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Introduction: Where We Are and Where We Can Be

Elementary particle physicists and astronomers work at different extremes, the very small and the very large. They approach the physical world differently. Particle physicists seek simplicity at the microscopic level, looking for mathematically elegant and precise rules that govern the fundamental particles. Astronomers seek to understand the great diversity of macroscopic objects present in the universe—from individual stars and black holes to the great walls of galaxies. There, far removed from the microscopic world, the inherent simplicity of the fundamental laws is rarely manifest.

Physicists have extended the current understanding of matter down to the level of the quarks that compose neutrons and protons and their equally fundamental partners the leptons (the electron, the muon, and the tau particle, along with their three neutrino partners). They have constructed an elegant and precise mathematical description of the forces that shape quarks and leptons into the matter that we see around us. While elementary particle physicists cannot predict all the properties of matter from first principles, their theories describe in some detail how neutrons and protons are constructed from quarks, how nuclei are formed from neutrons and protons, and how atoms are built from electrons and nuclei (see Box 1.1).

Astronomers’ accomplishments in the realm of the universe are no less impressive. They have shown that the universe is built of galaxies expanding from a big bang beginning. Giant telescopes can see across the universe back to the time when galaxies were born, a few billion years after the big bang. The discovery and the subsequent study of the cosmic microwave background (CMB) radiation (the echo of the big bang) provide a snapshot of the universe when it was only about a half million years old, long before the first stars and galaxies were born. Hydrogen, lithium, deuterium, and helium were produced in nuclear reactions that took place when the universe was seconds old, and their presence today in the quantities predicted by the big bang



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1 Introduction: Where We Are and Where We Can Be Elementary particle physicists and astronomers work at different extremes, the very small and the very large. They approach the physical world differently. Particle physicists seek simplicity at the microscopic level, looking for mathematically elegant and precise rules that govern the fundamental particles. Astronomers seek to understand the great diversity of macroscopic objects present in the universe—from individual stars and black holes to the great walls of galaxies. There, far removed from the microscopic world, the inherent simplicity of the fundamental laws is rarely manifest. Physicists have extended the current understanding of matter down to the level of the quarks that compose neutrons and protons and their equally fundamental partners the leptons (the electron, the muon, and the tau particle, along with their three neutrino partners). They have constructed an elegant and precise mathematical description of the forces that shape quarks and leptons into the matter that we see around us. While elementary particle physicists cannot predict all the properties of matter from first principles, their theories describe in some detail how neutrons and protons are constructed from quarks, how nuclei are formed from neutrons and protons, and how atoms are built from electrons and nuclei (see Box 1.1). Astronomers’ accomplishments in the realm of the universe are no less impressive. They have shown that the universe is built of galaxies expanding from a big bang beginning. Giant telescopes can see across the universe back to the time when galaxies were born, a few billion years after the big bang. The discovery and the subsequent study of the cosmic microwave background (CMB) radiation (the echo of the big bang) provide a snapshot of the universe when it was only about a half million years old, long before the first stars and galaxies were born. Hydrogen, lithium, deuterium, and helium were produced in nuclear reactions that took place when the universe was seconds old, and their presence today in the quantities predicted by the big bang

