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PHYSICS THROUGH THE 1990s Gravitation, Cosmology, and Cosmic-Ray Physics Pane} on Gravitation, Cosmology, and Cosmic-Ray Physics Physics Survey Committee Board on Physics and Astronomy Commission on Physical Sciences, Mathematics, and Resources National Research Council NATIONAL ACADEMY PRESS Washington, D.C. 1986

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NATIONAL ACADEMY PRESS 2101 Constitution Avenue, NW Washington, DC 20418 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The National Research Council was established by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and of advising the federal government. The Council operates in accordance with general policies determined by the Academy under the authority of its congressional charter of 1863, which establishes the Academy as a private, nonprofit, self- governing membership corporation. The Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in the conduct of their services to the government, the public, and the scientific and engineering communities. It is administered jointly by both Academies and the Institute of Medicine. The National Academy of Engineering and the Institute of Medicine were established in 1964 and 1970, respectively, under the charter of the National Academy of Sciences. The Board on Physics and Astronomy is pleased to acknowledge generous support for the Physics Survey from the Department of Energy, the National Science Foundation, the Department of Defense, the National Aeronautics and Space Administration, the Department of Commerce, the American Physical Society, Coherent (Laser Products Division), General Electric Company, General Motors Foundation, and International Business Machines Corporation. Library of Congress Cataloging-in-Publication Data Main entry under title: Gravitation, cosmology, and cosmic-ray physics. (Physics through the 1990s) Includes index. 1. Gravitation. 2. Cosmology. 3. Cosmic rays. I. National Research Council (U.S.). Panel on Gravitation, Cosmology, and Cosmic-Ray Physics. II. Series. QC178.G64 1986 531'.14 85-32019 ISBN 0-309-03579-1 Printed in the United States of America First Printing, March 19 86 Second Printing, August 1986 Third Printing, August 1987

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PANEL ON GRAVITATION, COSMOLOGY, AND COSMIC-RAY PHYSICS DAVID T. WILKINSON, Princeton University, Chairman PETER L. BENDER, University of Colorado DOUGLAS M. EARDLEY, University of California, Santa Barbara THOMAS K. GAISSER, University of Delaware JAMES B. HARTLE, University of California, Santa Barbara MARTIN H. ISRAEL, Washington University LAWRENCE W. JONES, University of Michigan R. BRUCE PARTRIDGE, Haverford College DAVID N. SCHRAMM, The University of Chicago IRWIN I. SHAPIRO, Harvard-Smithsonian Center for Astrophysics ROBERT F. C. VESSOT, Harvard-Smithsonian Center for Astrophysics ROBERT V. WAGONER, Stanford University

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PHYSICS SURVEY COMMITTEE WILLIAM F. BRINKMAN, Sandia National Laboratories, Chairman JOSEPH CERNY, University of California, Berkeley, and Lawrence Berkeley Laboratory RONALD c. DAVIDSON, Massachusetts Institute of Technology JOHN M. DAWSON, University of California, Los Angeles MILDRED s. DRESSEEHAUS, Massachusetts Institute of Technology VAL L. FITCH, Princeton University PAUL A. FLEURY, AT&T Bell Laboratories WILLIAM A. FOWLER, w. K. Kellogg Radiation Laboratory THEODOR w. HANSCH, Stanford University VINCENT JACCARINO, University of California, Santa Barbara DANIEL KEEPPNER, Massachusetts Institute of Technology AEEXE! A. MARADUDIN, University of California, Irvine PETER D. MACD. PARKER, Yale University MARTIN L. PERK, Stanford University WATT w. WEBB, Cornell University DAVID T. WILKINSON, Princeton University DONALD c. SHAPERO, Stay Director ROBERT L. RIEMER, Staff Officer CHARLES K. REED, Consultant 1V

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BOARD ON PHYSICS AND ASTRONOMY HANS FRAUENFEEDER, University of Illinois, Chairman FELIX H. BOEHM, California Institute of Technology RICHARD G. BREWER, IBM San Jose Research Laboratory DEAN E. EASTMAN, IBM T.J. Watson Research Center JAMES E. GUNN, Princeton University LEO P. KADANOFF, The University of Chicago w. CARE LINEBERGER, University of Colorado NORMAN F. RAMSEY, Harvard University MORTON s. ROBERTS, National Radio Astronomy Observatory MARSHALL N. ROSENBEUTH, University Of Texas at Austin WILLIAM p. SEICHTER, AT&T Bell Laboratories SAM B. TREIMAN, Princeton University DONALD c. SHAPERO, Sta~Director ROBERT L. RIEMER, Sta~O~icer HELENE PATTERSON, Staf fAssistant SUSAN WYATT, StaffAssistant

