Peebles_Evolution_of_Universe

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Unformatted text preview: A UNIVERSAL VIEW Copyright 1998 Scientific American, Inc. The Evolution of the ALAN DRESSLER Carnegie Institution/ National Aeronautics and Space Administration by P. James E. Peebles, David N. Schramm, Edwin L. Turner and Richard G. Kron Universe Some 12 billion years ago the universe emerged from a hot, dense sea of matter and energy. As the cosmos expanded and cooled, it spawned galaxies, stars, planets and life GALAXY CLUSTER is representative of what the universe looked like when it was 60 percent of its present age. The Hubble Space Telescope captured the image by focusing on the cluster as it completed 10 orbits. Several pairs of galaxies appear to be caught in one another’s gravitational field. Such interactions are rarely found in nearby clusters and are evidence that the universe is evolving. The Evolution of the Universe t a particular instant roughly 12 billion years ago, all the matter and energy we can observe, concentrated in a region smaller than a dime, began to expand and cool at an incredibly rapid rate. By the time the temperature had dropped to 100 million times that of the sun’s core, the forces of nature assumed their present properties, and the elementary particles known as quarks roamed freely in a sea of energy. When the universe had expanded an additional 1,000 times, all the matter we can measure filled a region the size of the solar system. At that time, the free quarks became confined in neutrons and protons. After the universe had grown by another factor of 1,000, protons and neutrons combined to form atomic nuclei, including most of the helium and deuterium present today. All of this occurred within the first minute of the expansion. Conditions were still too hot, however, for atomic nuclei to capture electrons. Neutral atoms appeared in abundance only after the expansion had continued for 300,000 years and the universe was 1,000 times smaller than it is now. The neutral atoms then began to coalesce into gas clouds, which later evolved into stars. By the time the universe had expanded to one fifth its present size, the stars had formed groups recognizable as young galaxies. When the universe was half its present size, nuclear reactions in stars had produced most of the heavy elements from which terrestrial planets were made. Our solar system is relatively young: it formed five billion years ago, when the universe was two thirds its present size. Over time the formation of stars has consumed the supply of gas in galaxies, and hence the population of stars is waning. Fifteen billion years from now stars like our sun will be relatively rare, making the universe a far less hospitable place for observers like us. Our understanding of the genesis and evolution of the universe is one of the great achievements of 20th-century science. This knowledge comes from decades of innovative experiments and theories. Modern telescopes on the ground and in space detect the light from galaxies billions of light-years away, showing us what the universe looked like when it was young. Particle accelerators probe the basic physics of the high-energy environment of the early universe. Satellites detect the cosmic background radiation left over from the early stages of expansion, providing an image of the universe on the largest scales we can observe. Our best efforts to explain this wealth of data are embodied in a theory known as the standard cosmological model or the big bang cosmology. The major claim of the theory is that in the large-scale average, the universe is expanding in a nearly homogeneous way from a dense early state. At present, there are no fundamental challenges to the big bang theory, although there are certainly unresolved issues within the theory itself. Astronomers are not sure, for example, how the galaxies were formed, but there is no reason to think the process did not occur within the framework of the big bang. Indeed, the predictions of the theory have survived all tests to date. Yet the big bang model goes only so far, and many fundamental mysteries remain. What was the universe like before it was expanding? (No observation we have made allows us to look back beyond the moment at which the expansion began.) What will happen in the distant future, when the last of the stars exhaust the supply of nuclear fuel? No one knows the answers yet. Our universe may be viewed in many lights—by mystics, theologians, philosophers or scientists. In science we adopt the plodding route: we accept only what is tested by experiment or observation. Albert Einstein gave us the now well-tested and accepted general theory of relativity, which establishes the relations between mass, energy, space and time. Einstein showed that a homogeneous distribution of matter in space fits nicely with his theory. He assumed without discussion that the universe is static, unchanging in the large-scale average [see “How Cosmology Became a Science,” by Stephen G. Brush; Scientific American, August 1992]. Magnificent Cosmos 87 A Copyright 1998 Scientific American, Inc. ubble’s measurements indicated that the redshift of a distant galaxy is greater than that of one closer to Earth. This relation, now known as Hubble’s law, is just what one would expect in a uniformly expanding universe. Hubble’s law says the recession velocity of a galaxy is equal to its distance multiplied by a quantity called Hubble’s constant. The redshift effect in nearby galaxies is relatively subtle, requiring good instrumentation to detect it. In contrast, the redshift of very distant objects—radio galaxies and quasars—is an awesome phenomenon; some appear to be moving away at greater than 90 percent of the speed of light. Hubble