4
Astrophysics

Looking Farther in Space and Time

For centuries by the practice of astronomy—first with the unaided human eye, then compass and sextant, finally with ever more powerful optical telescopes—humankind has undertaken to map the starlit skies, in hopes of better guiding navigation, constructing accurate calendars, and understanding natural cycles. Archaeological discoveries suggest that astronomy may be the oldest science, and Stonehenge one of the most prodigious, if not among the earliest, outdoor observatories. Employing a vast armamentarium of new imaging tools, the modern science of astrophysics begins with telescopic views of the visible sky. But astrophysicists are then able to use those observations to explore the physics of gravity, to observe the actions of fundamental particles, and to study other more exotic phenomena in space. From such explorations they develop theories about the origin, formation, structure, and future of the universe—the study of cosmology.

Guided by such theories, and using technology developed in the last several decades, astrophysicists probe phenomena beyond the range of vision. Beyond, that is, not only the farthest reach of optical telescopes (that are limited by the diameter of their mirrors and the sensitivity of their detecting devices) but also outside the relatively narrow range of the electromagnetic spectrum where visible-light rays reside, with wavelengths spanning from about 3000 to 9000 angstroms (1 Å = 10-8 cm). The modern astrophysicist harvests a much wider

NOTE: Addison Greenwood collaborated on this chapter with Marcia F. Bartusiak.



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Science at the Frontier: Volume I 4 Astrophysics Looking Farther in Space and Time For centuries by the practice of astronomy—first with the unaided human eye, then compass and sextant, finally with ever more powerful optical telescopes—humankind has undertaken to map the starlit skies, in hopes of better guiding navigation, constructing accurate calendars, and understanding natural cycles. Archaeological discoveries suggest that astronomy may be the oldest science, and Stonehenge one of the most prodigious, if not among the earliest, outdoor observatories. Employing a vast armamentarium of new imaging tools, the modern science of astrophysics begins with telescopic views of the visible sky. But astrophysicists are then able to use those observations to explore the physics of gravity, to observe the actions of fundamental particles, and to study other more exotic phenomena in space. From such explorations they develop theories about the origin, formation, structure, and future of the universe—the study of cosmology. Guided by such theories, and using technology developed in the last several decades, astrophysicists probe phenomena beyond the range of vision. Beyond, that is, not only the farthest reach of optical telescopes (that are limited by the diameter of their mirrors and the sensitivity of their detecting devices) but also outside the relatively narrow range of the electromagnetic spectrum where visible-light rays reside, with wavelengths spanning from about 3000 to 9000 angstroms (1 Å = 10-8 cm). The modern astrophysicist harvests a much wider NOTE: Addison Greenwood collaborated on this chapter with Marcia F. Bartusiak.

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Science at the Frontier: Volume I band of radiation from space, from radio signals with wavelengths meters in length to x rays and gamma rays, whose wavelengths are comparable to the size of an atomic nucleus (10-5 angstrom). And one of the most urgent and compelling questions in modern astrophysics concerns matter in space that apparently emits no radiation at all, and hence is referred to as dark matter. J. Anthony Tyson from AT&T Bell Laboratories told the Frontiers symposium about his deep-space imaging project and the search for the dark matter that most astrophysicists now believe constitutes over 90 percent of all of the physical matter in the universe. If there really is 10 times as much invisible matter as that contained in luminous sources (and maybe even more), most theories about the evolution of the universe and its present structure and behavior will be greatly affected. Ever since the 1930s—soon after modern theories about the universe's evolution, now embraced as the Big Bang paradigm, were first proposed—scientists have been speculating about this ''missing mass or missing light problem," explained Margaret Geller from the Harvard-Smithsonian Center for Astrophysics. The Big Bang model has been generally accepted for about three decades as the reigning view of the creation of the universe. The whimsical name Big Bang was coined by British cosmologist Fred Hoyle (ironically, one of the proponents of the competing steady-state theory of the universe) during a series of British Broadcasting Corporation radio programs in the late 1940s. The Big Bang hypothesis suggests that the universe—all of space and matter and time itself—was created in a single instant, some 10 billion to 20 billion years ago. This event remains a subject of central interest in modern cosmology and astrophysics, both because its complexities are not fully understood and also because the world ever since—according to the credo of science and the dictates of natural law—may be understood as its sequel. The event defies description, even in figurative language, but physicists have written an incredibly precise scenario of what probably occurred. There was not so much an explosion as a sudden expansion, away from an infinitely dense point that contained all of the matter of the universe. Both space and time, as currently conceived, did not exist before this moment but were created as the very fabric of the expansion. As space expanded at unimaginable speed, the matter it contained was carried along with it. Today's scientists can observe the resonant echos of this seminal event in many ways. The most significant consequence of the Big Bang is that nearly all objects in the universe are moving away from one another as the expansion continues, and the farther they are apart, the faster they are receding.

