CMB-S4 is an international scientific collaboration with the goal of unlocking some of the last great mysteries that shroud the origin of the cosmos. Funded by the U.S. Department of Energy and National Science Foundation, the collaboration comprises a suite of new instruments and new experiments that will allow researchers to peer farther into the universe’s past than ever before. These new capabilities allow CMB-S4 to gather data that promises not only to advance the frontiers of cosmology, but also to enlarge and enhance the science being done in multiple fields of astronomy.

The rich and diverse collection of scientific questions CMB-S4 was built to address can be organized into four key themes:

The primary mission of CMB-S4 is the search for primordial gravitational waves. The current leading explanation of the origin of large-scale structure in the universe centers on cosmic inflation, a period of accelerated expansion early in the universe’s history. Models of this period predict that it would have produced gravitational waves, ripples in the fabric of spacetime, that should be observable today in the polarization pattern of the Cosmic Microwave Background. The challenge lies in designing and building an instrument with the necessary sensitivity to detect these faint signals.

As the CMB light starts traveling freely through the universe at a few hundred thousand years after the big bang, it passes through the gravitational waves. As space stretches and compresses, it distorts the pattern of polarization, creating a signature called a B-mode. Detection of this signature is challenging due to its small amplitude (temperature changes as small as a billionth of a degree) and the presence of two distinct contaminants: B-modes from another cosmological process called gravitational lensing and B-modes from astrophysical processes in our own galaxy.

If primordial gravitational waves are detected, it would be one of the greatest breakthroughs in cosmology since the discovery of the CMB itself. Detection would not only provide observational confirmation of one of cosmology’s most important theoretical frameworks, it would also offer the first evidence for the quantization of gravity, and reveal something fundamental about physics at the highest energies, a discovery which may point the way toward a grand unified theory.

Beyond its role probing the earliest instants of time and the furthest reaches of space, the capabilities of CMB-S4 will offer the opportunity to do many other types of astronomy, including some much closer to home. Because our instruments are built to explore a very specific region of the electromagnetic spectrum, we can use them to scan that region for signals from non-cosmological sources.

In particular, CMB-S4 will allow us to capture our first dynamic images of the sky at millimeter wavelengths, capturing not just snapshots, but moving pictures, revealing phenomena that vary over time with a precision and clarity unmatched by any previous experiment at these wavelengths. Examples of these time-variable phenomena include blazars, x-ray binaries, core collapse supernovae, and more. In addition to these distant sources, time-variable data from these wavelengths can be used to look for thermal signals from asteroids, dwarf planets, or other large bodies here in our solar system, and the long baseline of data from CMB-S4 will enhance both current and future experiments in multi-messenger astronomy.

Many critical questions in cosmology rely on a detailed understanding of how matter is distributed throughout the universe. Yet observing that matter is complicated; over 80% of it is now known to be dark matter, and much of the rest is in the form of a diffuse ionized plasma, which, like water vapor in air, is almost invisible. CMB-S4 will enable the construction of detailed maps of the distribution of this matter throughout the cosmos, measuring both the distribution of dark matter through gravitational lensing of the CMB, and the density of the ionized plasma through Compton scattering.

Of particular relevance to this work is the era when galactic clusters were both still accreting new material, and actively creating the bulk of their stars. CMB-S4 increases the catalog of clusters from this era by an order of magnitude over the previous generation of experiments.

According to the standard model of cosmology, dark matter and dark energy together comprise 95% of the energy density in the universe. Yet numerous questions about dark matter and dark energy remain. Is there a large-scale structure to the distribution of matter? Does the dark matter interact with baryonic matter beyond gravitational influence? Most importantly, are dark matter and dark energy the only two ingredients, or could there be another we have not yet seen?

Many extensions to the standard model of particle physics lead to the production of light particles in the big bang, that would contribute to the total energy density and have observable consequences in both the temperature and polarization anisotropy of the CMB. While current instruments provide sensitivity to particles produced in the first tens of seconds, with CMB-S4 we will achieve sensitivity to particles produced much earlier, in the first billionths of a second, greatly expanding our discovery potential.