A collaboration led by scientists at Nagoya University and Institute for Cosmic Ray Research, the University of Tokyo in Japan has investigated the nature of dark matter surrounding galaxies seen as they were 12 billion years ago, billions of years further back in time than ever before. Their findings, published in Physical Review Letters, offer the tantalizing possibility that the fundamental rules of cosmology may differ when examining the early history of our universe.
How do we see something that happened so long ago? Because of the finite speed of light, we see distant galaxies not as they are today, but rather as they were billions of years in the past. But even more challenging, how do we see something like dark matter, that by its nature does not emit light? Consider a very distant source galaxy, even further away than the galaxy whose dark matter one wants to investigate. The gravitational pull of the foreground galaxy, including its dark matter, distorts the surrounding space and time, as predicted by Einstein’s theory of General Relativity. As the light from the source galaxy travels through this distortion, it bends, changing the apparent shape of the galaxy in the sky. The greater the amount of dark matter, the greater the distortion. Thus, scientists can measure the amount of dark matter around the foreground galaxy (the “lens” galaxy) from the distortion.
However, at this point, scientists encounter a problem. The galaxies in the deepest reaches of the universe are incredibly faint. As a result, the further away from Earth you look, the less effective this technique becomes. The lensing distortion is subtle and difficult to detect in most cases, so one needs many background galaxies to detect the signal . Most previous studies remained stuck at the same limits. Unable to detect enough distant source galaxies to measure the distortion, they could only analyze dark matter from no further back than 8-10 billion years ago. These limitations left open the question about the distribution of dark matter between this time and 13.7 billion years ago, around the beginnings of our universe.
To overcome these challenges and observe dark matter from the furthest reaches of the universe, a research team led by Nagoya University’s Hironao Miyatake, in collaboration with the University of Tokyo, National Astronomical Observatory of Japan, and Princeton University, used a different source of background light, the microwaves released from the Big Bang itself.
How did they do this? First, using data from the observations of the Subaru Hyper Suprime-Cam Survey, the team identified 1.5 million lens galaxies using visible light, selected to be seen 12 billion years ago. Next, to overcome the lack of galaxy light even further away, they employed microwaves from the cosmic microwave background (CMB), the radiation residue from the Big Bang. Using microwaves observed by the European Space Agency’s Planck satellite, the team measured how the dark matter around the lens galaxies distorted the microwaves.
“Look at dark matter around distant galaxies?” asks Professor Masami Ouchi of the University of Tokyo, who made many of the observations. “It was a crazy idea. No one realized we could do this. 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.”
“Most researchers use source galaxies to measure dark matter distribution from the present to eight billion years ago”, adds Assistant Professor Yuichi Harikane of the Institute for Cosmic Ray Research, University of Tokyo. “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 a preliminary analysis, the researchers soon realized 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, and thus these galaxies are seen soon after they first formed.
“I was happy that we opened a new window into that era,” Miyatake says. “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 comprise 100-1000 galaxies bound by gravity with large amounts of dark matter.
One of the most exciting of the researchers’ findings was related to the clumpiness of the 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 is excited about the possibilities. “Our finding is still uncertain”, he says. ”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 it could suggest – if the result holds after the uncertainties are reduced — an improvement of the model that may give insight into the nature of dark matter itself.” This study used data available from existing telescopes, including Planck and Subaru. The group has only reviewed a third of the Subaru Hyper Suprime-Cam Survey data taken thus far. The next step will be to analyze the entire data set, which should allow for a more precise measurement of the dark matter distribution. In the future, the team expects to use an advanced data set like the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) survey to explore more of the earliest parts of space. “LSST will allow us to observe half the sky,” Harikane says. “I don’t see any reason we couldn’t see the dark matter distribution 13 billion years ago next.”
Journal：Physical Review Letters
Title：First Identification of a CMB Lensing Signal Produced by 1.5 Million Galaxies at z∼4: Constraints on Matter Density Fluctuations at High Redshift
Authors：Hironao Miyatake (Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University; Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo) / Yuichi Harikane (Institute for Cosmic Ray Research, The University of Tokyo) / Masami Ouchi (National Astronomical Observatory of Japan; Institute for Cosmic Ray Research, The University of Tokyo; Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo) / Yoshiaki Ono (Institute for Cosmic Ray Research, The University of Tokyo) / Nanaka Yamamoto (Division of Physics and Astrophysical Science, Graduate School of Science, Nagoya University) / Atsushi J. Nishizawa (DX Center, Gifu Shotoku Gakuen University; Institute for Advanced Research, Nagoya University) / Nata Bahcall (Department of Astrophysical Sciences, Princeton University) / Satoshi Miyazaki (National Astronomical Observatory of Japan) Andrés A. Plazas Malagón (Department of Astrophysical Sciences, Princeton University)