"Beyond Einstein science" is a term that applies to a set of new scientific challenges at the intersection of physics and astrophysics. Observations of the cosmos now have the potential to extend our basic physical laws beyond where 20th-century research left them. Such observations can provide stringent new tests of Einstein's general theory of relativity, indicate how to extend the Standard Model of elementary-particle physics, and-if direct measurements of gravitational waves were to be made-give astrophysics an entirely new way of observing the universe. New physical understanding may be required to explain cosmological observations, and the challenge of investigating the laws of physics using astronomical techniques promises to bring higher precision, clarity, and completeness to many astrophysical investigations relating to galaxies, black holes, and the large-scale structure of the universe, among other areas.
In 2003, NASA, working with the astronomy and astrophysics communities, prepared a research roadmap entitled Beyond Einstein: From the Big Bang to Black Holes. This roadmap proposed that NASA undertake space missions in five areas in order to study dark energy, black holes, gravitational radiation, and the inflation of the early universe, and to test Einstein's theory of gravitation. Two of the five planned mission areas were Einstein Great Observatories: Constellation-X (Con-X) and the Laser Interferometer Space Antenna (LISA). The other three were planned as smaller Einstein Probes: the Black Hole Finder Probe (BHFP), the Inflation Probe (IP), and the Joint Dark Energy Mission (JDEM). Candidate missions for all of these mission areas are currently in various stages of definition and development.
Prompted by congressional language inserted in the formulation of the fiscal year (FY) 2007 budget, NASA and the Department of Energy (DOE) asked the National Research Council (NRC) to prepare a report reviewing NASA's Beyond Einstein Program. The NRC appointed the Committee on NASA's Beyond Einstein Program to carry out this study. The report was to assess the five proposed Beyond Einstein mission areas and recommend one mission area for first development and launch utilizing a Beyond Einstein Program funding wedge that will start in FY 2009. To accomplish this task, the committee assessed all five mission areas, using criteria that address both potential scientific impact and technical readiness. In addition, the report was to assess each mission in sufficient detail to provide input for decisions by NASA and for the NRC's next astronomy and astrophysics decadal survey regarding both the ordering of the remaining missions and the investment strategy for future technology development within the Beyond Einstein Program. In responding to this latter charge, the committee has attempted to indicate what next steps each of the missions would need to take in order to prepare for future assessments.
The criteria adopted by the committee in assessing the missions fell into two general categories. First, the committee looked at the potential scientific impact within the context of other existing and planned space-based and ground-based missions. Here the committee considered how directly each assessed mission would address the research goals of the Beyond Einstein research program, likely contributions to the broader field of astrophysics, the potential for revolutionary scientific discovery, the scientific risks and readiness of the mission, and its competition from other ground- and space-based instruments.
Second, the committee considered the realism of preliminary technology and management plans and of cost estimates. Criteria used by the committee included plans for the maturity of critical mission technology, technical performance margins, schedule margins, risk-mitigation plans, and the proposal's estimated costs versus independent probable cost estimates prepared by the committee.
The committee made its recommendations on the basis of the above criteria, but during its deliberations it identified several policy-related issues relevant to the Beyond Einstein Program. These issues included implications for U.S. science and technology leadership, program funding constraints, relationships in interagency and international partnerships, investments in underlying research and technology and supporting infrastructure, and the impact of International Traffic in Arms Regulations. The committee reviewed these issues in order to understand the broader context of the study.
Using the criteria described above, the committee performed extensive assessments for each mission. It is impossible to adequately summarize here all of the points that factored into the final mission selection. Rather, each of the missions reviewed by the committee is briefly described below, along with a summary of a few of the major points from the committee's assessment.
Science Impact and Technology Readiness
Black Hole Finder Probe (BHFP)
The two Black Hole Finder Probe mission concepts presented to the committee are called EXIST (Energetic X-ray Imaging Survey Telescope) and CASTER (Coded Aperture Survey Telescope for Energetic Radiation). Both of these concepts use wide-field coded-aperture hard x-ray telescopes, divided into arrays of subtelescopes at two different energy bands. With their arrays of subtelescopes, either would survey the entire sky between a few kiloelectronvolts and 600 keV during the course of their 95 minute orbits, providing information about source variability on time scales ranging from milliseconds to many days.
