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Unraveling the Mysteries of the Dark Matter in the Cosmos

Now, at the Atacama Cosmology Telescope (ACT) collaboration, researchers have submitted a set of papers to The Astrophysical Journal featuring an innovative new map of dark matter distributed throughout a quarter of the sky. This extends deep into the cosmos, which verifies Einstein’s theory of how huge structures grow and bend light over the universe’s 14-billion-year lifespan.

Unraveling the Mysteries of the Cosmos

A view of Stephan’s Quintet, a visual grouping of five galaxies from the James Webb Telescope. Image Credit: NASA, ESA, CSA, STScI

With the development of Albert Einstein’s theory of general relativity, modern cosmology dates back to the early 20th century.

The new map makes use of light coming from the cosmic microwave background (CMB) basically as a backlight to silhouette all the matter between humans and the Big Bang.

It’s a bit like silhouetting, but instead of just having black in the silhouette, you have texture and lumps of dark matter, as if the light were streaming through a fabric curtain that had lots of knots and bumps in it.

Suzanne Staggs, Director of ACT and Henry DeWolf Smyth Professor, Department of Physics, Princeton University

Staggs added, “The famous blue and yellow CMB image [from 2003] is a snapshot of what the universe was like in a single epoch, about 13 billion years ago, and now this is giving us the information about all the epochs since.”

It’s a thrill to be able to see the invisible, to uncover this scaffold of dark matter that holds our visible star-filled galaxies. In this new image, we can see directly the invisible cosmic web of dark matter that surrounds and connects galaxies,” stated Jo Dunkley, a professor of physics and astrophysical sciences, who leads the analysis for ACT.

Usually, astronomers can only measure light, so we see how galaxies are distributed across the universe; these observations reveal the distribution of mass, so primarily show how the dark matter is distributed through our universe,” stated David Spergel, Princeton’s Charles A. Young Professor of Astronomy on the Class of 1897 Foundation, Emeritus, and the president of the Simons Foundation.

We have mapped the invisible dark matter distribution across the sky, and it is just as our theories predict. This is stunning evidence that we understand the story of how structure in our universe formed over billions of years, from just after the Big Bang to today,” stated co-author Blake Sherwin, a 2013 Ph.D. alumnus of Princeton and a professor of cosmology at the University of Cambridge, where he leads a large group of ACT researchers.

Sherwin added, “Remarkably, 80% of the mass in the universe is invisible. By mapping the dark matter distribution across the sky to the largest distances, our ACT lensing measurements allow us to clearly see this invisible world.”

When we proposed this experiment in 2003, we had no idea the full extent of information that could be extracted from our 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.

Mark Devlin, Reese Flower Professor, Astronomy, University of Pennsylvania

Devlin, the deputy director of ACT, had previously been a Princeton postdoc from 1994 to 1995.

Despite accounting for the majority of the universe, detecting dark matter has proven difficult because it does not interact with light or other forms of electromagnetic radiation.

To uncover the mystery of dark matter, over 160 collaborators constructed and gathered data from the National Science Foundation's Atacama Cosmology Telescope situated in the high Chilean Andes. They observed light emitted after the Big Bang, the dawn of the universe's formation when it was just 380,000 years old. Cosmologists often refer to this diffuse CMB light, which fills the universe, as the "baby picture of the universe."

The research group tracked how the gravitational pull of immense dark matter structures could distort the CMB during its 14-billion-year journey to reach humans.

We’ve made a new mass map using distortions of light left over from the Big Bang,” stated Mathew Madhavacheril, a 2016-2018 Princeton postdoc who is the lead author of one of the papers and an assistant professor in physics and astronomy at the University of Pennsylvania.

Madhavacheril added, “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 added, “Our results also provide new insights into an ongoing debate some have called ‘The Crisis in Cosmology.’”

This “crisis” comes from recent measurements that make use of various background lights discharged from stars in galaxies instead of the CMB.

While earlier studies pointed to cracks in the standard cosmological model, our findings provide new reassurance that our fundamental theory of the universe holds true,” stated Frank Qu, lead author of one of the papers and a Cambridge graduate student as well as a former Princeton visiting researcher.

The CMB is famous already for its unparalleled measurements of the primordial state of the universe, so these lensing maps, describing its subsequent evolution, are almost an embarrassment of riches.

Suzanne Staggs, Director of ACT and Henry DeWolf Smyth Professor, Department of Physics, Princeton University

Staggs added, “We now have a second, very primordial map of the universe. Instead of a ‘crisis,’ I think we have an extraordinary opportunity to use these different data sets together. Our map includes all of the dark matter, going back to the Big Bang, and the other maps are looking back about 9 billion years, giving us a layer that is much closer to us.

We can compare the two to learn about the growth of structures in the universe. I think is going to turn out to be really interesting. That the two approaches are getting different measurements is fascinating,” continued Staggs.

Staggs led the team responsible for constructing the detectors that have collected data for the past five years.

The ACT had been operational for 15 years but was decommissioned in September 2022. However, more papers presenting the final set of observations are expected to be submitted soon. The Simons Observatory will continue to perform future observations at the same site, with a new telescope scheduled to commence operations in 2024. This new instrument will be capable of mapping the sky almost 10 times faster than the ACT.

The ACT team's series of papers had over 56 co-authors who are currently or have been Princeton researchers. Additionally, more than 20 junior scientists who worked on ACT while at Princeton are now staff or faculty scientists. Lyman Page, the James S. McDonnell Distinguished University Professor in Physics at Princeton, was the former principal investigator of ACT.

This study was financially 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. The members of the team at the University of Cambridge were assisted by the European Research Council.


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