Dark matter is mapped unseen
Dark matter is like a construction scaffold on a building, allowing visible matter to build stars and galaxies, according to the first three-dimensional map of the unseen stuff that is thought make up the majority of mass in the universe.
The map, created with images from the Hubble Space Telescope, confirms theories that dark matter accumulates in the same regions as visible matter, giving it a kind of shell, or exoskeleton, to work within.
“I like to think of visible matter as the olive in the martini of dark matter,” said Sean Carroll, a theoretical physicist at Caltech.
The research, appearing today in the online edition of the journal Nature, was led by Richard Massey and Nick Scoville of Caltech, and involved 70 astronomers from around the world.
Besides showing where dark matter is located, the three-dimensional map also showed that the structure of dark matter has changed over billions of years from a smooth configuration to a more “clumpy” material as it collapses from its own gravity.
“This is the first time we’ve mapped dark matter” over large areas of the universe, said astrophysicist Adam Riess, of Johns Hopkins University, who was not involved in the research. “This shows us that not only are galaxies distributed [in clusters] but dark matter is too.”
Dark matter, like dark energy, cannot be seen. It also does not interact directly with visible matter. Its existence has been hypothesized to explain the continuing expansion of the universe.
Cosmologists say dark matter and dark energy make up 96% of the universe, and without them, the cosmos would have collapsed from its own gravity.
To create the map, the research team focused Hubble’s camera on a 2-degree-wide area of the sky in the constellation Sextans. The project was given 600 orbits of Hubble time, the largest investment of viewing time for any single project since the space telescope was launched in 1990, Massey said.
Because dark matter is not visible, Massey said, the researchers used a technique called gravitational lensing to infer the location of clumps of dark matter. As Albert Einstein proved nearly a century ago, gravity bends light. Or rather, it bends space so that light passing through it appears to take a slight detour.
Massey compared the background stars and galaxies to wallpaper. “You look for deviations and distortions in the wallpaper,” Massey said.
Gravitational lensing has been used to search for dark matter before, but not with this degree of precision.
One of the difficulties facing the research team was sorting the tiny imperfections in the Hubble telescope from the imperfections caused by dark matter. Over the years, Hubble has been slightly damaged by constant bombardment from cosmic rays. That damage had to be teased out and corrected.
Another problem was that as Hubble orbits into and out of Earth’s shadow, the telescope’s optics warm up and cool down, introducing further distortions. “There were lots of difficulties to be overcome,” Massey said.
To produce a three-dimensional record of dark matter over billions of years, the team also sorted visible galaxies by their respective red-shifts. The farther away -- and further back in time -- something is, the greater its light is shifted toward the red end of the light spectrum. This allowed the researchers to show how dark matter changed over billions of years.
“It started out a lot smoother and became a lot more filamentary and clumpy,” Massey said.
The research still doesn’t show what dark matter is, but techniques like these might eventually solve that problem too, Carroll said.
