The findings, achieved by a collaboration led by researchers from Japan’s Nagoya University, suggest that Dark matter in the early universe is less “lumpy” 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 universe could have been different when the 13.7-billion-year-old universe it was only 1.7 billion years old. The key to mapping dark matter in the very early universe cosmic microwave background (CMB), a type of fossilized radiation left over from the Big Bang that is distributed throughout the universe. “Look at dark matter around distant galaxies? It was a crazy idea. Nobody realized we could do that,” University of Tokyo professor Masami Ouchi he said in a statement. “But after I gave a talk about a large distant galaxy sample, Hironao came to me and said that it might be possible to see the 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 is, the further light travels towards us and therefore 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.
Detection of ‘early’ dark matter
To “see” dark matter at all, astronomers must rely on its interaction with gravity. According Einstein’s theory of relativity, massive objects cause the curvature of spacetime. A common analogy is an elastic sheet of rubber that holds balls of increasing mass. The greater the mass, the greater the “dimple” it causes in the leaf. Likewise, the larger the cosmic object, the more extreme the distortion of spacetime it causes. Massive objects like galaxies cause space-time to bend so strongly that light from sources behind a galaxy is bent, just as the path of a marble rolled on a stretched sheet of rubber 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 through the “lens galaxy.” The more dark matter a lens galaxy contains, the greater the distortion of the 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 thinner and harder to see, and scientists need as many background sources as many early galaxies to to speckle lens of dark matter. This problem has limited mapping the distribution of dark matter in 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 the ubiquitous radiation created when the universe cooled enough to allow atoms to form, reducing the number of free electrons that scatter photons in a moment cosmologists call “the last scattering.” The reduction of 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 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 could look further back in time 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 almost from the earliest moments of the universe,” Harikane said. “12 billion years ago, things were very different. You see more galaxies in the process of forming than today; the first clusters of galaxies are also starting to form.” These clusters may consist of between 100 and 1,000 galaxies that are gravitationally bound to large amounts of dark matter.
Is dark matter clumped together?
One of the most important aspects of the team’s findings is the possibility that dark matter was less aggregated in the early universe than many current models suggest it should be. For example, the widely accepted Lambda-CDM model suggests that microscopic fluctuations in the CMB should have resulted in the creation of gravity in dense 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 inconclusive,” Harikane said. “But if true, it would suggest that the whole model is flawed as you go further back in time. This is exciting because if the result remains after reducing the uncertainties, it could suggest an improvement to the model that can provide insights into 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 whether the assumptions behind the model need to be revised. The data the team used to reach their findings came from the Subaru Hyper Suprime-Cam survey, which analyzes data from a telescope in Hawaii. But researchers have only used a third of that data so far, meaning a better map of the dark matter distribution could become available as the rest of the observations are integrated. The team is also eagerly awaiting input from the Vera C. Rubin ObservatoryThe Legacy Survey of Space and Time (LSST) which could allow researchers to examine dark matter even further back in time. “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 down the line.” The team’s research was published Aug. 1 in the journal Physical Review Letters. Follow us on Twitter @Spacedotcom and up Facebook.
