Dark matter is a hypothetical form of matter that should outweigh the regular stuff that makes us by five to one. It is invisible to our instruments and its effects can only be observed through gravity. A standard approach measures its distribution by looking at how its presence distorts the light of distant galaxies. This method, though effective, has limitations on how far back it can look into the past: up to 8 billion years ago in most cases. Now that has just been extended all the way to 12 billion years ago.
Massive objects warp space-time around them, bending light to act like a lens on distant objects behind them. The biggest ones can create spectacular lensed images of distant galaxies. Smaller ones create smaller distortions but by measuring them, we can reconstruct exactly the distribution of mass in a lensed galaxy. In this way, astronomers can see the invisible dark matter.
This approach works as long as you have a lot of bright background galaxies and the galaxies are lensing their light closer to us. For this reason, looking deeper into the universe – so further into the past – creates a limit: the first galaxies formed a few hundred million years after the Big Bang, and they weren’t all that bright.
Reporting in Physical Review Letters, a collaboration led by scientists at Nagoya University approached the same method in a new way, revealing the distribution of dark matter around galaxies a whopping 12 billion years ago. They didn’t look for the light of distant galaxies to be distorted but instead looked at the very first light in the universe: the cosmic microwave background (CMB).
The CMB is an emission that permeates all of the cosmos. About 380,000 years after the Big Bang, the universe was finally cool enough for light to move without being absorbed by matter; thus, this light was free. As the universe expanded, its wavelength has been stretched all the way to microwaves, but it is still affected by the gravity of massive objects. So by measuring lensing in the CMB, the researchers were able to push the distribution of dark matter further back in time and deeper into space.
“Look at dark matter around distant galaxies? “It was a crazy idea. No one realized we could do this,” Professor Masami Ouchi of the University of Tokyo, who made many of the observations, said in a statement.
"But after I gave a talk about a large distant galaxy sample, Hironao [Miyatake, research lead] 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 8 billion years ago”, added Assistant Professor Yuichi Harikane of the Institute for Cosmic Ray Research at the 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.”
The most intriguing find is the measurement of the clumpiness of dark matter. According to the standard model of cosmology or Lambda-CDM, which underpins our understanding of the universe, dark matter forms regions of overdensity where over time galaxies form. But, the clumpiness measurement in this study is lower than predicted by theory.
“Our finding is still uncertain," Miyatake, from Nagoya University, said. “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.”
“At this point, we will try to get better data to see if the Lambda-CDM model is actually able to explain the observations that we have in the universe,” said Andrés Plazas Malagón, associate research scholar at Princeton University. “And the consequence may be that we need to revisit the assumptions that went into this model.”
The team used data from the European Space Agency’s Planck observatory for the CMB and observations from the Subaru Hyper Suprime-Cam Survey (HSC). Only one-third of the HSC data has been analyzed so the team is now working to complete that.