CMB-S4 Science Book, First Edition

TL;DR

CMB-S4 aims to establish a definitive, large-scale ground-based CMB program with multi-site coverage and massive detector counts to push the boundaries of inflationary physics, neutrino properties, light relics, dark matter, and dark energy. The book presents a coherent design framework linking high-sensitivity, multi-frequency measurements and delensing strategies to robust forecasts for r, N_eff, Σmν, and beyond, while detailing the data-analysis, simulations, and forecasting pipelines needed to interpret the data. It emphasizes the crucial role of foreground characterization, instrument cross-calibration, and cross-survey synergy (e.g., with LSST, DESI) in extracting precise cosmological constraints and testing fundamental physics. Even in the absence of a primordial B-mode detection, CMB-S4 is positioned to dramatically tighten limits and falsify broad classes of inflationary models, while enabling unparalleled tests of gravity, neutrino physics, and the early Universe’s thermal history. The work also outlines the technical roadmap from Stage-3 to Stage-4, the necessary computational infrastructure, and the community-wide collaboration required to realize this transformative observatory.

Abstract

This book lays out the scientific goals to be addressed by the next-generation ground-based cosmic microwave background experiment, CMB-S4, envisioned to consist of dedicated telescopes at the South Pole, the high Chilean Atacama plateau and possibly a northern hemisphere site, all equipped with new superconducting cameras. CMB-S4 will dramatically advance cosmological studies by crossing critical thresholds in the search for the B-mode polarization signature of primordial gravitational waves, in the determination of the number and masses of the neutrinos, in the search for evidence of new light relics, in constraining the nature of dark energy, and in testing general relativity on large scales.

Figures (62)

• Figure 1-1: Current measurements of the angular power spectrum of the CMB temperature and polarization anisotropy. The horizontal axis is scaled logarithmically in multipole $\ell$ left of the vertical dashed line ($\ell < 30$) and as $\ell^{0.6}$ at higher multipole. Best-fit models of residual foregrounds plus primary CMB anisotropy power for TT datasets are also plotted. To illustrate the expected improvements with CMB-S4, the projections for a strawman instrumental configuration are shown in grey (binned with $\Delta\ell = 5$ for TT and EE spectra and $\Delta\ell = 30$ for BB) for a $\Lambda$CDM with $r =0$ cosmological model.
• Figure 1-2: Plot illustrating the evolution of the raw sensitivity of CMB experiments, which scales as the total number of bolometers. Ground-based CMB experiments are classified into Stages with Stage II experiments having $O$(1000) detectors, Stage III experiments having $O$(10,000) detectors, and a Stage IV experiment (such as CMB-S4) having $O$(100,000) detectors. Figure from Snowmass CF5 Neutrino planning document.
• Figure 1-3: Schematic timeline showing the expected increase in sensitivity ($\mu$K$^2$) and the corresponding improvement for a few of the key cosmological parameters for Stage-3, along with the threshold-crossing aspirational goals targeted for CMB-S4.
• Figure 1-4: Schematic chart showing how scientific findings (yellow circles), technical advances (blue circles) and satellite decisions by various agencies (green circles) would affect the science goals, the survey strategy and possibly the design of CMB-S4 (green boxes)
• Figure 2-5: Left and center panels: In an expanding universe, the distance between two separated points increases over time, simply due to the expansion of the space between them. The two panels here show the spacetime trajectories of two comoving points, A and B. For the decelerating expansion illustrated in the left panel, the separation rate is greater in the past and even exceeds the speed of light at sufficiently early times. Thus A and B go from being out of causal contact---unable to influence each other---to being in causal contact. In an accelerating universe, the separation rate is smaller in the past. The two points go from being in causal contact to being out of causal contact. In the inflationary universe scenario, an early epoch of acceleration---the inflationary era---smoothly maps onto a long period of deceleration. Thus two points can go from being in causal contact to out of causal contact and, much later, back into causal contact. Right panel: Fluctuations in the value of the inflaton field, which is responsible for the accelerating expansion of the cosmos, evolve differently, depending on whether their wavelength $\lambda$ is less than or greater than the horizon length $L = c/H$. When $\lambda \ll L$, the uncertainty principle limits how smooth the field can be. As a result, the amplitude of the fluctuation is inversely proportional to $\lambda$ and thus decreases as the Universe expands (and the influence of the uncertainty principle is reflected by the appearance of Planck's constant $\hbar$ in the expression for the amplitude). As $\lambda$ becomes larger than the horizon, the crests and troughs of the wave cease to be in causal contact, so the amplitude stops evolving. For superhorizon evolution, the asymptotic value of the amplitude corresponds to replacing the wavelength in the subhorizon case with $2\pi L$. Eventually, cosmic expansion stretches the fluctuations to astrophysically large length scales. Elsewhere in this document $\hbar$ and $c$ are set equal to unity.