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Wits CfA contributes to a new map of the universe’s cosmic growth, supporting Einstein

Research by the Atacama Cosmology Telescope collaboration has culminated in a groundbreaking new image that reveals the most detailed map of dark matter distributed across a quarter of the entire sky, reaching deep into the cosmos. Findings provide further support to Einstein’s theory of general relativity, which has been the foundation of the standard model of cosmology for more than a century, and offers new methods to demystify dark matter.

Figure 1: A new map of the dark matter made by the Atacama Cosmology Telescope. The orange regions show where there is more mass; purple where there is less. The typical features are hundreds of millions of light years across. The grey/white shows where contaminating light from dust in our Milky Way galaxy, measured by the Planck satellite, obscures a deeper view. (Credit: : ACT Collaboration).

Researchers from the Atacama Cosmology Telescope (ACT) collaboration have submitted a set of papers to The Astrophysical Journal, featuring a groundbreaking new map of dark matter distributed across a quarter of the entire sky, extending deep into the cosmos. What’s more, it confirms Einstein’s theory of how massive structures grow and bend light over the 14-billion-year life span of the universe.

Despite making up 85% of the matter in the universe and influencing its evolution, dark matter has been hard to detect because it doesn’t interact with light or other forms of electromagnetic radiation. As far as we know dark matter only interacts with gravity.

To track it down, the more than 160 collaborators who have built and gathered data from the National Science Foundation’s Atacama Cosmology Telescope in the high Chilean Andes observe light emanating following the dawn of the universe’s formation, the Big Bang—when the universe was only 380,000 years old. Cosmologists often refer to this diffuse light that fills our entire universe as the “baby picture of the universe,” but formally, it is known as the cosmic microwave background radiation (CMB).

The team tracks how the gravitational pull of large, heavy structures including dark matter warps the CMB on its 14-billion-year journey to us, like how a magnifying glass bends light as it passes through its lens.

Figure 2: The Atacama Cosmology Telescope. (Credit: Mark Devlin)

“We’ve made a new mass map using distortions of light left over from the Big Bang,” says Mathew Madhavacheril, lead author of one of the papers and assistant professor in the Department of Physics and Astronomy at the University of Pennsylvania. “Remarkably, it provides measurements that show that both the ‘lumpiness’ of the universe, and the rate at which it is growing after 14 billion years of evolution, are just what you’d expect from our standard model of cosmology based on Einstein's theory of gravity.”

“The CMB lensing data rivals more conventional surveys of the visible light from galaxies in their ability to trace the sum of what is out there,” says Suzanne Staggs, director of ACT and Henry DeWolf Smyth Professor of Physics at Princeton University. “Together, the CMB lensing and the best optical surveys are clarifying the evolution of all the mass in the universe.”

“When we proposed this experiment in 2003, we had no idea the full extent of information that could be extracted from our telescope, says Mark Devlin, the Reese Flower Professor of Astronomy at the University of Pennsylvania and the deputy director of ACT. “We owe this to the cleverness of the theorists, the many people who built new instruments to make our telescope more sensitive, and the new analysis techniques our team came up with.”

At Wits, Professor Matt Hilton (who is also an Honorary Professor at the University of KwaZulu-Natal), has been hunting for massive galaxy clusters in the ACT data. Galaxy clusters leave an imprint on images of the CMB through the Sunyaev-Zel'dovich effect that can contaminate the lensing signal. 'It turns out that some of the tools that we developed to find clusters in the ACT data and model their signals have been useful in helping the ACT lensing team to make such an amazingly precise measurement.' says Matt. Researchers at Wits, UKZN, and Rhodes University in South Africa are using the MeerKAT and SALT telescopes to follow-up massive clusters detected by ACT in pursuit of a variety of science goals.

ACT, which operated for 15 years, was decommissioned in September 2022. Nevertheless, more papers presenting results from the final set of observations are expected to be submitted soon, and the Simons Observatory will conduct future observations at the same site, with a new telescope slated to begin operations in 2024. This new instrument will be capable of mapping the sky almost 10 times faster than ACT.

The pre-print articles highlighted in this release are available on and will appear on the open-access They have been submitted to the Astrophysical Journal.

This work was supported by the U.S. National Science Foundation (AST-0408698, AST-0965625 and AST-1440226 for the ACT project, as well as awards PHY-0355328, PHY-0855887 and PHY-1214379), Princeton University, the University of Pennsylvania, and a Canada Foundation for Innovation award. Authors at the University of Cambridge were supported by the European Research Council.

Extra media

1-minute video of ACT (Credit: Debra Kellner).



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