into black holes. The rate and high-energy x-ray luminosity of these events are uncertain, but detection would be exciting and unique.

The Joint Dark Energy Mission and Constellation-X will make measurements that characterize the effect of dark energy on the geometry of the universe and/or on the growth of structure. This will yield the ratio of the dark energy pressure to its energy density as a function of time, enabling researchers to distinguish between a cosmological constant, a dynamical evolving field, a modification of general relativity, or some other new physics. The primary purpose of the JDEM missions is to employ at least two of the following three techniques for the exploration of dark energy: (1) using Type Ia supernovas as standard candles to determine the luminosity-distance versus redshift relation; (2) using weak lensing to measure the angular-diameter versus redshift relation, as well as the growth of structure; and (3) using baryon acoustic oscillations to measure angular diameter versus distance. ConX will use galaxy clusters in two different ways to measure the evolution of dark energy. The first is to determine cluster distances independent of redshift (assuming the gas mass fraction is redshift-independent) and compare these distances to the measured redshift. The second is to measure the effect of dark energy on the growth of structure by determining the mass distribution of clusters as a function of redshift. For the latter measurement, Con-X relies on wide-area cluster surveys from other experiments and will provide the follow-up observations required to accurately determine the cluster masses.

LISA also has the potential to measure the dark energy equation of state, along with the Hubble constant and other cosmological parameters. Through gravitational-waveform measurements, LISA can determine the luminosity distance of sources directly. If any of these sources can be detected and identified as infrared, optical, or x-ray transients and if their redshift can be measured, this would revolutionize cosmography by determining the distance scale of the universe in a precise, calibration-free measurement.

The science risk of the JDEM and Con-X dark energy evolution measurements is the uncertainty in the level of precision and control of the systematic effects. At the present time, weak lensing and baryon acoustic oscillation measurements appear most likely to provide the requisite factor-of-10 improvement over currently available constraints, and each of the proposed JDEM missions employs one of these techniques. The complex astrophysics associated with clusters makes the understanding of systematic effects particularly challenging for this measurement; however, it is possible that detailed x-ray observations of individual clusters with Con-X will improve theoretical understanding sufficiently to allow a precision measurement of w. It is important to use several independent methods of measurement, since they can lead to almost orthogonal constraints and have very different uncertainties. However, because of the importance of controlling systematics, the committee favors the JDEM missions over Con-X for this measurement.

One risk to the success of cosmography with gravitational waves from merging supermassive black holes is the uncertain merger rate. Also at the present time researchers do not know if it will be possible to determine optical counterparts in order to measure redshifts. While the prospect is very exciting, since it would be precise and free of systematic uncertainties, it may not be achievable if, for example, counterparts do not exist. The committee notes that both a wide-FOV near-IR space telescope, such as JDEM, and the Con-X mission would enhance the prospects of counterpart identification if they flew simultaneously with LISA.

All of the JDEM dark energy measurements are being pursued by other experiments. Ground-based telescopes are currently improving statistics of the supernova and baryon oscillation measurements, and future wide-field telescopes will make progress on weak lensing. Space measurements are, however, unique for access to the near-IR, redshift coverage, and stable point spread function, all of which are important for the control of systematics crucial for these measurements. For cluster studies, the eROSITA x-ray mission and ground-based Sunyaev-Zeldovich experiments will significantly improve the dark energy measurements, but it is unlikely that the ultimate precision will be reached without Constellation-X’s spectroscopic capability.

Improving measurements of the amount of dark and baryonic matter in the universe is also essential to understanding the amount of dark energy. All of the JDEM mission concepts can contribute to this goal. Wide FOV optical and NIR imaging telescopes can study the large-scale distribution of mass via weak lensing and clarify

Front Matter (R1-R12)

Executive Summary (1-8)

1 Introduction (9-14)

2 Science Impact (15-65)

3 Mission Readiness and Cost Assessment (66-114)

4 Policy Issues (115-117)

5 Findings and Recommendations (118-132)

Appendixes (133-134)

Appendix A: Letter of Request (135-136)

Appendix B: Background and Statement of Task (137-138)

Appendix C: Input from the Broader Astronomy and Astrophysics Community (139-142)

Appendix D: List of Briefings to the Committee (143-145)

Appendix E: Request for Information to Mission Teams (146-151)

Appendix F: Mission Teams' Technology Funding Inputs to the Committee (152-153)

Appendix G: Acronyms (154-157)

Appendix H: Glossary (158-167)

Appendix I: Biographies of Committee Members and Staff (168-174)