contributed to another crucial part of the picture. He counted the number of visible galaxies in different directions in the sky and found that they appear to be rather uniformly distributed. The value of Hubble’s constant seemed to be the same in all directions, a necessary consequence of uniform expansion. Modern surveys confirm the fundamental tenet that the universe is homogeneous on large scales. Although maps of the distribution of the nearby galaxies display clumpiness, deeper surveys reveal considerable uniformity. The Milky Way, for instance, resides in a knot of two dozen galaxies; these in turn are part of a complex of galaxies that protrudes from the so-called local supercluster. The hierarchy of clustering has been traced up to dimensions of about 500 million light-years. The fluctuations in the average density of matter diminish as the scale of the structure being investigated increases. In maps that cover distances that reach close to the observable limit, the average density of matter changes by less than a tenth of a percent. To test Hubble’s law, astronomers need to measure distances to galaxies. MULTIPLE IMAGES of a distant galaxy, which appear as faint blue ovals, are the result of an effect One method for gauging distance is to known as gravitational lensing. The effect occurs when light from a distant body is bent by the observe the apparent brightness of a gravitational field of an intervening object. In this case, the cluster of red galaxies, concentrated galaxy. If one galaxy is four times faintin the center of the picture, produces distorted images of the more distant galaxy, which lies far er than an otherwise comparable galbehind the red galaxies. The photograph was produced using the Hubble Space Telescope. W. N. COLLEY, J. A. TYSON AND E. L. TURNER In 1922 the Russian theorist Alexander A. Friedmann realized that Einstein’s universe is unstable; the slightest perturbation would cause it to expand or contract. At that time, Vesto M. Slipher of Lowell Observatory was collecting the first evidence that galaxies are actually moving apart. Then, in 1929, the eminent astronomer Edwin P. Hubble showed that the rate a galaxy is moving away from us is roughly proportional to its distance from us. The existence of an expanding universe implies that the cosmos has evolved from a dense concentration of matter into the present broadly spread distribution of galaxies. Fred Hoyle, an English cosmologist, was the first to call this process the big bang. Hoyle intended to disparage the theory, but the name was so catchy it gained popularity. It is somewhat misleading, however, to describe the expansion as some type of explosion of matter away from some particular point in space. That is not the picture at all: in Einstein’s universe the concept of space and the distribution of matter are intimately linked; the observed expansion of the system of galaxies reveals the unfolding of space itself. An essential feature of the theory is that the average density in space declines as the universe expands; the distribution of matter forms no observable edge. In an explosion the fastest particles move out into empty space, but in the big bang cosmology, particles uniformly fill all space. The expansion of the universe has had little influence on the size of galaxies or even clusters of galaxies that are bound by gravity; space is simply opening up between them. In this sense, the expansion is similar to a rising loaf of raisin bread. The dough is analogous to space, and the raisins, to clusters of galaxies. As the dough expands, the raisins move apart. Moreover, the speed with which any two raisins move apart is directly and positively related to the amount of dough separating them. The evidence for the expansion of the universe has been accumulating for some 60 years. The first important clue is the redshift. A galaxy emits or absorbs some wavelengths of light more strongly than others. If the galaxy is moving away from us, these emission and absorption features are shifted to longer wavelengths—that is, they become redder as the recession velocity increases. Hubble’s Law H 88 Scientific American Presents The Evolution of the Universe Copyright 1998 Scientific American, Inc. axy, then it can be estimated to be twice as far away. This expectation has now been tested over the whole of the visible range of distances. Some critics of the theory have pointed out that a galaxy that appears to be smaller and fainter might not actually be more distant. Fortunately, there is a direct indication that objects whose redshifts are larger really are more distant. The evidence comes from observations of an effect known as gravitational lensing [see illustration on opposite page]. An object as massive and compact as a galaxy can act as a crude lens, producing a distorted, magnified image (or even many images) of any background radiation source that lies behind it. Such an object does so by bending the paths of light rays and other electromagnetic radiation. So if a galaxy sits in the line of sight between Earth and some distant object, it will bend the light rays from the object so that they are observable [see “Gravitational Lenses,” by Edwin L. Turner; Scientific American, July 1988]. During the past decade, astronomers have discovered about two dozen gravitational lenses. The object behind the lens is always found to have a higher redshift than the lens itself, confirming the qualitative prediction of Hubble’s law. Hubble’s law has great significance not only because it describes the expansion of the universe but also because it can be used to calculate the age of the cosmos. To be precise, the time elapsed since the big bang is a function of the present value of Hubble’s constant and its rate of change. Astronomers