A new survey of the night sky has observed dark matter — the mysterious substance that makes up more than a quarter of the universe but emits no light of its own — as it existed 12 billion years ago, not long after the universe began. This oldest-ever snapshot of dark matter in the universe offers the tantalizing possibility that the fundamental rules of cosmology may have differed during the early history of our universe.

To observe dark matter this far back in time, a research team led by Nagoya University’s Hironao Miyatake, in collaboration with the University of Tokyo, National Astronomical Observatory of Japan, and Princeton University, relied on a different source of background light, the microwaves released from the Big Bang at the origin of the universe. The study, the first to observe dark matter so far back in time using the Cosmic Microwave Background (CMB), was published August 1, 2022 in Physical Review Letters.

“Look at dark matter around distant galaxies?” asked Professor Masami Ouchi of the University of Tokyo, who made many of the observations. “It was a crazy idea. No one realized we could do this. But after I gave a talk about a large distant galaxy sample, Hironao came to me and said it may be possible to look at dark matter around these galaxies with the CMB.”

Typically, researchers detect dark matter by observing how light bends due to the gravitational pull that this invisible matter exerts on light emitted from background galaxies located far away. Due to the amount of time it takes for light to travel, distant galaxies appear to us on Earth as they existed billions of years ago.

Until now, observing dark matter around distant galaxies seemed impossible because light traveling from the far reaches of the universe is incredibly faint. The researchers were able to overcome this challenge by using the CMB as a light source, instead of using the typically used background galaxies.

Using data from the observations of the Subaru Hyper Suprime-Cam Survey (HSC) at the Subaru Telescope in Hawaii, the team identified 1.5 million galaxies that could be seen by the faintly visible light they emitted 12 billion years ago. Next, to overcome the lack of background galaxy light even further away, the team employed cosmic microwaves observed by the European Space Agency’s Planck satellite to measure how the dark matter around the galaxies distorted the microwaves.

“Most researchers use background source galaxies to measure dark matter distribution from the present to eight billion years ago,” said Assistant Professor Yuichi Harikane of the Institute for Cosmic Ray Research, University of Tokyo. “However, we could look further back into the past because we used the more distant CMB as a background source to measure the dark matter around distant galaxies. For the first time, we were measuring dark matter from almost the earliest moments of the universe.”

After a preliminary analysis, the researchers realized they had a large enough sample to detect the distribution of dark matter around these galaxies. Combining the large distant galaxy sample and the lensing distortions in CMB, they detected dark matter further back in time, from 12 billion years ago. This is only 1.7 billion years after the beginning of the universe, and thus these galaxies are seen soon after they first formed.

“I was happy that we opened a new window into that era,” said Miyatake, who was a postdoctoral researcher at Princeton University from 2012 to 2015. "Twelve billion years ago, things were very different. You see more galaxies that are in the process of formation than at the present; the first galaxy clusters are starting to form as well.” Galaxy clusters comprise 100 to 1000 galaxies bound by gravity with large amounts of dark matter.

“This result gives a very consistent picture of galaxies and their evolution, as well as the dark matter in and around galaxies, and how these galaxies evolve with time,” said Neta Bahcall, Eugene Higgins Professor of Astronomy, professor of astrophysical sciences, and director of undergraduate studies in astrophysics at Princeton University.

The study provided hints that the fundamental rules of cosmology may have differed during the early history of our universe. One of the most exciting of the researchers’ findings was related to the clumpiness of the galaxies and their dark matter distribution. According to the standard theory of cosmology, the Lambda-CDM model, subtle fluctuations in the CMB form pools of densely packed matter by attracting surrounding matter through gravity. This creates inhomogeneous clumps that form stars and galaxies in these dense regions. The group’s findings suggest that their clumpiness measurement was lower than predicted by the Lambda-CDM model.

Miyatake is excited about the possibilities. “Our finding is still uncertain,” he said. ”But if it is true, it would suggest that the entire model is flawed as you go further back in time. This is exciting, because it could suggest – if the result holds after the uncertainties are reduced -- an improvement of the model that may give insight into the nature of dark matter itself.”

“At this point, we will try to get better data to see if the Lambda-CDM model is actually able to explain the observations that we have in the universe,” said Andrés Plazas Malagón, associate research scholar at Princeton University. “And the consequence may be that we need to revisit the assumptions that went into this model.”

“One of the strengths of looking at the universe using large scale surveys such as the ones used in this research is that you can study everything that you see in the resulting images, from nearby asteroids in our solar system to the most distant galaxies from the early universe. You can use the same data to explore a lot of new questions,” said Michael Strauss, professor and chair of the Department of Astrophysical Sciences at Princeton University.

This study used data available from existing telescopes, including Planck and Subaru. The group has only reviewed a third of the Subaru Hyper Suprime-Cam Survey data taken thus far. The next step will be to analyze the entire data set, which should allow for a more precise measurement of the dark matter distribution. In the future, the team expects to use an advanced data set like the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) survey to explore more of the earliest parts of space. “LSST will allow us to observe half the sky,” Harikane said. “I don’t see any reason we couldn’t see the dark matter distribution 13 billion years ago next.”

The study, “First identification of a CMB lensing signal produced by 1.5 million galaxies at z ~4: Constraints on matter density fluctuations at high redshift,” by Hironao Miyatake, Yuichi Harikane, Masami Ouchi, Yoshiaki Ono, Nanaka Yamamoto, Atsushi J. Nishizawa, Neta Bahcall, Satoshi Miyazaki and Andrés A. Plazas Malagón, was published in the journal Physical Review Letters on August 1, 2022. 10.1103/PhysRevLett.129.061301

The Hyper Suprime-Cam (HSC) collaboration includes the astronomical communities of Japan and Taiwan, and Princeton University. The HSC instrumentation and software were developed by the National Astronomical Observatory of Japan (NAOJ), the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), the University of Tokyo, the High Energy Accelerator Research Organization (KEK), the Academia Sinica Institute for Astronomy and Astrophysics in Taiwan (ASIAA), and Princeton University. Funding was contributed by the FIRST program from the Japanese Cabinet, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japan Society for the Promotion of Science (JSPS), Japan Science and Technology Agency (JST), the Toray Science Foundation, NAOJ, Kavli IPMU, KEK, ASIAA, and Princeton University.