July 30, 2008
A team of astronomers at the University of Hawaii Institute for Astronomy (IfA) led by Istvan Szapudi has found direct evidence for the existence of dark energy. Dark energy works against the tendency of gravity to pull galaxies together and so causes the universe's expansion to speed up. The nature of dark energy (what precisely it is, and why it exists) is one of the biggest puzzles in modern science.
This is arguably the clearest detection to date of dark energy's stretching effect on vast cosmic structures: there is only a one in 200,000 chance that the detection would occur by chance.
"We were able to image dark energy in action, as it stretches huge supervoids and superclusters of galaxies," Szapudi says. Superclusters are vast regions of space, half a billion light-years across, that contain an unusually high concentration of galaxies, while supervoids are similarly sized regions with a below-average number of galaxies. They are the largest structures known in the universe. The team made the discovery by measuring the subtle imprints that superclusters and supervoids leave in microwaves that pass through them.
"When a microwave enters a supercluster, it gains some gravitational energy, and therefore vibrates slightly faster," explains Szapudi. "Later, as it leaves the supercluster, it should lose exactly the same amount of energy. But if dark energy causes the universe to stretch out at a faster rate, the supercluster flattens out in the half-billion years it takes the microwave to cross it. Thus, the wave gets to keep some of the energy it gained as it entered the supercluster."
"Dark energy sort of gives microwaves a memory of where they've been recently," says team member Mark Neyrinck. The team also includes Benjamin Granett, the first author on the paper, which will be published in the Astrophysical Journal Letters in August or September.
The team compared an existing database of galaxies with a map of the cosmic microwave background radiation (CMB), the faint hiss of microwaves left over from the Big Bang. As predicted, they found that the microwaves were a bit stronger if they had passed through a supercluster, and a bit weaker if they passed through a supervoid.
"With this method, for the first time we can actually see what supervoids and superclusters do to microwaves passing through them," Granett says.
The signal is difficult to detect, since ripples in the primordial CMB are larger than the imprints of individual superclusters and supervoids. To extract a signal, the team averaged together patches of the CMB map around the 50 largest supervoids and the 50 largest superclusters that they detected in extremely bright galaxies drawn from the Sloan Digital Sky Survey, a project that mapped the distribution of galaxies over a quarter of the sky.