Several reports published recently in consultation with the entire astronomy and astrophysics community have identified key questions that confront the discipline in the coming decade. These made the task of identifying the major challenges in astrophysics much simpler for the committee. In particular, the NRC decadal survey of astronomy and astrophysics, Astronomy and Astrophysics in the New Millennium, published in 2001 (often referred to as the McKee-Taylor report), the NRC report Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century, published in 2002 (also called the Turner report), and the National Science and Technology Council (NSTC) report The Physics of the Universe: A Strategic Plan for Federal Research at the Intersection of Physics and Astronomy, published in 2004, were instrumental in developing the list of questions summarized in this section. At its first meeting, the committee heard a presentation from Chris McKee, coauthor of the McKee-Taylor report, and at a later meeting it heard presentations from Tom Abel, Eve Ostriker, Ed Seidel, and Alex Szalay on topics related to the identification of the major challenges and the potential impact of HECC on them.
Committee members found themselves in complete agreement with the consistent set of major challenges identified in each of these three published reports. The challenges take the form of questions that are driving astrophysics and that are compelling because our current state of knowledge appears to make the challenges amenable to attack:
What is dark matter?
What is the nature of dark energy?
How did galaxies, quasars, and supermassive black holes form from the initial conditions in the early Universe observed by WMAP and COBE, and how have they evolved since then?
How do stars and planets form, and how do they evolve?
What is the mechanism for supernovae and gamma-ray bursts, the most energetic events in the known Universe?
Can we predict what the Universe will look like when observed in gravitational waves?
As in other fields of science, astrophysicists adopt four modes of investigation: observation, experiment, theory, and computation. Astronomy is characterized by its reliance on observation over experimentation, and this clearly affects the information available to astrophysicists. Virtually all that we know about the Universe beyond the solar system comes from electromagnetic radiation detected on Earth and in space. To push the frontiers of our knowledge, astronomers build ever-larger telescopes that operate over ever-wider bands of the electromagnetic spectrum and equip them with more efficient and more sensitive digital detectors and spectrographs. This is leading to an explosion of data in digital form, a point to which we return below.
Experimentation has a long and distinguished history in astrophysics. Although it is of course impossible to build a star in the laboratory and perform experiments on it, it is possible to measure basic physical processes important in stars and other astrophysical systems in the laboratory. For example, the cross sections for nuclear reactions of relevance to astrophysics have been the subject of laboratory measurements for many decades, as have the cross sections for the interaction of astrophysically abundant ions with light. More recently, the construction of high-energy-density laser and plasma fusion devices have enabled experiments on the dynamics of plasmas at the pressures and temperatures relevant to a variety of astrophysical systems.
Theory is primarily concerned with the application of known physical laws to develop a mathematical