Electronic version with figures: http://www.ifa.hawaii.edu/info/press-releases/szapudi-7-08/
Hawaii Scientists Find Direct Evidence of "Dark Energy" in Supervoids and Superclusters
A team of astronomers at the University of Hawaii Institute for Astronomy (IfA) led by Dr. 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 of 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 said. 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," 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."
"Dark energy sort of gives microwaves a memory of where they've been recently," postdoctoral scientist Mark Neyrinck said. The team also includes graduate student 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 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," Granett said.
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.
Paper preprint available at http://arxiv.org/abs/0805.3695
Less technical preprint at http://arxiv.org/abs/0805.2974
Pictures and movies at http://www.ifa.hawaii.edu/cosmowave/supervoids
Figure 1: The UH team compared directions in the sky where they found superclusters (red circles) and supervoids (blue circles) with the strength of the Cosmic Microwave Background. Superclusters are more likely to coincide with directions where microwaves are unusually strong (red or orange coloring) and supervoids with directions where the microwaves are unusually weak (blue coloring). Credit: B. Granett, M. Neyrinck, I. Szapudi
Figure 2: From left to right, Istvan Szapudi, Mark Neyrinck, and Benjamin Granett. Photo by Andrea Gaspar.
Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.
Established in 1907 and fully accredited by the Western Association of Schools and Colleges, the University of Hawaii is the state's sole public system of higher education. The UH System provides an array of undergraduate, graduate, and professional degrees and community programs on 10 campuses and through educational, training, and research centers across the state. UH enrolls more than 50,000 students from Hawaii, the U.S. mainland, and around the world.