The radiative remnant of the Big Bang, warped by dark matter 12 billion years ago. Credit: Reiko Matsushita
Scientists probed the nature of dark matter around galaxies seen as they were 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 studying the early history of our universe. The collaboration was led by scientists at Nagoya University in Japan, and the findings were published today (August 1) in the journal Physical Review Letters.
Seeing something that happened so long ago is a challenge. Because the speed of light is limited, we see distant galaxies not as they are today, but as they were billions of years ago. But dark matter, which does not emit light, is even more difficult to observe.
“It was a crazy idea. No one realized we could do this. — Professor Masami Ouchi
Consider a distant source galaxy, even more distant than the target galaxy whose dark matter you want to study. 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 passes through this warp in spacetime, it bends, changing the apparent shape of the galaxy. The greater the amount of dark matter, the greater the resulting distortion. Astronomers can therefore measure the amount of dark matter around the foreground galaxy (the “lensing” galaxy) from the distortion.
However, after a certain threshold, scientists run into a problem. In the deepest parts of the universe, galaxies are incredibly faint. As a result, the farther from Earth we look, the less effective the gravitational lensing technique becomes. Since lensing distortion is subtle and difficult to detect in most cases, many background galaxies are required to detect the signal.
Most previous studies have stayed within the same limits. Unable to find source galaxies far enough away to measure the distortion, they can 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 that time and 13.7 billion years ago, around the beginning of our universe.
To overcome these challenges and observe dark matter from the farthest reaches of the universe, a team of researchers led by Hironao Miyatake of Nagoya University, in collaboration with the University of Tokyo, the National Astronomical Observatory of Japan and Princeton University, used a different source in 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 have been seen 12 billion years ago.
Then, to overcome the lack of galactic light even further, they used microwaves from the cosmic microwave background (CMB), the radiative remnant of the Big Bang. Using microwaves observed by the European Space Agency’s Planck satellite, the team measured how dark matter around lensed galaxies distorts the microwaves.
“Look at the 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. Nobody realized we could do that. But after I gave a lecture on a large sample of a distant galaxy, 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 can look further back in time because we used the more distant CMB to measure dark matter. For the first time, we have measured dark matter from almost the earliest moments of the universe.
After a preliminary analysis, the scientists soon realized that they had a large enough sample to detect the distribution of dark matter. Combining the large sample of a distant galaxy and lensing distortions in the CMB, they found dark matter even further back in time, 12 billion years ago. This is only 1.7 billion years after the beginning of the universe, and thus these galaxies are visible soon after their initial formation.
“I was happy that we opened a new window into that era,” Miyatake said. “12 billion years ago, things were very different. You see more galaxies in the process of forming than you do now; the first galaxy clusters also begin to form. Galaxy clusters consist of 100-1000 galaxies bound together by gravity with large amounts of dark matter.
“This result provides a very consistent picture of galaxies and their evolution, as well as the dark matter in and around galaxies and how that picture evolves over time,” said Netha Bacall, the Eugene Higgins Professor of Astronomy, Professor of Astrophysical Sciences, and director of undergraduate studies at Princeton University.
One of the most exciting findings from the study has to do with the clumping of dark matter. According to the standard theory of cosmology, the Lambda-CDM model, subtle fluctuations in the CMB form pools of tightly packed matter by gravitationally pulling on surrounding matter. This creates inhomogeneous clumps that form stars and galaxies in these dense regions. The group’s findings suggest that their clump measurement is lower than predicted by the Lambda-CDM model.
Miyatake is enthusiastic about the possibilities. “Our finding is still uncertain,” he said. “But if it’s true, it would mean that the whole model is wrong when you go further back in time. This is exciting because if the result holds after reducing the uncertainty, it could offer an improvement to the model that could provide insight into the nature of dark matter itself.
“At this point, we’re going to try to get better data to see if the Lambda-CDM model is actually able to explain the observations we have in the universe,” said Andres Plazas Malagon, an associate researcher at Princeton University. “And the consequence may be that we have to reexamine the assumptions that went into that model.”
“One of the strengths of looking at the universe using large-scale surveys like those used in this study is that you can study everything you see in the resulting images, from nearby asteroids in our solar system to the most distant galaxies in the early universe. You can use the same data to investigate many new questions,” said Michael Strauss, professor and chair of the Department of Astrophysical Sciences at Princeton University.
This study used available data from existing telescopes, including Planck and Subaru. The group reviewed only a third of the data from the Subaru Hyper Suprime-Cam Survey. The next step will be to analyze the entire data set, which should 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 Observatory’s Legacy Survey of Space and Time (LSST) to study more of the earliest parts of the cosmos. “The 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.”
Reference: “First Identification of 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, 1 August 2022, Physical Review Letters.DOI: 10.1103/PhysRevLett.129.061301
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