New findings reveal the most detailed mass map of dark matter

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.

The Big Bang (left), the wavy lines illustrate the distortion; the warped light received by the ACT (right); the new Dark Matter map (lower left). Credit: Lucy Reading-Ikkanda/Simons Foundation/ACT Collaboration

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 light captured by the ACT was used to produce a cosmic microwave background lensing mass map, a visualization of the distribution of dark matter in our sky. (Image: The Atacama Cosmology Telescope collaboration)

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).

Image: Lucy Reading-Ikkanda/Simons Foundation

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.”

References:

Frank J. Qu et al, The Atacama Cosmology Telescope: A Measurement of the DR6 CMB Lensing Power Spectrum and its Implications for Structure Growth, arXiv (2023). DOI: 10.48550/arxiv.2304.05202

Niall MacCrann et al, The Atacama Cosmology Telescope: Mitigating the impact of extragalactic foregrounds for the DR6 CMB lensing analysis, arXiv (2023). DOI: 10.48550/arxiv.2304.05196

Mathew S. Madhavacheril et al, The Atacama Cosmology Telescope: DR6 Gravitational Lensing Map and Cosmological Parameters, arXiv (2023). DOI: 10.48550/arxiv.2304.05203