Priority III: How Did the Universe Begin?
Measurements of the cosmic microwave background (CMB) form the cornerstone of modern cosmology. These data determine the age, shape, contents, and initial conditions of the universe. CMB experiments establish the amounts of atomic matter, dark matter, and dark energy. Precise measurement of the CMB will also enable the determination of the number and type of neutrino species and the sum of their masses. Gravitational waves from the proposed inflationary origin of our universe make unique polarization patterns of the CMB—a smoking gun of the first tiny fraction of a second in the history of the universe. Such a detection probes the energy scales of what is called Grand Unification, where the fundamental interactions of quantum mechanics—the electrical and the strong and weak nuclear forces—merge. The detection of this incredibly small CMB polarization signal, or at least the setting of strong limits on inflationary models, drives major CMB programs around the world.
Antarctic CMB research, including growth to incorporate an experimental program termed Stage 4 (CMB-S4), was identified as Strategic Priority III in NASEM (2015): “How did the universe begin and what are the underlying physical laws that govern its evolution and ultimate fate? A next-generation cosmic microwave background (CMB) program.” CMB-S4 is an international scientific collaboration to achieve CMB measurements that will be an order of magnitude more sensitive than the current set of experiments. CMB-S4 focuses on two sites identified as providing excellent conditions for observing the CMB: the South Pole plateau and the high-altitude Atacama Desert in Chile. At the South Pole, CMB-S4 requires the construction of multiple telescopes—both small- and large-aperture instruments working together, which build on existing South Pole research and complement key components of CMB-S4 in Chile. Other observation programs, such as SPIDER, a balloon-borne observation program that performs measurements at shorter wavelengths and reaches larger angular scales than ground-based observatories enhance the interpretation of the ground-based CMB data (SPIDER Collaboration, 2021). In this chapter, the committee reviews progress in advancing this NASEM (2015) strategic priority and discusses implementation issues and opportunities to improve progress.
EVALUATION OF PROGRESS
The current CMB experiments at the South Pole continue to provide world-leading results and are on track to lead seamlessly into the long-term CMB-S4 program, both scientifically and technically. The CMB-S4 project has made good
progress in planning over the past 5 years. In this section, the committee evaluates progress toward scientific goals and the adequacy of funding support, as well as scientific community support for this priority.
Progress Toward Scientific Goals
CMB-S4 builds on the successful CMB experiments currently at the South Pole, including the South Pole Telescope (SPT; see Figure 4-1), Background Imaging of Cosmic Extragalactic Polarization (BICEP)/Keck, whose science teams and ongoing operations are mainly supported by the NSF Office of Polar Programs (OPP). The SPT has produced world-leading CMB results over the past decade, and the most sensitive polarization limits by a wide margin come from BICEP/Keck and from the SPT. Both programs are active and expanding their efforts, consistent with NASEM (2015), which highlights that the scientific and technical developments from both experiments are providing a critical foundation for a next-generation ground-based CMB experimental program.
Important new scientific results in the period under review have been published from work with both the SPT and BICEP/Keck. These results rely heavily on NSF support for these programs in the decade before NASEM (2015). The SPT results include competitive constraints on the Lambda Cold Dark Matter model (Balkenhol et al., 2021), the standard cosmological model that explains the properties
of the universe, including constraints on the Hubble constant, the parameter that sets the current expansion rate of the universe (e.g., Freedman, 2017). Ancillary science results from the SPT include using the thermal Sunyaev-Zel’dovich effect to discover new galaxy clusters at high redshifts (Bleem et al., 2020), the discovery of strongly gravitationally lensed high-redshift dusty star–forming galaxies, and participation in the Event Horizon Telescope, an international collaboration that links radio dishes across the globe to create an Earth-sized interferometer (Doeleman, 2017). The Event Horizon Telescope collaboration has measured the size of the emission regions of supermassive black holes (Event Horizon Telescope Collaboration, 2019); the SPT team shared the Breakthrough Prize in Fundamental Physics in 2020 for this work.
The SPT team installed a third-generation polarization receiver (SPT-3G) during the period of this review, tripling the sensitivity of the telescope (Montgomery et al., 2021). In 2018, the team began a new survey with SPT-3G, which will run until 2023. After the completion of the SPT-3G survey, the plan is to install a new camera that will use new microwave kinetic inductance detector technology, which will improve the sensitivity of the SPT in higher-frequency bands and help constrain inflation through measures of primordial non-Gaussianity.1 The SPT team has also recently been awarded a 1-year NSF Mid-Scale Research Infrastructure (MSRI) design award for a new wide-field three-mirror anastigmatic (TMA) 5-meter telescope. This new instrument, which is also envisioned to serve as a key part of CMB-S4, will be used to remove the effects of lensing in data from the BICEP/Keck experiments (BICEP/Keck and SPTpol Collaborations, 2021). The NSF MSRI program is relatively new and seems to be an effective strategy to fill a funding need between the Major Research Infrastructure program and the Major Research Equipment and Facility Construction program.