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Science at the Frontier: Volume I GRAVITY'S SHADOW: THE HUNT FOR OMEGA Evidence for a dark, unknown matter permeating the universe is rooted in Newton's basic law of gravitation. In what may be the most influential science document ever written, the Principia Mathematica, the illustrious British scientist Isaac Newton in 1687 showed mathematically that there exists a force of attraction between any two bodies. Although Newton was working from observations made by the Danish astronomer Tycho Brahe and calculations earlier completed by Brahe's German assistant, Johannes Kepler, he extrapolated this work on celestial bodies to any two objects with mass, such as the apocryphal apple falling on his head. The attractive force between any two bodies, Newton demonstrated, increased as the product of their masses and decreased as the square of the distance between them. This revelation led to a systematic way of calculating the velocities and masses of objects hurtling through space. Indeed, it was by applying Newton's formulae to certain observational data in the 1930s that Fritz Zwicky, a Swiss-born astronomer working at the California Institute of Technology, obtained astronomy's first inkling that dark matter was present in the universe. Concern about dark matter, noted Geller, "is not a new issue." By analyzing the velocities of the galaxies in the famous Coma cluster, Zwicky noticed that they were moving around within the cluster at a fairly rapid pace. Adding up all the light being emitted by the Coma galaxies, he realized that there was not enough visible, or luminous, matter to gravitationally bind the speeding galaxies to one another. He had to assume that some kind of dark, unseen matter pervaded the cluster to provide the additional gravitational glue. "Zwicky's dynamical estimates—based on Newton's laws which we think work everywhere in the universe—indicated that there was at least 10 times as much mass in the cluster of galaxies as he could account for from the amount of light he saw," said Geller. "The problem he posed we haven't yet solved, but we have made it grander by giving it a new name: Now we call it the dark matter problem.'' Over the last 15 years, astronomer Vera Rubin with the Carnegie Institution of Washington has brought the dark matter problem closer to home through her telescopic study of the rotations of dozens of spiral galaxies. The fast spins she is measuring, indicating rates higher than anyone expected, suggest that individual galaxies are enshrouded in dark haloes of matter as well; otherwise each galaxy would fly apart. One of the reasons astrophysicists refer to the dark matter as a problem is related to the potential gravitational effect of all that dark matter on the fate of the cosmos, a question that arises when the

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Science at the Frontier: Volume I equations of Einstein's theory of general relativity are applied to the universe at large. General relativity taught cosmologists that there is an intimate relationship between matter, gravity, and the curvature of space-time. Matter, said Einstein, causes space to warp and bend. Thus if there is not enough matter in the cosmos to exert the gravitational muscle needed to halt the expansion of space-time, then our universe will remain "open," destined to expand for all eternity. A mass-poor space curves out like a saddle whose edges go off to infinity, fated never to meet. The Big Bang thus would become the Big Chill. On the other hand, a higher density would provide enough gravity to lasso the speeding galaxies—slowing them down at first, then drawing them inward until space-time curls back up in a fiery Big Crunch. Here space-time is "closed," encompassing a finite volume and yet having no boundaries. With a very special amount of matter in the universe, what astrophysicists call a critical density, the universe would stand poised between open and closed. It would be a geometrically flat universe. Scientists refer to this Scylla and Charybdis dilemma by the Greek letter Ω. Omega is the ratio of the universe's true density, the amount of mass per volume in the universe, to the critical density, the amount needed to achieve a flat universe and to just overcome the expansion. An Ω of 1 indicates a flat universe, greater than 1 a closed universe, and less than 1 an open universe. By counting up all the luminous matter visible in the heavens, astronomers arrive at a value for Ω of far less than 1; it ranges between 0.005 and 0.01. When astronomers take into account the dark matter measured dynamically, though, the amounts needed to keep galaxies and clusters stable, Ω increases to about 0.1 to 0.2. But theoretically, Ω may actually be much higher. In 1980 physicist Alan Guth, now with the Massachusetts Institute of Technology, introduced the idea that the universe, about 10-36 second after its birth, may have experienced a fleeting burst of hyperexpansion—an inflation—that pushed the universe's curvature to flatness, to the brink between open and closed. And with the geometry of the universe so intimately linked with its density, this suggests that there could be 100 times more matter in the cosmos than that currently viewed through telescopes. THE TEXTURE OF THE UNIVERSE Despite Guth's prediction, astronomical observations continue to suggest that the universe is open, without boundaries. As David Koo from the University of California, Santa Cruz, concluded, "If omega is less than one, the universe will not collapse. Presumably, we will