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COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS, AND RESOURCES HERBERT FRIEDMAN, National Research Council, Chairman THOMAS D. BARROW, Standard Oil Company (Retired) ELKAN R. BLOUT, Harvard Medical School WILLIAM BROWDER, Princeton University BERNARD F. BURKE, Massachusetts Institute of Technology GEORGE F. CARRIER, Harvard University CHARLES L. DRAKE, Dartmouth College MILDRED s. DRESSEEHAUS, Massachusetts Institute of Technology JOSEPH L. FISHER, Office of the Governor, Commonwealth of Virginia JAMES c. FLETCHER, University of Pittsburgh WILLIAM A. FOWLER, California Institute of Technology GERHART FRIEDEANDER, Brookhaven National Laboratory EDWARD D. GOLDBERG, Scripps Institution of Oceanography MARY L. GOOD, Signal Research Center J. Ross MACDONALD, University of North Carolina THOMAS F. MALONE, Saint Joseph College CHARLES J. MANKIN, Oklahoma Geological Survey PERRY L. MCCARTY, Stanford University WILLIAM D. PHILLIPS, Mallinckrodt, Inc. ROBERT E. SIEVERS, University of Colorado JOHN D. SPENGEER, Harvard School of Public Health GEORGE w. WETHERIEE, Carnegie Institution of Washington RAPHAEL G. KASPER, Executive Director LAWRENCE E. MCCRAY, Associate Executive Director V1

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Preface Gravitation, cosmology, and cosmic-ray physics are often regarded as subfields of astrophysics, as well as physics, because they are practiced by using physical techniques in an astronomical setting. However, this report makes no pretense of surveying all of astrophys- ics; that enormous task was excellently done by the Astronomy Survey Committee (George B. Field, chairman). Their report, Astronomy and Astrophysics for the 1980's (National Academy Press, Washington, D.C., 1982), has been widely circulated, and its recommendations are currently being considered and implemented. We have restricted our review to the above-named three areas of physics and astrophysics currently of particular interest to physicists. Gravitation was explicitly not considered in the Field report and thus becomes a focus of this report. Cosmology has been an active area of astronomy for 60 years, and the many successes and opportunities of astronomical techniques are eloquently described in the Field report. The cosmology part of this report attempts to supplement the report of the Astronomy Survey Committee by emphasizing new results and ideas, particularly those triggered by recent contributions from other areas of physics. There is also some overlap between this report and the Field report in the area of cosmic rays; however, the vast scope of the earlier report allowed only cursory treatment. The study of cosmic rays, developed and practiced mainly by physicists, is an appropriate topic for the present report. Choosing which areas of astrophysics not . . V11

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Vlll PREFACE to emphasize in this study was more difficult. Related areas that could logically have been included are x-ray and gamma-ray astrophysics, most topics in theoretical astrophysics, nuclear astrophysics, solar physics, atomic and molecular astrophysics, and astrophysical plas- mas. The interconnectedness of astrophysics leads to some discussion in our report of all of these active areas. Also, reviews and recommen- dations concerning some of these areas can be found in the Astronomy Survey Committee report and in the reports of other panels of the Physics Survey Committee. In this report we have tried to characterize the fields by reporting some recent successes (Highlights) and by discussing some open questions that are guiding current research (Opportunities). The level and style of the presentation were chosen assuming that the reader is a student or a colleague not currently active in these fields. Experts will no doubt find regrettable omissions and technical errors; we did put clarity and perspective above completeness and detailed accuracy when it seemed that a choice was necessary. Our hardest task, how- ever, was to attempt to look into the future and chart a reasonable course (Recommendations). At best one can extrapolate ahead the most promising current research and ideas, hoping that work on this predictable program will best facilitate discoveries and new directions. Indeed, we wish to emphasize that all three of these research areas are developing rapidly and that flexibility will be needed to respond effectively to new ideas and discoveries. We expect that some of our recommendations will appear quite foolish 10 years from now because of unanticipated new developments. Our activities began with the formation of the panel in September 1983. In October about 90 "Dear Colleague" letters solicited advice from physicists and astronomers active in gravitation and cosmology. The letters requested views on facilities or major instrumentation needs, promising new areas, and a draft outline of this report. Based on that advice a meeting was called in December to consider proposed initiatives in gravitation. A list of participants and the agenda were widely circulated before the meeting. No panel meetings were held in cosmology or cosmic rays as responses to our solicitations did not indicate that meetings were needed. In these areas we relied on letters from colleagues and the comments, criticism, and advice of readers. We are particularly indebted to an active group of expert, critical readers. Their extensive comments on our first draft and guidance on the recommendations have substantially affected the content and conclusions of this report. We thank the readers: Marc Davis, Univer- sity of California, Berkeley; Stanley Deser, Brandeis University;