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BOX 1.1 OUR COSMIC ROOTS An amazing chain of events was unleashed by the big bang, culminating some 13 billion years later in molecules, life, planets, and everything we see around us. Running the expansion of our universe in reverse, back to the big bang, we can be confident there was a time when it was so hot that the universe was just a soup of the elementary particles. Researchers are beginning to speculate about even earlier times when particles did not even exist and our universe was a quantum mechanical soup of strange forms of energy in a bizarre world of fluctuating geometry and unknown symmetries and even an unknown number of spatial dimensions. The journey to the universe we know today is depicted in Figure 1.1.1. It began at the end of inflation, when vacuum energy and quantum fuzziness became a slightly lumpy soup of quarks, leptons, and other elementary particles. Ten microseconds later quarks formed into neutrons and protons. Minutes later the cooling fireball cooked the familiar lighter elements of deuterium, helium, helium-3, and lithium (the rest of the periodic table of chemical elements was to be produced in stars a few billion years later). Atoms, with their electrons bound to nuclei, came into existence only a half million years or so later. The cosmic microwave background is a messenger from that era when atoms were formed. Along the way, dark matter particles and neutrinos escaped annihilation because of the weakness of their interactions, and for that reason they are still here today. The slight lumpiness of the dark matter— a legacy of the quantum fuzziness that characterized inflation—triggered the beginning of the formation of the structure that we see today. Starting some 30,000 years after the beginning, the action of gravity slowly, but relentlessly, amplified the primeval lumpiness in the dark matter. This amplification culminated in the formation of the first stars when the universe was 30 million years old, the first galaxies when the universe was a few hundred million years old, and the first clusters of galaxies when the universe was a few billion years old. As the dark matter clumped, the ordinary matter followed, clumping because of the larger gravitational pull of the more massive dark matter. Ordinary matter would get the final word, as its atomic interactions would eventually allow it to sink deeper and form objects made primarily of atoms—stars and planets—leaving dark matter to dominate the scene in galaxies and larger objects. This gulf of time between the decoupling of matter and radiation and the formation of the first stars is aptly referred to as the “dark ages.” Mountain-top observatories on Earth and the Hubble Space Telescope reveal evidence of the model confirms that the universe began from a soup of elementary particles. Einstein’s magnificent theory of space and time describes gravity, the force that holds the universe together and controls its fate. Using the laws of gravity, nuclear physics, and electromagnetism, astronomers have developed a basic understanding of essentially all the objects they have found in the universe, and a detailed understanding of many.

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dark ages: Probe deeply enough into space and back in time with a big telescope, and the result is fewer and fewer galaxies. As stars and galaxies evolved, enriching their protoplanetary gas clouds and eventually planets with the chemical products of stellar evolution, new possibilities for complexity arose: the chemical and molecular conditions for life. Our cosmic roots are in the stars and what came long before. It is possible now to trace those roots back to the quark soup, but it should be possible to trace them back even further to the quantum fuzziness that might have been their origin during inflation. FIGURE 1.1.1 The universe today is the product of a long, long chain of events, as shown in this artist’s conception of cosmological evolution beginning with the big bang. Scientists are exploring not only the chronological relationships between these events but also the causal connections. Image courtesy of NASA. These advances owe much to new technology. Optical astronomy has witnessed a millionfold gain in sensitivity since 1900, and a hundredfold gain since 1970. Gains in the ability to view the subatomic world of elementary particles through new accelerators and detectors have been similarly impressive. The exponential growth in computing speed and in information storage capability has helped to translate these detector advances

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into science breakthroughs. Technology has extended researchers’ vision across the entire electromagnetic spectrum, giving them eyes on the universe from radio waves to gamma rays, and new forms of “vision” using neutrinos and gravitational waves may reveal more cosmic surprises. Entirely new detectors never dreamed of before are making possible the search for new kinds of particles. In pursuing their own frontiers at opposite extremes, astronomers and physicists have been drawn into closer collaboration than ever before. They have found that the profound questions about the very large and the very small that they seek to answer are inextricably connected. Physicists want to know if there are new particles in addition to the familiar quarks and leptons. Astronomers are excited to know, too, because these new particles may be the substance of the dark matter that holds all structures in the universe together—including our own Milky Way galaxy. The path of discovery for astronomers now includes accelerators and other laboratory experiments, and the path for physicists now includes telescopes both on the ground and in space. In their quest for further simplicity and unity in the subatomic world, particle theorists have postulated the existence of additional space-time dimensions. These putative new dimensions in space might explain why the expansion of the universe seems to be speeding up rather than slowing down and might provide the underlying mechanism for the tremendous burst of growth known as inflation that astronomers believe occurred during the earliest moments of creation. If they exist, these new dimensions are well hidden, and the hunt for them will involve both astronomers and physicists. Even in the testing of well-established laws of nature—such as those of electromagnetism, gravity, and nuclear physics—physicists are joining with astronomers to use the universe as a laboratory to probe regimes of high temperature, high density, and strong gravity that cannot be studied on Earth. Both astronomers and physicists have a stake in knowing whether or not nature’s black holes are described accurately by Einstein’s theory of gravity and to find the answers, they will have to work together. More than ever before, breakthrough discoveries in astronomy and physics are occurring at the boundary of the two disciplines. For example, in 1998 physicists working with astronomers and using telescopes announced evidence that the expansion of the universe is speeding up, not slowing down, as had been expected. If the expansion is indeed accelerating, it must be because of dark energy, a mysterious form of energy heretofore unknown. Determining the nature of the dark energy is key to understanding the fate of the universe and may well be important to understanding

