Cosmic Microwave Background—Stage 4 (CMB-S4)

The fourth-generation ground-based cosmic microwave background (CMB) experiment, or CMB-S4, consists of several dedicated telescopes equipped with highly sensitive superconducting cameras. These telescopes will spend seven years listening to the microwave sky at two locations already recognized for their suitability: the South Pole, which will host several telescopes of varying sizes that will observe across a wide range of microwave frequencies; and the Atacama Plateau in Chile, a high-desert site that will host two large telescopes that can also observe several different frequencies. The South Pole telescopes will conduct an ultra-deep survey of 3% of the sky, while the Atacama telescopes will conduct a complementary ultra-wide and deep survey of 70% of the sky. Together, the two sites promise to 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 have four scientific goals:

  1. Search for primordial gravitational waves and other evidence of inflation;
  2. Study the dark Universe;
  3. Map matter in the cosmos;
  4. Explore the time-variable millimeter-wave sky.

Search for primordial gravitational waves and other evidence of inflation

CMB-S4 will be able to detect the signature of primordial gravitational waves set in motion by quantum fluctuations magnified by the rapidly inflating Universe, as predicted by some of the best-motivated models of our cosmic origins.

With an order of magnitude more detectors than previous ground-based CMB experiments and exquisite control of systematic errors, CMB-S4 will reduce the limits on earlier observations by a factor of five, enabling either the direct detection of primordial gravitational waves or ruling out large classes of inflationary models and dramatically impacting current thought on cosmic inflation.

Study the dark Universe

The sensitivity of CMB-S4 detectors will enable searches for previously undiscovered light relic particles, which may have formed as the initial extreme energies of the early Universe cooled, provides an opportunity for researchers to fine-tune their totals of dark matter versus baryonic matter. CMB-S4 can also contribute to the ongoing effort to map all the matter in the Universe (see below), a vital part of tracing its expansion history and from there tracing the changing influence of dark energy during the past 13.75 billion years.

Map matter in the cosmos

According to observations, there is roughly five times more dark matter than baryonic matter and most of that baryonic matter is in the form of hot ionized gas rather than cold gas or stars. CMB-S4 will be able to map out normal and dark matter separately by measuring the fluctuations in the total mass density (using gravitational lensing) and the ionized gas density (using Compton scattering).

In addition, the precise 2-D map of total matter distribution provided by CMB-S4 lensing data will provide important constraints on dark energy, modified gravity, and neutrino masses, while the maps of ionized gas will provide insights into galactic evolution, especially during the heyday of star and galaxy formation around three billion years after the Big Bang.

 Explore the time-variable millimeter wave sky

Targeted follow-up observations of gamma-ray bursts, core-collapse supernovae, tidal disruption events, classical novae, X-ray binaries, and stellar flares have found a wealth of transient events that would be detectable by CMB-S4, but to date there has been only one systematic survey of the variable sky in millimeter wavelengths. In addition to such far-distant events, thermal emissions from a variety of solar system bodies provide another target for the CMB-S4 survey, bringing its contributions to astrophysics and cosmology a little closer to home.

The legacy of CMB-S4

CMB-S4 will join a long line of ground-based CMB experiments with KIPAC involvement, including several BICEP experiments, the Simons Observatory, and the South Pole Telescope, but the science it promises to deliver will place it in a class by itself.

The first two goals of CMB-S4 deal with the deepest, most fundamental aspects of our Universe at its inception. But science goals 3 and 4 will provide a wealth of data for researchers studying all phases and time periods of cosmic evolution, making CMB-S4 a valuable partner in multimessenger experiments.

With a goal of beginning construction in 2023 and science operations beginning in 2029, CMB-S4 could take its place among the great surveys that continue to broaden our view of our Universe.

For more information about the CMB-S4 project, see CMB-S4: Next Generation CMB Experiment.