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1 Introduction PHYSICS AND ASTRONOMY IN EINSTEIN’S CENTURY As the 20th century came to a close, astronomers discovered something that challenged the basic understand- ing of physics developed over the entire century: dark energy. The 20th century opened with the development of Albert Einstein’s theory of special relativity, published in 1905, which gave science a radically new way of understanding how space and time are related through the propagation of light. Between 1911 and 1916, Einstein generalized his theory to include gravity. This immediately became science’s best tool for understanding the large-scale structure of the universe. In 1929, Edwin Hubble found that distant galaxies are all moving away from us. Einstein’s theory of general relativity provided a natural framework for astronomers’ observations of the completely unexpected recession of galaxies in the distant uni- verse—space itself is expanding. While those advances were occurring in astronomy, physicists were developing quantum theory, an entirely new way of viewing the world of the extremely small: of atoms, nuclei, and electrons. By 1930, the basic struc- ture of atoms and molecules was understood, and physicists were moving on to probe the even smaller nucleus. Astronomers and physicists were making discoveries at opposite ends of the size and distance scales, and they were not in a position to see the connections between their research at that time. Just before World War II, physicists developed high-energy accelerators to probe deeper inside atomic nuclei and, later, inside the building blocks of the nucleus—protons and neutrons. A series of successively higher-energy accelerator experiments discovered many new “elementary” particles in the succeeding decades. By the 1970s, a “standard model” of what some people had called a “zoo” of new particles had been formulated. The Standard Model still summarizes nearly everything that has been learned experimentally about elementary particles to date. In the decades after World War II, astronomers got their first indications of an exotic consequence of Einstein’s relativity: collapsed stars and galactic cores called black holes—black because their gravity is so intense that no light can escape from them. Astronomers now know that black holes are found in nearly every galaxy, including our own. They also inferred from observing neutron stars in orbit that the gravitational waves predicted by Einstein must exist, although scientists still have not detected them directly. Astronomers also began to suspect that their telescopes were not seeing all the kinds of matter in the universe. They inferred that there must be unseen “dark” matter that exerts a gravitational pull on the motions of stars in galaxies and on the motion of galaxies in galaxy clusters. Dark matter is now thought to be ubiquitous, comprising over 20 percent of the mass-energy density of 

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10 NASA’S BEYOND EINSTEIN PROGRAM the observed universe. If the dark matter is an elementary particle, it cannot be one of those in the Standard Model. The Standard Model is incomplete. Astronomy was beginning to pose challenges to physics. By the 1980s, the expanding universe had gotten its own theoretical model, which is by now so well verified that it deserves also to be called standard: the so-called Inflationary Big Bang Model. At its earliest moments, our entire universe was unimaginably small and unimaginably hot. It suddenly “inflated” in a “big bang.” The Infla- tionary Big Bang Model naturally explains why the universe appears to us so nearly flat and so nearly uniform in all directions. The universe continued to expand after the brief episode of rapid inflation ended; this evolved into the expansion that Edwin Hubble first observed. As the universe expanded it cooled, and its composition changed from particles only found by accelerators today to the hydrogen and helium atoms currently observed. This pro- cess was completed after about 300,000 years, when electrically charged electrons and ions first combined into these atoms. In the 1990s, astronomers showed with exquisite precision that microwaves almost uniformly filling the universe in all directions today are the light radiation created when atoms first formed. This cosmic background radiation was subsequently shifted to longer wavelengths by the ongoing expansion of the universe. Precision observations of the microwave background not only confirmed many aspects of the Inflationary Big Bang Model but also became an important tool for learning more about both physics and astronomy. A NEW ERA IN PHYSICS AND ASTRONOMY By 1995, astronomers and physicists could be reasonably satisfied with their century’s work. Einstein’s theory of relativity, the Standard Model of particle physics, and the Inflationary Big Bang Model had passed the experi- mental tests that science had devised in the past two generations. With the exception of the dark matter puzzle, the foundations seemed reasonably solid, as far as they went. However, Einstein’s relativity and basic particle physics were now irretrievably entangled with one another, and understanding the relationship between the two had become one of the central unanswered questions of contemporary science. In 1998, astronomy again shook the foundations of physics. Astronomers, by relating the apparent brightness of Type Ia supernovas in distant galaxies to their speed of recession, found that the expansion of the universe—of space itself—is speeding up. The speedup implies the existence of a new kind of energy, “dark energy,” that com- prises 70 percent of the total mass-energy density in the universe. Einstein’s equations for an expanding universe