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Scientists have mapped the dark matter around some of the earliest and most distant galaxies ever.
1.5 million galaxies look the same as they did 12 billion years ago, less than 2 billion years after the Big Bang. These galaxies distort the cosmic microwave background radiation (light emitted in earlier times in the universe) as seen from Earth. This distortion, called gravitational lensing, reveals the distribution of dark matter around these galaxies, the scientists report in his August 5 physical review letter. By understanding how dark matter accumulated around galaxies in the early history of the universe, scientists can learn more about the mysterious matter. Lens technology could also help scientists unravel the mystery of how matter clumps together in the universe.
Dark matter is an unknown mass of matter that surrounds galaxies. Scientists have never detected dark matter directly, but they can observe its gravitational effects on the universe (SN: 07/22/22). One of these effects is the gravitational lensing effect. When light passes through a galaxy, its mass bends it like a lens. How much the light bends reveals the mass of galaxies containing dark matter.
Mapping the dark matter around such distant galaxies is difficult, says cosmologist Hironao Miyatake of Nagoya University in Japan. This is because scientists need a source of light farther than the galaxy to act as a lens. Scientists usually use even more distant galaxies as this source of light. But if you look deep into space, these galaxies are hard to spot. Instead, Miyatake and colleagues turned to the cosmic microwave background, the oldest light in the universe. The team used lens measurements of the cosmic microwave background radiation from the Planck satellite in combination with various distant galaxies observed by the Subaru Telescope in Hawaii (SN: 07/24/18). “We need many lensed galaxies because the gravitational lensing effect is so small,” says Miyatake. The distribution of dark matter around the galaxy was as expected, researchers report.
The researchers also estimated a quantity called Sigma-8, a measure of how “massive” the matter in the universe is. Over the years, scientists have found evidence that various measurements of Sigma-8 disagree (SN: 8/10/20). This could indicate that there is something wrong with the scientists’ theories about the universe. However, the evidence is inconclusive.
“One of the most interesting things in cosmology right now is whether this tension is real,” says her Risa Wechsler, a cosmologist at Stanford University. “This is one really good example of her technique that helps shed some light.”
Measuring sigma-8 from early distant galaxies could help reveal what’s going on. Hendrik Hildebrand, a cosmologist at Ruhr University Bochum in Germany, who was not involved in the study, said: “We want to measure this quantity, this Sigma-8, from as many perspectives as possible.
If estimates for different epochs of the universe disagree, it could help physicists develop new theories that can better explain the universe. New measurements of Sigma-8 aren’t accurate enough to end the debate, but future projects like the Ruby Observatory in Chile may improve estimates (SN: 1/10/20)