CMB-S4 Science Book, First Edition

Abstract

The next generation "Stage-4" ground-based cosmic microwave background (CMB) experiment, CMB-S4, consisting of dedicated telescopes equipped with highly sensitive superconducting cameras operating at the South Pole, the high Chilean Atacama plateau, and possibly northern hemisphere sites, will provide a dramatic leap forward in our understanding of the fundamental nature of space and time and the evolution of the Universe. CMB-S4 will be designed to cross critical thresholds in testing inflation, determining the number and masses of the neutrinos, constraining possible new light relic particles, providing precise constraints on the nature of dark energy, and testing general relativity on large scales.CMB-S4 is intended to be the definitive ground-based CMB project. It will deliver a highly constraining data set with which any model for the origin of the primordial fluctuations-be it inflation or an alternative theory-and their evolution to the structure seen in the Universe today must be consistent. While we have learned a great deal from CMB measurements, including discoveries that have pointed the way to new physics, we have only begun to tap the information encoded in CMB polarization, CMB lensing and other secondary effects. The discovery space from these and other yet to be imagined effects will be maximized by designing CMB-S4 to produce high-fidelity maps, which will also ensure enormous legacy value for CMB-S4. CMB-S4 is the logical successor to the Stage-3 CMB projects which will operate over the next few years. For maximum impact, CMB-S4 should be implemented on a schedule that allows a transition from Stage 3 to Stage 4 that is as seamless and as timely as possible, preserving the expertise in the community and ensuring a continued stream of CMB science results. This timing is also necessary to ensure the optimum synergistic enhancement of the science return from contemporaneous optical surveys (e.g., LSST, DESI, Euclid and WFIRST). Information learned from the ongoing Stage-3 experiments can be easily incorporated into CMB-S4 with little or no impact on its design. In particular, additional information on the properties of Galactic foregrounds would inform the detailed distribution of detectors among frequency bands in CMB-S4. The sensitivity and fidelity of the multiple band foreground measurements needed to realize the goals of CMB-S4 will be provided by CMB-S4 itself, at frequencies just below and above those of the main CMB channels. This timeline is possible because CMB-S4 will use proven existing technology that has been developed and demonstrated by the CMB experimental groups over the last decade. There are, to be sure, considerable technical challenges presented by the required scaling-up of the instrumentation and by the scope and complexity of the data analysis and interpretation. CMB-S4 will require: scaled-up superconducting detector arrays with well-understood and robust material properties and processing techniques; high-throughput mmwave telescopes and optics with unprecedented precisi...

Improved limits on the tensor-to-scalar ratio using BICEP and Planck

“…We note that future re-analyses of the Planck data are not anticipated to provide much improvement, while ground-based experiments (such as BICEP/Keck, the Simons Observatory [30], and later CMB-S4 [31]) will observe the sky with ever deeper sensitivity, placing even stronger constraints on the tensor-to-scalar ratio r (or detecting primordial B modes of course). However, improved measurements of the reionization optical depth require very large scales, which are very difficult to measure from ground.…”

MUSE: Marginal Unbiased Score Expansion and Application to CMB Lensing

“…Specifically, we will consider a 1500 deg 2 region with the noise levels and masking expected for the upcoming SPT-3G survey [36, specification given in Table . I]. This is also similar in size to the region expected to be probed (to deeper noise levels) with the CMB-S4 survey for the main purpose of primordial gravitational wave detection [14]. We continue to work in the flat-sky approximation, which begins to break down near patches of sky of this size.…”

“…However, as first shown by Hirata and Seljak [11,12] and Seljak and Hirata [13], at noise thresholds which are currently being crossed by the most sensitive experiments, the QE ceases to be near-optimal and Bayesian methods which fully extract all-orders information from the data can yield significantly better results. At the noise levels of the planned CMB-S4 experiment [14], this includes reconstructing the gravitational lensing field to ∼ 10 times lower noise levels [15] and yielding delensed maps of B modes which allow ∼ 3 times better constraints on the amplitude of primordial gravitational waves, r [16].…”

Gravitational dark matter: free streaming and phase space distribution

“…Regarding the visible components, we have acquired and inculcated a great deal of knowledge about its very existence and fundamental properties. However, apart from the existential evidences through multiple observations such as galaxy rotation curve, large scale structure, CMB [1,[9][10][11][12][13], invisible components are far from our present understanding. Dark matter is one of the invisible components which attracts lot of attention due to its seemingly unavoidable entente with the visible components in quantum field theoretic framework .…”