It's a weird, weird, weird universe we live in. Cosmologists and astronomers know that only 5% of it consists of ordinary matter of the sort found in stars and planets. Another 23% consists of mysterious dark matter that (so far) manifests itself only through its gravity. And a whopping 72% of the universe consists of bizarre, space-stretching dark energy which is speeding up the expansion of the universe. Scientists don't know exactly what dark matter and dark energy are. But now they've pulled off a bit of black magic and used the subtle effects of one to study the other.
Dark matter gives structure to the cosmos. Space is filled with a vast "cosmic web" of strands and clumps of dark matter, which have grown from microscopic variations in the original, nearly smooth distribution dark matter after the big bang. Through their gravity, the clumps draw in ordinary matter, so the galaxies form and reside within these clumps. Responding to their own gravity, the clumps and strands also grow denser and more compact. At the same time, dark energy stretches the very fabric of space. So if scientists can study the evolution of the cosmic web, they ought to be able to see the effects of dark energy setting in and slightly slowing the growth and coalescence of the clumps.
And that's what astrophysicist Tim Schrabback of the Leiden Observatory in the Netherlands, and colleagues have done. Their study uses data from the Cosmic Evolution Survey, or COSMOS, the largest galaxy survey ever conducted with NASA's orbiting Hubble Space Telescope. Thanks to an improved algorithm to analyze the images, Schrabback's team could study in great detail the shapes of over 446,000 galaxies in a 1.64 square degree patch of sky. During its journey to Earth, the light from these faint galaxies must pass through the lumps and filaments of dark matter in the cosmic web. The gravity exerted by the clumps bends the paths of light rays and distorts the images of the galaxies, so rather than appearing as randomly oriented ellipses on the sky, neighboring galaxies align a bit like fish in a school. The distortions are tiny, but starting in 2000, astronomers managed to detect the effect, which is known as cosmological weak lensing, in surveys of thousands of galaxies. In recent years, they've even begun to trace, at least crudely, the three-dimensional structure of the cosmic web.
Now, Shrabback and colleagues have gone a key step further and have actually detected the effects of dark energy on the evolution of the cosmic web. This was possible thanks to additional ground-based data which allowed them to estimate distances to almost half of the observed galaxies. Together with the shape measurements, the distances helped the researchers produce a 3D "picture" of the distribution of the dark matter in the cosmic web. This tomographic approach is the cosmic analogous of the "reconstruction of the skeleton from a CT scan," adds team member F. William High from Harvard University.
This 3D view on the "cosmic web" shows how many clumps are to be found at different distances from us, and how massive they are. Comparing the results obtained on the different distance slices, the team recorded a slowing of the growth of cosmic structures, a sign that the dark energy is driving the universe to expand faster and faster. The excellent agreement between these and many other measurements indicates that "cosmologists seem to be on the right track on their quest to understand the properties and evolution of the Universe," notes Schrabback.
"This important result shows the power of weak lensing studies from space to track down dark energy," says Yun Wang, a cosmologist at the University of Oklahoma, Norman. She also points out that, in order to fully exploit this method and attempt to understand what dark energy actually is, "a much wider survey is necessary," such as those planned for the future space-telescope missions such as the United States proposed Joint Dark Energy Mission and Europe's proposed Euclid satellite. "This study demonstrates that the method can work, and anticipates the success of these future experiments," she says.