Scientists have discovered dark matter around galaxies that existed about 12 billion years ago, the earliest detection yet of this mysterious substance that dominates the universe.
The findings, achieved by a collaboration led by researchers from Japan's Nagoya University, suggest that dark matter in the early universe is less 'clumpy' than predicted by many current cosmological models. If further work confirms this theory, it could change scientists' understanding of how galaxies evolve and suggest that the fundamental rules governing the cosmos could have been different when the 13.7 billion-year-old universe was just 1.7 billion years old.
The key to mapping dark matter in the very early universe the cosmic microwave background (CMB), a sort of fossil radiation left over from the Big Bang that is distributed throughout the entire cosmos.
"Look at dark matter around distant galaxies? It was a crazy idea. No one realized we could do this," University of Tokyo professor Masami Ouchi said in a statement. "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."
Because light takes a finite time to travel from distant objects to Earth, astronomers see other galaxies as they existed when the observed light left them. The more distant a galaxy, the longer the light has been traveling to us and thus the further back in time we see them, so we see the most distant galaxies as they were billions of years ago, in the infant universe.
Observing dark matter is even trickier. Dark matter is the mysterious substance that makes up around 85% of the total mass of the universe. It doesn't interact with matter and light like the everyday matter made of protons and neutrons that fills stars, planets and us.
Detecting 'early' dark matter
To 'see' dark matter at all, astronomers must rely on its interaction with gravity.
According to Einstein's theory of relativity, objects of tremendous mass cause the curvature of space-time. A common analogy is a stretchy rubber sheet holding balls of increasing mass. The greater the mass, the larger the 'dent' it causes in the sheet. Likewise, the larger the cosmic object, the more extreme the warping of space-time it causes.
Massive objects like galaxies cause space-time to curve so strongly that light from sources behind a galaxy is curved, just like the path of a marble rolled across the stretched rubber sheet would deviate. This effect shifts the position of the light source in the sky, a phenomenon called gravitational lensing.
To study the distribution of dark matter in a galaxy, astronomers can observe how light from a source behind that galaxy is changed as it passes the 'lens galaxy.' The more dark matter a lens galaxy contains,s the greater the distortion of the light passing it.
But the technique has limitations.
Because the earliest and most distant galaxies are very faint, as astronomers look deeper into the universe and further back in time, the lensing effect becomes more subtle and difficult to see and scientists need both a lot of background sources and a lot of early galaxies to spot lensing by dark matter. This problem has limited the mapping of dark matter distribution to galaxies that are around 8 to 10 billion years old.
But the CMB provides a more ancient light source than any galaxy. The CMB is ubiquitous radiation that was created when the universe cooled enough to allow atoms to form, reducing the number of photon-scattering free electrons in a moment cosmologists call 'the last scattering.' The reduction in free electrons allowed photons to travel freely, meaning that the universe suddenly stopped being opaque and became transparent to light.
And just like light from other distant sources, the CMB can be distorted by galaxies with dark matter due to gravitational lensing.
"Most researchers use source galaxies to measure dark matter distribution from the present to 8 billion years ago," University of Tokyo assistant professor Yuichi Harikane said in the statement. "However, we could look further back into the past because we used the more distant CMB to measure dark matter."
The team combined lensing distortions of a large sample of ancient galaxies with those of the CMB to detect dark matter dating back to when the universe was just 1.7 billion years old. And this ancient dark matter paints a very different cosmic picture.
"For the first time, we were measuring dark matter from almost the earliest moments of the universe," Harikane said. "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."
These clusters can be comprised of between 100 and 1,000 galaxies bound to large amounts of dark matter by gravity.
Is dark matter clumpy?
One of the most significant aspects of the team's findings is the possibility that dark matter is less clumpy in the early universe than many current models suggest it should be.
For example, the widely accepted Lambda-CDM model suggests that tiny fluctuations in the CMB should have resulted in gravity creating densely packed pockets of matter. These fluctuations eventually lead matter to collapse to form galaxies, stars and planets, and should also result in dense pockets of dark matter.
"Our finding is still uncertain," Harikane 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."
The team will continue to collect data to assess whether the Lambda-CDM model conforms to observations of dark matter in the early universe or if the assumptions behind the model need to be revised.
The data used by the team to reach their findings originated from the Subaru Hyper Suprime-Cam Survey, which analyzes data from a telescope in Hawai'i. But the researchers have used only a third of this data thus far, meaning that a better dark matter distribution map could be available as the rest of the observations are incorporated.
The team is also looking forward to data from the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) which could allow the researchers to look at dark matter even further back in time.
"LSST will allow us to observe half the sky," Harikane said. "I don't see any reason we couldn't see the dark matter distribution 13 billion years ago next."
The team's research was published Aug. 1 in the journal Physical Review Letters.
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