Science Importance and Readiness BHFP is designed to find black holes on all scales, from one to billions of solar masses. It will observe high-energy x-ray emission from accreting black holes and explosive transients and will address the question of how black holes form and grow.
BHFP will be unique among current or planned missions in high-energy x-ray sensitivity combined with large field of view and frequent coverage of the sky. The resulting hard x-ray sky maps, temporal variability data, and the large number of short-lived transient detections will have a direct impact on a number of important astrophysical questions. BHFP will provide a unique window into the properties and evolution of astronomical objects whose physics is dominated by strong gravity.
The committee found the science risk for BHFP mission candidates to be rather high. Although a census of massive black holes in galaxies can be achieved, only very-high-luminosity and very-high-mass black holes will be seen at high redshifts. In addition, the very uncertain conversion from x-ray luminosity to black hole growth rate implies that BHFP will not provide a unique value (to better than a factor of 10) of the black hole growth rate (e.g., in solar masses per year) in any individual galaxy or even in the entire universe. Finally, the difficulty in identifying host galaxies also yields significant risk in the interpretation of BHFP results. Both multiwavelength observational data and theoretical advances (e.g., in black hole accretion modeling) will be necessary in order for BHFP to realize its full scientific potential.
Technology Readiness The two BHFP mission candidates differ primarily in their selection of detector material. CASTER faces more technology maturity challenges, as the detector technology in general is at lower Technology Readiness Levels (TRLs) than that of EXIST, as discussed in Chapter 3 of this report. The estimated costs for both mission concepts are higher than projected in the original Beyond Einstein roadmap: there the Einstein Probes had been envisioned as medium-scale missions that could be executed much more rapidly and cheaply than the flagship LISA and Con-X missions. However, the BHFP probe concepts now have costs that the mission teams estimate are in the vicinity of a billion dollars. This report's independent assessment (Chapter 3) also finds probable costs inconsistent with the original Einstein Probe cost range. The committee suggests that judicious trade-offs among sensitivity, detector area, and observing time may enable a smaller telescope to carry out the most important BHFP science at lower cost.
The Constellation-X mission has been designed to be a general-purpose astrophysical observatory. Its primary new capability is very-high-spectral-resolution, high-throughput x-ray spectroscopy, representing an increase in these capabilities of roughly two orders of magnitude over missions currently flying.
Science Importance and Readiness Con-X will make the broadest and most diverse contributions to astronomy of any of the candidate Beyond Einstein missions. The committee understands that Con-X has the potential to make strong contributions to Beyond Einstein science through the study of the evolution of supermassive black holes and the mapping of the dynamics of clusters of galaxies. However, other Beyond Einstein missions will address both the measurement of dark energy parameters and tests of strong-field general relativity in a more focused and definitive manner and, as a result, the committee did not choose Con-X as one of the highest priorities for Beyond Einstein funding. The committee concluded that the merits of Con-X can be fully assessed only when it is judged as a major astrophysics mission in a context broader than that of the Beyond Einstein Program. Given that Con-X was ranked second only to the James Webb Space Telescope in the NRC's 2001 astronomy and astrophysics decadal survey, Astronomy and Astrophysics in the New Millennium, NASA's characterization of it as a Beyond Einstein mission understates its significance to general astronomy.
Technology Readiness Con-X is one of the best studied and tested of the missions presented to the committee. Aside from the well-known risks of satellite implementation, a number of technical risks have been called out by the Con-X mission team and are discussed in Chapter 3. Chief among these is the achievement of the needed mirror angular resolution and the development of the position-sensitive microcalorimeters. The Con-X team has reasonable plans to mature both of these technologies, and, given adequate resources and time, there is little reason to expect that these technologies will limit the main science goals of the observatory.
Con-X development activities need to continue aggressively in areas such as achieving the mirror angular resolution, cooling technology, and x-ray microcalorimeter arrays to improve the Con-X mission's readiness for consideration in the next astronomy and astrophysics decadal survey. The committee, however, does not believe that the current Beyond Einstein funding wedge should fund these activities. Beyond Einstein is not the sole justification for Con-X, as its primary science capabilities support a much broader research program.