have determined the approximate rate of the expansion, but no one has yet been able to measure the second value precisely. Still, one can estimate this quantity from knowledge of the universe’s average density. One expects that because gravity exerts a force that opposes expansion, galaxies would tend to move apart more slowly now than they did in the past. The rate of change in expansion is thus related to the gravitational pull of the universe set by its average density. If the density is that of just the visible material in and around galaxies, the age of the universe probably lies between 10 and 15 billion years. (The range allows for the uncertainty in the rate of expansion.) Yet many researchers believe the density is greater than this minimum value. So-called dark matter would make up the difference. A strongly defended argument holds that the universe is just dense enough that in the remote future the expansion will slow almost to zero. Under this assumption, the age of the universe decreases to the range of seven to 13 billion years. To improve these estimates, many astronomers are involved in intensive research to measure both the distances to galaxies and the density of the universe. Estimates of the expansion time provide an important test for the big bang model of the universe. If the theory is correct, everything in the visible universe should be younger than the expansion time computed from Hubble’s law. These two timescales do appear to be in at least rough concordance. For example, the oldest stars in the disk of the Milky Way galaxy are about nine billion years old—an estimate derived from the rate of cooling of white dwarf stars. The stars in the halo of the Milky Way are somewhat older, about 12 billion years—a value derived from the rate of nuclear fuel consumption in the cores of these stars. The ages of the oldest known chemical elements are also approximately 12 billion years—a number that comes from radioactive dating techniques. Workers in laboratories have derived these age estimates from atomic and nuclear physics. It is noteworthy that their results agree, at least approximately, with the age that The Evolution of the Universe HOMOGENEOUS DISTRIBUTION of galaxies is apparent in a map that includes objects from 300 million to 1,000 million light-years away. The only inhomogeneity, a gap near the center line, occurs because part of the sky is obscured by the Milky Way. Michael Strauss, now at Princeton University, created the map using data from the Infrared Astronomical Satellite. astronomers have derived by measuring cosmic expansion. Another theory, the steady-state theory, also succeeds in accounting for the expansion and homogeneity of the universe. In 1946 three physicists in England—Hoyle, Hermann Bondi and Thomas Gold—proposed such a cosmology. In their theory the universe is forever expanding, and matter is created spontaneously to fill the voids. As this material accumulates, they suggested, it forms new stars to replace the old. This steady-state hypothesis predicts that ensembles of galaxies close to us should look statistically the same as those far away. The big bang cosmology makes a different prediction: if galaxies were all formed long ago, distant galaxies should look younger than those nearby because light from them requires a longer time to reach us. Such galaxies should contain more short-lived stars and more gas out of which future generations of stars will form. Testing the Steady-State Hypothesis he test is simple conceptually, but it took decades for astronomers to develop detectors sensitive enough to study distant galaxies in detail. When astronomers examine nearby galaxies that are powerful emitters of radio wavelengths, they see, at optical wavelengths, relatively round systems of stars. Distant radio galaxies, on the other hand, appear to have elongated and sometimes irregular structures. Moreover, in most distant radio galaxies, unlike the ones nearby, the distribution of light tends to be aligned with the pattern of the radio emission. Likewise, when astronomers study the population of massive, dense clusters of galaxies, they find differences between those that are close and those far away. Distant clusters contain bluish galaxies that show evidence of ongoing star formation. Similar clusters that are nearby contain reddish galaxies in which active star formation ceased long ago. Observations made with the Hubble Space Telescope confirm that at least some of the enhanced star formation in these younger clusters may be the result of collisions between their member galaxies, a process that is much rarer in the present epoch. So if galaxies are all moving away from one another and are evolving from earlier forms, it seems logical that they were once crowded together in some dense sea of matter and energy. Indeed, in 1927, before much was known about distant galaxies, a Belgian cosmologist and priest, Georges Magnificent Cosmos 89 T Copyright 1998 Scientific American, Inc. JOHNNY JOHNSON, AFTER PAT McCARTHY Carnegie Institution; NIVR, NASA AND SERC the Cosmic Background Explorer (COBE) satellite and a number of rocket-launched, balloon-borne and ground-based experiments. The cosmic background radiation has two distinctive properties. First, it is nearly the same in all directions. (As the COBE team, led by John Mather of the National Aeronautics and Space Administration Goddard Space Flight Center, showed in 1992, the variation is just one part per 100,000.) The interpretation is that the radiation uniformly fills space, as predicted in the big bang cosmology. Second, the spectrum is very close to that of an object in thermal equilibrium at 2.726 kelvins above absolute zero. To be sure, the cosmic background radiation was produced when the