Dark matter distribution

The static speed of light ensures that astronomers see distant galaxies not as they are, but as they existed a long time ago. The same is true of the matter (ordinary or dark) that surrounds these ancient galaxies.

Astronomers recently used this knowledge — as well as a cosmic signal sent shortly after the Big Bang — to map how dark matter was distributed around galaxies about 12 billion years ago. In short, they found that dark matter is less “clumsy” than expected, which, if confirmed, suggests that many accepted models of cosmology need to be revised.

The new findings are presented in a study published on August 1 in Physical examination letters.

Back to the beginning

Although it is difficult to measure “invisible” matter, the typical approach involves two galaxies, one in the foreground and one in the background. According to Einstein, the massive gravity of the foreground galaxy is actually warping the fabric of spacetime near it. Thus, as light from the background galaxy passes the foreground galaxy, it is bent as if by an optical lens. This causes the background galaxy to be both highly distorted and magnified, a phenomenon called gravitational lensing.

Because the background galaxy (or “source galaxy”) appears more distorted when the foreground galaxy (a “lensing galaxy”) has a large mass, astronomers can analyze the distortions to determine the distribution of matter—including dark matter— around the lens galaxy.

Gravitational lensing occurs when light from a distant object bends around intervening matter, such as a galaxy cluster. The result is a magnified, distorted image of the background object. (Credit: NASA/ESA)

But this method only works when the source galaxy is bright enough to shed light on the lensing galaxy. And because extremely distant galaxies are extremely faint, astronomers have so far been unable to estimate the dark matter in galaxies from about 8 billion to 10 billion years ago. This has left them largely ignorant of the true structure of the early universe.

In order to overcome this obstacle, a team of astronomers recently changed the approach. Instead of using two galaxies, they chose to use a more distant light source instead of a source galaxy: cosmic microwave background (CMB) emitted when the universe was only 300,000 years old.

“It was a crazy idea,” said Masami Ouchi, the study’s author and an astronomer at the University of Tokyo. press release. “No one realized we could do this.”

“Most researchers use source galaxies to measure the distribution of dark matter from the present to 8 billion years ago,” added study author Yuichi Harikane, an astronomer at the University of Tokyo. “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.

Illuminating dark matter

To use their new approach, the astronomers selected 1.5 million galaxies — all seen as they were about 12 billion years ago — to collectively serve as a gravitational lens. And with the more distant CMB serving as a background light source, the team was able to measure how much dark matter is scattered around these lensed galaxies.

“I was happy that we opened a new window into this era,” Hironao Miyatake, another author of the study and an astronomer at Nagoya University, said in a press release. “12 billion years ago, things were very different.”

According to the team, their results could challenge prevailing theories of cosmology, including the idea that small variations in the CMB are what initially gave rise to the earliest clumps of matter that eventually formed into stars and galaxies. As it turns out, the team says, the universe was much more homogeneous in its early years than previously thought.

“Our finding is still uncertain,” Miyatake said in a press release. “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.

Ultimately, the team says their approach will provide astronomers with more precise measurements of the amount and distribution of dark matter in ancient galaxies. It will also allow astronomers to probe other aspects of the early universe.

“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,” said Michael Strauss, a professor of astrophysical sciences at Princeton University who was not involved in the research, in a press release. “You can use the same data to investigate many new questions.”

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