and NASA. Funding opportunities and the changing needs of larger programs sometimes require advice on significantly shorter intervals than the long-term advice provided here on program balance. In the past decade, for example, changing priorities at NASA overall, combined with the Columbia disaster, resulted in an abrupt funding redistribution that ultimately led to a significant imbalance in NASA’s astrophysics program, which in turn created issues with continuity of small-scale funding.4 For such unforeseen changes in circumstance, the AAAC can, as discussed in Chapter 3, provide tactical advice to DOE, NASA, and NSF on the support of individual and group grants funding, including the balance between grants programs, mission/facility operations, and the design and development of new missions/facilities.
The role of theory in astrophysics has evolved in ways that reflect the increasing complexity of observations. Today, theoretical astrophysicists use analytical methods to devise speculative scenarios that account for new observations, they carry out detailed computational simulations of complex systems, and they develop new methods and frameworks for testing models against observational data. Together these methods propel progress, often in unforeseen ways. For example, the discussion of gravitational microlensing in the 1980s led to new observational constraints on the nature of dark matter in the 1990s and now provides a powerful pathway to the discovery of exoplanets. Similarly, recent observations of the cosmic microwave background have provided precision measurements of the age and content of the universe, but only because the theoretical framework had been developed over the preceding several decades, starting with new, bold theories about the exponential expansion rate of universe in its first few moments. Moreover, theory informed the design of experiments and enabled measurements to be extracted. The result is a spectacularly successful “standard model” of the universe, which experiments recommended in this report will test even more stringently.
Several important trends are increasing the scope of theoretical activity and enhancing the roles of theorists:
The boundary between astrophysics theory and high-energy physics theory has become increasingly blurred as astrophysical observations play a growing role in particle physics phenomenology. Much of the information we