Scientists reveal distribution of dark matter around galaxies 12 billion years ago – further back in time than ever before

A collaboration led by researchers from Nagoya University in Japan has studied the nature of dark matter around galaxies as it was 12 billion years ago, billions of years further back in the universe than ever before. Their results, published in Physical examination lettersoffer the tantalizing possibility that the basic rules of cosmology may be different when we examine the ancient history of our universe.

It’s hard to watch something that happened so long ago. Because of the limited speed of light, 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 dark matter, which does not emit light.

Consider a distant source galaxy, even more distant than the galaxy whose dark matter we wish to study. The foreground galaxy’s gravity, including its dark matter, distorts the surrounding space and time, as predicted by Einstein’s theory of general relativity. When the light from the source galaxy passes through this distortion, it bends, changing the apparent shape of the galaxy. The greater the amount of dark matter, the greater the distortion. Thus, scientists can measure the amount of dark matter around the foreground galaxy (the “lensing galaxy”) from the distortion.

But beyond a certain point, scientists run into a problem. The galaxies in the depths of the universe are incredibly faint. Therefore, the further we see from Earth, the less effective this technique becomes. Lensing distortion is subtle and difficult to detect in most cases, so many background galaxies are needed to detect the signal.

Most previous studies have stuck to 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 constraints 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 research team led by Hironao Miyatake of Nagoya University, in collaboration with the University of Tokyo, the National Astronomical Observatory of Japan and ‘Princeton University, used another source. of background light, the microwaves emitted by the Big Bang itself.

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

Then, to overcome the lack of light from even more distant galaxies, they used microwaves from the cosmic microwave background (CMB), the radiation left over from the Big Bang. Using microwaves observed by the European Space Agency’s Planck satellite, the team measured how dark matter around lenticular galaxies distorted 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. Nobody realized we could do that. But after giving a lecture on a large sample of distant galaxies, Hironao came to me and said that it 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. “But we could see further into the past because we used the more distant CMB to measure dark matter. For the first time, we have measured dark matter since almost the earliest moments of the universe. »

After a preliminary analysis, the researchers quickly 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 the CMB, they discovered dark matter even further back in time, 12 billion years ago. This is only 1.7 billion years after the beginning of the universe, which is why these galaxies are seen shortly after they are formed.

“I was happy that we opened a new window to that era,” Miyatake said. “12 billion years ago, things were very different. You see more galaxies forming than today; the first clusters of galaxies are also starting to form. Clusters of galaxies include 100 to 1000 gravitationally bound galaxies with large amounts of dark matter.

“This result provides a very coherent picture of galaxies and their evolution, as well as dark matter in and around galaxies and how this picture changes over time,” said Neta Bahcall, Eugene Higgins Professor of Astronomy, Professor of Astrophysical Sciences and Director of undergraduate studies at Princeton University.

One of the researchers’ most interesting discoveries was related to 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 gravitationally pulling in the surrounding matter. 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, it suggests that the whole model is flawed when you go back in time. It’s exciting because if the result holds after reducing uncertainties, it could indicate an improvement in 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 research scientist at Princeton University. “And the consequence may be that we have to revise the assumptions that were in this model. »

“One of the strengths of observing the universe using large surveys like 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 enable a more precise measurement of the distribution of dark matter. In the future, the team plans to use an advanced dataset such as 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 see half the sky,” Harikane said. “I see no reason why we couldn’t see the distribution of dark matter 13 billion years ago. »

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