Science

Dark matter 12 billion years old discovered for the first time

Scientists have discovered dark matter around galaxies that existed about 12 billion years ago, making it the earliest discovery of this mysterious substance dominating the universe.

The findings, from a collaboration led by researchers at Japan’s Nagoya University, suggest that dark matter in the early universe is less “clumpy” than many current cosmological models predict. If further work confirms this theory, it could change scientists’ understanding of how galaxies evolve and suggest that the fundamental laws that govern the cosmos may have been different when the 13.7 billion-year-old universe was only 1.7 billion years old.

The key to mapping dark matter in the very early universe is the cosmic microwave background (CMB), a kind of fossil radiation left over from the Big Bang that propagates throughout the cosmos.

“Look at the dark matter around distant galaxies? It was a crazy idea. Nobody knew we could do it,” University of Tokyo professor Masami Ouchi said in a statement. “But after I gave a talk about a large sample of a distant galaxy, Hironao approached me and said that it was possible to study the dark matter around these galaxies using the CMB.”

Related: With the James Webb Space Telescope at full capacity, scientists hope to discover the earliest galaxies

Because light takes a finite time to travel from distant objects to Earth, astronomers see other galaxies as they were when the observed light left them. The farther away the galaxy, the longer the light went to us and, therefore, the farther in time we see them, so we see the most distant galaxies as they were billions of years ago, in the rudimentary Universe.

Observing dark matter is even more difficult. Dark matter is a mysterious substance that makes up about 85% of the total mass of the universe. It does not interact with matter and light, like the ordinary matter, consisting of protons and neutrons, which fills the stars, planets and us.

Detection of “early” dark matter

To “see” dark matter at all, astronomers must rely on its interaction with gravity.

According to Einstein’s theory of relativity, objects of enormous mass cause the curvature of space-time. A common analogy is an elastic rubber sheet holding balls of increasing mass. The larger the mass, the more “dent” it leaves on the sheet. In the same way, the larger the space object, the more severe the curvature of space-time it causes.

Massive objects such as galaxies cause space-time to warp so much that light from sources behind the galaxy bends just like a ball rolling on a stretched rubber sheet bends. This effect changes the position of the light source in the sky, a phenomenon called gravitational lensing.

To study the distribution of dark matter in a galaxy, astronomers can observe how light from a source behind that galaxy changes as it passes through the lens galaxy. The more dark matter the lens galaxy contains, the greater the distortion of the light passing through it.

But the technology has its limitations.

Since the earliest and most distant galaxies are very dim, as astronomers look deeper into the universe and further into the past, the lensing effect becomes more subtle and difficult to observe, and scientists need both many background sources and many early galaxies. to point lensing by dark matter. This problem has limited mapping of the distribution of dark matter to galaxies that are between 8 and 10 billion years old.

But the CMB is an older source of light than any galaxy. The CMB is the ubiquitous radiation that came into being when the universe cooled enough to allow atoms to form, reducing the number of free electrons that scatter photons, in what cosmologists call the “final scattering.” The reduction in the number of free electrons allowed photons to travel freely, meaning that the universe suddenly ceased to be opaque and became transparent to light.

And just like light from other distant sources, the CMB can be distorted by dark matter galaxies due to gravitational lensing.

“Most researchers use source galaxies to measure the distribution of dark matter from present to 8 billion years ago,” Yuichi Harikane, associate professor at the University of Tokyo, said in a statement. “However, we could look even further into the past because we used the more distant CMB to measure dark matter.”

The team combined the lens distortions of a large sample of ancient galaxies with CMB distortions to detect dark matter dating back to when the universe was only 1.7 billion years old. And this ancient dark matter paints a very different cosmic picture.

“For the first time, we have measured dark matter almost from the earliest moments of the universe,” Harikane said. “12 billion years ago, things were very different. You see more galaxies that are in the process of formation than at present; the first clusters of galaxies also begin to form.”

These clusters can include from 100 to 1000 galaxies bound by gravity with a large amount of dark matter.

Is dark matter lumpy?

One of the most important aspects of the team’s discovery is the possibility that dark matter in the early universe is less clumpy than many current models suggest.

For example, the widely accepted Lambda-CDM model suggests that tiny fluctuations in the CMB should have resulted in gravity creating densely packed pockets of matter. These fluctuations eventually lead to the collapse of matter with the formation of galaxies, stars and planets, and should also lead to the formation of dense pockets of dark matter.

“Our discovery is still undetermined,” Harikane said. “But if this is true, it suggests that the whole model is wrong if you go back in time. This is interesting because if the result holds after the uncertainties are reduced, it may offer an improvement in the model that insights can provide. into the nature of dark matter itself.”

The team will continue to collect data to evaluate whether the Lambda-CDM model is consistent with observations of dark matter in the early universe or whether the assumptions underlying the model need to be revisited.

The data the team used to make their findings came from the Subaru Hyper Suprime-Cam Survey, which analyzes data from a telescope in Hawaii. But so far, researchers have used only a third of that data, meaning that as more observations are included, a better map of the distribution of dark matter may be available.

The team is also looking forward to data from the Vera S. Rubin Legacy of Space and Time Observatory (LSST), which could allow researchers to look even further back into dark matter.

“LSST will allow us to observe half of the sky,” Harikane said. “I don’t see any reason why we couldn’t see the distribution of dark matter 13 billion years ago.”

The team’s research was published in August. 1 in Physical Review Letters.

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