universe was far hotter than 2.726 kelvins, yet researchers anticipated correctly that the apparent temperature of the radiation would be low. In the 1930s Richard C. Tolman of the California Institute of Technology showed that the temperature of the cosmic background would diminish because of the universe’s expansion. The cosmic background radiation provides direct evidence that the universe did expand from a dense, DISTANT GALAXIES are visible in this blowup of a Hubble Deep Field image. The hot state, for this is the condition needed to produce configuration in the box is 10.6 billion light-years away and thus appears as it did the radiation. In the dense, hot early universe thermowhen the universe was only 12 percent of its present age. Some of the other gal- nuclear reactions produced elements heavier than axies shown here are closer to Earth, so this one image contains many galaxies at hydrogen, including deuterium, helium and lithium. widely different distances, stacked up along the line of sight. Pictures such as this It is striking that the computed mix of the light eleone provide important information about how galaxies evolve from being loose, ments agrees with the observed abundances. That is, irregular forms in the past into more regular shapes in the present epoch. (As- all evidence indicates that the light elements were protronomers often look at negative images like this one, in which the background is duced in the hot young universe, whereas the heavier elements appeared later, as products of the therlight and the stars are dark, because weak features are easier to see.) monuclear reactions that power stars. The theory for the origin of the light elements Lemaître, proposed that the expansion of the universe might emerged from the burst of research that followed the end of be traced to an exceedingly dense state he called the primeval World War II. George Gamow and graduate student Ralph “super-atom.” It might even be possible, he thought, to de- A. Alpher of George Washington University and Robert Hertect remnant radiation from the primeval atom. But what man of the Johns Hopkins University Applied Physics Labowould this radiation signature look like? ratory and others used nuclear physics data from the war efWhen the universe was very young and hot, radiation fort to predict what kind of nuclear processes might have occould not travel very far without being absorbed and emitted curred in the early universe and what elements might have by some particle. This continuous exchange of energy main- been produced. Alpher and Herman also realized that a remtained a state of thermal equilibrium; any particular region nant of the original expansion would still be detectable in the was unlikely to be much hotter or cooler than the average. existing universe. When matter and energy settle to such a state, the result is a Despite the fact that significant details of this pioneering so-called thermal spectrum, where the intensity of radiation work were in error, it forged a link between nuclear physics at each wavelength is a definite function of the temperature. and cosmology. The workers demonstrated that the early uniHence, radiation originating in the hot big bang is recogniz- verse could be viewed as a type of thermonuclear reactor. As able by its spectrum. a result, physicists have now precisely calculated the abunIn fact, this thermal cosmic background radiation has been dances of light elements produced in the big bang and how detected. While working on the development of radar in the those quantities have changed because of subsequent events 1940s, Robert H. Dicke, then at the Massachusetts Institute in the interstellar medium and nuclear processes in stars. of Technology, invented the microwave radiometer—a device capable of detecting low levels of radiation. In the 1960s Bell Putting the Puzzle Together Laboratories used a radiometer in a telescope that would ur grasp of the conditions that prevailed in the early track the early communications satellites Echo-1 and Telstar. universe does not translate into a full understanding The engineer who built this instrument found that it was deof how galaxies formed. Nevertheless, we do have quite tecting unexpected radiation. Arno A. Penzias and Robert W. Wilson identified the signal as the cosmic background radia- a few pieces of the puzzle. Gravity causes the growth of dention. It is interesting that Penzias and Wilson were led to this sity fluctuations in the distribution of matter, because it more idea by the news that Dicke had suggested that one ought to strongly slows the expansion of denser regions, making them grow still denser. This process is observed in the growth of use a radiometer to search for the cosmic background. Astronomers have studied this radiation in great detail using nearby clusters of galaxies, and the galaxies themselves were O SPACE TELESCOPE SCIENCE INSTITUTE AND NASA 90 Scientific American Presents The Evolution of the Universe Copyright 1998 Scientific American, Inc. 1.0 probably assembled by the same verse. New telescopes such as process on a smaller scale. the twin 10-meter Keck teleThe growth of structure in the scopes in Hawaii and the 2.510–2 early universe was prevented by meter Hubble Space Telescope, HELIUM radiation pressure, but that other new telescopes at the changed when the universe had South Pole and new satellites expanded to about 0.1 percent looking at background radiation 10–4 of its present size. At that point, as well as new physics experiHELIUM 3 the temperature was about ments searching for “dark mat–6 3,000 kelvins, cool enough to ter” may allow us to see how the 10 allow the ions and electrons to mass of the universe affects the combine to form neutral hydrocurvature of space-time, which