For millennia, humans have been fascinated by the mysteries of the cosmos. From ancient civilizations such as the Babylonians, Greeks, and Egyptians to modern-day astronomers, the allure of the starry sky has inspired countless quests to unravel the secrets of the universe.
Although models explaining the cosmos have been around for centuries, the field of cosmology, in which scientists employ quantitative methods to gain insights into the universe’s evolution and structure, is comparatively nascent. Its foundation was established in the early 20th century with the development of Albert Einstein’s theory of general relativity, which now serves as the basis for the standard model of cosmology.
Now, a set of papers submitted to The Astrophysical Journal by researchers from the Atacama Cosmology Telescope (ACT) collaboration has revealed a groundbreaking new image that shows the most detailed map of matter distributed across a quarter of the entire sky, reaching deep into the cosmos. It confirms Einstein’s theory about how massive structures grow and bend light, with a test that spans the entire age of the universe.
“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 authors note that the lumpiness quality is attributed to the uneven distribution of dark matter throughout the universe and that its growth has remained consistent with earlier predictions. And, despite making up 85% of 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.
Funded by the National Science Foundation, the ACT was built by Penn and Princeton University and started observations to track down the elusive dark matter in 2007. The more than 160 collaborators who have built and gathered data from ACT, which is situated 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 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.
“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 Penn 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.”
Penn researchers Gary Bernstein and Bhuvnesh Jain have led research mapping dark matter by using visible light emitted from relatively nearby galaxies as opposed to light from the CMB. “Interestingly, we found matter to be a little less lumpy than the simplest theory predicts,” Jain says “However, Mark and Mathew’s beautiful work on the CMB agrees perfectly with the theory.”
“The stunning ACT dark matter maps severely narrow down the times and places where the simplest theory could be going wrong,” Bernstein says. “One speculation is that a new feature of gravity or dark energy is appearing just in the last few billion years, after the era ACT is measuring.”
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.
Mathew Madhavacheril is an assistant professor in the Department of Physics and Astronomy in the School of Arts & Sciences at the University of Pennsylvania.
Mark Devlin is the Reese W. Flower Professor of Astronomy and Astrophysics in the Department of Physics and Astronomy in the School of Arts & Sciences at Penn.
Gary Bernstein is the Reese W. Flower Professor of Astronomy and Astrophysics in the Department of Physics and Astronomy in Penn’s School of Arts & Sciences.
Bhuvnesh Jain is the Walter H. and Leonore C. Annenberg Professor in the Natural Sciences and co-director of the Center for Particle Cosmology in the Department of Physics and Astronomy at Penn’s School of Arts & Sciences.
The pre-print articles highlighted in this release are available on https://act.princeton.edu/ and will appear on the open-access arXiv.org. 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. Team members at the University of Cambridge were supported by the European Research Council.