The earliest light in the universe, known as the cosmic microwave background, has been traveling since the dawn of the universe over 14 billion years ago.
A group of scientists from 30 institutions has released the sharpest and clearest map ever made of the mass in the universe, created by measuring how this ancient light was bent as it traveled.
The data came from the South Pole Telescope, which takes advantage of the extraordinarily clear skies of Antarctica to record the cosmic microwave background. Funded primarily by the National Science Foundation and the U.S. Department of Energy and run by a collaboration led by the University of Chicago, the telescope is located at the NSF Amundsen-Scott South Pole Station.
“This is the deepest map of the universe we have so far—it’s like taking a photograph with very low background noise exposure,” said Yuuki Omori, research scientist at UChicago and lead author of the paper, which was released Sept. 1 on arXiv.
The results offer a new perspective on ongoing questions in cosmology, such as the structure of the universe and how it evolved.
“What an amazing thing to be able to see all the mass in the universe,” said John Carlstrom, the Subramanyan Chandrasekhar Distinguished Service Professor of Astronomy and Astrophysics at UChicago and the South Pole Telescope project director. “We are entering the new era of cosmology promised by these instruments.”
Testing open questions
The cosmic microwave background, sometimes called the “afterglow of the Big Bang,” has been traveling across the universe since it was created. It can tell scientists about the beginning of the universe, but it can also tell us about later epochs, thanks to a happy quirk of gravity.
The light of the cosmic microwave background travels steadily, except if it is bent ever so slightly by the enormous gravitational pull of a galaxy or cluster of galaxies.
By carefully cataloguing this effect, known as gravitational lensing, scientists can reconstruct a map of all the matter between us and where this light was emitted.
“It’s like cartography—we are making a literal map of the universe,” said Omori.
The latest mass map made from the South Pole Telescope data is the most sensitive measurement of the gravitational lensing of the cosmic microwave background to date, and enables a number of tests of open cosmological questions.
One of the key measurements is how “clumpy” the universe is—that is, where and how matter is lumped together. This tells scientists about how the universe grew over time and opens a window into the relatively mysterious middle years of the history of the universe.
“Scientists want to tell a coherent story that can explain everything about the universe,” said Omori. “We have a pretty good story about the beginning of the universe and a pretty good one about today, but less about the middle years.”
Those years weren’t uneventful, either; that’s when stars and galaxies began to form in earnest, and much of the structure in the universe today can be traced back there.
When combined with data from the Dark Energy Survey, the new South Pole Telescope results are the best yet at narrowing down how the structure of the universe was formed over time.
“This provides a window to the younger state of the universe that we have few other ways to see,” said Wai Ling Kimmy Wu, assistant professor at Caltech and co-author on the study.
Scientists can also compare this map to other measurements of the cosmic microwave background, in case some unknown factor is causing an error in one experiment. The new data agrees very well with other microwave background experiments such as the Planck satellite and the Atacama Cosmology Telescope—which boosts scientists’ confidence in the results.
The data can also be compared to a different measurement known as baryon acoustic oscillations, which can be used to measure the expansion of the universe over time; this comparison can help find potential holes in our current best models of the universe.
All the mass in the universe
The new data is also a step on the road towards two major goals for the South Pole Telescope.
First, the scientists hope that the cosmic microwave background data could someday reveal new insights about neutrinos—the fundamental but mysterious particles that zip through us all the time but barely ever touch us.
“By weighing all of the mass in the universe and studying the structure, we can probe the effects neutrinos had on the growth of structure,” said Wu. “It’s a unique probe of neutrinos, and could even give us a way to measure the mass of neutrinos very precisely.”
Second, the results are also progress towards an ambitious milestone: being able to detect the ripples in gravity from the Big Bang itself.
Known as “primordial gravitational waves,” these ripples date from the first nanoseconds of the existence of the universe, and they could provide the first concrete experimental measurement of cosmic inflation. They could also perhaps point the way towards a longstanding puzzle in physics: reconciling the laws of gravity with the laws of quantum mechanics.
To find these waves, the South Pole Telescope’s data will be used to clean data from the BICEP experiment, also funded by the National Science Foundation and located at the South Pole.
“Essentially, our map can be used to reduce the noise they see and locate the true signal,” said Omori.
The collaboration is working on analyzing a larger set of six years’ worth of data from the South Pole Telescope 3G project, which will make progress on both fronts.
“Any grand unified theory of the universe would have to predict what this signal looks like,” said UChicago research professor Tom Crawford, deputy director of the South Pole Telescope. “Our next upgrade will be even more powerful—so we’ll have an even better shot at detecting this signal. It’s an exciting time.”
Citation: "SPT-3G D1: Quadratic-Estimator CMB Lensing Reconstruction and Cosmology." Omori et al, published to arXiv Sept. 1, 2026.
