By closely measuring the distorted path taken by the universe’s oldest light, researchers with the Atacama Cosmology Telescope (ACT) collaboration have mapped out the distribution of dark matter in the cosmos with unparalleled precision. The results shore up Einstein’s theory of gravity, provide new insights into the nature of dark matter and expose critical information about the universe’s fundamental properties.
“This result is another victory for Einstein’s theory of gravity,” says ACT scientist Adriaan Duivenvoorden, a researcher in the Simons Foundation’s CMB analysis and simulation group. “The measured distribution of matter in the late-time universe agrees with what we expect from our understanding of the early universe. The fact that we are able to correctly predict how the matter moves and evolves over such a long period of time is a testament to our understanding of cosmology.”
The researchers present their methods and findings in a set of papers submitted to The Astrophysical Journal and available now on arXiv.org.
“We’ve made a new mass map using distortions of light left over from the Big Bang,” says Mathew Madhavacheril, an assistant professor in the department of physics and astronomy at the University of Pennsylvania and lead author of one of the papers. “Remarkably, it provides measurements that show that both the ‘lumpiness’ of the universe and the rate at which it is growing after [nearly] 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 new map “confirms our now-standard picture of dark matter and ordinary matter tracing the same large-scale structure,” says astrophysicist and ACT scientist David Spergel, president of the Simons Foundation and head of the CMB analysis and simulation group.
Although dark matter makes up 85 percent of the universe’s mass and has shaped its evolution, the mysterious material remains 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 down the location of that dark matter, the more than 160 collaborators behind the National Science Foundation’s ACT located in the Chilean Andes focused on a quarter of the sky and closely observed microwave radiation emitted soon after the universe’s birth 13.7 billion years ago. That light originated about 380,000 years after the Big Bang and has been traveling toward us from all directions ever since. Cosmologists have whimsically dubbed this light the “baby picture of the universe,” but more formally, they call it the cosmic microwave background, or CMB.
The CMB didn’t follow a straight path from its origin to Earth. The intense gravitational tug of massive objects such as galaxies distorts the fabric of space-time, curving the light’s path the way a magnifying glass bends light passing through its lens. By analyzing the distortions, scientists can work out how much mass lurks between us and the light’s origin. Because we can see ordinary matter, any unaccounted-for mass must be dark matter.
“When we proposed [ACT] in 2003, we had no idea the full extent of information that could be extracted from our telescope,” says Mark Devlin, the Reese W. 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.”
Scientists have made such matter maps before — the first ones were done by ACT in 2013, followed by undertakings using the South Pole Telescope and the Planck satellite. This time around, though, the ACT team managed far higher accuracy thanks to new data and improved analysis techniques. As ACT is a ground-based telescope, the researchers had to account for light emitted by atoms and molecules in Earth’s atmosphere.
“A lot of effort went in to finding a model that accurately describes the noise contribution from the atmosphere and ensuring that our results are robust to any remaining mis-modeling of the noise,” says Duivenvoorden, who worked on the study’s observations and analysis and is also a research fellow at the Center for Computational Astrophysics at the Simons Foundation’s Flatiron Institute in New York City.
“Ten years later, we have a much better picture with more sensitivity and less noise,” says ACT scientist Simone Aiola, a data and pipeline project leader in the Simons Foundation’s CMB analysis and simulation group.
The improved accuracy and reduced noise “provide new insights into an ongoing debate some have called ‘the crisis in cosmology,’” says Blake Sherwin, a professor of cosmology at the University of Cambridge, where he leads a group of ACT researchers. That crisis arose when researchers used the light from stars in galaxies rather than the CMB to map matter. These projects produced results suggesting that dark matter is not lumped together enough under the standard model of cosmology. That mismatch raised concerns that the standard model was wrong and that Einstein’s theory of gravity wasn’t cutting it. However, the team’s latest results from ACT were able to precisely assess that the vast lumps seen in the matter map are exactly the right size.
“We aren’t seeing the departures from gravity that would suggest new physics,” Aiola says. “These new results wash away a lot of the non-Einsteinian gravity models.”
The results came as a relief to many. “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 graduate student Frank Qu, lead author of one of the new papers. “It will be interesting to see how this possible discrepancy between different measurements will be resolved.”
The new results serve as a powerful tool for understanding the cosmos, says Suzanne Staggs, director of ACT and the Henry DeWolf Smyth professor of physics at Princeton University. “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.”
In addition to mapping matter and revealing information about dark matter, the new results also help constrain the properties of mysterious particles called neutrinos. Scientists initially thought neutrinos had no mass, but they now know that the odd particles have a very, very small mass — at most a few millionths that of an electron.
After 15 years of operation, ACT was decommissioned in September 2022. Even so, more papers presenting results from the final set of observations are expected to be submitted soon.
Scientists are also eagerly awaiting the launch of the Simons Observatory, located near the ACT site in the Chilean Andes and funded by the Simons Foundation and participating universities. A new telescope at the observatory slated to begin operations in 2024 will map the CMB with even greater precision.