Washington, July 30 : A team of astronomers at the University of Hawaii Institute for Astronomy (IfA) has found direct evidence for the existence of "dark energy" in supervoids and superclusters.
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 is one of the biggest puzzles of modern science.
The team from the University of Hawaii IfA made the discovery of dark energy by measuring the subtle imprints that superclusters and supervoids leave in microwaves that pass through them.
"We were able to image dark energy in action, as it stretches huge supervoids and superclusters of galaxies," said Dr. Istvan Szapudi.
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.
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.
"When a microwave enters a supercluster, it gains some gravitational energy, and therefore vibrates slightly faster," explained 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," he added.
"Dark energy sort of gives microwaves a memory of where they've been recently," said postdoctoral scientist Mark Neyrinck.
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 had 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," said Granett.
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.