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PREFACE 1X Francis Everitt, Stanford University; George Field, Center for Astro- physics; Alan Guth, Massachusetts Institute of Technology; Peter Michelson, Stanford University; Ezra T. Newman, University of Pittsburgh; James Peebles, Princeton Universiy; Jean-Paul Richard, University of Maryland; Joseph Silk, University of California, Berkeley; Joseph Taylor, Princeton University; Kip Thorne, California Institute of Technology; V. K. Balasubrahmanyan, Goddard Space- flight Center; Rainer Weiss, Massachusetts Institute of Technology; Clifford Will, Washington University; and Gaurang B. Yodh, Univer- sity of Maryland. The gravitation part of this report benefits greatly from the earlier report of the Space Science Board's Committee on Gravitational Physics (Irwin I. Shapiro, chairman): Strategy for Space Research in Gravitational Physics in the 1980's. Also, the authors of the cosmic-ray portion of this report (Thomas Gaisser, Martin Israel, and Lawrence Jones) acknowledge the assistance of the reports of NASA's Cosmic- Ray Program Working Group (1982, 19854. The Panel is indebted to Donald C. Shapero for providing advice and services throughout this project and to Robert L. Riemer for oversee- ing publication of the report. Finally, we acknowledge the assistance and patience of Marion Fugill (Princeton), who held us together and made order out of the chaos of many drafts of this report.

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Contents I SUMMARY OF PRINCIPAL RECOMMENDATIONS Recommendations on Gravitational Physics, 3 Space Program in Gravitation, 3 Ground-Based Studies in Gravitation, 4 Gravitation Theory, 4 Recommendations on Cosmology, 5 Space Program in Cosmology, 5 Ground-Based Studies in Cosmology, 5 Growth in Cosmology Research, 6 Recommendations on Cosmic-Ray Physics, 6 Space Program in Cosmic Rays, 6 Ground-Based Cosmic-Ray Studies, 7 II GRAVITATION 1 EXPERIMENTAL TESTS OF GENERAL RELATIVITY: INTRODUCTION.... 2 EXPERIMENTAL TESTS OF GENERAL RELATIVITY: HIGHLIGHTS ...... Equivalence Principle, Eotvos to Lunar Laser Ranging, 15 X1 11 15

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X11 CONTENTS Gravitational Redshift, Mossbauer to Rocketborne Maser, 17 Light Deflection, Eclipses to Radio Interferometry, 19 Signal Retardation, Newest and Most Accurate Test, 19 Perihelion Advance, Einstein's only Handle, 21 Changing Gravitational Constant, Solar-System Time Versus Atomic Time, 21 Laboratory Testing of Gravitation, Searching for the Unexpected, 22 3 EXPERIMENTAL TESTS OF GENERAL RELATIVITY: OPPORTUNITIES . . . Tests for "Magnetic" Gravitational Effects 24 Relativity Gyroscope Experiment, 24 Black-Hole Jets, 26 Ranging to the Moon and Inner Planets, 27 Radar Ranging, 28 Ranging to Planetary Landers and Orbiters, 28 Lunar Laser Ranging, 30 Measurement of Second-Order Solar-System Effects, 31 Gravitational Quadrupole Moment of the Sun, 33 Systems of Compact Stars, 34 4 SEARCH FOR GRAVITATIONAL WAVES: INTRODUCTION Theory, 37 Sources, 38 Detectors, 40 SEARCH FOR GRAVITATIONAL WAVES: HIGHLIGHTS. Binary Pulsar, 42 Bar Detectors, 43 Interferometric Detectors, 44 Pulsar Timing and Millisecond Pulsars, 46 24 36 42

