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Mapping Dark Matter Like Never Before

Act site
Act site
The Atacama Cosmology Telescope, Credit: Debra Kellner

A new groundbreaking image from one of the world’s most powerful telescopes that reveals the most detailed map of dark matter distributed across one quarter of the sky, and deep into the cosmos, offers scientists a perspective that may lead to new methods to demystify dark matter.

The research that led to the image, completed by the Atacama Cosmology Telescope (ACT) collaboration, also provides 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. Details explaining the scientific method behind the new image are articulated in a set of three papers that are posted to the ACT website, and will be published in the Astrophysical Journal.

Although dark matter makes up a large chunk of the universe, approximately 85 percent, it has remained hard to detect because dark matter does not interact with light or other forms of electromagnetic radiation. Scientists believe dark matter may only interact with gravity.

To track dark matter down, more than 160 collaborating scientists worldwide have built and gathered data from the National Science Foundation’s Atacama Cosmology Telescope in the high Chilean Andes. They observe light emanating from 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 international 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; this phenomenon is called “gravitational lensing.” The work by the ACT team further supports Einstein’s theory about how massive structures grow and bend light, with a test that spans the entire age of the universe.

Neelima Sehgal, associate professor in the Department of Physics and Astronomy, is one of the scientists involved in the project and is co-lead of the working group within the ACT collaboration that focuses on measuring the gravitational lensing of the CMB.

Act graphic long“This ACT result showcases the precision that can be obtained with measurements of the gravitational lensing of the microwave background, as well as the promise of future more sensitive CMB experiments in terms of furthering our understanding of the physics of the Universe,” said Sehgal.

“We’ve made a new mass map using distortions of light left over from the Big Bang,” said Mathew Madhavacheril, lead author of one of the papers and assistant professor in the Department of Physics and Astronomy at the University of Pennsylvania, who also earned his doctorate at Stony Brook University. “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.”

“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,” said Cambridge PhD student Frank Qu, lead author of one of the new papers. A co-author on the same paper with Qu is Dongwon Han, who contributed significantly to the project and who also earned his PhD from Stony Brook University.

Mark Devlin, the Reece Flower Professor of Astronomy at the University of Pennsylvania and the deputy director of ACT, noted that 20 years ago when plans for experiments with this telescope were made, the team had no idea of the full extent of information that could be extracted from the telescope.

“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,” says Devlin.

Further scientific papers highlighting other results from work with the telescope are slated for submission later this year. ACT was decommissioned in late 2022 to make room for new observations that will continue at the site with the Simons Observatory, a new set of telescopes due to begin operating in 2024, which can map the sky at a rate 10 times faster than ACT.

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