Humans have been fascinated by the secrets of the cosmos for millennia. Modern cosmologists, unlike ancient philosophers who imagined the universe’s origins, employ mathematical tools to acquire insights into the universe’s evolution and structure.
Modern cosmology began with the formulation of Albert Einstein’s theory of general relativity in the early 20th century.
The Atacama Cosmology Telescope (ACT) group has now developed a revolutionary new image that depicts the most precise map of dark matter dispersed across a fifth of the entire sky, extending deep into the cosmos. Furthermore, it confirms Einstein’s theory of how massive structures grow and bend light over the universe’s entire 14-billion-year life span.
“We have mapped the invisible dark matter across the sky to the largest distances, and clearly see features of this invisible world that are hundreds of millions of light-years across,” says Blake Sherwin, professor of cosmology at the University of Cambridge, where he leads a group of ACT researchers. “It looks just as our theories predict.”
Dark matter has been difficult to detect despite accounting for 85% of the universe and influencing its evolution because it does not interact with light or other kinds of electromagnetic energy. As far as we know dark matter only interacts with gravity.
To find it, the more than 160 collaborators who built and collected data from the National Science Foundation’s Atacama Cosmology Telescope in the high Chilean Andes observe light emitted after the Big Bang, when the universe was only 380,000 years old.
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. Together, the CMB lensing and the best optical surveys are clarifying the evolution of all the mass in the universe.
Suzanne Staggs
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 studies how the gravitational pull of huge, heavy things, including dark matter, warps the CMB on its 14-billion-year journey to us, similar to how light bends when it travels through a magnifying glass’s lens.
“We’ve made a new mass map using distortions of light left over from the Big Bang,” says Mathew Madhavacheril, 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.”
Sherwin adds, “our results also provide new insights into an ongoing debate some have called ‘The Crisis in Cosmology,’” explaining that this crisis stems from recent measurements that use a different background light, one emitted from stars in galaxies rather than the CMB.
These have produced results indicating that dark matter was not lumpy enough under the standard model of cosmology, raising concerns that the model may be broken. However, the team’s most recent ACT results allowed them to precisely assess that the vast lumps visible in this image are the correct size.
“When I first saw them, our measurements were in such good agreement with the underlying theory that it took me a moment to process the results,” says Cambridge Ph.D. student Frank Qu, part of the research team. “It will be interesting to see how this possible discrepancy between different measurements will be resolved.”
“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.”
ACT, which operated for 15 years, was decommissioned in September 2022. Nonetheless, more papers presenting results from the final set of observations are expected to be submitted soon, and future observations will be conducted at the same site, with a new telescope set to begin operations in 2024. This new equipment will be able to map the sky about ten times faster than ACT.