allow a so-called cosmological constant that acts the same everywhere and, within today’s observational limits, could account for the speedup. However, basic physics theories have no natural explanation for the size of the observed acceleration rate. The discovery of dark energy has caused huge excitement. The questions fly thick and heavy. Can an under- standing of dark energy (and dark matter) teach us ways in which particle physics models should be extended? Should we not test the degree to which dark energy is exactly constant, as Einstein’s cosmological constant predicts? Or, going beyond Einstein, will we find that it varies? Can the distant universe be observed with the exquisite precision needed to detect its variation? If dark energy does vary, what would be learned about the physics of particles? Either answer—constant or varying—has profound implications for both physics and astronomy. There is renewed interest in testing Einstein’s theory. Should we not now investigate general relativity experimentally where it has never been tested before—in the so-called strong-field regime? Can we do this by observing the gravitational waves generated when two black holes merge. Will this be how we first detect gravitational waves directly? What will be learned when we do detect them directly? Will it be found that there are deviations from Einstein’s general relativity? Can we detect the gravitational waves generated at the moment of inflation? If so, will we learn about particle energy scales vastly higher than those attainable in accelerators? Do atoms behave in unexpected ways when they are at the high temperatures and pressures associated with the strong gravitational field near a black hole? Shouldn’t a more complete census of black holes be made? Amidst all the new questions awaiting answers, two points emerge with clarity. The first is that the 21st century in astronomy and physics will be very different from the 20th. We are going to have to go beyond Einstein. Second, the understanding of the inflationary big bang universe is sufficiently secure that scientists can use the universe

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INTRODUCTION 11 itself the way they use accelerators: to explore the most basic laws of physics. Astronomers and physicists will be working together in mutually supportive ways from now on. EVOLUTION OF THE U.S. STRATEGY FOR MOVING “BEYOND EINSTEIN” NASA did not wait long to explore the implications of the new discoveries. In 1999 NASA asked the National Research Council’s (NRC’s) Board on Physics and Astronomy to identify the most exciting science at the interface of physics and astronomy. The resulting report, Connecting Quarks with the Cosmos, was developed by physicists and astronomers working together. Connecting Quarks with the Cosmos became one of two foundational NRC documents for NASA’s Beyond Einstein Program, the other being the NRC decadal survey of astronomy and astrophysics, Astronomy and Astrophysics in the New Millennium. The 2001 decadal survey began by laying out the fundamental goal of the field: “to understand how the uni- verse and its constituent galaxies, stars, and planets formed, how they evolved, and what their destiny will be.” To achieve this goal, the report said, astronomers must pursue a balanced strategy with several elements:  • Survey the universe and its constituents, including galaxies as they evolve through cosmic time, stars and planets as they form out of collapsing interstellar clouds in our galaxy, interstellar and intergalactic gas as it accumulates the elements created in stars and supernovae, and the mysterious dark matter and perhaps dark energy that so strongly influence the large-scale structure and dynamics of the universe. • Use the universe as a unique laboratory for probing the laws of physics in regimes not accessible on Earth, such as the very early universe or near the event horizon of a black hole. • Search for life beyond Earth, and if it is found, determine its nature and its distribution. • Develop a conceptual framework that accounts for all that astronomers have observed. The first and second elements above relate to the issues taken up in this report. Connecting Quarks with the Cosmos contrasted the approaches of physicists and astronomers to science: 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. Blending these two ways of looking at nature has already put us on the verge of very important new insights, and we are only at the beginning. After discussing the scientific opportunities immediately ahead and taking into account the decadal survey, Connecting Quarks with the Cosmos recommended a suite of space missions with the following goals, among others: • Measure the polarization of the cosmic microwave background with the goal of detecting the signature of inflation. The committee recommends that NASA, NSF, and DOE undertake research and development to bring the needed experiments to fruition. . . . • Determine the properties of dark energy. The committee supports the Large Synoptic Survey Telescope project, which has significant promise for shedding light on the dark energy. The committee further recommends that NASA and DOE work together to construct a wide-field telescope in space to determine the expansion history of the universe and fully probe the nature of dark energy. . . . National Research Council, Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century, The National Academies   Press, Washington, D.C., 2003. National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.   National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001, p. 3.   National Research Council, Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century, The National Academies   Press, Washington, D.C., 2003, p. 9.

