A collaboration led by scientists at Nagoya University in Japan has investigated the nature of the dark matter surrounding galaxies as they were seen 12 billion years ago.
Their findings, published in Physical Review Lettersraises the possibility that the fundamental rules of cosmology are different when examining the early history of our Universe.
Seeing something that happened so long ago is difficult. Because of the finite speed of light, we see distant galaxies not as they are today, but as they were billions of years ago. But even more difficult is the observation of dark matter, which does not emit light.
Consider a distant source galaxy, even more distant than the galaxy whose dark matter is to be investigated. 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.
The fundamental rules of cosmology might be different
As light from the source galaxy travels through this distortion, it bends, changing the apparent shape of the galaxy. 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 “lensing” galaxy) as a function of the distortion.
However, beyond a certain point, scientists run into a problem. Galaxies in the deepest regions of the Universe are incredibly faint. Therefore, the further we get from Earth, the less effective this technique becomes. Lensing distortion is subtle and difficult to detect in most cases, so many background galaxies are needed to detect the signal.
Most previous studies have stuck to the same limits. Unable to detect enough distant source galaxies to measure the distortion, they could only analyze dark matter from at most 8-10 billion years ago. These limitations left open the question of the distribution of dark matter between now and 13.7 billion years ago, around the beginning of our Universe.
Clues to dark matter
To overcome these challenges and observe dark matter, a team of researchers 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 , the microwaves released by the Big Bang itself.
First, using data from the Subaru Hyper Suprime-Cam Survey (HSC) observations, the team identified 1.5 million lens galaxies using visible light, selected to be seen 12 billion years ago.
Then, to overcome the lack of light from even more distant galaxies, they used microwaves from the cosmic microwave background (CMB), the remnant radiation from the Big Bang. Using microwaves observed by the European Space Agency’s Planck satellite, the team measured how dark matter around lensing galaxies distorted the microwaves.
One of the most interesting discoveries of astronomers
“Most researchers use source galaxies to measure the distribution of dark matter from the present to eight billion years ago,” added Assistant Professor Yuichi Harikane from the Institute for Cosmic Ray Research, University of Tokyo. “However, we were able to look further into the past because we used the more distant CMB to measure dark matter. For the first time, we measure dark matter from almost the first 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. By combining the large sample of distant galaxies and the lensing distortions in the CMB, they detected dark matter even further back in time, 12 billion years ago. This is only 1.7 billion years since the beginning of the Universe, and therefore these galaxies are seen shortly after they formed.
One of the researchers’ most interesting discoveries was related to the clumping of dark matter. According to the standard theory of cosmology, the Lambda-CDM model, subtle fluctuations in the CMB form clumps of dense matter by gravitationally pulling on the surrounding matter.
“Our discovery is still uncertain”
This creates inhomogeneous clumps that form stars and galaxies in these dense regions. The group’s findings suggest that their measurement of crowding was smaller than predicted by the Lambda-CDM model.
“Our discovery is still uncertain,” Miyatake said. “But if true, it would suggest that the whole model is flawed as we move further back in time. This is interesting because if the result holds after the uncertainties are reduced, it could suggest an improvement to the model that could provide insights into the nature of dark matter itself.”
“One of the strengths of observing the Universe with large-scale surveys such as those used in this research is that you can study everything you see in the resulting images, from nearby asteroids to the most distant galaxies in The early universe. You can use the same data to explore a lot of new questions,” said Michael Strauss, professor and chair of the Department of Astrophysical Sciences at Princeton University, according to EurekAlert.