First measurement of dark matter since almost the start of the universe

Aug. 1 () –

Scientists have investigated the nature of the dark matter surrounding galaxies seen as they were 12 billion years ago. billions of years further back in time than ever before.

Their findings, published in the journal ‘Physical Review Letters’offer the tantalizing possibility that the fundamental rules of cosmology differ when examining the early history of our universe.

The study authors remind that seeing something that happened so long ago is difficult. Due to the finite speed of light, we see distant galaxies not as they are today, but as they were billions of years ago, but even more difficult is observing the dark matter, which does not emit light.

In a distant source galaxy, even more distant than the galaxy whose dark matter is to be investigated, its gravitational pull on the source galaxy, including its dark matter, distorts the surrounding space and time, as predicted by Einstein’s theory of general relativity . As light from the source galaxy travels 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 surrounding the foreground galaxy (the “lens” galaxy) from the distortion.

However, after a certain point scientists run into a problem. The galaxies in the deepest parts of the universe are incredibly faint. Therefore, the further from Earth we look, the less effective this technique will be. Lens 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 limits. Unable to detect enough distant source galaxies to measure the distortion, they could only analyze dark matter from no more than 8-10 billion years ago. These limitations 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 problems 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 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 visible-light lensing galaxies, selected for viewing 12 billion years ago.

Next, to overcome the lack of light from even more distant galaxies, 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 dark matter around target galaxies distorted microwaves.

Professor Masami Ouchi of the University of Tokyo, who made many of the observations, acknowledges that trying to look at dark matter around distant galaxies “was a crazy idea. Nobody realized we could do it,” he admits. But after giving a talk about a large sample of distant galaxies, Hironao came to see me and said that it might be possible to observe 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,” adds Associate Professor Yuichi Harikane of the University of Tokyo Cosmic Ray Research Institute. However, we were able to look further back into the past because we used the more distant CMB to measure dark matter. For the first time, we measured dark matter from almost the earliest moments of the universe.”

After preliminary analysis, the researchers soon realized they had a large enough sample to detect the distribution of dark matter. Combining the large sample of distant galaxies and lensing distortions in the CMB, they detected dark matter even further back in time, as far back as 12 billion years. This is only 1.7 billion years after the beginning of the universe, so these galaxies are seen shortly after their formation.

“I’m glad we’ve opened a new window on that time,” Miyatake says. “Twelve billion years ago, things were very different. You see more galaxies forming than today; the first galaxies are also starting to form.” galaxy clusters”. Galaxy clusters comprise between 100 and 1,000 galaxies held 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 this picture evolves over time,” said Neta Bahcall, Eugene Professor of Astronomy. Higgins, professor of astrophysical sciences and director of university studies at Princeton University (United States).

One of the most interesting findings of the researchers is related to the agglomeration of dark matter. According to the standard theory of cosmology, the Lambda-CDM model, subtle fluctuations in the CMB form clumps of densely packed matter by attracting surrounding matter through gravity. This creates inhomogeneous clusters that form stars and galaxies in these dense regions. The group’s findings suggest that their measurement of crowding was lower than that predicted by the Lambda-CDM model.

Miyatake is excited about the possibilities. “Our finding is still uncertain,” he acknowledges, “but if true, it would suggest that the entire model is flawed as you go back in time. This is exciting because if the result holds after uncertainties are reduced, could suggest an improvement to the model that could provide insights into the nature of dark matter itself.”

“Right now, we will try to get better data to see if the Lambda-CDM model is really capable of explaining the observations we have in the universe,” said Andrés Plazas Malagón, a research associate at Princeton University. “And the consequence it may be that we need to revise the assumptions that were made in this model.”

“One of the strengths of looking at the universe with large-scale surveys like the ones 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 largest galaxies. far from the early universe. The same data can be used to explore many new questions,” says Michael Strauss, professor and director of the Department of Astrophysical Sciences at Princeton University.