Scientists from Nagoya University in Japan led a group that looked at what dark matter was like around galaxies 12 billion years ago. This is billions of years further back in time than any other study has gone.

According to their research, which was published in Physical Review Letters, it’s possible that the fundamental rules of cosmology might be different when looking at the early history of our universe.

It’s challenging to remember something that happened so long ago. We observe distant galaxies not as they are today but as they were billions of years ago due to the limited speed of light. Observing dark matter, which does not emit light, is considerably more difficult.

Think about a source galaxy that is even farther away than the galaxy whose dark matter you want to learn more about.

Einstein’s theory of general relativity says that the gravitational pull of the foreground galaxy, which includes its dark matter, warps the space and time around it.

When passing through this distortion, the light from the source galaxy bends and appears to change the galaxy’s apparent shape.

The distortion increases with the amount of dark matter. Because of the distortion, researchers can calculate the amount of dark matter in the vicinity of the foreground galaxy (also known as the “lens” galaxy).

But after a certain threshold, scientists run into trouble. In the farthest reaches of the universe, galaxies are exceedingly dim. As a result, this strategy gets less successful as we look farther away from Earth.

To pick up the signal, you need several galaxies in the background to mask the lensing distortion.

The majority of earlier investigations stayed within the same parameters. They could only investigate dark matter from a time period of no more than 8–10 billion years ago due to their inability to find enough distant source galaxies to assess the distortion.

For this reason, researchers were unable to answer the question of how dark matter was distributed between this time and the beginning of our universe.

A research team led by Hironao Miyatake from Nagoya University, in association with the University of Tokyo, the National Astronomical Observatory of Japan, and Princeton University, overcame these difficulties and observed dark matter from the farthest reaches of the universe by using a different source of background light, the microwaves released from the Big Bang itself.

First, the team used data from the Subaru Hyper Suprime-Cam Survey (HSC) to find 1.5 million lens galaxies that could be seen with visible light and were around 12 billion years ago.

They then used microwaves from the cosmic microwave background (CMB), the radiation leftover from the Big Bang, to overcome the lack of galaxy light even further out.

With the help of the Planck satellite from the European Space Agency, the team measured how the dark matter around the lens galaxies bent the microwaves.

Looking at dark matter around distant galaxies “was a crazy idea. No one realized we could do this,” says Professor Masami Ouchi of the University of Tokyo, who made many of the observations. 

“But after I gave a talk about a large distant galaxy sample, Hironao came to me and said it may be possible to look at dark matter around these galaxies with the CMB.”  

According to Assistant Professor Yuichi Harikane of the Institute for Cosmic Ray Research at the University of Tokyo, “Most researchers use source galaxies to measure dark matter distribution from the present to eight billion years ago. 

“However, we could look further back into the past because we used the more distant CMB to measure dark matter. For the first time, we were measuring dark matter from almost the earliest moments of the universe.” 

After doing a preliminary investigation, the researchers quickly determined that their sample size was sufficient to detect the dispersion of dark matter. Using the enormous sample of distant galaxies and the lensing distortions in the CMB, scientists identified dark matter from 12 billion years ago. This is barely 1.7 billion years after the Big Bang, hence these galaxies are observed shortly after their formation.

“I was happy that we opened a new window into that era,” adds Miyatake. “12 billion years ago, things were very different. You see more galaxies that are in the process of formation than at the present; the first galaxy clusters are starting to form as well.” 

Galaxy clusters are made up of 100–1000 galaxies that are gravitationally bound and have a lot of dark matter.

According to Neta Bahcall, the Eugene Higgins Professor of Astronomy, Professor of Astrophysical Sciences, and Director of Undergraduate Studies at Princeton University, “This result gives a very consistent picture of galaxies and their evolution, as well as the dark matter in and around galaxies, and how this picture evolves with time.”

One of the most interesting things the researchers found was that dark matter sticks together. The Lambda-CDM model, the accepted explanation of cosmology, states that minute variations in the CMB attract nearby matter by gravity to produce pools of densely packed matter.

In these crowded areas, this leads to inhomogeneous aggregates that eventually give rise to stars and galaxies. According to the group’s findings, their clumpiness measurement was less than what the Lambda-CDM model anticipated.

Miyatake is excited about the potential. He says, “Our finding is still uncertain. But if it is true, it would suggest that the entire model is flawed as you go further back in time. This is exciting because if the result holds after the uncertainties are reduced, it could suggest an improvement of the model that may provide insight into the nature of dark matter itself.”

ndrés Plazas Malagón, an associate research fellow at Princeton University, adds, “At this point, we will try to get better data to see if the Lambda-CDM model is actually able to explain the observations that we have in the universe. And the consequence may be that we need to revisit the assumptions that went into this model.” 

Large-scale surveys, like the ones employed in this work, have the advantage of allowing for the study of everything visible in the resulting images, from nearby asteroids in our solar system to the most distant galaxies from the early cosmos. According to Princeton University professor and department chair Michael Strauss, you can investigate a variety of new issues with the same data.

This research utilized data from existing telescopes, such as Planck and Subaru. The crew has only gone over a third of the data from the Subaru Hyper Suprime-Cam Survey.

Next, the complete data set will be analyzed, which should provide a more precise measurement of the dark matter distribution.

The group intends to investigate more of the earliest regions of space in the future using a sophisticated data set, such as the Legacy Survey of Space and Time (LSST) from the Vera C. Rubin Observatory. 

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

Image Credit: REIKO MATSUSHITA

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