Amid simmering anxiety about the future of federally funded science, the U.S. government has quietly withdrawn support for cosmology’s next premier project, an experiment that would have given us the best read yet of the strangest chapter in our cosmic origin story.
Called CMB-S4—or Cosmic Microwave Background Stage 4—the project would have used a suite of new radio telescopes, constructed in Antarctica and Chile, to search the big bang’s faint, ancient afterglow for new clues about the universe’s earliest moments. First conceived in 2013 and repeatedly ranked as a top priority by the nation’s astronomers and physicists, the project carried an estimated $900-million price tag, which was set to be roughly split between U.S. Department of Energy and the National Science Foundation (NSF).
Yet in a terse, unsigned statement to project leaders on July 10, the two agencies declared they had “jointly decided that they can no longer support the CMB-S4 project.”
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“We knew things weren’t looking good,” says John Carlstrom of the University of Chicago, the project’s principal investigator. “They had warned us that it was not the time to start any big projects, given all the budget areas and all the uncertainty. But whether they would continue to drag us out or have a clean break or try to do something—that was unknown.”
Without federal support, Carlstrom says, the project is essentially canceled. Although abrupt, the decision follows years of concern about the decay of U.S. Antarctic scientific infrastructure, exacerbated by hesitations from both agencies about starting big new projects in the face of ongoing federal budget uncertainty.
Still, the decision “is a tremendous loss for science and also for the U.S. as the leader in science. It’s a big blow to the community,” says Hitoshi Murayama, a physicist at the University of California, Berkeley, and chair of the once-a-decade Particle Physics Project Prioritization Panel, which in 2023 ranked CMB-S4 as the highest priority for construction. The project also received a high ranking in 2021 from the U.S. astronomy and astrophysics community via a similar but separate process, the Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020).
“If the agencies are really saying it’s over, scientifically, that’s awful,” says Joel Parriott, the American Astronomical Society’s director of external affairs and public policy. “And for the people involved, that’s devastating.”
Ripples in the Dark
The universe burst into existence some 13.8 billion years ago, and the unfolding sweep of cosmic evolution eventually led to us.
“What kind of universe created intelligent beings that can go and look at the first instants and understand how everything evolved?” Carlstrom says.
Perhaps the most curious aspect of that beginning is the wealth of circumstantial evidence suggesting that in the first minuscule moments after the big bang, the universe underwent an almost inconceivably exponential expansion—a ballooning so violent it shook the fabric of the newborn cosmos. This “cosmic inflation” would have left indelible ripples in spacetime called primordial gravitational waves. Today they would be visible as subtle, swirly fingerprints in the big bang’s relict radiation. Known as the cosmic microwave background, or CMB, that radiation is a diffuse light that permeates all of space. CMB-S4’s flagship objective was to detect those signature swirls in the CMB and clinch the case for cosmic inflation.
“There are a few different signatures that would be consistent with inflation, and some of them we’ve already seen,” says Jo Dunkley, an astrophysicist at Princeton University. “But the tensor fluctuation—these gravitational waves—they would be much stronger evidence than we currently have.”
First observed in 1965, the CMB has become a treasure trove for cosmologists, who use it to look back in time and study the characteristics of the very early universe. But even after 60 years of observations, we’ve only scratched the surface of what’s possible to see in this faint light. With CMB-S4 and other next-generation observatories, scientists have aimed for multiple breakthroughs, ranging from precisely measuring the energies at play in the first instants of creation to constraining the properties of dark energy, the still-mysterious force that drives the universe’s accelerating expansion. Along the way, almost as a bonus, advanced CMB studies could also reveal new particles, clarify the nature of known ones (such as neutrinos) and identify the earliest seeds of sprawling galaxy clusters and other large-scale cosmic structures.
A key signal of cosmic inflation would be patterns of swirling ripples imprinted on the cosmic microwave background by primordial gravitational waves. The swirls seen here, however, in data from the BICEP2 telescope, ultimately proved to be contamination from dust in our own galaxy.
Science History Images/Alamy Stock Photo
“The CMB is a way that we might learn about physics at scales that are completely inaccessible in any other way,” says David Spergel, a theoretical astrophysicist and president of the Simons Foundation.
