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Latest data from South Pole Telescope signal 'new era' for measuring the first light in the universe

The earliest light in the universe has been traveling across space since just after the Big Bang. Known as the cosmic microwave background, it is imperceptible to the human eye. But if scientists can capture it, using some of the most sensitive detectors ever made, it can tell us about how our universe formed and evolved over time.

Researchers have released unprecedentedly sensitive measurements of the cosmic microwave background from two years of observations using an upgraded camera on the South Pole Telescope. The telescope, located at the National Science Foundation's Amundsen-Scott South Pole Station, and funded jointly by the National Science Foundation and the U.S. Department of Energy, was designed specifically to map the very faint light from the microwave background.

The , released on June 25, are impressive—the precision on the fine details of the cosmic microwave background exceeds that of all previous measurements, even those taken from space. When combined with data from other ground-based telescopes, it offers a new benchmark to constrain the possible answers to major questions about the universe.

"This is a watershed moment for cosmic microwave background cosmology," said Tom Crawford, deputy director of the South Pole Telescope and research professor at the University of Chicago. "It ushers in a new era, in which our understanding of the universe will be advanced in large part by ground-based cosmic microwave background experiments."

The new readings offer a cross-check on our fundamental model of the universe. As more data is released, it will sharpen several tests of major outstanding questions in cosmology, such as the nature of dark energy and the rate at which the universe is expanding.

Cosmic constraints

The cosmic microwave background, sometimes referred to as the afterglow of the Big Bang, dates back over 13 billion years, to the period just after our universe formed. This makes it an incredibly rich source of information—if you can catch it, that is.

This light is extremely faint, and the variations in it are even fainter. To even have a chance of capturing it, you need a very clear sky and perfectly dry viewing conditions, both of which can be found in Antarctica.

The South Pole Telescope, run by a collaboration led by the University of Chicago, has been mapping this light since 2007. There have been several cameras installed in the telescope over the years, but the latest, , has an order of magnitude more detectors than previous versions. The data in the newest result were taken in 2019 and 2020, and represent the first two years of full-power SPT-3G observations. They cover about 1/25th of the sky, mapping it in more detail than any other measurement of this kind.

One of the main uses for this data is to place constraints on the many possible answers to our questions about the universe, such as how it formed and the fundamental laws that govern its evolution. Data provided by the cosmic microwave background helps guide the quest to fit everything together into a cohesive picture.

The current best model explaining the formation of the cosmos is known as Lambda-CDM. However, recent studies have come back with that Lamba-CDM may not be the whole picture. There is also an ongoing debate on the rate of expansion of the universe, known as the "Hubble tension," which would have significant ramifications for our understanding of the universe and in which the cosmic microwave background plays a key role.

The new constraints from the South Pole Telescope, released in a paper co-led by Etienne Camphuis, a postdoctoral researcher with Silva Galli's team at the Institut d'Astrophysique de Paris/CNRS Terre et Univers, and Wei Quan (Ph.D.'24) of Argonne National Laboratory, sharpen this picture significantly.

The findings confirm the Hubble tension independently at very high statistical significance, the group said, while remaining consistent with other cosmic microwave background constraints, including those from the and the in Chile.

They also sharpen a newly appearing anomaly in our cosmological picture—the disagreement between cosmic microwave background constraints and those from large-scale surveys of the movements of galaxies (particularly from the ).

As more of the data from SPT-3G comes online, it will continue to provide an ever more powerful independent way to test hypotheses.

"If there really is a departure from the standard model, we'll be able to see it much more strongly with these upcoming datasets," said Quan. "If it's a real signal, it will be magnified."

Gold standard from the ground

Previously, the gold standard for cosmic microwave background measurements was the data from the Planck satellite, taken more than a decade ago. Now the new South Pole Telescope data, when combined with data from Atacama Cosmology Telescope, set a new standard—a moment many in the field have been waiting for, the scientists said.

Space-based telescopes such as Planck have the advantage of clearer sight, since Earth's atmosphere isn't gumming up the view.

But it's substantially easier to operate a telescope from the ground. Creating a complex instrument to run even in a place as harsh as Antarctica is far easier than designing something that has to survive a rocket launch and conditions in space. "If something breaks on a ground-based telescope, you can walk over and fix it," said Brad Benson, associate professor of astronomy and astrophysics at UChicago and operations director of the South Pole Telescope. "You can't do that in space."

Advances in detectors and designs are finally allowing ground-based telescopes to equal or rival Planck's data.

"For years, Planck was effectively defining our cosmological model by itself," said Camphuis. "However, in science, it's important to confirm measurements. With the South Pole Telescope and Atacama Cosmology Telescope, we now have an almost fully independent set of measurements with similar constraining power."

These new results represent less than a quarter of the data taken with SPT-3G on the South Pole Telescope.

"This is just the beginning," said Crawford. "The picture is only going to get more interesting."