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Science at the Frontier: Volume I die by ice rather than by fire." Yet astrophysicists look eagerly for hints of further amounts of dark matter by studying the various structures observed in the celestial sky. How the universe forged its present structures—galaxies grouping to form clusters, and the clusters themselves aggregating into superclusters—is significantly dependent on both the nature and amounts of dark matter filling the universe. The dark matter provides vital information, said Edmund Bertschinger of the Massachusetts Institute of Technology, "for the reigning cosmological paradigm of gravitational instability." Had there not been matter of a certain mass several hundred thousand years after the Big Bang, the universe would have been smooth rather than "clumpy," and the interactions between aggregations of particles necessary to form structures would not have taken place. S. George Djorgovski from the California Institute of Technology agreed that the ''grand unifying theme of the discussion at the symposium—and for that matter in most of the cosmology today—is the origin, the formation, and the evolution of the large-scale structure of the universe,'' which, he said, "clearly must be dominated by the dark matter." Geller and another guest at the symposium, Simon White from Cambridge University in Great Britain, have for years been involved in pioneering research to elucidate this large-scale structure. Until the 1980s, the best map of the universe at large was based on a vast catalog of galaxy counts, around 1 million, published in 1967 by Donald Shane and Carl Wirtanen from the Lick Observatory in California. They compiled their data from an extensive set of photographs taken of the entire celestial sky as seen from the Lick Observatory. Shane and Wirtanen spent 12 years counting each and every galaxy on the plates, and from their catalog P. James E. Peebles and his colleagues at Princeton University later derived the first major map of the universe, albeit a two-dimensional one. Peebles' 1-million-galaxy map was striking: The universe appeared to be filled with a network of knotlike galaxy clusters, filamentary superclusters, and vast regions devoid of galaxies. But was that only an illusion? The raw input for Peebles' map did not include redshift data—data for establishing the third point of reference, the absolute distances of the various galaxies. How does one know, from a two-dimensional view alone, that the apparent shape of a group of stars (or galaxies) is not merely an accidental juxtaposition, the superposition of widely scattered points of light (along that particular line of sight) into a pattern discernible only because a human being is stationed at the apex? Without a parallax or triangulated view, or some other indicator of the depth of the picture being sketched, many unsupported suppositions were made, abetted perhaps by the inherent

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Science at the Frontier: Volume I tendency of the human brain to impose a pattern on the heterogeneous input it receives from the eye. The universe does display inhomogeneity; the question is how much. Soon after galaxies were discovered by Hubble in the 1920s, the world's leading extragalactic surveyors noticed that many of the galaxies gathered, in proximity and movement, into small groups and even larger clusters. The Milky Way and another large spiraling galaxy called Andromeda serve as gravitational anchors for about 20 other smaller galaxies that form an association called the Local Group, some 4 million light-years in width (a light-year being the distance light travels in a year, about 6 trillion miles). Other clusters are dramatically larger than ours, with over 1000 distinct galaxies moving about together. Furthermore, our Local Group is caught at the edge of an even larger assembly of galaxies, known as the Local Supercluster. Superclusters, which can span some 108 light-years across, constitute one of the largest structures discernible in astronomical surveys to date. In mapping specific regions of the celestial sky, astronomers in the 1970s began to report that many galaxies and clusters appeared to be strung out along lengthy curved chains separated by vast regions of galaxyless space called voids. Hints of such structures also emerged when Marc Davis and John Huchra at the Harvard-Smithsonian Center for Astrophysics completed the first comprehensive red-shift survey of the heavens in 1981, which suggested a "frothiness" to the universe's structure, a pattern that dramatically came into focus when Geller and Huchra extended the redshift survey, starting in 1985. Probing nearly two times farther into space than the first survey, the second effort has now pegged the locations of thousands of additional galaxies in a series of narrow wedges, each 6 degrees thick, that stretch across the celestial sky. Geller and her associates found, to their surprise, that galaxies were not linked with one another to form lacy threads, as previous evidence had been suggesting. Rather, galaxies appear to congregate along the surfaces of gigantic, nested bubbles, which Geller immediately likened to a "kitchen sink full of soapsuds." The huge voids that astronomers had been sighting were simply the interiors of these immense, sharply defined spherical shells of galaxies. Each bubble stretches several hundreds of millions of light-years across. White and his collaborators, hunting constantly for clues to this bubbly large-scale structure, have developed a powerful computer simulation of its formation. This domain allows them to probe the nonequilibrium, nonlinear dynamics of gravitating systems. They have applied a great deal of theoretical work to the dark matter prob-