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CONTENTS Xiii Sources of Gravitational Waves Recent Developments, 47 6 SEARCH FOR GRAVITATIONAL WAVES: OPPORTUNITIES . Laser Interferometer Detector with 5-Kilometer Baseline, 49 Bar Detector Sensitivity and Bandwidth, 52 Observations with Bar Detectors, 54 Pulsar Searches, 55 Spacecraft Tracking, 55 Space Interferometers, 56 Event Rates and Source Calculations, 57 Computation, 58 7 GRAVITATION THEORY: INTRODUCTION 8 GRAVITATION THEORY: HIGHLIGHTS Neutron Stars, 61 Gravitational Collapse and Black Holes, 62 Quantum Particle Creation by Black Holes, 64 Quantum Effects in the Early Universe, 64 Alternative Theories, 65 Exact Solutions of the Einstein Equations, 65 Asymptotic Properties of Space-Time, 66 Numerical Relativity, 67 Emission of Gravitational Radiation, 67 The Positive Energy Theorem, 68 Quantum Field Theory in Curved Space-Time, 69 Quantum Gravity, 69 Supergravity, 71 Kaluza-Klein Theories, 71 9 GRAVITATION THEORY: OPPORTUNITIES Classical Gravitation, Singularities, Asymptotic Structure, 72 Quantum Gravity, 73 Astrophysical Properties of Neutron Stars and Black Holes, 75 49 59 61 72

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Xiv CONTENTS Computation, 77 New Kinds of Experimental Tests, 77 Communication with Other Subfields: Gravitation Experiment, Astronomy and Astrophysics, Field Theory and Elementary-Particle Physics, Pure Mathematics, 78 10 RECOMMENDATIONS . . Space Techniques, 80 Ground-Based Techniques, 81 Gravitation Theory, 81 III COSMOLOGY 11 INTRODUCTION THE STANDARD MODEL 12 HIGHLIGHTS............. Big-Bang Nucleosynthesis, 90 Large-Scale Properties of the Universe, 92 Structure in the Universe, 94 Invisible Mass, 96 Cosmology and Grand Unification, 98 The Inflationary Universe, 99 Gravitational Lenses, 100 13 OPPORTUNITIES ........... Observations from Space, 101 Continued Ground-Based Observations, 104 Particle Physics and Cosmology, 106 Theory, 106 14 RECOMMENDATIONS. Space Program, 108 Ground-Based Program, 109 Human and Computational Resources, 109 . IV COSMIC RAYS 15 OVERVIEW 80 . 87 90 101 108 115

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CONTENTS XV 16 HIGHLIGHTS Nucieosynthesis, 123 Isotope Ratios, 125 Abundances of Heavy Elements, 125 Solar Neutrinos, 126 Acceleration, 127 Shock Acceleration, 128 Acceleration Fractionation, 129 Termination of Acceleration Mechanism, 130 High-Energy Gamma Rays, 13 Anomalous Component, 132 Galactic Cosmic-Ray Transport and the Interstellar Medium, 132 Energy Dependence of Escape from Galaxy, 133 Correlation Between Anisotropy and Energy, 135 Secondaries from Light Nuclei, 135 Propagation in Galactic Halo, 136 Connection with Gamma and Radio Astronomy, 137 High-Energy Nuclear and Particle Physics, 137 Nucleon Decay Experiments as Cosmic-Ray Detectors, 138 NucIeus-Nucleus Collisions, 139 Cross Sections, Spectra, Anisotropies, and Composition of Primary Cosmic Rays Above )0~7 Electron Volts, 140 Magnetic Monopoles, 141 17 OPPORTUNITIES Spaceborne Experiments, 143 Isotopes, 144 Galactic Cosmic-Ray Isotopes, 144; Solar-Flare Isotopes, 145 Ultraheavy Elements, 145 High-Energy Composition and Spectra, 146 Positrons, Antiprotons, Deuterium, and 3He, 147 Antimatter, 148 Nucleus-NucIeus Interactions, 148 Solar Modulation of Cosmic Rays, 149 121 143

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Xvi CONTENTS Ground-Based Experiments, 149 Gamma-Ray Astronomy, 149 Air-Shower Detectors, 150 Neutrino Astronomy, 151 Magnetic Monopoles, 153 Nucleon Decay Detectors, 153 Solar Neutrinos, 154 Future Opportunities, 155 Theory, 155 18 RECOMMENDATIONS. . Spaceborne Experiments, 157 Major New Programs, 158 Continuing Programs, 159 Studies for the Future, 160 Ground-Based Experiments, 161 Gamma-Ray Astronomy, 162 Highest-Energy Cosmic Rays and Extensive Air Showers, 162 High-Energy Neutrino Astronomy, 163 Magnetic Monopoles, 163 Large Underground Detectors, 163 Solar Neutrinos, 164 Theory, 164 INDEX 157 165