Capturing the Universe's Oldest Light as Never Before

Through eons of time and across billions of light-years of space, it still whispers to us: the Big Bang, the origin of all things. We first learned of these whispers quite by accident in 1964, when researchers originally assumed that an unexplained noise detected from all over the sky was caused by bird droppings spattered on their receiver antenna. Upon further investigation, the noise equated to omnipresent microwave energy, just a whisker above absolute zero. We have since come to refer to this energy as the cosmic microwave background, or CMB. Observations and theory have aligned in revealing that the CMB is the earliest light produced in the universe—the relic radiation of the Big Bang.

Increasingly sophisticated studies of the CMB have teased out ever more precise details about the Big Bang's unfolding and the composition of the cosmos. Researchers are now planning to take the science ever further with a project called CMB-S4. It represents a fourth generation of ground-based CMB telescopes, building upon the insights of its predecessors and expected to deliver potentially breakthrough findings. Those predecessors include experiments such as the South Pole Telescope (SPT, and which has Kavli astrophysics institute researcher involvement) erected in the heart of Antarctica, as well as telescopes in the Atacama Desert in Chile. With their clear skies, high altitudes, and ultra-low atmospheric water vapor, these sites are among the best places on the planet for astronomy and especially for microwave observations.

The CMB-S4 project calls for placing more than 500,000 cryogenically cooled superconducting detectors at 21 new telescopes at the South Pole and in Chile. The former site will focus on just 3% of the sky, drilling deep to stack observations and gather faint CMB signals, while the latter will more shallowly cover a huge swath—about 70%—of the firmament, compiling statistics about the CMB in many directions and complementing the South Pole dataset.

"We're really looking forward to what CMB-S4 can do," says John Carlstrom, a senior member of the Kavli Institute for Cosmological Physics (KICP) at the University of Chicago. Carlstrom is the Project Scientist and has served as a co-spokesperson for CMB-S4. "We know just how good the South Pole is for digging deep," Carlstrom adds, "and Chile for conducting an ultra-wide survey. These will be wonderful datasets to combine."

The science case for CMB-S4 is extremely compelling, says Carlstrom, commenting that "there's a whole book written on the science you can do with CMB-S4." The project was accordingly ranked highly in the most recent Astronomy and Astrophysics Decadal Survey, prepared every 10 years by the United States National Academies of Sciences, Engineering, and Medicine and reflects the priorities of the broad astronomy and astrophysics scientific community. Development is continuing apace on the project, which has been in the works for approximately a decade and is aiming for first observations early in the 2030s.

The natural scientific starting place for CMB-S4 is unprecedently probing the Big Bang itself. The project would specifically look for evidence of a well-regarded, yet speculative theory called cosmic inflation. This theory posits that the universe, some mere 10-36 seconds into its formation, entered a fleeting epoch of exponential expansion, rapidly growing in size with space expanding faster than the speed of light before resuming a more gradual blossoming. If this inflation occurred, it would neatly explain numerous observational realities, including the large-scale structure of the cosmos, the isotropy (sameness) of the universe in all directions, and the relatively even distribution of the CMB.

The "smoking gun" of cosmic inflation would be certain gravitational waves signatures imprinted on the CMB. With 10 times the number of detectors as third-generation ground-based CMB telescopes, CMB-S4 would provide the most stringent search yet for these signatures, called B-modes. Carlstrom explains that CMB-S4 can rule out entire classes of the models for inflation with non-detections, though the hope is that the B-modes are directly seen. "Either we detect [B-modes] or something quite different is going on and we're back to the blackboard," says Carlstrom.

Characterizing the CMB with greater precision will also help nail down the composition and evolution of the cosmos. "By determining the energy density of the early universe precisely and accurately with CMB measurements," says Carlstrom, "we can constrain models of how the universe evolved and what stuff's in it." Such precise measurements could ferret out the existence of hypothesized ultralight particles that could in turn comprise dark matter. "A lot of physics models essentially predict new particles," says Carlstrom. "If any of these new particles were naturally created in the early universe, it will impact what we measure for the CMB. You can't hide them."

Over its intended seven-year operational lifetime, CMB-S4 will build up a rich map of the matter (both normal and dark) strewn throughout the cosmos. The data will advance our understanding of the evolution of structure formation in the universe. In this way, CMB-S4 will track the influence over cosmic history of dark energy, an even more substantial and mysterious entity than dark matter that is accelerating the universe's expansion.

The science returns don't stop there. Innovatively, by frequently gathering data at its two sites and in collaboration with existing and other planned instruments, CMB-S4 will capture "transient" or short-lived astrophysical events at millimeter wavelengths. To an extent, the project would therefore do for the millimeter sky what the Legacy Survey of Space and Time (LSST)—slated to run at the Vera Rubin Observatory in coming years—will do for the optical sky. "Doing all these millimeter-wave measurements of the same sky every day allows us to open up a new window the transient universe, like the Rubin Observatory’s LSST does at optical wavelengths," says Carlstrom.

More than 300 international scientists have joined the CMB-S4 collaboration across a range of astrophysical specialties, indicating the project's expansive scientific portfolio.

"CMB-S4 will revolutionize and totally transform the CMB by really benefitting the whole community," says Carlstrom. "It's super-exciting, but we have a lot of work to do."