Scientists have probed the nature of the dark matter surrounding galaxies as it was 12 billion years ago, billions of years further back in time than ever before. Their findings offer the tantalizing possibility that the fundamental rules of cosmology may differ when considering the early history of our universe. The collaboration was carried out by scientists at Nagoya University in Japan, and the findings were published today (August 1) in the journal Physical Review Letters. Watching something that happened so long ago is challenging. Because the speed of light is finite, we see distant galaxies not as they are today, but as they were billions of years ago. But even more difficult is the observation of dark matter, which does not emit light. “It was a crazy idea. Nobody realized we could do that.” — Professor Masami Ouchi Consider a distant source galaxy, even further away than the target galaxy whose dark matter is to be probed. As predicted by Einstein’s theory of general relativity, the gravitational pull of the foreground galaxy, including its dark matter, distorts the surrounding space and time. As light from the source galaxy travels through this distortion in spacetime, it bends, changing the apparent shape of the galaxy. The greater the amount of dark matter, the greater the resulting distortion. Therefore, astronomers can measure the amount of dark matter around the foreground galaxy (the “lens” galaxy) from the distortion. However, beyond a certain limit, scientists face a problem. In the deepest parts of the universe, galaxies are incredibly faint. As a result, the farther we look from Earth, the less effective the gravitational lensing technique becomes. Because lensing distortion is subtle and difficult to detect in most cases, many background galaxies are required to detect the signal. Most previous studies stuck to the same limits. Unable to detect enough distant galaxies to measure the distortion, they could only analyze dark matter from no more than 8-10 billion years ago. These constraints left open the question of the distribution of dark matter between this time and 13.7 billion years ago, around the beginning of our universe. To overcome these challenges and observe dark matter from the most distant regions of the universe, a team of researchers led by Hironao Miyatake from Nagoya University, in collaboration with the University of Tokyo, the National Astronomical Observatory of Japan and the University Princeton, they used a different source of background light, the microwaves released by the Big Bang itself. First, using data from the Subaru Hyper Suprime-Cam Survey (HSC) observations, the team identified 1.5 million lensed galaxies using visible light, selected to be seen 12 billion years ago. Then, to overcome the lack of galactic light even further away, they used microwaves from the cosmic microwave background (CMB), the leftover radiation from the Big Bang. Using microwaves observed by the European Space Agency’s Planck satellite, the team measured how the dark matter around the lens galaxies distorted the microwaves. “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 understood that we could do this. But after I gave a talk about a large distant galaxy sample, Hironao came to me and said that it might be possible to look at the dark matter around these galaxies with the CMB.” “Most researchers use source galaxies to measure the distribution of dark matter from the present to eight billion years ago,” added Assistant Professor Yuichi Harikane of the University of Tokyo’s Cosmic Ray Research Institute. “However, we could look further back in time because we used the more distant CMB to measure dark matter. For the first time, we measured dark matter almost from the earliest moments of the universe.” After a preliminary analysis, the scientists soon realized that they had a large enough sample to trace the distribution of dark matter. By combining the large distant galaxy sample and lensing distortions in the CMB, they traced dark matter even further back in time to 12 billion years ago. That’s only 1.7 billion years after the beginning of the universe, and so these galaxies look just after they were first formed. “I was happy that we opened a new window on that era,” Miyatake said. “12 billion years ago, things were very different. You see more galaxies in the process of forming than you do today. the first clusters of galaxies also begin to form.” Clusters of galaxies contain 100-1000 gravitationally bound galaxies 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 that picture evolves with time,” said Neta Bahcall, Eugene Higgins Professor of Astronomy, Professor of Astrophysics and Director of Undergraduate Studies at Princeton University. One of the most exciting discoveries from the study was related to the accumulation of dark matter. According to the standard theory of cosmology, the Lambda-CDM model, subtle fluctuations in the CMB form pools of dense matter by pulling the surrounding matter through gravity. This creates inhomogeneous clumps that form stars and galaxies in these dense regions. The team’s findings suggest that the measure of aggregation was lower than predicted by the Lambda-CDM model. Miyatake is enthusiastic about the possibilities. “Our finding is still uncertain,” he said. “But if true, it would suggest that the whole model is flawed as you go further back in time. This is exciting because if the result remains after reducing the uncertainties, it could indicate an improvement in the model that may provide 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 really able to explain the observations we have in the universe,” said Andrés Plazas Malagón, a research associate at Princeton University. “And the consequence may be that we have to rethink the assumptions that went into that model.” “One of the strengths of examining the universe using large-scale surveys like those used in this research is that you can study everything you see in the resulting images, from nearby asteroids in our solar system to the most distant galaxies from early universe. You can use the same data to explore many 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 team has only looked at a third of the Subaru Hyper Suprime-Cam Survey data. The next step will be to analyze the entire data set, which will allow a more precise measurement of the distribution of dark matter. In the future, the research team expects to use an advanced dataset such as the Vera C. Rubin’s Legacy Survey of Space and Time (LSST) Observatory to explore more of the early parts of space. “LSST will allow us to observe half the sky,” Harikane said. “I see no reason why we can’t see the distribution of dark matter 13 billion years ago down the line.” Citation: “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 Ya. waza Atshi, Nanaka Yamamoto, Neta Bahcall, Satoshi Miyazaki, and Andrés A. Plazas Malagón, August 1, 2022, Physical Review Letters.DOI: 10.1103/PhysRevLett.129.061301
