The US Department of Energy (DOE), Office of High Energy Physics (HEP), and the National Science Foundation (NSF) Divisions of Astronomical Sciences (AST), Physics (PHY), and Polar Programs (PLR) request that the Astronomy and Astrophysics Advisory Committee (AAAC) establish a Cosmic Microwave Background Stage 4 Concept Definition Task force (CMB-S4 CDT) as a subcommittee in order to develop a concept for a CMB-S4 experiment.

Ground-based CMB generally falls under the purview of NSF and DOE, while NASA supports CMB projects within its long-duration balloon program and space missions. In its 2016 report, the interagency AAAC recommended the following: “We encourage DOE, NSF, and the university community to continue working toward a plan for a future (Stage 4) ground based CMB experiment.” Following that recommendation and other community input, NSF and DOE are requesting that the AAAC establish this CDT. The creation of the CDT also is in response to the favorable comments or recommendations on CMB science that have been made by community advisory groups over the past decade:

• 2010, Astronomy and Astrophysics Decadal Survey (charged by NASA Astrophysics, NSF-AST, and DOE- HEP): CMB projects are among the “projects thought compelling for a [competed] Mid-Scale Innovations Program” in NSF and ''The committee recommends... continuing steps consistent with the DOE mission''
  
• 2014, Particle Physics Project Prioritization Panel (P5) of the High Energy Physics Advisory Panel (HEPAP) (charged by DOE-HEP and NSF Directorate for Mathematical and Physical Sciences, which includes NSF- PHY and NSF-AST): “Support CMB experiments as part of the core particle physics program. The multidisciplinary nature of the science warrants continued multi-agency support”
  
• 2015, National Academies report on “A Strategic Vision for NSF Investments in Antarctic and Southern Ocean Research” (charged by NSF-PLR): Identified continuation of studies of the Cosmic Microwave Background as one out of three strategic priorities
  

These community reports identify CMB as an important scientific priority for consideration by DOE-HEP, NSF- AST, NSF-PHY, and NSF-PLR, hence providing additional rationale for the CDT activity.

The Snowmass 2013 process brought together the U.S.-based CMB community, including many of the current experimental teams, and began to define a coordinated next generation experiment which was termed CMB- S4.

The CMB-S4 community-based collaboration has held semiannual meetings and produced a substantial CMB Science Book justifying the CMB-S4 science case.

DOE-HEP national laboratories and some university groups are already actively engaged in technology development for CMB-S4.

The CMB-S4 CDT is asked to develop a concept for implementing a ground-based CMB-S4 experiment. The CDT will take as input the community CMB-S4 Science Book and any further community information as appropriate, and will consider the global landscape of CMB experiments (including ground, balloons, and space).

Specifically, the CDT is asked to deliver:

• A summary of the Science Requirements and their rationale
  
• Technical Requirements derived from the Science Requirements
  
• Project Strawman Concept
  
• Options and Alternatives (prioritized to the extent possible) for:
  
  
  
  • Concept design (e.g. sites, telescopes, detectors)
    
  • Concept staging and schedule
    
  • Collaboration and Data models and interfaces
    
  
  
• R&D development needed, with priorities, to demonstrate technical readiness
  
• Cost ranges for the strawman concept, including explanations for how they were developed.
  

The CDT should provide a progress report to the AAAC by May 2017 and a final report to AAAC by August 2017 for consideration and approval. In accordance with Federal Advisory Committee Act (FACA) rules, this report will be discussed and approved by the AAAC before formal transmittal to the agencies.

Report [PDF]

This report presents a concept for implementing a ground-based Cosmic Microwave Background Stage-4 experiment (CMB-S4) designed to tackle questions about fundamental physics from the earliest moments in the Universe through to the epoch of dark energy domination. CMB-S4 will search for in ation at one end of the energy scale, and for sub-eV particles at the other, including neutrinos and as-yet-undiscovered light relics. CMB-S4 data will trace out the behavior of gravity across cosmological distances and anchor our understanding of how structures form under the gravitational collapse of dark matter moderated by the in uence of baryons. These aims require CMB measurements with sensitivity, precision, and accuracy far beyond those obtained to date, roughly two orders of magnitude more sensitive than the summation of all measurements acquired so far, and an order of magnitude more sensitive than Stage-3 experiments. To address this challenge, the CMB community has come together to advocate a single comprehensive CMB-S4 experiment, which has been endorsed by the 2014 report of the Particle Physics Project Prioritization Panel (P5) Building for Discovery and the 2015 NAS/NRC report A Strategic Vision for NSF Investments in Antarctic and Southern Ocean Research.Report [PDF]