REPORT OF THE DARK ENERGY TASK FORCE Andreas Albrecht, University of California, Davis Gary Bernstein, University of Pennsylvania Robert Cahn, Lawrence Berkeley National Laboratory Wendy L. Freedman, Carnegie Observatories Jacqueline Hewitt, Massachusetts Institute of Technology Wayne Hu, University of Chicago John Huth, Harvard University Marc Kamionkowski, California Institute of Technology Edward W. Kolb, Fermi National Accelerator Laboratory and The University of Chicago Lloyd Knox, University of California, Davis John C. Mather, Goddard Space Flight Center Suzanne Staggs, Princeton University Nicholas B. Suntzeff, Texas A&M University Dark energy appears to be the dominant component of the physical Universe, yet there is no persuasive theoretical explanation for its existence or magnitude. The acceleration of the Universe is, along with dark matter, the observed phenomenon that most directly demonstrates that our theories of fundamental particles and gravity are either incorrect or incomplete. Most experts believe that nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. For these reasons, the nature of dark energy ranks among the very most compelling of all outstanding problems in physical science. These circumstances demand an ambitious observational program to determine the dark energy properties as well as possible. The Dark Energy Task Force (DETF) was established by the Astronomy and Astrophysics Advisory Committee (AAAC) and the High Energy Physics Advisory Panel (HEPAP) as a joint sub-committee to advise the Department of Energy, the National Aeronautics and Space Administration, and the National Science Foundation on future dark energy research.iiiii I. Executive Summary……………………………………………………………..1 II. Dark Energy in Context…………..……………………………………………..5 III. Goals and Methodology for Studying Dark Energy………………………….....7 IV. Findings of the Dark Energy Task Force……………………………………….11 V. Recommendations of the Dark Energy Task Force………………………….....21 VI. A Dark Energy Primer………………………………………………………….27 VII. DETF Fiducial Model and Figure of merit…………………………..................39 VIII. Staging Stage IV from the Ground and/or Space……...……..………………....45 IX. DETF Technique Performance Projections…………………………..................53 1. BAO………………………….....................................................................54 2. CL…………………………........................................................................60 3. SN…………………………........................................................................65 4. WL………………………….......................................................................70 5. Table of models...........................................................................................77 X. Dark Energy Projects (Present and Future) ………………...…………….….....79 XI. References…………………………………........................................................89 XII. Acknowledgments…………………………........................................................91 XIII. Technical Appendix...…………………………...................................................93 XIV. Logistical Appendix…..………………………..................................................123iv1I. Executive Summary Over the last several years scientists have accumulated conclusive evidence that the Universe is expanding ever more rapidly. Within the framework of the standard cosmological model, this implies that 70% of the universe is composed of a new, mysterious dark energy, which unlike any known form of matter or energy, counters the attractive force of gravity. Dark energy ranks as one of the most important discoveries in cosmology, with profound implications for astronomy, high-energy theory, general relativity, and string theory. One possible explanation for dark energy may be Einstein’s famous cosmological constant. Alternatively, dark energy may be an exotic form of matter called quintessence, or the acceleration of the Universe may even signify the breakdown of Einstein’s Theory of General Relativity. With any of these options, there are significant implications for fundamental physics. The problem of understanding the dark energy is called out prominently in major policy documents such as the Quantum Universe Report and Connecting Quarks with the Cosmos, and it is no surprise that it is featured as number one in Science magazine’s list of the top ten science problems of our time. To date, there are no compelling theoretical explanations for the dark energy. In the absence of useful theoretical guidance, observational exploration must be the focus of our efforts to understand what the Universe is made of. Although there is currently conclusive observational evidence for the existence of dark energy, we know very little about its basic properties. It is not at present possible, even with the latest results from ground and space observations, to determine whether a cosmological constant, a dynamical fluid, or a modification of general relativity is the correct explanation. We cannot yet even say whether dark energy evolves with time. Fortunately, the extraordinary scientific challenge of the dark energy has generated outstanding ideas for an observational program that can greatly impact our understanding. A properly executed dark energy program should have as its goals to 1. Determine as well as possible whether the accelerating expansion is consistent with a cosmological constant. 2. Measure as well as possible any time evolution of the dark energy. 3. Search for a possible failure of general relativity through comparison of the effect of dark energy on cosmic expansion with the effect of dark energy on the growth of cosmological structures like galaxies or galaxy clusters. To recommend a program to reach these goals, the Dark Energy Task Force first requested input from the community. The community responded with fifty impressive white papers outlining