Dark Energy Signs Seen in Giant Clusters and Voids

posted: 4 August 2008 7:00 a.m. ET

: The researchers 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

Scientists have found more intriguing evidence for dark energy — one of nature's most befuddling phenomena.

Dark energy is thought to make up about 74 percent of the universe, while dark matter — a mysterious form of matter that scientists can only detect by noting its gravitational pull on things — makes up about 22 percent. That leaves only 4 percent of the universe composed of things we can see and touch: the normal protons, electrons and neutrons called baryonic matter.

Scientists don't know what dark energy is, but they observe its tugging effect, which causes the expansion of the universe to accelerate. Now they have seen this mysterious force in some of the largest known features of the cosmos, called superclusters and supervoids.

The former are particularly crowded areas of space, each with a lot of galaxies huddled in a region half a billion light-years across, while the latter are the opposite, rather barren expanses notably lacking galaxies.

Astronomers led by István Szapudi of the University of Hawaii Institute for Astronomy observed dark energy stretching out these areas by detecting changes in rays of microwave light before and after they passed through the regions.

"When a microwave enters a supercluster, it gains some gravitational energy, and therefore vibrates slightly faster," Szapudi said. "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."

Szapudi, with University of Hawaii postdoctoral researcher Mark Neyrinck and graduate student Benjamin Granett, analyzed a map of the varying strength of the microwave radiation left over from the Big Bang, called the cosmic microwave background radiation (CMB), across the universe. They matched this data to a map of the universe with the 50 largest supervoids and the 50 largest superclusters plotted, based on information from the Sloan Digital Sky Survey, a project that mapped the distribution of galaxies over a quarter of the sky.

As the researchers predicted, 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 team will detail their findings in the Astrophysical Journal Letters in August or September.