The mission suite designed in the Beyond Einstein roadmap to carry out the program’s research goals consists of two flagship missions, the Laser Interferometer Space Antenna (LISA) and Constellation-X (Con-X), as well as three smaller missions known as the Einstein Probes. The flagship missions are well defined and mature in their scientific formulation. The Einstein Probes—the Cosmic Inflation Probe (CIP), the Black Hole Finder Probe (BHFP), and the Joint Dark Energy Mission (JDEM)—are typically smaller in scale, and multiple technical or observational approaches are being considered for their implementation. A competitive review will determine which of the implementation approaches of a given probe concept will be selected. The committee considered the scientific questions for each class of probe, as well as all proposed observational approaches, in reaching its conclusions about each mission area.

As one of its overall criteria for evaluating the Beyond Einstein missions, the committee formulated a set of five criteria for use in assessing the scientific content and quality of the mission candidates. These criteria characterize the scientific readiness, risk, and progress that each mission promises relative to the Beyond Einstein science goals. These science goals are well conceived and are traceable through numerous strategy and planning documents, and the committee has therefore chosen to adopt them as well.

  • Advancement of Beyond Einstein research goals. The primary assessment criterion is how directly and unambiguously a mission candidate addresses the Beyond Einstein research goals.

  • Broader science contributions. Many of the mission candidates in the Beyond Einstein portfolio can provide data that are central to other astrophysical investigations not identified as part of the Beyond Einstein research goals.

  • Potential for revolutionary discovery. Will the mission candidate’s measurements truly alter current paradigms, or discover new and unexpected phenomena?

  • Science risk and readiness.3 Considering the mission candidate as designed, how much risk is there that the measurements will not answer the questions posed? This risk could be due either to systematic effects associated with astronomical phenomena not easily addressed with theory, or to uncertainties in the levels of the signal to be measured or the number of accessible astronomical sources. Are the theoretical frameworks for understanding the measurements in place? Are there foundational measurements that need to be made first (e.g., characterization of astronomical backgrounds, wide-field surveys to find targets, and so on)?

  • Uniqueness of the mission candidate for addressing the scientific questions. Are there other projects, either space- or ground-based, that are likely to compete in addressing Beyond Einstein questions before the completion of the mission in question? How essential is the vantage point of space for the proposed science?

This chapter describes the science goals, potential impact, and scientific readiness of each of the five mission candidates. Note that because the current state of development varies greatly among missions, the level of detail in the following mission discussions varies as well. The chapter concludes with a comparative assessment of progress to be made against each of the three Beyond Einstein questions.



The Black Hole Finder Probe is one of the three Einstein Probes discussed in the Beyond Einstein roadmap. BHFP is designed to find black holes on all scales, from one to billions of solar masses. BHFP will address the question “How did black holes form and grow?” by observing high-energy x-ray emissions from accreting black holes and explosive transients. With a very wide field of view, BHFP can detect variable sources and bursts of x-rays that herald the birth of new black holes and map high-energy x-ray sources over the entire sky. By operating in the hard x-ray band (a few to 600 keV), BHFP can detect accreting black holes that are surrounded by obscuring


This criterion is focused on scientific challenges inherent in the investigation, assuming that the technology challenges are or can be met. The technology challenges for each mission are addressed in Chapter 3.

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