
Observations during the mid-twentieth century showed that, on average, all galaxies in the nearby universe, including our own, are very old. Some of the Milky Way's oldest stars, which congregate in dense spherical systems called globular clusters, are about 13 billion years old, almost as old as the universe itself (a fact that once made theorists who held with the standard big-bang model of the universe distinctly uncomfortable). Distant elliptical galaxies that formed when the universe was roughly half its present age also contain very old stars, meaning either that they formed very soon after the big bang or that their stars matured more rapidly than in other galaxies.
By the 1980s it was clear to astronomers that their only hope of finding significant evolutionary differences in galaxies was to observe lookback times that were greater than half the age of the universe, something ground-based telescopes were ill equipped to do. Theoretical models predicted that if and when a population of "primeval galaxies" were found, they would likely look considerably different from the ones in the modern universe. But that was about all astronomers agreed upon. The galaxies' colors, sizes, numbers, and peak formation period remained questionable. As future observations would show, some astronomers' predictions were spot on, while others were considerably off the mark.
"A short version of the story," says Caltech's S. George Djorgovski, a pioneer in the search for primeval galaxies since the mid-1980s, "is that just about every conceivable combination of luminosity, color, size, and so forth was proposed for primeval galaxies over the years, and pretty much all of them corresponded to some formative stage in the evolutionary history of galaxies of some type or other."
A major question clouding the search for primeval galaxies was whether they would be shrouded in dust. Many deep surveys conducted into the early 1990s had failed to turn up any definitive candidates for primeval galaxies. If they were obscured by intervening curtains of dust, that could explain the lack of unambiguous detections. But dust was not something spontaneously created out of the big bang. Rather, dust was a product of generations of stars that had formed enough metals to "pollute" their environments. The first stars, however, were metal-free, or nearly so, meaning that if the first galaxies were obscured by dust, then perhaps they weren't that young after all—or even first.
Some astronomers, Djorgovski among them, speculated that perhaps a few primeval galaxies had already been found. The first such candidate was discovered as early as 1963 by Caltech astronomer Maarten Schmidt. Except for a slight ray of light projecting from it, the object, a radio source known as 3C 273, looked like an ordinary star in the 200-inch Hale Telescope on Palomar Mountain. Its spectrum, however, did not look like any stellar spectrum Schmidt had ever seen. After puzzling over it, he finally realized that the star's spectral lines were displaced over 15 percent from their standard laboratory position toward the red end of the spectrum, a phenomenon known as a redshift. Because the universe is expanding, light coming from sources at progressively greater cosmological distances is stretched like pulled taffy toward longer, or redder, wavelengths. The greater the redshift, the more distant the object and the farther back in time it lies. When applied to 3C 273, Schmidt was astonished to realize that this "star" must lie well over a billion light-years away. How could a star be that bright from that distance? The answer was that it couldn't, unless it was actually closer than its redshift implied and its abnormal spectrum was the result of some intrinsic pathology—an imploded supernova, perhaps, or a supermassive star with an intense gravitational field. Such explanations were quickly ruled out, however, and astronomers had to finally admit that they were dealing with a new type of astrophysical creature.
By the 1970s astronomers conjectured that the intense luminosity of these sources—called quasi-stellar objects, or quasars—was probably sustained by some extremely energetic process going on in their cores, most likely matter funneling into a supermassive black hole. The centers of most giant galaxies, however, are one hundred times less luminous than those of quasars, and if you removed them to any substantial distance, they would rapidly dim. As such, the possibility stood that quasars and galaxies with active quasarlike nuclei might not represent primeval galaxies as a class.
Another quandary was the size of primeval galaxies. The size of early galaxies was based on two notions of galaxy formation—the "top-down" and "bottom-up" theories. The top-down theory argued that galaxies formed from huge pancake-shaped masses of gas and then broke apart into smaller objects. The bottom-up theory reversed the process: smaller protogalaxies merged to form larger galaxies. Would primeval galaxies, then, appear as large, low-surface-brightness objects, or would they appear smaller and bluer, owing to their first generation of hot stars turning on? Even more uncertain were the roles that initial formation conditions and environment played in shaping the different types of galaxies. Some astronomers thought these galaxies would be highly clustered because they would have initially formed where the density of matter was greatest. Others disagreed, holding that the first galaxies would be isolated fragments, some of which might be undergoing mergers.
Excerpted from Chasing Hubble's Shadows by Jeff Kanipe. Copyright © 2006 Jeff Kanipe. Excerpted by permission of Farrar, Straus and Giroux.
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Table of Contents
1 - HUNTING FOR SNARKS,
2 - THE RACE FOR THE MOST DISTANT GALAXIES,
3 - TO THE END OF THE BEGINNING,
4 - COSMIC CANVAS,
5 - THE EGG BEFORE THE CHICKEN,
6 - IT STINKS, BUT IT ROCKS,
7 - KECK: SAILING ACROSS AN UNKNOWN OCEAN,
8 - PECULIAR UNIVERSE,
9 - DISTANT REFLECTIONS IN A NEARBY UNIVERSE,
10 - MORE OF EVERYTHING,