Scientists have discovered dark matter around galaxies that existed around 12 billion years ago, the earliest detection yet of this mysterious substance that dominates the universe.
Findings from a collaboration led by researchers at Japan’s Nagoya University show that dark matter in the early universe is less “clumsy” than predicted by many current cosmological models. If further work confirms this theory, it could change scientists’ understanding of how galaxies develop and suggest that the basic rules governing the cosmos may have been different when 13.7 billion years ago universe is only 1.7 billion years old.
The key to mapping dark matter in the very early universe is cosmic microwave background (CMB), a kind of fossil radiation left over from the Big Bang that spreads throughout space.
“See dark matter around distant galaxies? It was a crazy idea. Nobody realized we could do this,” Masami Ouchi, a professor at the University of Tokyo it said in a statement. “But after I gave a lecture on a large sample of a distant galaxy, Hironao came to me and said that it might be possible to look at the dark matter around these galaxies with the CMB.”
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Because light takes a finite time to travel from distant objects to The Earth, astronomers see other galaxies as they existed when the observed light left them. The more distant a galaxy is, the longer the light has traveled 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 more difficult. Dark matter is the mysterious substance that makes up about 85% of the total mass of the universe. It does not interact with matter and light like the everyday matter made up of protons and neutrons that fills stars, planets and us.
Discovery of “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 enormous mass cause the curvature of space-time. A common analogy is a stretchy rubber sheet that holds balls of increasing mass. The bigger the mass, the bigger the “dent” it causes in the leaf. Likewise, the larger the space object, the more extreme space-time distortion it causes.
Massive objects like galaxies cause space-time to warp so much that light from sources behind a galaxy is bent, just as the path of a marble rolled on a stretched rubber sheet would be deflected. 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 this galaxy changes as it passes the “lensing galaxy.” The more dark matter a lensed galaxy contains, the greater the distortion of light passing through 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 subtler and harder to see, and scientists need both many background sources , as well as from very early galaxies to point-lens from dark matter. This problem has limited mapping the distribution of dark matter to galaxies that are about 8 to 10 billion years old.
But the CMB provides a more ancient source of light than any galaxy. The CMB is a ubiquitous emission that is created when the universe cools enough to allow atoms to form, reducing the number of photon-scattering free electrons in what cosmologists call the “last scattering.” Reduction of free electrons is 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 dark matter galaxies due to gravitational lensing.
“Most researchers use source galaxies to measure the distribution of dark matter from the present to 8 billion years ago,” University of Tokyo assistant professor Yuichi Harikane said in the statement. “However, we can look further 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 only 1.7 billion years old. And this ancient dark matter paints a very different cosmic picture.
“For the first time, we have measured 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 forming than now; the first galaxy clusters are also starting to form.”
These clusters can consist of between 100 and 1,000 galaxies bound together by large amounts of dark matter by gravity.
Is dark matter lumpy?
One of the most significant aspects of the team’s findings is the possibility that dark matter was less bulky in the early universe than many current models suggest.
For example, the widely accepted Lambda-CDM model suggests that small fluctuations in the CMB must have resulted in gravity creating tightly packed pockets of matter. These fluctuations eventually cause matter to collapse to form galaxies, stars and planets, and should also lead to dense pockets of dark matter.
“Our finding is still uncertain,” Harikane said. “But if true, it would mean that the whole model is flawed as you go further back in time. This is exciting because if the result holds after the uncertainties are reduced, it may offer an improvement to 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 matches observations of dark matter in the early universe, or if the assumptions behind the model need to be revised.
The data the team used to arrive at their findings came from the Subaru Hyper Suprime-Cam Survey, which analyzed data from a telescope in Hawaii. But researchers have used only a third of that data so far, meaning a better map of the distribution of dark matter may be available as the remaining observations are incorporated.
The team is also looking forward to data from Vera C. Rubin ObservatoryLegacy Survey of Space and Time (LSST), which could allow researchers to look at dark matter even further back in time.
“The LSST will allow us to observe half the sky,” Harikane said. “I see no reason why we can’t see the distribution of dark matter 13 billion years ago.”
The team’s research was published Aug. 1 in the journal Nature Communications Physical examination letters.
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