Scientists have investigated the nature of dark matter around galaxies that are 12 billion years old and older than ever before. Their findings offer the tantalizing possibility that the basic rules of cosmology may be different when studying the early history of our universe. The collaboration was led by scientists at Nagoya University in Japan, and the results of the study have been published recently in Physical Review Letters.
It is challenging to see what happened so long ago. Because the speed of light is finite, we see distant galaxies not as they are today, but as they were billions of years ago. But even more difficult is to observe dark matter – because it doesn’t emit light.
Consider a distant source galaxy, even more distant than the target galaxy, whose dark matter we want to study. As Einstein’s theory of general relativity predicts, the gravitational force of the foreground galaxy – including its dark matter – distorts the surrounding space and time. When light from the source galaxy passes through this distorted space-time, it bends and changes the surface shape of the galaxy. The greater the amount of dark matter, the greater the resulting distortion. Thus, astronomers can measure the amount of dark matter around foreground galaxies from the distortions.
Beyond a certain threshold, however, scientists run into a problem. At the deepest parts of the universe, galaxies are very faint. Therefore, the farther away from Earth we look, the less effective the gravitational lensing technique becomes. Because the distortion of the lensing is subtle and hard to detect in most cases, many background galaxies are needed to detect the signal.
Most previous studies have stayed at the same limit. Unable to detect source galaxies distant enough to measure distortions, they could only analyze dark matter no more than 8-10 billion years old. These limits leave the distribution of dark matter from this time to 13.7 billion years ago, around the beginning of our universe, unresolved.
To overcome these challenges and observe dark matter from the farthest 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 background light source, namely microwaves released by the Big Bang itself.
First, the team used visible light to identify 1.5 million lensed galaxies, selected to be 12 billion years old, by using observations from the Subaru Super Super Superluminova Camera Survey (HSC).
Next, to overcome the lack of light from more distant galaxies, they used microwaves from the cosmic microwave background (CMB), the radiative remnant of the Big Bang. By using microwaves observed by the ESA Planck satellite, the team measured how dark matter around lenticular galaxies distorts microwaves.
“A look at dark matter around distant galaxies?” Professor Masami Ouchi of the University of Tokyo asked, “It’s a crazy idea. No one realized we could do that. But after I gave a talk on a sample of large distant galaxies, Hironao came to me and said maybe we could use the CMB to look at the dark matter around these galaxies.”
“Most researchers use source galaxies to measure the distribution of dark matter from the present to 8 billion years ago,” added Assistant Professor Yuichi Harikane of the University of Tokyo’s Institute of Cosmic Ray Research, “however, we can go further back in time because we use the more distant CMB to measure dark matter. For the first time, we are measuring dark matter almost from the earliest moments of the universe.”
After initial analysis, the scientists soon realized that they had a large enough sample to detect the distribution of dark matter. Combining the large sample of distant galaxies with the lensing distortions of the CMB, they detected dark matter even further back in time – from 12 billion years ago. Since this is only 1.7 billion years after the beginning of the universe, these galaxies would have been seen shortly after they first formed.
“I’m glad we’ve opened a new window on that era,” Miyatake said, “12 billion years ago, things were very different. You see more galaxies in the process of forming than you do now; the first galaxy clusters are also starting to form. The galaxy clusters consisted of 100-1000 gravitationally bound galaxies and a lot of dark matter.”
Neta Bahcall, the Eugene Higgins Professor of Astronomy, Professor of Astrophysical Sciences and Director of Undergraduate Research from Princeton University, noted, “This result gives a very consistent picture of galaxies and their evolution and the dark matter in and around them and how this picture has evolved over time.”
One of the most exciting findings in this study has to do with the clumping nature of dark matter. According to the standard theory of cosmology, known as the Lambda-CDM model, subtle fluctuations in the CMB form dense pools of matter by gravitationally attracting surrounding matter. This produces inhomogeneous clumps, and in these dense regions stars and galaxies are formed. The group’s findings suggest that their measurements of clumps are lower than predicted by the Lambda-CDM model.
Miyatake is enthusiastic about this possibility. “Our finding is still uncertain. But if it is true, it will show that the whole model is flawed when you go further back in time. That’s exciting because if the result still holds after the uncertainty is reduced, it could suggest improvements to the model and possibly lead to an understanding of the nature of dark matter itself.”
“At this point, we will try to get better data to see if the Lambda-CDM model can really explain our observations in the universe,” said Andrés Plazas Malagón, an associate research scholar at Princeton University, “and the results may be that we need to revisit the assumptions in this model.”
“One of the advantages of using a large-scale survey to look at the universe, such as the one used in this study, is that you can study everything you see in the images you get, from nearby asteroids in our solar system to the most distant galaxies in the early universe,” said Michael Strauss, professor and chair of the Department of Astrophysics from Princeton University Michael Strauss said, “You can use the same data to explore a lot of new questions.”
The study used data from existing telescopes, including Planck and Sparrow. The team reviewed only 1/3 of the data from the Subaru Super Ultra Microscope survey. Next they will analyze the entire data set, which will allow them to make more precise measurements of the dark matter distribution. In the future, the team expects to use advanced datasets like the Vera C. Rubin Observatory’s Legacy Survey in Space and Time (LSST) to explore more of the earliest parts of space. “The LSST will allow us to observe half of the sky. I don’t see any reason why we can’t next see the distribution of dark matter 13 billion years ago,” Harikane said.