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Science at the Frontier: Volume I lem, helping to develop current ideas about the collapse of protogalaxies, how filaments and voids form in the large-scale distribution of clusters, and how galaxy collisions and mergers may have contributed to the evolution of structure in the present universe. They have used their numerical simulation technique to explore the types of dark matter that might give rise to such structures, such as neutrinos (stable, elementary particles with possibly a small rest mass, no charge, and an extreme tendency to avoid detection or interaction with matter) and other kinds of weakly interacting particles that have been hypothesized but not yet discovered. They link various explanations of dark-matter formation and distribution to the large-scale galaxy clustering that seems—as astrophysicists reach farther into deep space—to be ubiquitous at all scales. But cosmologists look at the universe from many different perspectives, and many astrophysicists interested in galaxy formation and structure energetically search the skies in hopes of finding a galaxy at its initial stage of creation. Djorgovski goes so far as to call the search for a new galaxy aborning "the holy grail of modern cosmology. Though I suspect the situation is far more complicated, and I am not expecting the discovery of primeval galaxies." Rather, he predicted, "we will just learn slowly how they form," by uncovering pieces of the puzzle from various observations. One such effort he calls "paleontocosmology: since we can bring much more detail to our studies of nearby [and therefore older] galaxies than those far away, we can try to deduce from the systematics of their observed properties how they may have been created." This ability to look back in time is possible because of the vast distances the radiation from the galaxies under observation must travel to reach earthbound observers. All electromagnetic radiation travels at a speed of approximately 300,000 kilometers per second (in a year about 9 trillion kilometers or 6 trillion miles, 1 light-year). Actually, the distances are more often described by astrophysicists with another measure, the parallax-second, or parsec, which equals 3.26 light-years. This is the distance at which a star would have a parallax equal to 1 second of arc on the sky. Thus 1 megaparsec is a convenient way of referring to 3.26 million light-years. The laws of physics, as far as is known, limit the velocity of visible-light photons or any other electromagnetic radiation that is emitted in the universe. By the time that radiation has traveled a certain distance, e.g., 150 million light-years, into the range of our detection, it has taken 150 million years—as time is measured on Earth—to do so. Thus this radiation represents information about the state of its emitting source that far back in time. The limit of this

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Science at the Frontier: Volume I view obviously is set at the distance light can have traveled in the time elapsed since the Big Bang. Astrophysicists refer to this limit as the observable universe, which provides a different prospect from any given point in the universe but emanates to a horizon from that point in all directions. Cosmologists study the stages of cosmic evolution by looking at the state of matter throughout the universe as the photons radiating from that matter arrive at Earth. As they look farther away, what they tend to see is hotter and is moving faster, both characteristics indicative of closer proximity to the initial event. The Big Bang paradigm allows cosmologists to establish points of reference in order to measure distances in deep space. As the universe ages, the horizon as viewed from Earth expands because light that was emitted earlier in time will have traveled the necessary greater distance to reach our view. At present, the horizon extends out to a distance of about 15 billion light-years. This represents roughly 50 times the average span of the voids situated between the richest superclusters recently observed. These superclusters are several tens of millions of light-years in extent, nearly 1000 times the size of our Milky Way galaxy. Our star, the Sun, is about 2 million light-years from its galactic neighbor, Andromeda. Nine orders of magnitude closer are the planets of its own solar system, and the Sun's light takes a mere 8 minutes to reach Earth. Tyson's surveys excite astrophysicists because they provide data for galaxies more distant than those previously imaged, ergo, a window further back in time. Nonetheless, Jill Bechtold from the University of Arizona reminded symposium participants that "quasars are the most luminous objects we know, and we can see them at much greater distances than any of the galaxies in the pictures that Tony Tyson showed. Thus, you can use them to probe the distribution of the universe in retrospect," what astrophysicists call "look-back" time. Quasar is the term coined to indicate a quasistellar radio source, an object first recognized nearly 30 years ago. Quasars are thought to be the result of high-energy events in the nuclei of distant galaxies, and thus produce a luminance that can be seen much further than any individual star or galaxy. Tyson and his fellow scientists took the symposium on a journey into deep space, showing pictures of some of the farthest galaxies ever seen. While these are the newest galaxies to be observed, their greater distance from Earth also means they are among the oldest ever seen, reckoned by cosmological, or lookback time. The astrophysicists described and explained the pictures and the myriad other data they are collecting with powerful new imaging tools, and also talked about the simulations and analyses they are performing on all