Inflation Probe (IP)
The Inflation Probe mission effort seeks to study for the first time the conditions that existed during the crucial phase of exponential expansion in the early history of the universe. Four IP mission concepts have been proposed to date. Three propose to study the signal impressed on the polarization of the cosmic microwave background (CMB) radiation by gravity waves induced during the inflationary period. The fourth proposes to measure the structure in the universe on various length scales, arising from the primordial density fluctuations induced by inflation.
Science Importance and Readiness Understanding inflation is an important goal of the Beyond Einstein Program. The exponential expansion during the era of inflation may have similarities with the much more slowly accelerating expansion occurring today that is attributed to the presence of dark energy. A deeper understanding of both inflation and dark energy is needed in order to explore that similarity. Studying inflation may also lead to understanding the source of the largest structures in the universe, which appear to be linked to quantum fluctuations and phenomena at the smallest scales. The theoretical framework for understanding the results of both the CMB and high- redshift galaxy observations is already in place.
Technology Readiness One of the four mission concepts, the Cosmic Inflation Probe (CIP), has a mission design that is a modification of existing missions. Although the state of CIP technology is more advanced than that of the polarization missions, it would benefit from advances in grating technologies. NASA's Astrophysics Research Grants Program is already in place to fund these types of investigations. However, it should be noted that the scope of this program may need to be changed to accommodate aggressive IP development.
The three CMB polarization Inflation Probes collectively are in a stage of development earlier than that of the CIP. The three CMB proposals outline detector and instrument concepts that are extrapolations from existing experiments. The CMB polarization experiments-Experimental Probe of Inflationary Cosmology (EPIC-F), Einstein Polarization Interferometer for Cosmology (EPIC-I), and CMB Polarimeter (CMBPol)-all require extremely sensitive millimeter-wave continuum detectors and extremely effective rejection of the common-mode noise from the anisotropy signal. All three of these missions have proposed to use state-of-the-art detectors to reach the required high sensitivity. If the European Space Agency's (ESA's) Planck mission is successful, it will go a long way but not all the way toward proving the readiness of the detector technology. Along with the continued investment in grating technology required to continue to mature CIP, significant continued support of detector and ultracool cryocoolers (sub-100 mK) is needed to push the three polarization missions along. Given the missions' state of development, it is not necessary to provide direct technology development support to each of the mission teams. Although the state of CIP technology is more advanced than the technology of the polarization missions, CIP would benefit from intensive theoretical investigations as well as further refinement of grating technologies.
Joint Dark Energy Mission (JDEM)
The Joint Dark Energy Mission is a partner mission between NASA and DOE that would use an optical-to-near-infrared wide-field survey telescope to investigate the distribution of dark energy. Three concepts for a JDEM have thus far been proposed: the Supernova Acceleration Probe (SNAP), the Dark Energy Space Telescope (DESTINY), and the Advanced Dark Energy Physics Telescope (ADEPT).
Science Importance and Readiness Understanding the nature of dark energy is one of the most important scientific endeavors of this era. A central goal of JDEM is a precision measurement of the expansion history of the universe to determine whether the contribution of dark energy to the expansion rate varies with time. A discovery that the expansion history is not consistent with Einstein's cosmological constant would have a fundamental impact on physics and astronomy.
JDEM will significantly advance both dark energy and general astrophysical research. The wide-field optical and near-infrared surveys required for dark energy studies will create large, rich data sets useful for many other astrophysics studies, enlarging an already significant discovery potential. A full-sky, near-infrared spectroscopic survey, such as proposed by ADEPT, has never been performed, and no comparable mission is planned. This survey would open the emission-line universe, providing new probes of star formation during the epoch when galaxies grew, along with data for many other astrophysics studies. A low-background, wide-field imaging survey, such as proposed by DESTINY and SNAP, would provide a much larger diffraction-limited near-infrared survey than would be available otherwise. Such a survey would revolutionize the understanding of how and when galaxies acquire their mass, as well as providing copious data for many other astrophysics studies.
Excerpted from NASA's BEYOND EINSTEIN PROGRAM Copyright © 2007 by National Academy of Sciences. Excerpted by permission.
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