Distribution of dark matter around galaxies 12 billion years ago revealed

Cosmic Microwave Background (CMB)

The Big Bang radiation residue, distorted by dark matter 12 billion years ago. Credit: Reiko Matsushita

Scientists have studied the nature of dark matter surrounding galaxies as they were 12 billion years ago, billions of years further back in time than ever before. Their findings offer the tantalizing possibility that the fundamental rules of cosmology may differ when examining the ancient history of our universe. The collaboration was led by scientists from[{” attribute=””>Nagoya University in Japan and the findings were published today (August 1) in the journal Physical Review Letters.

Seeing something that happened such a long time ago is challenging. Because of 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 observing dark matter, which does not emit light.

“It was a crazy idea. No one realized we could do this.” — Professor Masami Ouchi

Consider a distant source galaxy, even farther away than the target galaxy whose dark matter one wants to investigate. As predicted by Einstein’s theory of general relativity, the gravitational attraction of the foreground galaxy, including its dark matter, distorts the surrounding space and time. As the light from the source galaxy travels through this distortion in spacetime, it bends, changing the apparent shape of the galaxy. The greater the amount of dark matter, the greater the resulting distortion. Therefore, astronomers can measure the amount of dark matter around the foreground galaxy (the “lens” galaxy) from the distortion.

However, beyond a certain threshold, scientists encounter a problem. In the deepest reaches of the universe, the galaxies are incredibly faint. As a result, the farther away from Earth we look, the less effective the gravitational lensing technique becomes. Because the lensing distortion is subtle and difficult to detect in most cases, many background galaxies are needed to detect the signal.

Most previous studies have 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 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.

To overcome these challenges and observe dark matter from the farthest reaches of the universe, a team of researchers led by Hironao Miyatake from Nagoya University, in collaboration with the University of Tokyo, the National Astronomical Observatory of Japan, and Princeton University, used a different source of background light, the microwaves released from the Big Bang itself.

First, using data from the observations of the Subaru Hyper Suprime-Cam Survey (HSC), 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 farther away, they employed microwaves from the cosmic microwave background (CMB), the radiation residue from the DOI: 10.1103/PhysRevLett.129.061301

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