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Science at the Frontier: Volume I of these with powerful new computing approaches. Along the way, the history of modern cosmology was sketched in: exploratory efforts to reach out into space, and back in time, toward the moment of creation some 10 billion to 20 billion years ago, and to build a set of cosmological theories that will explain how the universe got from there to here, and where—perhaps—it may be destined. In less than two decades, scientists have extended their probes to detect the radiation lying beyond the comparatively narrow spectrum of light, the only medium available to optical astronomers for centuries. The pace of theoretical and experimental advance is accelerating dramatically. The sense of more major discoveries is imminent. THE BIG BANG PICTURE OF THE UNIVERSE "The big bang," says Joseph Silk in his treatise The Big Bang, "is the modern version of creation" (Silk, 1989, p. 1). Silk, a professor of astronomy with the University of California at Berkeley, knits together most of the major issues in modern cosmology—the study of the large-scale structure and evolution of the universe—within the framework of the signal event most astrophysicists now believe gave birth to the universe around 15 billion years ago. Any such universal theory will rest on a fundamental cosmological principle, and Silk traces the roots of the Big Bang theory to Copernicus, who in 1543 placed the Sun at the center of the universe, displacing Earth from its long-held pivotal position. With this displacement of Earth from a preferred point in space came the recognition that our vantage point for regarding the universe is not central or in any way significant. Since observations show the arrangement of galaxies in space to vary little, regardless of direction or distance, scientists believe the universe is isotropic. This regularity is then imposed over two other basic observations: everything seems to be moving away from everything else, and the objects farthest removed from Earth are moving away proportionally faster. Taken together, these phenomena invite time into the universe and allow theorists to "run the film backwards" and deduce from their observations of current phenomena how the universe probably originated, in order to have evolved into what is manifest today. From this line of thinking, it follows that the universe began from a single point of infinitely dense mass, which underwent an unimaginable expansion some 10 billion to 20 billion years ago. Although the astronomical data currently available favor a Big Bang cosmology, predictions based on the Big Bang model are continually examined. Perhaps the most convincing single test was accomplished in 1965, when radio astronomers Arno Penzias and Rob-

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Science at the Frontier: Volume I ert Wilson, using a highly sensitive receiver designed at their Bell Laboratories facility in New Jersey, detected what has come to be called the cosmic microwave background radiation. Spread uniformly across the celestial sky, this sea of microwaves represents the reverberating echo of the primordial explosion, a find that later garnered the Nobel Prize for Penzias and Wilson. Subsequent studies of this background radiation show it to be uniform to better than 1 part in 10,000. Such isotropy could arise only if the source were at the farthest reaches of the universe. Moreover, measurements of the radiation taken at different wavelengths, says Silk, indicate that it "originates from a state of perfect equilibrium: when matter and radiation are in equilibrium with one another, the temperatures of both must be identical" (Silk, 1989, p. 84). Cosmologists call the point at the very beginning of time a singularity. Before it, classical gravitational physics can say or prove nothing, leaving all speculation to the metaphysicians. Big Bang theory encompasses a series of events that occurred thereafter, which conform to two continuing constraints: first, the laws of physics, which are believed to be universal, and second, data from observations that are continually probing farther in space, and therefore further back in time toward the event itself. This series of events cosmologists can "date," using either lookback time from the present or cosmic time. When measuring time forwards, singularity (the moment of creation) represents time zero on the cosmic calendar. The events occurring during the first second have been hypothesized in great subatomic detail—and necessarily so—for at that point the temperature and pressure were greater than the forces that bind particles together into atoms. As time ensues, space expands, and matter thus thins out; therefore, temperature and pressure decrease. This "freezing" permits the forces that generate reactions between particles to accomplish their effects, in essence leading to the manufacture of a more complex universe. Complexity then becomes entwined with evolution, and eventually Earth and the universe reach their present state. Where the state of universal evolution has reached, cosmologists can only speculate. Since their observations rely on time machine data that only go backwards, they extrapolate according to the laws of physics into the future. At the moment of creation, all four forces were indistinguishable; the most advanced theories in physics currently suggest that the forces were united as one ancestral force. But as the universe cooled and evolved, each force emerged on its own. At 10-43 second, gravity first separated, precipitating reactions between particles at unimaginably intense pressures and temperatures. At 10-36 second, the strong