To definitively detect—or rule out—those inflation-scrawled swirls, scientists need to make very deep and detailed observations of the CMB. That’s something best done from space, above Earth’s turbulent, troublesome atmosphere, following in the footsteps of a few earlier CMB-focused satellites from NASA and the European Space Agency. Today such a mission would cost billions of dollars, however, and wouldn’t be amenable to upgrades or enduring operations. Seeking the swirls is feasible (and cheaper) from Earth—presuming you’re able to stare for a long time at the same patch of sky through an exceptionally stable and dry column of air.
Antarctica, despite its logistical challenges, is one of the very few places on the planet where that’s possible. “The South Pole is particularly outstanding in that regard,” says Rachel Mandelbaum, a physicist at Carnegie Mellon University, who served on multiple high-profile expert panels that recommended prioritizing CMB-S4. “If you’re at the pole, as the Earth is rotating, you’re still looking at the same patch of sky.”
That would make the U.S.’s Amundsen-Scott South Pole Station a natural hosting site for CMB-S4. And in fact, it is already home to a couple of other CMB projects: the South Pole Telescope and the BICEP Array. (Incidentally, the BICEP Array predecessor BICEP2 made its own claim of detecting smoking-gun swirls in the CMB in 2014, although those putative features were soon shown to instead be the work of contaminating dust in our own Milky Way.)
At the South Pole, with a new five-meter aperture microwave telescope plus an array of nine smaller telescopes and state-of-the-art detectors, CMB-S4 would take an ultradeep look at roughly 3 percent of the sky. It would be much more sensitive than all of its predecessors and more easily able to cut through any contaminating dust.
To add to CMB-S4’s utility and allure, the project also planned to include two new six-meter telescopes on the summit of Cerro Toco in Chile’s Atacama Desert. High, dry and with a stable atmosphere above, that site is already home to the Simons Observatory, a newly operational set of telescopes that are conducting similar cosmological observations. The additional CMB-S4 telescopes would make nightly observations of huge swaths of galaxy-studded sky in an effort to map visible matter, better understand the dark universe and catch astrophysical transient events in action.
Altogether, there would have been 550,000 detectors spread between CMB-S4’s two sites, giving the project an unprecedented chance to hunt for clues of cosmic inflation in the universe’s oldest light.
“I’m kind of mesmerized by how much science there still is to get from the CMB,” says Suzanne Staggs, a physicist at Princeton University and co-director of the Simons Observatory. “It provides a unique opportunity to understand the early universe.”
With such a compelling science case—and such strong support from multiple authoritative panels planning the near future of U.S. research—CMB-S4 seemed almost inevitable for a time. “This project has scientifically been through about every appraisal that it could be, with glowing reviews,” Parriott says.
Optimistically, the team hoped it might be able to start construction at the two sites in the near future. Then delays started piling up, and a series of rude awakenings began.
The Big Chill
Despite its status as a scientific priority, CMB-S4’s not-so-glowing fate may have been foretold years ago. Chief among its challenges was that prized South Pole location because, while the pole might be ideal for astronomy, it’s not the easiest place on Earth to build and operate sophisticated science facilities. Antarctica is the coldest, driest, windiest, most remote continent on Earth; it demolishes infrastructure without even trying.
And NSF, which manages the U.S. Antarctic program on behalf of the government, has known for more than a decade that the existing facilities are in desperate need of maintenance. “If someone hasn’t been following this for a while, they might assume that this is the government pulling back from all kinds of projects,” says Mitch Ambrose, director of science policy news at the American Institute of Physics. “But in the case of CMB-S4, I think there’s a longer history in terms of the challenges with the infrastructure in Antarctica that have been brewing.”
In 2011 the White House and NSF convened a panel to evaluate the logistical challenges associated with maintaining U.S. scientific leadership in Antarctica. The panel’s report, released after visits to three Antarctic research stations, noted that activities there “are very well managed but suffer from an aging infrastructure” and are hamstrung by “the lack of a capital budget,” which it described as “a situation that no successful corporation would ever permit to persist.”
“The status quo is simply not an option,” the report continued, after noting such deficiencies as a warehouse where forklifts fall through the floor, buildings with gaps so large that snow blows inside and the repeated forced choice between repairing a roof or conducting a science experiment.
A report from the U.S. National Academy of Sciences followed and also identified the need to shore up crucial Antarctic infrastructure. NSF, correspondingly and with a limited budget, began planning some upgrades. Then 2020 and the COVID pandemic came along, with disruptions to site access and supply chain issues that sent price tags through those crumbling roofs.
