Scientists have mapped the dark matter around some of the oldest and most distant galaxies so far.
The 1.5 million galaxies appear as they were 12 billion years ago, or less than 2 billion years after the Big Bang. Those galaxies distort the cosmic microwave background (light emitted during an even earlier era of the universe) as seen from Earth. That distortion, called gravitational lensing, reveals the dark matter distribution around those galaxies, scientists report in the August 5 Physical Review Letters.
Understanding how dark matter accumulates around galaxies early in the universe’s history could give scientists more information about the mysterious substance. And in the future, this lensing technique could also help scientists unravel a mystery about how matter clumps together in the universe.
Dark matter is an unknown massive substance that surrounds galaxies. scientists have never directly detected dark matterbut they can observe its gravitational effects in the cosmos (Serial number: 7/22/22). One such effect is gravitational lensing: when light passes through a galaxy, its mass deflects the light like a lens. How much light is bent reveals the galaxy’s mass, including its dark matter.
It’s hard to map the dark matter around such distant galaxies, says cosmologist Hironao Miyatake of Nagoya University in Japan. That’s because scientists need a light source that is farther away than the lensing galaxy. Scientists typically use even more distant galaxies as the source of that light. But when you look that deep into space, those galaxies are hard to find.
So instead, Miyatake and his colleagues turned to the cosmic microwave background, the oldest light in the universe. The team used lens measurements of the cosmic microwave background from the Planck Satellitecombined with a multitude of distant galaxies observed by the Subaru Telescope in Hawaii (Serial number: 7/24/18). “The gravitational lensing effect is very small, so we need a lot of lensed galaxies,” says Miyatake. The distribution of dark matter around the galaxies matched expectations, the researchers report.
The researchers also estimated a quantity called sigma-8, a measure of how “lumpy” matter is in the cosmos. For years, scientists have found evidence that different sigma-8 measures disagree with each otherSerial number: 8/10/20). That could be a hint that something is wrong with scientific theories of the universe. But the evidence is not conclusive.
“One of the most interesting things in cosmology right now is whether or not that tension is real,” says cosmologist Risa Wechsler of Stanford University, who was not involved in the study. “This is a very good example of one of the techniques that will help shed light on that.”
Measuring sigma-8 using early and distant galaxies could help reveal what’s going on. “You want to measure this amount, this 8-sigma, from as many perspectives as possible,” says cosmologist Hendrik Hildebrandt of Ruhr University Bochum in Germany, who was not involved in the study.
If estimates from different eras of the universe don’t agree with each other, that could help physicists come up with a new theory that could better explain the cosmos. While the new 8-sigma measurement is not precise enough to settle the debate, future projects, such as the rubin observatory in Chile, could improve the estimate (Serial number: 01/10/20).