Astronomy and Astrophysics in the New Millennium is the most recent in a series of surveys of the field conducted once every 10 years by committees of leading astronomers. The current survey, authored by the Astronomy and Astrophysics Survey Committee, provides recommendations for a research program for the decade that addresses the key scientific questions confronting

Cover Of The Current Survey Report

astronomers and astrophysicists today. The explosion of knowledge in recent years, made possible by current facilities, has in turn raised a wealth of intriguing new questions about the universe and its constituent galaxies, stars, and planets. To answer them, astronomers will need a diverse array of increasingly capable 21st-century tools. Current challenges in astronomy and astrophysics, and the research initiatives that the survey report recommends to address them (see high-priority projects in astronomy and astrophysics), are the subject of this web site.

One millennium ago, the few astronomers working on Earth — in Asia (particularly in China), in the Middle East, and in Mesoamerica — knew of only six of the nine planets that orbit the Sun. Although they studied the stars, they did not understand that these points of light were as mighty as our own Sun, nor could they imagine the vast distances that separate these stars from Earth. One millennium later humanity's astronomical horizons, enlarged by observations made from every part of our planet and above it, had expanded to include the entire universe. Today we know that the Sun is but one of 300 billion stars in the Milky Way Galaxy, which itself is but one of trillions of galaxies within the visible universe. By peering billions of light-years into space, telescopes look billions of years into the past to observe the cosmos when it was young. Astronomers can now interpret what they see within the framework of a well- tested model, called the inflationary Big Bang theory. This theory describes how the cosmos has evolved since the first 10 –36 second of cosmic time, the moment of the Big Bang that began the universe. The universe has been expanding ever since that moment. During the first billion years after the Big Bang, galaxies and galaxy clusters began to emerge from a relatively featureless cosmos. Most of the matter in the universe exists in the form of dark matter, whose nature remains a mystery but whose existence is convincingly deduced from its gravitational effect on visible matter. Startling new observational evidence points to an even more mysterious dark energy that pervades the universe, driving the expansion to ever-greater velocities as time goes on.

One cannot, of course, predict what astronomers will tell us in the year 3000 A.D., or even in 2100 A.D. For the foreseeable future of the next few decades, we can, however, summarize the defining issues for astronomy and astrophysics by posing five fundamental questions:

• How did the universe begin, how did it evolve from a primordial soup of elementary particles into the complex structures we see today, and what fate lies in store for the cosmos?
• How do galaxies first arise and mature?
• How are stars born and how do they live and die?
• How do planets form and change as they age?
• Does life exist elsewhere in the universe?

	Fourteen billion years of cosmic evolution since the Big Bang have spawned the mighty zoo of cosmic objects that populate the universe today. On one of these objects lives a species on the verge of fitting together the pieces of this puzzle of how the universe came to be the way it is. We humans are about to understand how we came to inhabit a small rocky planet in orbit around a rather average star in the outer arms of the spiral galaxy we call the Milky Way. That understanding will reveal to us the ultimate fate of the universe.

Researchers have now begun to gather the fundamental observational data that will one day answer all of these questions. For only one do we already have a fairly complete answer: We know about the lives of stars. The development and observational validation of the theory of

what astronomers call stellar evolution was arguably the greatest accomplishment of 20th-century

astrophysics. For the new century, astronomers' long-term goal is to assemble a detailed picture of the formation, evolution, and destiny of the universe, and of its constituent galaxies, stars, and planets, which include the Milky Way, the Sun, and Earth.

To achieve this goal, the Astronomy and Astrophysics Survey Committee believes that astronomers should carry out the following program of observational and theoretical research:

• Map the distribution of galaxies, gas, and dark matter in the universe, and survey the stars and planets in the Milky Way. Mapping the distant universe will help to reveal the formation of galaxies in the early universe and their maturation to the present, the evolution of primordial hydrogen and helium gas created in the Big Bang into gas enriched with almost all of the elements found in the periodic table, and the distribution and nature of the mysterious dark matter that constitutes most of the matter in the universe. Surveys within the Milky Way will help to reveal how stars and planets are created in collapsing clouds of gas and dust and the variety and abundance of planetary systems.
• Search for life beyond Earth, and, if it is found, determine its nature and its distribution in the Milky Way Galaxy. This goal is so challenging and of such importance that it could occupy astronomers for the foreseeable future. The search for evidence of life beyond Earth through remote observation is a major focus of the new interdisciplinary field of astrobiology.
• Use the universe as a unique laboratory to test the known laws of physics in regimes that are not accessible on Earth and to search for entirely new physics. It is remarkable that the laws of physics developed on Earth appear to be consistent with phenomena occurring billions of light-years away and under extreme conditions radically different from those under which the laws were derived and tested. Researchers have only begun to probe the conditions near the event horizons of black holes or in the very early universe. In these environments, the tests of the laws of physics will be much more stringent. New physical processes may be revealed that shed light on the unification of the forces and particles of nature.
• Develop a conceptual framework that accounts for the complete range of astronomical observations. As with all scientific theories, such a framework must be subject to continual checks by further observation.

For the new decade, astronomers are poised to make progress in five particular areas:

1. Determining the large-scale properties of the universe:its age, the types of matter and energy that it contains, and the history of its expansion;
2. Studying the dawn of the modern universe, when the first stars and galaxies formed;
3. Understanding the formation and growth of black holes of all sizes;
4. Studying the formation of stars and their planetary systems, including the birth and evolution of giant and terrestrial planets; and
5. Understanding the effects of the astronomical environment on Earth.