gen and helium. The neutral in turn influences our observa10–8 LITHIUM matter was able to slip through tions of distant galaxies. the radiation and to form gas We will also continue to study clouds that could collapse into issues that the big bang cosmol–10 10 star clusters. Observations show ogy does not address. We do not DEUTERIUM 1.0 0.001 0.01 0.1 that by the time the universe was know why there was a big bang DENSITY one fifth its present size, matter or what may have existed behad gathered into gas clouds DENSITY OF NEUTRONS AND PROTONS in the universe deter- fore. We do not know whether large enough to be called young mined the abundances of certain elements. For a higher-density our universe has siblings—other universe, the computed helium abundance is little different, and expanding regions well removed galaxies. A pressing challenge now is to the computed abundance of deuterium is considerably lower. from what we can observe. We reconcile the apparent uniformi- The shaded region is consistent with the observations, ranging do not understand why the funty of the early universe with the from an abundance of 24 percent for helium to one part in 1010 damental constants of nature lumpy distribution of galaxies in for the lithium isotope. This quantitative agreement of theory have the values they do. Adthe present universe. Astrono- and observation is a prime success of the big bang cosmology. vances in particle physics suggest some interesting ways these mers know that the density of the early universe did not vary by much, because they ob- questions might be answered; the challenge is to find experiserve only slight irregularities in the cosmic background radi- mental tests of the ideas. In following the debate on such matters of cosmology, one ation. So far it has been easy to develop theories that are consistent with the available measurements, but more critical should bear in mind that all physical theories are approximatests are in progress. In particular, different theories for gal- tions of reality that can fail if pushed too far. Physical science axy formation predict quite different fluctuations in the cos- advances by incorporating earlier theories that are experimenmic background radiation on angular scales less than about tally supported into larger, more encompassing frameworks. one degree. Measurements of such tiny fluctuations have not The big bang theory is supported by a wealth of evidence: it yet been done, but they might be accomplished in the genera- explains the cosmic background radiation, the abundances of tion of experiments now under way. It will be exciting to light elements and the Hubble expansion. Thus, any new coslearn whether any of the theories of galaxy formation now mology surely will include the big bang picture. Whatever developments the coming decades may bring, cosmology has under consideration survive these tests. The present-day universe has provided ample opportunity moved from a branch of philosophy to a physical science where for the development of life as we know it—there are some hypotheses meet the test of observation and experiment. SA 100 billion billion stars similar to the sun in the part of the universe we can observe. The big bang cosmology implies, however, that life is possible only for a bounded span of time: The Authors the universe was too hot in the distant past, and it has limited P. JAMES E. PEEBLES, DAVID N. SCHRAMM, EDWIN L. resources for the future. Most galaxies are still producing new TURNER and RICHARD G. KRON have individually earned stars, but many others have already exhausted their supply of top honors for their work on the evolution of the universe. Peegas. Thirty billion years from now, galaxies will be much bles is professor of physics at Princeton University, where in 1958 darker and filled with dead or dying stars, so there will be far he began an illustrious career in gravitational physics. Most of his free time is spent with his three grandchildren. Turner is chair fewer planets capable of supporting life as it now exists. of astrophysical sciences at Princeton and director of the 3.5-meThe universe may expand forever, in which case all the galaxter ARC telescope in New Mexico. He has a personal, cultural ies and stars will eventually grow dark and cold. The alternaand religious interest in Japan. Since 1978 Kron has served on the tive to this big chill is a big crunch. If the mass of the universe faculty of the department of astronomy and astrophysics at the is large enough, gravity will eventually reverse the expansion, University of Chicago, and he is also a member of the experimenand all matter and energy will be reunited. During the next tal astrophysics group at Fermi National Accelerator Laboratodecade, as researchers improve techniques for measuring the ry. He enjoys observing distant galaxies almost as much as directing Yerkes Observatory near Lake Geneva, Wis. Schramm, mass of the universe, we may learn whether the present expanwho was Louis Block Distinguished Service Professor in the sion is headed toward a big chill or a big crunch. Physical Sciences and vice president for research at the UniversiIn the near future, we expect new experiments to provide a ty of Chicago, died in a tragic airplane accident while this special better understanding of the big bang. New measurements of issue was being prepared for publication. This article updates a the expansion rate and the ages of stars are beginning to conversion that appeared in Scientific American in October 1994. firm that the stars are indeed younger than the expanding uni- The Evolution of the Universe RELATIVE ABUNDANCE GEORGE RETSECK Magnificent Cosmos 91 Copyright 1998 Scientific American, Inc. ...
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