“A lot of that planning really went off the rails during the pandemic in a major way,” Parriott says. “As somebody who’s spent a lot of time thinking about the U.S. Antarctic program, it’s kind of heartbreaking to see what’s become of it.”
Since then NSF has struggled to make the required upgrades—a situation that became an ominous portent for projects like CMB-S4. In 2023 the agency paused new projects at the South Pole. In May 2024 NSF definitively told CMB-S4 that the South Pole was off-limits; buildings were sinking into the snow, electrical power was insufficient, and there wasn’t enough room to house essential personnel. As a result, NSF officially declined to move the project toward its next design milestone.
“When the announcement came out a year ago, I was completely shocked,” Staggs says.
Afterward NSF and the DOE had a simple question for CMB-S4: Could the project proceed without the South Pole site? What if Chile was the only option?
Charting a New Course
On June 4 the collaboration submitted a proposal to both agencies that outlined a path forward in Chile at roughly half the cost of the original plan. By constructing one large telescope plus a smaller array of dishes at Cerro Toco and leaning heavily on data-sharing and collaborations with the South Pole Telescope, the Simons Observatory and others, the CMB-S4 collaboration reckoned it could still achieve its scientific objectives, albeit more slowly and less robustly.
“In the June plan, the idea was: ‘Okay, we’re scaling back; we’re working with these other experiments so that allows us to build less.’ And the expectation was that we could get telescopes on the air as early as 2032 … with combined results in 2040, 2041,” Carlstrom says. “You know, when I started this [in 2013], I thought, ‘This is going to be great; we’ll get on the air in 2020, and I can retire in 2025.”
Staggs, the Simons Observatory’s co-director, says both projects’ leaders met multiple times over the past year to talk about the revised plan.
“Even prior to that, because there was always a plan for part of the CMB-S4 to be in Chile, we had envisioned that eventually the two projects would be working very closely together, at least operationally, but with no details laid out yet,” Staggs says. “And we were sort of hoping we would be starting on that right around now—because, with the news that they would need to move to Chile, it seemed it was going to be a good opportunity for us to work together more.”
But under intense and mounting budgetary pressures, a balance sheet filled with fixed costs for operating cherished existing facilities and a backlog of other projects awaiting construction, the agencies decided to withdraw anyway.
The agencies “just had really hard choices to make,” Ambrose says. “This is the biggest tension point here: the community seems to really want this thing, and yet the agencies aren’t willing to do it.”
Knocked down hard, scientists who had planned on CMB-S4’s success are now focused on getting back up—and charting a new path forward.
“It’s not that the search for primordial gravitational waves won’t happen; it just won’t advance as rapidly as we had hoped,” Spergel says. “I hope this ends up being an opportunity to rethink how we do the science and not a decision to step away from doing what is really exciting and compelling science.”
In a statement sent to Scientific American, a DOE spokesperson reiterated that “the scientific case for CMB exploration is strong and compelling” and said that the agency “plans to continue supporting CMB research,” which is described as a core component of the DOE’s high-energy physics program. That includes investigating opportunities to make near-term upgrades to existing experiments at the South Pole and in Chile. (NSF declined to provide comment.)
“If these existing projects weren’t there at all, that would be also a different situation,” Dunkley says. “We’ll have to see how things evolve on that front: How much upgrading or continuation of the projects that are already running can be achieved?”
One possible solution, Spergel says, is to build as much as possible in Chile to do as much science as possible from there—and then pivot to the South Pole if needed.
Another possibility that most U.S. researchers seem less eager to mention is to effectively cede leadership in CMB studies to other nations. Japan’s space agency, for instance, is leading development of LiteBIRD (Light Satellite for the Study of B-mode Polarization and Inflation from Cosmic Background Radiation Detection), a space-based CMB mission, for launch in the early 2030s. And on the Tibetan Plateau, China’s Ali Cosmic Microwave Background Polarization Telescope (AliCPT) has recently completed the first of two planned construction phases, with scientific observations soon to begin. The U.S. is involved in both efforts, chiefly via hardware contributions from the federal National Institute of Standards and Technology, but only plays a supporting role. Despite continued U.S. support for CMB experiments in Chile, perhaps the long-sought confirmation of the strangest chapter of cosmic history will come from elsewhere.
“We’ll get there eventually,” Carlstrom says. “It’s just going to be much harder to do without the South Pole, much harder to do without substantial new instrumentation wherever you are, including Chile.”