Space & Science

Distribution of dark matter around galaxies 12 billion years ago revealed

Cosmic Microwave Background (CMB)

The radiation residue from the Big Bang, distorted by dark matter 12 billion years ago. Credit: Reiko Matsushita

Scientists have studied the nature of dark matter surrounding galaxies 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 examining the ancient history of our universe. The collaboration was led by scientists from Nagoya University in Japan and the results were published today (August 1) in the journal Physical examination letters.

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Seeing something that happened so long ago is difficult. As the speed of light is finite, we do not see distant galaxies as they are today, but as they were billions of years ago. But it is even more difficult to observe black matterwhich 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 we want to study. As predicted by Einstein’s theory of general relativity, the gravitational pull of the foreground galaxy, including its dark matter, distorts 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 threshold, scientists run into a problem. In the depths of the universe, galaxies are incredibly faint. As a result, the further away from Earth we look, the less effective the gravitational lensing technique becomes. Because lens distortion is subtle and difficult to detect in most cases, many background galaxies are needed to detect the signal.

Most previous studies have stuck with the same limitations. Unable to detect enough distant source galaxies to measure the distortion, they could only analyze dark matter from no more than 8 to 10 billion years ago. These limitations leave 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 far reaches of the universe, a team of researchers led by Hironao Miyatake of Nagoya Universityin collaboration with the University of Tokyothe National Astronomical Observatory of Japanand princeton universityused another source of background light, the microwaves emitted by the big Bang himself.

First, using data from Subaru Hyper Suprime-Cam Survey (HSC) observations, the team identified 1.5 million lens galaxies using visible light, selected to be seen 12 billion years ago. ‘years.

Then, to overcome the lack of light from even more distant galaxies, they used microwaves from the cosmic microwave background (CMB), the radiation residue from the big Bang. Using microwaves observed by the European Space Agency’s Planck satellite, the team measured how dark matter around lens galaxies distorted microwaves.

“Look at dark matter around distant galaxies?” asked Professor Masami Ouchi of the University of Tokyo, who made many observations. “It was a crazy idea. No one realized we could do this. But after giving a talk about a large sample of a distant galaxy, Hironao came to me and said he might be possible to observe 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 Cosmic Ray Research Institute. from the University of Tokyo. “However, we were able to look further into the past because we used the more distant CMB to measure dark matter. For the first time, we have been measuring dark matter since 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. By combining the large sample of distant galaxies and the lensing distortions in CMB, they detected 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 so these galaxies are seen shortly after they formed.

“I was happy that we opened a new window to that era,” Miyatake said. “Twelve billion years ago things were very different. You see more galaxies being formed than you do now; the first clusters of galaxies also begin to form. Galaxy clusters include 100 to 1000 gravity-bound galaxies with large amounts of dark matter.

“This result gives a very coherent picture of galaxies and their evolution, as well as dark matter in and around galaxies, and how that picture changes over time,” said Neta Bahcall, professor of astronomy Eugene Higgins, professor of Astrophysical Sciences, and Director of Undergraduate Studies at princeton university.

One of the most exciting findings of the study was related to the clumping 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 in surrounding matter by gravity. This creates inhomogeneous clusters that form stars and galaxies in these dense regions. The group’s results suggest that their clumping measurement was lower than predicted by the Lambda-CDM model.

Miyatake is excited about the possibilities. “Our discovery is still uncertain,” he said. “But if true, that would suggest the whole model is flawed as you go further back in time. That’s exciting because if the result holds after reducing uncertainties, that might suggest improvement. of the model that could 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, associate researcher at the University. from Princeton. “And the consequence may be that we have to revisit the assumptions that went into this model.”

“One of the strengths of observing the universe using large-scale surveys, such as those used in this research, is that you can study everything you see in the resulting images, from asteroids close to our solar system to the most distant galaxies in the 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 group only looked at a third of the Subaru Hyper Suprime-Cam survey data. The next step will be to analyze the data set, which should allow a more precise measurement of the distribution of dark matter. In the future, the research team plans to use an advanced dataset like the Legacy Survey of Space and Time (LSST) from the Vera C. Rubin Observatory to further explore the earliest parts of space. “LSST will allow us to observe half the sky,” Harikane said. “I see no reason why we couldn’t see the distribution of dark matter 13 billion years ago next.”

Reference: “First Identification of a CMB Lensing Signal Produced by 1.5 Million Galaxies at z~4: Constraints on High Redshift Matter Density Fluctuations” by Hironao Miyatake, Yuichi Harikane, Masami Ouchi, Yoshiaki Ono, Nanaka Yamamoto, Atsushi J. Nishizawa, Neta Bahcall, Satoshi Miyazaki and Andrés A. Plazas Malagón, August 1, 2022, Physical examination letters.
DOI: 10.1103/PhysRevLett.129.061301

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