Detailed map of matter in the cosmos confirms Einstein's theory of general relativity, astronomers say

Researchers from the Atacama Cosmology Telescope (ACT) collaboration have submitted a set of papers to the Astrophysical Journal featuring a groundbreaking new map of dark matter distributed across a quarter of the entire sky and extending deep into the cosmos.

The result confirms Albert Einstein’s theory of general relativity – which predicts how massive structures grow and bend light – with a test that spans the 14-billion-year life of the universe.

“We have used the cosmic microwave background (CMB), the oldest light in the universe emitted soon after the Big Bang, to measure how dark matter – the invisible stuff that makes up the majority of the matter in the universe – is distributed on large scales,” says Adam Hincks, an assistant professor in the University of Toronto’s David A. Dunlap department of astronomy and astrophysics in the Faculty of Arts & Science.

“The distribution agrees very well with theoretical predictions. It's a really satisfying result scientifically because it shows we have a robust understanding of how our universe grows and evolves. The fact that we can successfully explain how our cosmos works with this level of precision is amazing.”

In addition to Hincks, the international collaboration includes fellow U of T astrophysicists Richard Bond from the Canadian Institute for Theoretical Astrophysics (CITA) and Renée Hložek from the Dunlap Institute for Astronomy & Astrophysics.

The U of T team also includes post-doctoral researcher Yilun Guan (Dunlap Institute and David. A. Dunlap department), who played a leading role in pre-processing and calibrating the raw data of the telescope; post-doctoral researcher Zack Li (CITA) who worked on models of how different astronomical objects appear at our wavelengths that were heavily used to ensure that our results were not contaminated by other signals from the sky. Several students also participated.

“I started working on ACT as a graduate student in 2008 while at Oxford University, and am really excited to come full circle, supervising and working with talented students and post-doctoral researchers on ACT,” says Hložek.

“We saw the first detection of small-scale lensing from the ground with ACT in 2011 and now have a pristine measurement of these tiny deflections caused by the matter between us and this last scattering surface of the CMB. It is such a privilege to have worked with this data for almost 15 years and to do so with so many expert colleagues and the support of Canadian institutions like CIFAR and the Digital Research Alliance.”

Although dark matter makes up roughly 85 per cent of the matter in the universe and has shaped its evolution, it has remained hard to detect because it doesn’t interact with light or other forms of electromagnetic radiation. As far as astronomers know, dark matter only interacts with gravity.

To track it down, the more than 160 collaborators who have built and gathered data from ACT in the high Chilean Andes observed 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 a “baby picture of the universe.” The team tracked how the gravitational pull of large, massive structures – including those comprising dark matter – warps the CMB on its 14-billion-year journey to us, in the same way a magnifying glass bends light as it passes through the lens.

The measurements show that the “lumpiness” of the universe and the rate at which the cosmos is expanding after 14 billion years of evolution are just what you’d expect from our standard model of cosmology based on general relativity.

New insights into 'the crisis in cosmology'

The results also provide new insights into an ongoing debate referred to as “the crisis in cosmology.” The crisis stems from recent measurements that use a different background light – one emitted from stars in galaxies rather than the cosmic microwave background. These have produced results that suggest the dark matter was not lumpy enough under the standard model of cosmology and led to concerns that the model may be broken. However, the team’s latest results from ACT were able to precisely assess that the vast lumps seen in this image are the exact right size.

“This quite different path to the cosmological parameters from CMB science is spot-on with what we got with the Planck satellite – another collaboration with significant U of T contribution – and with ACT. That the results are so accurate using this lensing method is amazing,” Bond says.

“It also feeds into a major set of much-debated cosmological topics that may indicate that inferences about the universe from CMB and other early time observations don’t agree with more nearby measures with different data sets.”

U of T’s contribution to the ACT collaboration also included the expertise and enormous computing power to process a vast amount of raw data and turn them into maps of the sky. The Niagara supercomputer, located at the SciNet supercomputer centre at U of T, was an essential resource for these results. SciNet is funded by the CFI under the auspices of Compute Canada, the Government of Ontario, the Ontario Research Fund–Research Excellence and U of T.

ACT operated for 15 years before being decommissioned in September 2022. Further 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 ten times faster than ACT.

“In cosmology, as in all of science, having independent measurements that test the same theoretical model is really important,” says Hincks.

“Not only is our result at the forefront of sensitivity in using the Cosmic Microwave Background to probe the largest structures in the universe, it is complementary to measurements that look at different periods in the universe's history using different techniques. What we're finding is that the overall picture we have about the evolution of the cosmos is consistent. At the same time, our increasingly better measurements are allowing us to scrutinize the details of that picture when these different probes show apparent discrepancies.”

The research was supported by the U.S. National Science Foundation, Princeton University, the University of Pennsylvania and a Canada Foundation for Innovation award.

With files from the Atacama Cosmology Telescope collaboration