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Science at the Frontier: Volume I and the electroweak forces uncoupled, which in turn permitted the fundamental particles of nature, quarks and leptons, to take on their own identity. At 10-10 second, the four forces of the universe were at last distinct; as the weak force separated from the electromagnetic force, conditions deintensified to the point where they can be simulated by today's particle physicists exploring how matter behaves in giant particle accelerators. At 10-5 second, the quarks coalesced to form the elementary particles—called baryons—and the strong forces that bind the nucleus together came to dominate. By 1 second after singularity, the weak nuclear force allowed the decay of free neutrons into protons and the leptons, or light particles, the electrons, and neutrinos. Within 3 minutes, the protons and neutrons could join to form the light nuclei that even today make up the bulk of the universe, primarily hydrogen and helium. These nuclei joined electrons to form atoms within the next 1 million years, and only thereafter, through the evolution of particular stars, were these elements combined into the heavier elements described in the periodic table. From the earliest moment of this scenario, the universe was expanding outward. The use of the term Big Bang unfortunately suggests the metaphor of an explosion on Earth, where mass particles hurtle outward from a central source, soon to be counteracted by the comparatively massive effects of the planet's gravity. In the universe according to Einstein, however, it is the fabric of space itself that is expanding—carrying with it all that matter and energy that emerged during creation's first microseconds, as described above. Gravity assumes a central role, according to Einstein's conception of the universe as an infinite fabric that evolves in a four-dimensional world called space-time. The mass that may be conceptualized as part of the fabric of space-time has a distorting effect on its shape, ultimately curving it. Einstein's ideas came to transform the face of science and the universe, but other astronomers and cosmologists, using general relativity as a touchstone, were the progenitors of the Big Bang conception that has come to define the modern view. BASIC MEASUREMENTS IN SPACE Beginning early in the 20th century, cosmology was revolutionized by astronomers making observations and developing theories that—in the context of first special and then general relativity—led in turn to more penetrating theories about the evolution of the universe. Such theories often required detection and measurement of stellar phenomena at greater and greater distances. These have been forthcoming, furthered by the development of larger optical telescopes

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Science at the Frontier: Volume I of gas in the early universe with only hydrogen and helium that later formed stars. The stars then created elements like carbon, nitrogen, and oxygen through nuclear synthesis," but these "forests" of Lyman-alpha lines may provide the portrait of a protogalaxy that Djorgovski said astronomers eagerly seek (Figure 4.1). While definitive confirmation of the full implications of the Lyman-alpha forest is in the future, Bechtold explained that the Lyman-alpha absorption lines possess an indisputable value for cosmology today. "Because they are so numerous, they can serve as tracers of large-scale structure very well, in fact, in some ways better than galaxies." Their distribution throughout the universe is "much more uniform than galaxies in the present day," and thus they provide a possible precursor or link in the causal chain that might explain the Figure 4.1 An optical spectrum of the quasar 0913+072 obtained with the Multiple Mirror Telescope on Mt. Hopkins, Arizona, a joint facility of the Smithsonian Institution and the University of Arizona. The dashed line indicates the quasar continuum; the deep absorption lines originate in galaxies along the line of sight, or intergalactic Lyman-alpha forest clouds. (Courtesy of J. Bechtold.)

