how galaxies and clusters acquired their mass through both weak lensing and optical photometric surveys. Alternatively, the full-sky NIR spectroscopic survey could revolutionize the understanding of how and when star formation occurred in galaxies.
Constellation-X will make important contributions by detecting and characterizing the warm hot intergalactic medium, believed to contain most of the atoms in the present-day universe. Existing measurements of the baryon content in the early universe from the CMB would allow determination of the present-day distribution of baryonic matter. Con-X also has the potential to probe the nature of dark matter, which constitutes most of the mass of the universe, by observing its effect in galaxy clusters.
The JDEM and Con-X measurements of the matter content and distribution in the universe would be synergistic with the many other efforts in this area being pursued by other ground- and space-based facilities.
As a whole, the suite of five Beyond Einstein missions has tremendous potential to unambiguously answer the three fundamental questions at the core of the program. In its consideration of which mission should fly first, the committee’s primary science-evaluation criterion was how directly and unambiguously the different missions would answer one or more of the three questions put forward in NASA’s Beyond Einstein roadmap. This evaluation involved balancing breadth, depth, and scientific risk. The committee gave priority to those missions that promise significant advances, even if on a single question, over missions providing more incremental but broader progress touching on many areas, although both sets of contributions were valued. The committee determined that Inflation Probe is the candidate offering the greatest potential for progress in addressing the question, What powered the big bang? JDEM is the mission providing the measurements most likely to determine the nature of dark energy, and LISA provides the most direct and cleanest probe of spacetime near a black hole. Constellation-X, in contrast, provides measurements promising progress on at least two of the three Beyond Einstein questions, but does not provide the most direct, cleanest measurement on any of them. It is, however, an outstanding general astrophysics observatory that will make important advances on other questions set forth in NASA’s Beyond Einstein roadmap. The Black Hole Finder Probe will contribute to a black hole census, but it provides less direct measurements of black hole properties than LISA measurements. It was the committee’s judgment that for a focused program like Beyond Einstein, it is most important to provide the definitive measurement against at least one of the questions.
With any bold scientific venture there is always risk. For Inflation Probe, the scientific risk is, at the current time, unacceptably high for an investment of the scale of the proposed missions. Uncertain signal levels, foregrounds, and measurement sensitivities suggest that it is premature to proceed with an IP at this time. However, progress from the ground and suborbital platforms will likely be rapid in the next few years, and the maturation of theory and observation in this area will likely make it an exciting future opportunity. JDEM provides the best constraints on the nature of dark energy; however, there is risk that the systematic uncertainties associated with astronomical phenomena will limit the ultimate precision at a level less constraining than what the missions currently estimate, representing less of an advance over ground-based measurements than would be desirable for an investment of this scale. However, it is certainly the case that the ultimate precision and best control of systematics in constraining the DE equation of state will be achieved by space-based observations. Also mitigating the overall scientific risk of the mission is the fact that JDEM is guaranteed to make advances in other areas of Beyond Einstein science, such as the evolution of black holes and matter content of the universe. These two factors, in the committee’s view, make a strong case for a JDEM in spite of the risk posed by uncertain systematic effects. On purely scientific grounds, LISA is the mission that is most promising and least scientifically risky. Even with pessimistic assumptions about event rates, it should provide unambiguous and clean tests of the theory of general relativity in the strong-field dynamical regime and be able to make detailed maps of spacetime near black holes. Thus, the committee gave LISA its highest scientific ranking.