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12 NASA’S BEYOND EINSTEIN PROGRAM • Use space to probe the basic laws of physics. The committee supports the Constellation-X and Laser Interferom- eter Space Antenna missions, which hold great promise for studying black holes and for testing Einstein’s theory in new regimes. The committee further recommends that the agencies proceed with an advanced technology program to develop instruments capable of detecting gravitational waves from the early universe.  NASA then asked its Space Science Advisory Committee (SSAC) to formulate a program plan in light of the decadal survey and Connecting Quarks with the Cosmos. SSAC turned to its Structure and Evolution of the Universe Subcommittee (SEUS) and asked it to develop an implementation roadmap as part of the 2003 strategic plan of NASA required under the Government Performance Results Act of 1993 (Public Law No. 103-62). SEUS coined the term “Beyond Einstein”; its report, Beyond Einstein: From the Big Bang to Black Holes, proposed the following program architecture: The Beyond Einstein program has three linked elements. . . . The central element is a pair of Einstein Great Obser- vatories, Constellation-X and LISA. . . . The second element is a series of competitively selected Einstein Probes, each focused on one of the science questions, (a joint dark energy mission (JDEM), an inflation probe, and a black hole finder). The third element is a program of technology development, theoretical studies, and education, to support the Probes and the vision missions. . . . In 2004, the White House Office of Science and Technology Policy (OSTP) released Physics of the Universe, an interagency study that illustrates and defines the mutual interest in Beyond Einstein science shared by the Department of Energy (DOE), the nation’s principal sponsor of elementary-particle physics; NASA, the nation’s principal sponsor of space astronomy; and the National Science Foundation (NSF), the nation’s principal sponsor of general science. As one consequence, NASA and DOE agreed to develop a Joint Dark Energy Mission (JDEM), one of the five mission areas included in NASA’s Beyond Einstein roadmap. THE CHARGE TO THE COMMITTEE The present assessment was prompted by congressional language inserted in the formulation of the fiscal year (FY) 2007 budget. According to Senate Report 109-274 accompanying the Energy and Water Appropriations Bill of 2007: The Committee is concerned that the joint mission between the Department of Energy and NASA is untenable because of NASA’s reorganization and change in focus toward manned space flight. The Committee directs the Department (of Energy) to immediately begin planning for a single-agency space-based dark energy mission. Similar language was inserted in House Report 109-474 accompanying the Energy and Water Development Appropriations Bill of 2007: NASA has failed to budget and program for launch services for JDEM. Unfortunately, in spite of best intentions, the multi-agency aspect of this initiative poses insurmountable problems that imperil its future. Therefore, the Com- mittee directs the Department [of Energy] to begin planning for a single-agency dark energy mission with a launch in fiscal year 2013. The administration responded with the approach described here: In August 2006, OSTP Director John H. Marburger convened a meeting of the NASA administrator, the DOE science undersecretary, and the respective chairs of the NRC Space Studies Board, the NRC Board on Physics and Astronomy, and the joint DOE/NASA/NSF Astronomy and Astrophysics Advisory Committee. The purpose of this meeting was to encourage a fair, joint- agency process for going forward on a Beyond Einstein mission. National Research Council, 2003, Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century, The National   Academies Press, Washington, D.C., pp. 6-7. National Aeronautics and Space Administration, 2003, Beyond Einstein: From the Big Bang to Black Holes, Washington, D.C., January.  

