Scientists are reeling from an unexpected blow after news that a much anticipated future observatory—designed to decipher the earliest moments of cosmic history—won’t be able to proceed with its crucial outpost at the South Pole.
The project, dubbed CMB-S4 (Cosmic Microwave Background Stage 4), would offer tantalizing observations of the big bang’s afterglow, an all-sky echo of light emitted when the universe was in its infancy. Both astronomers and particle physicists have enthusiastically endorsed the project in their formal long-term planning processes. And scientists had hoped that construction on the observatory, both at the South Pole and at a second site in Chile, could begin in the early 2030s. But those hopes may have been dashed earlier this month when the U.S National Science Foundation (NSF) declared that it can’t currently support the observatory’s Antarctic outpost because of concerns about maintaining infrastructure and science operations at the South Pole.
Now the scientists who have rallied around CMB-S4 face a challenge much less appealing than decoding the oldest light in the universe: finding a way to keep their dream observatory from being put on ice. “I was definitely surprised, and it’s definitely a setback,” says Kevin Huffenberger, an astrophysicist at Florida State University and co-spokesperson for the CMB-S4 collaboration, which involves hundreds of researchers. “It’s a big thing that we have to deal with.”
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Ancient Light for Modern Science
CMB-S4 draws its name from its status as the premier planned fourth-generation facility for studying the cosmic microwave background, or CMB, which is sometimes nicknamed the universe’s baby picture.
Light takes time to travel through space, so astronomers can glimpse earlier cosmic epochs by looking farther away from Earth. The CMB is the very most distant—and therefore the very oldest—light that can be seen. It’s visible in every direction from Earth and dates to just 378,000 years after the big bang. That makes it essentially “14-billion-year-old fossil light,” says John Carlstrom, project scientist for CMB-S4 and an astrophysicist at the University of Chicago.
CMB science had an unassuming beginning six decades ago, when scientists in New Jersey spied a signal so weird that they at first believed it was electromagnetic interference caused by pigeon poop on their radio telescope. Now the CMB is considered the bedrock of our cosmic understanding, the keystone source that confirms the reality of the big bang and reveals the curious mixture of dark energy and dark matter—with a dash of “normal” matter—that makes up our universe. But its transformative potential reaches beyond cosmology to offer fresh insights for a wide range of sciences.
“You go from our solar system all the way out to the first instants of the universe,” says Mark Devlin, an astrophysicist at the University of Pennsylvania and spokesperson for the Simons Observatory, a CMB-S4 precursor that he says is just beginning a four-year science campaign in Chile. “It’s very hard to think of any other measurement you can do that would cover quite the distance.”
Most simply, perhaps, is that modern CMB studies require making a detailed survey of the sky in millimeter-wavelength light—which means CMB-S4 would gather data about asteroids, supernovas and a host of other dramatic celestial happenings as a natural by-product of its observations.
In addition, every photon of the CMB that scientists detect has traversed half of the universe—and has been changed by that journey. Each encounter with material it has passed is encoded into each ancient speck of light in a way that scientists can decrypt to understand cosmic structure on the grandest of scales. “There are layers of science that you can pull out as the light comes to you from the background,” Huffenberger says.
One of the more surprising types of science available in CMB studies is particle physics. “I didn’t anticipate that the CMB-S4 would come as a top priority in this process,” says Hitoshi Murayama, a physicist at the University of California, Berkeley, who led the latest iteration of a once-per-decade planning exercise, the 2023 Particle Physics Project Prioritization Panel. But CMB-S4 should pin down elusive questions about the mass of neutrinos while also potentially helping to discover new particles.
A Polar Problem
Perhaps the most tantalizing prospect of CMB-S4 is that it would hunt for primordial gravitational waves—ripples in spacetime that scientists hypothesize arose in the first split seconds of cosmic history, when a poorly understood phenomenon called inflation is thought to have briefly but profoundly supercharged the expansion of the universe. CMB-S4 is fine-tuned to find these primordial gravitational waves—and, with them, the best evidence that inflation is more than a cosmological fairy tale. “We’re going for this big question,” says Jeff McMahon, an astrophysicist at the University of Chicago and co-spokesperson for CMB-S4. “If we see this, it’ll just be magnificent. It’ll be a really big deal in physics.”
There’s just one problem: The signal of these primordial gravitational waves, if it exists, is predicted to be incredibly faint, which means scientists will need a lot of detectors, a lot of time and a very good viewpoint. Despite its logistical challenges, the South Pole—where the atmosphere is very dry and stable and astronomers can constantly stare at a single patch of the heavens—offers, by far, the best view available on Earth.
“Essentially, every committee that has looked at this has said, ‘The way to do that is to go the pole,’” says Risa Wechsler, a cosmologist at Stanford University, who sits on the National Academy of Sciences’ Board on Physics and Astronomy.
At that group’s May 7 meeting, the NSF’s interim division director for astronomical sciences, Robert C. Smith, announced that CMB-S4 won’t be moving forward “in its current form, with both South Pole and Chile elements.” The decision, he said, doesn’t reflect any change in the sky-high scientific merit of CMB-S4 or overall CMB science. Instead it is a product of ongoing and serious logistical and maintenance challenges at the remote Amundsen-Scott South Pole Station, the research station managed by the NSF that is the only permanent outpost located within 600 miles of the pole.
“We paused new science projects while we work on recapitalization and refurbishment of physical infrastructure, which are necessary to ensure that the South Pole Station remains a world-leading scientific platform that supports groundbreaking scientific research into the future,” says an NSF spokesperson. For example, they noted, several buildings are at risk of being buried by the aggressive local snow drifts. The agency is also concerned about snow piling up on roofs and causing structural damage.
But the announcement that CMB-S4 would suffer as a result came as a surprise to scientists on the project and beyond. “We knew that they needed to do some infrastructure improvements,” Huffenberger says. “But we had hoped to be able to work with the NSF to stage that and work on the same schedule so that both of the things could happen together.”
He and McMahon say they still don’t understand how the NSF made its decision and are working to determine if there’s any chance the agency will reconsider. In the meantime, the scientists who have been participating in the CMB-S4 collaboration are reckoning with what the NSF’s decision means for their dream project.
Focusing on Chile
As originally designed, CMB-S4 would include two large-aperture microwave telescopes in Chile’s high-altitude Atacama Desert, plus a third such instrument and nine small-aperture microwave telescopes at the South Pole. All told, the observatory would include 550,000 detectors spread between the two top-tier astronomy sites—vital technology for gathering cutting-edge observations that are sensitive enough to answer the scientists’ big questions about the universe.
To get equivalent data from only the Chile site, the researchers will need to rejigger that configuration, likely increasing the total number of telescopes to offset the suboptimal observing conditions, which would probably increase the project’s price tag as well. “Not being able to site the small-aperture telescopes at the [South] Pole isn’t a fatal blow by any stretch of the imagination, but it means we have to look back and understand how the experimental configuration will likely have to grow,” McMahon says.
He and his colleagues don’t intend to give up on the observations they dreamed up, however. “It’s a beautiful, rich science case that touches on the whole history of the universe, particle physics and astrophysics,” McMahon says. “I just want to do it.”