cosmic order, and frontiers of knowledge. These questions resulted from the careful surveying of the current state of research in astronomy and astrophysics done by Astro2010’s five Science Frontiers Panels (SFPs), later synthesized by the committee.¹ An assessment of the readiness of the astronomy and astrophysics enterprise to answer these questions led directly to the science program described in later chapters.

Scientific progress often follows predictable paths. Through keen insight and diligent pursuit, questions are asked and answered, and knowledge is recorded. But many of the most revolutionary discoveries in science are made when a new way of perceiving or thinking about the universe evaporates the fog that had obscured our view and reveals an unimagined cosmic landscape all around us. The history of astronomy is replete with these revelatory moments. This capacity of the universe to astonish us was certainly evident during the past decade. Here the committee lists just a few of the most far-reaching examples.

The surprising discovery in 1998 that the expansion of the universe is accelerating rather than slowing, due to the repulsive gravity of dark energy, has changed the way we think about the evolution and destiny of the universe and has challenged our understanding of physics at the most fundamental level. In the coming decade, an optimized and coordinated set of facilities on the ground and in space will test whether the simplest hypothesis—dark energy is the quantum energy of