Recent BICEP/Keck science results include the tightest constraints yet on primordial gravitational waves (BICEP2/Keck Collaboration, 2018), precise measurements of lensing of the CMB (see Figure 4-2), and limits on axions (BICEP/Keck Collaboration, 2021), a hypothetical particle billions of times lighter than the electron that is a currently favored candidate for cold dark matter (e.g., Di Luzio et al., 2020). The presence of axions would solve major symmetry puzzles in fundamental particle physics.
The next step in sensitivity for the BICEP/Keck program of large angular-scale polarimetry is a new instrument called the BICEP Array. It has 30,000 detector elements compared to 2,500 in the Keck Array. The telescope mount has been deployed, and the first of four receivers is currently operating. As of March 2020, all four BICEP Array receivers were scheduled to be deployed by the 2022 season,2 but the COVID-19 pandemic has disrupted implementation schedules.
1 The study of fluctuations in the CMB from a typical “Gaussian distribution” is important for evaluating inflationary models of the early universe.
2 See https://docushare.wipac.wisc.edu/dsweb/Get/Version-104468/CMB%20Presentation%20for%20ICNO%20Meeting%2018-Mar-2020.pdf.
The CMB-S4 project is extremely important for understanding the physics of the early universe and will significantly advance U.S. research capabilities and help maintain its leadership in this area. The CMB-S4 project seamlessly crosses boundaries between astronomy and physics, and support for the project to date has come from both NSF and the U.S. Department of Energy (DOE). Although CMB-S4’s construction and operations are mostly not contained within the period being evaluated in this report, direct progress has been made in planning for CMB-S4. The community produced a CMB-S4 Science Book (Abazajian et al., 2016) and a CMB-S4 Technology Book (Abitbol et al., 2017), which were the basis for convening a joint NSF-DOE Concept Definition Task Force, a subpanel of the Astronomy and Astrophysics Advisory Committee that advises DOE, the National Aeronautics and Space Administration (NASA), and NSF. The CMB-S4 Concept Definition Task Force (2017) report provided guidance on the science goals and measurement requirements for CMB-S4, in addition to instrument design, schedule, and cost, which were accepted by the Astronomy and Astrophysics Advisory Committee in
2017. The two key elements of the Concept Definition Task concept are that CMB-S4 will (1) require multiple cameras and telescopes distributed across two sites and (2) be undertaken by a single collaboration and run as one project. The two sites are the South Pole, with 18 new small-aperture telescopes and 2 new large-aperture telescopes, and Chile, with 2 new large-aperture telescopes (Abazajian et al., 2019). The scientific community has set a target schedule for completion of CMB-S4 near the end of this decade, followed by 7 years of survey operations. Recent developments include MSRI design awards for CMB-S4 and for a 5-meter TMA telescope, which would be deployed shortly and then its use could be continued in CMB-S4.
Until 2020, the two major ongoing experiments at the pole received sufficient financial support to continue their instrument development programs and to analyze the data they acquired. However, recently a number of CMB instrumentation proposals that were scientifically highly rated have been turned down for what may be logistics considerations. There is concern that logistics considerations have been allowed to overrule science decisions in recent funding decisions.
Analysis of OPP funding data for CMB-related research provided by NSF shows highly variable year-to-year funding, but a generally flat funding trend over time (2011-2020). Over the period 2016-2020, NSF awarded $4.3 million/year on average for CMB research (in actual year dollars). NSF reports that OPP funding for CMB research reflects about 7 percent of the total Antarctic Sciences budget, on average. Within NSF, the Physics and Astronomy Divisions also contribute research funding to CMB research projects, although less than OPP (P. Cutler, NSF, personal communication, 2020). Support for logistics and facilities associated with equipment deployment, installation, and field-based research at the South Pole is also the responsibility of NSF OPP. Per-project logistics budget information is not available.
