ing—what is the nature of the “dark energy” causing the acceleration? Most of the matter in the universe is invisible, and the nature of this dark matter remains a mystery. At present there is a vast gap in our knowledge of the evolution of the universe between the time at which the cosmic background radiation was produced, about 300,000 years after the Big Bang, and the time at which the most distant known galaxies emitted the light we see today, about a billion years later. This span of time includes the “dark ages,” when the only radiation was the glow left over from the Big Bang, and the dawn of the modern universe, when the first stars and galaxies formed. Researchers believe that supermassive black holes formed at about the same time that galaxies did, but nothing is known about how this occurred. Much smaller black holes are forming even today by processes that are poorly understood. Star formation drives the evolution of galaxies and leads to planet formation, yet there are far more questions than answers about how this process works. The discovery of extrasolar planets in the past decade has presented many new mysteries, since all the planetary systems observed so far are completely different from our solar system. How did such planetary systems form and evolve? What is their relation to our solar system, and which is the norm? Finally, when we look at our own Sun, we find that its light varies slightly with time. These variations may have significant effects on Earth’s climate, yet they are not now understood. To address these pressing scientific issues, and many others, the committee developed a comprehensive set of new initiatives in astronomy and astrophysics that will both vastly increase our knowledge of the universe and lead to many new discoveries. More important, these initiatives should enable us to achieve a greater understanding of the complex phenomena leading from the origin of the universe in the Big Bang to the existence of a life-bearing planet like Earth.

• The Astronomy and Astrophysics Survey Committee recommends the approval and funding of the prioritized new initiatives listed in Table 1.1.

Table 1.1 presents the priorities for initiatives for the decade 2000 to 2010. The priorities are listed separately for ground-based and space-based initiatives, both because the funding agencies are different (NSF, DOE, and DOD primarily for ground; NASA primarily for space) and because space facilities are intrinsically more expensive. Major and moderate projects are prioritized separately. The small initiatives consist