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Science at the Frontier: Volume I present structure. "They don't cluster. They don't have voids," pointed out Bechtold, which "supports the idea that they are protogalactic or pregalactic clouds that have collapsed to galaxies and later the galaxies collapsed to form clusters of galaxies." Thus quasars provide penetrating probes into the past because of their distance, which, but for their intense luminosity, would preclude optical detection. This fact, together with the phenomenon of a distant source of radiation picking up information about intermediate points along the path to Earth, provides a parallel with another recent and dramatic development in astrophysics. 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

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Science at the Frontier: Volume I 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

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Science at the Frontier: Volume I 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

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Science at the Frontier: Volume I 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. Gravity's Role as a Lens and a Cosmological Force Tyson and his colleagues were energetically conducting their surveys of this blue "curtain" even as other astronomers were discovering faint blue arcs, thought to be the result of gravitational lensing. The blue curtain is a much more efficient probe of foreground dark matter because the curtain is far enough away to enhance the chances of foreground lensing and rich enough to provide a canvas—rather than a point of light coming from a quasar—to observe the distortion. After subtracting the gravitational effects of those luminous galaxies in the foreground that they could observe, Tyson's team could nonetheless still observe fairly dramatic distortion in the blue curtain galaxy images. They realized they had taken an image—albeit an indirect one—of the dark, nonluminous matter in the foreground cluster of galaxies (Figure 4.2). With quasar studies, the foreground object was often a very specific galaxy or star, and the light was emanating from the discrete quasar source. Tyson's system works by the same principles of gravity, but instead of an identifiable body that one can see, the only evidence of the foreground object is the gravitational pull it exerts on the path of the distant light and the resulting tell-tale distortion of the distant galaxy's image (Figure 4.3). By enhancing FOCAS and developing other programs to simulate

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Science at the Frontier: Volume I Figure 4.2 The gravitational effect of dark matter in a foreground cluster of galaxies. The images of the foreground galaxies have been eliminated, revealing faint arcs. The clump of dark matter is 4 billion light-years away. (Courtesy of J.A. Tyson.) the possible shape and composition of dark matter, Tyson has been able to use his blue curtain as a vast new experimental database. Characteristic distortions in the previously too faint images have become observable as the deep CCD optical imaging surveys and analyses continue. A portrait of dark matter is emerging that allows other theories about the evolution of the universe to be tested—galaxy cluster formation, for one. "We have found that there is a morphological similarity between the distribution of the bright cluster galaxies and the distribution of the dark matter in the cluster, but the

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Science at the Frontier: Volume I dark matter is distributed relatively uniformly on the scale of an individual galaxy in the cluster," said Tyson. Their lens technique is better at determining the mass within a radius, to within about 10 percent accuracy, than the size of the inferred object. "Less accurate," added Tyson, "is the determination of the core size of this distribution. That is a little bit slippery, and for better determination of that we simply have to have more galaxies in the background. But Figure 4.3 A diagram showing the bending of a light path by the mass in a gravitational lens and the apparent change in the position of the source and the resulting distortion of its image. (Courtesy of J. A. Tyson.)

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Science at the Frontier: Volume I since we are already going to around 30th magnitude or so in some of these fields, that is going to be hard to do." "A very interesting question that is unanswered currently because of the finite size of our CCD imagers is how this dark matter really falls off with radius. Is the dark matter pattern that of an isothermal distribution of particles bound in their own gravitational potential well, in which case it would go on forever?" Tyson wondered. This is, in part, the impetus for Tyson to develop a larger CCD device. Results from Tyson's work may aid astronomers in their quest to understand how galaxies formed. In order to create the lumps that eventually developed into galaxies, most theories of galaxy formation start with the premise that the Big Bang somehow sent a series of waves rippling through the newly born sea of particles, both large and small fluctuations in the density of gas. According to one popular theory of galaxy formation, small knots of matter, pushed and squeezed by those ripples, would be the first to coalesce. These lumps, once they evolve into separate galaxies, would then gravitationally gather into clusters and later superclusters as the universe evolves. This process has been tagged, appropriately enough, the "bottom-up" model. Conversely, a competing theory of galaxy formation, known as the "top-down" model, essentially reverses the scale of collapse, with the largest, supercluster-sized structures collapsing first and most rapidly along the shorter axis, producing what astrophysicists call a pancake structure, which would then fragment and produce cellular and filamentary structures. Bertschinger explained that gravitational instability causes "different rates of expansion depending on the local gravitational field strength. Gravity retards expansion more in regions of above-average density, so that they become even more dense relative to their surroundings. The opposite occurs in low-density regions." Following earlier computer models simulating the evolution of a universe filled with cold dark matter, especially a model pioneered by Simon White and his associates in the 1980s, Bertschinger and his colleagues have explored how dark matter may cluster and how this development may parallel or diverge from galaxy clustering: "The gravitational field can be computed by a variety of techniques from Poisson's equation, and then each particle is advanced in time according to Newtonian physics." The complexity comes in when they try to capture the effects of the explosive microsecond of Guth's inflation era. For this they need to model the quantum mechanical fluctuations—essentially the noise—hypothesized to extend its im