The CMB-S4 program office in 2019 estimated the need for approximately $250 million (including 35 percent contingency and in-kind contributions) from NSF between 2019 and 2029, envisioned to come largely through the Major Research Equipment and Facilities Construction (MREFC) program.3 Several steps remain for approval of NSF major facility funding, including admission to the Design Stage, passing Conceptual, Preliminary, and Final Design Reviews, authorization by the NSF National Science Board for inclusion in a budget request to Congress, and appropriation from Congress (NSF, 2019). DOE is expected to be a major funding contributor for the CMB-S4 project, and the CMB-S4 program office estimated the need for $350 million (including contingency and in-kind contributions) from DOE through 2029.2 The fiscal year 2021 DOE budget includes $6 million in appropriations
3 See https://science.osti.gov/-/media/hep/hepap/pdf/201911/11-Carlstrom_CMB-S4.pdf?la=en&hash=26D7C9BE6D718A9F98197B2DF3805361142EAD47.
for research and development (R&D) and project management, including $1 million in initial equipment funding. DOE staff noted that its “slow ramp up of funding compared to [the] project’s request … limits the planned R&D, especially on detectors and readout.” DOE also noted long-term challenges ahead in synchronizing NSF and DOE support for the project.4
Scientific Community and Partnerships
CMB research successes have been supported by highly effective collaboration and self-organization within the research community. Large, collaborative research efforts are common within cosmology because of the nature of the equipment requirements. The BICEP/Keck program is a collaboration of 70 researchers, ranging from graduate students to senior faculty, at 10 universities and laboratories. The SPT collaboration consists of 120 researchers employed at 20 different universities and labs. A recent joint analysis effort in which SPT data are used to remove the effects of gravitational lensing from the BICEP/Keck maps provides an example of the collaborative research environment that is planned for CMB-S4 (BICEP/Keck and SPTpol Collaborations, 2021).
The CMB-S4 developed out of the Snowmass planning process in 2013, a year-long, community-wide study to identify exciting opportunities in their field of particle physics organized by the American Physical Society’s Division of Particles and Fields. The CMB-S4 effort gained momentum when recommended by the Particle Physics Project Prioritization Panel (P5, 2014). The CMB-S4 collaboration was officially established in 2018, along with an Integrated Project Office. As of December 2020, the collaboration included more than 200 scientists and engineers across 14 countries and more than 80 institutions.5 Collaboration members include scientists across several national laboratories and federal agencies, such as the National Institute of Standards and Technology and NASA.
Although the CMB research community has similar diversity, equity, and inclusion challenges as the overall scientific community, it is working actively to engage junior researchers and provide them with positions of leadership. For example, the SPT project has established a junior scientist advancement committee and a mentoring program where senior researchers are randomly paired with junior researchers (A. Bender, Argonne National Laboratory, personal communication, 2021).
Multiple agencies are working to collectively support CMB-S4. As discussed previously, DOE is an important funding contributor in support of CMB-S4. DOE recently named Lawrence Berkeley National Laboratory as the lead laboratory to carry out the DOE responsibilities (Roberts, 2020). NASA has also supported efforts to improve detection capabilities.
4 See https://nsf.gov/attachments/300822/public/2_DOE_HEP_Programs_and_Budget_Update_Kathy_Turner.pdf.
5 See https://cmb-s4.uchicago.edu/wiki/index.php/Current_Membership_List.
KEY IMPLEMENTATION CHALLENGES
As noted above, good progress has been achieved in CMB research, but program managers reported that more progress could have been achieved were it not for infrastructure and logistics issues and constraints. Overall, the South Pole CMB project participants noted that logistics support for CMB projects has declined since the 2015 report. Over the past several years, there has been a considerable reduction in LC-130 flights to the pole (see Box 1-2), which researchers note is challenging for the current pace of implementation. Researchers also noted that the limited time window provided to CMB balloon experimenters does not allow sufficient time to fully prepare and test payloads before launch (see Figure 4-3). Other specific concerns include aging facilities and equipment (e.g., power plant) and delayed maintenance at the South Pole. For example, requests to raise the Martin A. Pomerantz Observatory (MAPO) building, including in NASEM (2015), have gone unmet for a decade, causing increased expenditures for snow removal to maintain access.
There are several logistics challenges ahead. Because the tower and MAPO building on which the existing BICEP Array receiver is mounted are being buried in snow, NSF funding was secured to design and build the BICEP Array Replacement Tower (BART). Importantly, the BART team has not been included in logistic discussions regarding raising MAPO, with which it is directly linked. Inclusion of the BART team in early logistical conversations ensures that planning for both projects is consistent and properly coordinated.
The COVID-19 pandemic has caused 1 year of delay in both instrument readiness and deployment of the BICEP Array, and with OPP logistics resources spread thin, the project team is concerned about further delays in the schedule for deploying the replacement tower and the remaining receivers.
Once BART is deployed, a second tower will be built that will be connected to the MAPO building. This is the first of a number of towers that will need to be built for CMB-S4. The MAPO building will need to be raised before foundational work for CMB-S4 telescope installation can be completed, making it a critical step in the CMB-S4 construction timeline. However, the timeline for raising the MAPO building remains uncertain. The MAPO building, which contains essential computers and laboratory space, will continue to be a very important building for the current CMB experiments and, along with the construction of a second building, is required for CMB-S4.
