Universe that interacts gravitationally but not with electromagnetic radiation. Precise measurements of the expansion history of the Universe, made using distances determined from Type Ia supernovae, have recently indicated the existence of dark energy, which is responsible for an acceleration with time in the expansion rate of the Universe that counteracts the deceleration produced by gravity. In the past year, the Wilkinson Microwave Anisotropy Probe (WMAP), a satellite launched by NASA in 2001, has provided the most accurate observations of the fluctuations in the cosmic microwave background radiation that were first discovered by the Cosmic Background Explorer (COBE), another NASA mission, launched in 1989. These fluctuations are the imprint of structure in the Universe that existed about 300,000 years after the Big Bang, structure that eventually collapsed into the galaxies, stars, and planets we see today. These observations not only challenge astrophysicists to explain how these fluctuations grew and evolved into the structure we see today, but they also provide firm evidence that verifies the reality of both dark matter and dark energy. Discovering the nature of dark matter and dark energy are perhaps the two most compelling challenges that face astrophysicists today.
Exosolar planets. In the past 10 years, over 200 new planets have been discovered orbiting stars other than the Sun. Most of these planets are dramatically different from those in our solar system. For example, a significant fraction consist of large, Jupiter-like bodies that orbit only a few stellar radii from their host stars. There is no analog to such planets in our own solar system. Indeed, our current theory of planet formation predicts that Jupiter-like planets can form only in the outer regions of planetary systems, at distances several times the Earth-Sun separation. Thus, the existence of these new planetary systems challenges our understanding of planet formation and the dynamical evolution of planetary systems. As our techniques for detecting exosolar planets (also known as exoplanets) improve, astronomers are finding more and more planets closer in size to and with properties similar to those of Earth. (As of the time of this report, several planets with masses only a few times that of Earth have been found.) Of course, this raises the question of whether any of them harbor life.
Unambiguous evidence for the existence of black holes. It has long been known that there is a compact object at the center of our galaxy responsible for producing powerful nonthermal emission at a variety of wavelengths. One way to delimit the nature of this object is to measure its size and mass: If the inferred mass density is sufficiently high, then it must be a black hole. Using diffraction-limited images in the infrared to measure the motions of stars within 1 light-year of this object over the past 10 years, two research groups have been able to limit its mass to at least 3 million solar masses. There is no known object in the Universe, other than a supermassive black hole, that could contain this much mass in such a small volume. Similar observations of the motions of stars and gas near the centers of several other galaxies, notably NGC4258, also known as Messier106, have also provided unambiguous evidence for the existence of supermassive black holes.
The purpose of this chapter is to assess the potential impact of HECC on the progress of research in astrophysics. Using the discoveries of the past decade, along with sources such as prior decadal surveys of the field and input from outside experts, the committee formulated a list of the major challenges facing astrophysics today, identified the subset of challenges that require computation, and investigated the current state of the art and the future impact of HECC on progress in facing these challenges.