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Science at the Frontier: Volume I pact through time by the medium of acoustic waves. In their model, said Bertschinger, "most of the dark matter does end up in lumps associated with galaxies. The lumps are something like 1 million light-years" in extent, but it is not conclusive that they correspond to the haloes found beyond the luminous edges of galaxies. "It is plausible," he continued, "that luminous galaxies evolve in the center of these lumps," but to say so more definitively would involve simulating the dynamics of the stellar gases, which was beyond the scope of their model. But dark matter does seem to cluster, and in a manner similar to galaxies. Most of it is found within 1 million light-years of a galaxy. These conclusions are tentative, in that they emerged largely from simulations, but they are consistent with many of the observed data, including the limits found with respect to the anisotropy of the cosmic microwave background. THE FORMATION OF STARS Djorgovski is another astrophysicist, like Bechtold and Tyson, whose studies about one phenomenon—the formation and evolution of galaxies—often touch on larger cosmological questions. On the assumption that dark matter constitutes 90 percent or more of the mass in the universe, "large-scale structure is clearly dominated" by it, he said. But galaxies are the components of this large-scale structure, and their structure "is a much more complicated business." Astronomers approach the subject in two ways, he said. Looking at large redshifts, they hope to "come upon a primeval galaxy that will hopefully be a normal example undergoing its first major burst of star formation." Another approach, called paleontocosmology, looks at nearby, older galaxies that can be studied in some detail. Systematic properties may yield scaling laws and correlations that in turn allow strong inferences to be made about the early life and formation of galaxies (Figure 4.4). ''Galaxies are interesting per se," said Djorgovski, but better understanding their evolution could provide astrophysicists "tools to probe the global geometry, or the large-scale velocity field.'' Galaxy formation can be seen as two distinct types of events, said Djorgovski: assembling the mass (which is the business of the dark matter and gravity) and "converting the primordial [hydrogen] gas, once assembled, into stars that shine." The energy for this second step comes from two sources, he explained. "First of all, when you look at galaxies, you find out that they are about 1000 times denser than the large-scale structure surrounding them. That tells you that they must have collapsed out of their surrounding density field by

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Science at the Frontier: Volume I Figure 4.4 Computer simulation of a density field in the early universe. Shades of gray indicate different densities in a fairly arbitrary way. (Courtesy of S.G. Djorgovski.) about a factor of 10, and that involves dissipation of energy. When you add up how much that is, it comes to about 1059 ergs per galaxy." Once formed, stars engage in nuclear fission to make the helium and heavier metals that come to compose their cores, and they generate about 100 times more energy, 1061 ergs per galaxy. These events are thought to happen in the era around a redshift of 2, perhaps a little greater, "but we don't know exactly where or when," said Djorgovski, suggesting that the process probably evolves over a considerable period of time. But it will be the emission lines in its spectra that confirm a star in the process of forming, should

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Science at the Frontier: Volume I one be found. Djorgovski showed the scientists some pictures of one candidate "which has the romantic name of 3C 326.1, at a redshift of nearly 2, two-thirds of the way to the Big Bang." Whether this is a primeval galaxy remains a subject for debate, as is the case with many such candidates, because it is associated with intense radio emissions and its signals may be confounded by an active nucleus it contains. "We know about a dozen objects of this sort," said Djorgovski, "but what you really want to find are just ordinary galaxies forming with large redshifts. And that nobody has found so far." BIBLIOGRAPHY Flamsteed, Sam. 1991. Probing the edge of the universe. Discover 12(7):40–47. Guth, Alan H., and Paul J. Steinhardt. 1984. The inflationary universe. Scientific American 250(May):116–128. Kristian, Jerome, and Morley Blouke. 1982. Charge-coupled devices in astronomy. Scientific American 247(October):66–74. Silk, Joseph. 1989. The Big Bang. Freeman, New York. Turner, Edwin. 1988. Gravitational lenses. Scientific American 259(July):54–60. RECOMMENDED READING Harrison, Edward 1981. Cosmology: The Science of the Universe. Cambridge University Press, Cambridge.