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the quantum nature of gravity as well. While the nature of the dark energy is a “physics” question, astronomers are very interested in the answer, and their telescopes will likely play the critical role. We stand poised to make great progress in our understanding of the universe and the laws that govern it by connecting quarks with the cosmos. To do so we will need an integrated approach, both interdisciplinary and interagency. Parsing the science into the traditional categories of physics and astronomy and working narrowly within agencies and without coordination and cooperation will not realize the full science potential. In fact, it is important to note that in practice the physicist and the astronomer are often the same individual, and that the boundaries between the disciplines are generally indistinct. These boundaries are particularly difficult to apply to the practitioners of the interfacial science that is the subject of this report. There are encouraging signs that existing disciplinary and organizational obstacles can be overcome. Physicists and astronomers, and NASA and DOE, are working together on the Gamma-ray Large Area Space Telescope (GLAST), an instrument that will search for evidence of dark-matter annihilations and additional space-time dimensions as well as supermassive black holes and pulsars. The Cryogenic Dark Matter Search (CDMS), whose goal is to detect the dark matter particles that hold our own galaxy together, is supported by both the Division of Physics and the Division of Astronomical Sciences at NSF and by the Office of High Energy and Nuclear Physics at DOE. But there have been missed opportunities. While many of the pioneering ideas and experiments at the interface of physics and astronomy originated in the United States, many of the most important discoveries occurred elsewhere. For instance, in spite of the fact that the prototypes for the large underground detectors located in Europe and in Japan—which have shown that neutrinos may have enough mass to account for some of the dark matter—were developed in the United States, U.S. scientists and institutions did not lead these exciting and important discoveries. Just as it will take the combined efforts of astronomers and physicists to realize these opportunities, so also each of the three agencies has an important and unique role to play in the scientific adventure that links the extremely large and the extremely small. In this report the Committee on the Physics of the Universe identifies the most important and timely science opportunities at the intersection of physics with astronomy. Because of the interconnectedness of the science, which is an integral part of its richness, organizing the report into linear chapters was a challenge—no approach would allow each chapter to stand

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as a discrete element independent of the other chapters. The idea that elementary particles may constitute the bulk of the matter in the universe arises in several contexts—in discussions of both the evolution of the universe and the quest to unify the forces and particles, and in a chapter devoted to dark matter and dark energy. The committee hopes that readers of its report will thereby come to appreciate the many threads that connect the science of the quarks and the science of the cosmos. Chapter 2, “Foundations: Matter, Space, and Time,” provides the intellectual foundation for the four chapters that follow and is by far the most challenging chapter for nonexperts. Chapter 3 addresses opportunities for deepening researchers’ understanding of the fundamental forces and particles and of how gravity can be taken beyond Einstein. Chapter 4 deals with the earliest beginnings of the universe. Scientists are poised not only to extend current understanding of the universe back to a time when even the largest structures in the universe were subatomic quantum fluctuations, but also to make profound advances in how matter, space, and time are viewed. The bulk of the stuff in the universe—dark matter and dark energy—lies between the stars and galaxies and is mysterious. As Chapter 5 discusses, the solution to the dark matter problem very likely involves one (or more) new particles of nature, and astronomers and physicists are now poised to solve this 70-year-old puzzle. At the same time, a joint effort is needed to tackle the dark energy problem. Chapter 6 deals with the opportunities that lie ahead to use the universe as a laboratory to study the physical laws—of nuclear physics, gravity, and electromagnetism—in regimes beyond the reach of terrestrial laboratories, and even, possibly, to discover new laws. Chapter 7, the final chapter, summarizes the scientific opportunities identified by the committee in the form of 11 questions that are deep in their content, crosscutting, and ripe for answering. The chapter goes on to recommend a strategy for realizing the opportunities. The strategy is summarized in the committee’s seven recommendations at the end of the chapter. Appendix D is a glossary that also contains definitions of acronyms. This is a special moment. If we can take advantage of the opportunities that exist, we stand to make truly fundamental advances in our understanding of how the universe began as well as of the basic nature of matter, space, and time. Because of the deep and profound connections between quarks and the cosmos, advances in both are inextricably connected and taking will require a new approach that lies at the boundary of physics and astronomy.