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INTRODUCTION 13 NASA and DOE then requested that the NRC assess NASA’s Beyond Einstein Program plan according to the following charge, and the NRC established the Committee on NASA’s Beyond Einstein Program: An Architecture for Implementation to carry out the charge: 1. Assess the five proposed Beyond Einstein missions (Constellation-X, Laser Interferometer Space Antenna, Joint Dark Energy Mission, Inflation Probe, and Black Hole Finder Probe) and recommend which of these five should be developed and launched first [emphasis added], using a funding wedge that is expected to begin in FY 2009. The criteria for these assessments include: • Potential scientific impact within the context of other existing and planned space-based and ground-based mis- sions; and • Realism of preliminary technology and management plans, and cost estimates. 2. Assess the Beyond Einstein missions sufficiently so that they can act as input for any future decisions by NASA or the next Astronomy and Astrophysics Decadal Survey on the ordering of the remaining missions. This second task element will assist NASA in its investment strategy for future technology development within the Beyond Einstein Program prior to the results of the Decadal Survey. This report adopts terminology that is slightly different from that employed in the charge. Constellation-X (Con-X) and the Laser Interferometer Space Antenna (LISA) are specific single-mission candidates, whereas three candidates for the Joint Dark Energy Mission, four for the Inflation Probe (IP), and two for the Black Hole Finder Probe (BHFP) were submitted to this committee for assessment. The committee distinguishes between the five mission areas listed in the charge above and the 11 individual mission candidates. The committee also notes that both the LISA and the JDEM mission areas are proposed as NASA collaborations (with the European Space Agency and the DOE’s Office of Science, respectively) and that there is the possibility that the other mission areas may also involve partner organizations in the future. The committee assumes that organizations interested in partnering with NASA on such missions would be motivated by similar scientific and technical goals. THE COMMITTEE’S APPROACH TO ITS CHARGE The committee started with a systematic consideration of each of the 11 mission candidates identified thus far in the five mission areas in the Beyond Einstein Program plan. The committee invited presentations from agency leaders in NASA, DOE, and the European Space Agency, and at least two presentations from each team proposing a mission candidate. Additionally, the committee heard presentations from individual scientific leaders and listened to the broader scientific community in town hall meetings across the United States (see Appendixes C and D). Subsequently, the committee asked clarifying questions of each team and included their written responses in the committee’s assessment process (see Appendixes E and F). Using these inputs, the committee then assessed each mission candidate for its scientific excellence, its response to Beyond Einstein goals, its broad contributions to science, its competition from other space- and ground-based projects in the United States and abroad, and its scientific readiness. On the implementation side, the committee assessed each mission candidate for its cost, complexity, schedule, and related programmatic implica- tions and for its stage of development and overall technical readiness, and identified pertinent individual factors. The committee carried out these steps before any formal discussion of Task 1 of its charge, and only then did it begin a comparative discussion to identify the main competitors for the Task 1 recommendation. See Appendix A for the letter of request and Appendix B for the full statement of task. 7  See Appendix I for committee member biographies. 8 

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14 NASA’S BEYOND EINSTEIN PROGRAM OUTLINE OF THE REPORT Chapter 2 assesses the contribution that each mission candidate would make to three central Beyond Einstein science questions: What powered the big bang? How do black holes manipulate space, time, and matter? and What is the mysterious dark energy pulling the universe apart? Chapter 2 summarizes the strengths, scientific uncertain- ties, readiness, and uniqueness of the mission candidates and associated scientific programs. It should be noted that since the mission candidates vary greatly in their stage of development, the mission assessments necessarily differ in their level of detail in both Chapters 2 and 3. Chapter 3 examines the technical and programmatic challenges presented by each of the 11 Beyond Einstein mission candidates. The committee assessed team organization, project management, technology readiness and difficulty, cost and schedule risks, and technical and cost margins; it also identified special challenges particular to each mission candidate. Chapter 3 can be used as a summary reference for Beyond Einstein mission readiness as of FY 2007. The committee’s intent has been to provide a basis for judging the readiness of each mission to proceed to a Formulation Phase A (conceptual design phase) in FY 2009, and to support its advice on how best to prepare each mission area for consideration by the NRC’s forthcoming decadal survey. Chapter 4 discusses policy and overarching programmatic issues associated with the Beyond Einstein Program, including implications for U.S. science and technology leadership, program funding constraints, the role of inter- agency and international partnerships, the broader uses of investments in research and technology infrastructure, and the implications of International Traffic in Arms Regulations. Such issues are representative of those faced by most cutting-edge space science programs. Chapter 5 contains the committee’s eight major findings and three principal recommendations. The findings are not listed in priority order; their order expresses the progression of reasoning that led to the three principal conclusions. The chapter also summarizes the committee’s overall assessments of each of the Beyond Einstein mission candidates and provides advice on how to move each mission area forward until it can be considered by the decadal survey.