Another groundbreaking discovery by Nagoya University‘s six Nobel Prize winners looks back into parts of space further than ever before. In collaboration with the University of Tokyo and Princeton University, researchers revealed how they observed the formation of dark matter around galaxies 12 billion years ago, using radiation residue from the Big Bang.
It can be challenging to see events that happened so long ago. Due to the limited speed of light, the team observed faraway galaxies in their pre-billion-year history rather than their present state. Observing dark matter, which doesn’t produce light, is more challenging still.
Consider a faraway source galaxy that is even more remote than the target galaxy for studying its dark matter. As predicted by Einstein’s theory of general relativity, the gravitational pull of the foreground galaxy, including its dark matter, distorts the surrounding space and time. The apparent shape of the galaxy is altered as a result of the light from the source galaxy bending as it passes through the distortion. 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).
Beyond a certain point, a problem arises: Galaxies are exceedingly dim in the farthest reaches of the universe. As a result, this strategy gets less successful as we look farther away from Earth. There must be many background galaxies to identify the signal because the lensing distortion is typically modest and challenging to detect.
Most of the studies are stuck at the same limits. Besides being unable to identify enough distant source galaxies to measure the distortion, scientists could only analyze the 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.
Researchers in this study get around this problem by using data from the Subaru Hyper Supreme-Cam Survey (HSC) observations. They could detect 1.5 million lens galaxies using visible light, selected to be seen 12 billion years ago.
Next, they used microwaves from the cosmic microwave background (CMB) to address the lack of galaxy light further away. They especially used microwaves observed by the European Space Agency’s Planck satellite to quantify the dark matter around the lens galaxies distorted by the microwaves.
Professor Masami Ouchi of the University of Tokyo said, “Look at dark matter around distant galaxies? It was a crazy idea. No one realized we could do this. But after I talked 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.”
Assistant Professor Yuichi Harikane of the Institute for Cosmic Ray Research, University of Tokyo, said, “Most researchers use source galaxies to measure dark matter distribution from the present to eight billion years ago. However, we could look further 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 a preliminary analysis, the researchers soon realized that they had a large enough sample to detect the distribution of dark matter. Combining the large distant galaxy sample and the lensing distortions in CMB, they detected dark matter even further back in time, from 12 billion years ago. This is only 1.7 billion years after the beginning of the universe; thus, these galaxies are seen soon after they first formed.
KMI Designated Assistant Professor Hironao Miyatake said, “I was happy that we opened a new window into that era. 12 billion years ago, things were very different. You see more galaxies in the formation process than at present; the first galaxy clusters are also starting to form. Galaxy clusters comprise 100-1000 galaxies bound by gravity with large amounts of dark matter.”
Neta Bahcall, Eugene Higgins Professor of Astronomy, professor of astrophysical sciences, and director of undergraduate studies at Princeton University, said, “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 exciting findings of the researchers was related to the clumpiness of dark matter. According to the standard theory of cosmology, the Lambda-CDM model, subtle fluctuations in the CMB form pools of densely packed matter by attracting surrounding matter through gravity. This creates inhomogeneous clumps that form stars and galaxies in these dense regions. The group’s findings suggest that their clumpiness measurement was lower than predicted by the Lambda-CDM model.
Miyatake said, “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.”
Andrés Plazas Malagón, an associate research scholar at Princeton University, said, “At this point, we will try to get better data to see if the Lambda-CDM model can explain our observations in the universe. And the consequence may be that we need to revisit the assumptions that went into this model.”
Michael Strauss, a professor, and chair of the Department of Astrophysical Sciences at Princeton University, said, “One of the strengths of looking at the universe using large-scale surveys, such as the ones used in this research, is that you can study everything that you see in the resulting images, from nearby asteroids in our solar system to the most distant galaxies from the early universe. You can use the same data to explore many new questions.”
- Hironao Miyatake, Yuichi Harikane, et al. First Identification of a CMB Lensing Signal Produced by 1.5 Million Galaxies at z∼4: Constraints on Matter Density Fluctuations at High Redshift. Phys. Rev. Lett. 129, 061301 – Published 1 August 2022. DOI: 10.1103/PhysRevLett.129.061301