The new telescope that the SPT collaboration has designed (i.e., TMA), which will also serve as part of the CMB-S4 facility, employs monolithic 5-meter-diameter mirrors. Getting these mirrors to the South Pole is a logistics challenge. The solution may well involve the new capacity of overland traverse, currently limited to nonscience payloads. Researchers expressed the concern that the deployment needs for CMB-S4 will exceed NSF’s logistics support capacity and implementation schedules will be delayed.
The instruments and the instrumentation envisioned for CMB-S4 would also put a strain on the system beyond the capacity of the present, aging infrastructure. For example, power upgrades will be needed at the South Pole Station, but the scientists report that they have not been invited to play a role in forming the plans for the South Pole Station, despite the large load that the CMB experiments are expected to place on the aging infrastructure.
OPPORTUNITIES TO IMPROVE PROGRESS
Transportation is identified as one of the major areas where there are opportunities for improvement. Researchers involved in the projects are concerned that the number of LC-130 flights to the South Pole will be insufficient to support deployment of infrastructure according to their planned schedule and that limited logistics capacity will delay the implementation of CMB-S4. Development of a science traverse to remote field sites could benefit a wide variety of projects. A traverse might also be able to operate during conditions that prevent aircraft operations. However, initiating a science traverse will require considerable planning and coordination with various research activities located in remote regions.
Currently, principal investigators have a poor understanding of how NSF prioritizes logistics operations and scheduling. Improved two-way communication between NSF OPP operations and CMB scientists during project development can
help both sides understand the implementation alternatives, which will help ease constraints. Maintenance and logistics for a very large project such as CMB-S4 can be a large perturbation of the total South Pole Station effort, and early communication of capacity, costs, and schedule is crucial.
Given the large infrastructure requirements of CMB-S4, it is critical that science teams be informed in the ongoing drafting of the South Pole Master Plan, even as the MREFC process unfolds. CMB science teams should also be consulted in planning for upgrades and maintenance of shared capacity at the South Pole Station for items such as the power generator. Opportunities to improve logistics across all priorities are discussed further in Chapter 6.
CONCLUSIONS AND RECOMMENDATIONS
CMB research at the South Pole has been flourishing for many years, contributing to our understanding of the origin of the universe and strengthening the scientific leadership of the United States in this field. There are two major existing CMB experimental programs at the South Pole: the SPT and BICEP/Keck. NSF has succeeded in enabling groundbreaking research from one of the primary CMB sites in the world. High-impact results are anticipated from the continuation of these experiments. The BICEP Array, which is midway through construction, will provide a substantial step forward in sensitivity, and coordinated data analysis with the SPT will further enhance this sensitivity by reducing distortion of light by gravitational fields. A replacement telescope for the SPT has been proposed, called the TMA, which is designed specifically to enhance a coordinated data analysis effort between the TMA and the BICEP/Keck collaborations.
The scientific community has made significant progress in the design of CMB-S4, considered the global future of CMB research. The next-generation CMB experiment, CMB-S4, which is a central component of Priority III, includes telescopes at both the South Pole and in Chile. CMB-S4 would provide another order-of-magnitude step forward in the search for the imprints of gravitational waves from the expected inflationary origin of the universe. The technological advances in telescopes and instrumentation that have been made by the SPT and BICEP Array experiments are paving the way for CMB-S4. This is a long-range project; the scientific community has set an installation target of 2029, and NSF has played a critical role in supporting the design.
Inadequate levels of funding and planning for logistical support pose a threat to the rate of progress of ongoing Antarctic CMB research and the pace of future construction of the Antarctic CMB-S4 components. The leaders of both major ground-based efforts report scaling back their plans each season to match the transport and personnel allowances offered to them by NSF. Additional design work and effort by the scientific team are expended to work around delays in maintenance and upgrades to the existing towers and buildings. These issues are an even larger concern for CMB-S4, which would require the efficient deployment of new towers
and telescopes and substantial upgrades to the South Pole power plant. Details of how to transport the key components of this program to the South Pole and how to assemble them are a necessary part of the CMB-S4 planning and design process. Any uncertainty in the ability of the South Pole Station to support this effort or the approaches for equipment transportation could be a major hindrance of the CMB-S4 development process. Effective communication and collaboration between NSF logistics and the CMB-S4 planning team will be essential for future timely implementation. Principal researchers of the ground-based Antarctic CMB experiments and of the CMB balloon efforts report that the logistics discussions are not adequate, which hampers planning. NSF should support timely two-way communication between NSF logistics planning teams and scientists, in which tradeoffs between support options are explored. These discussions need to occur earlier in the experiment planning process so that effects on research plans and designs can be incorporated in a timely way.