Supervoids and superclusters point to dark energy
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 30, 2008

By studying regions of space with an above and below average concentration of galaxies – superclusters and supervoids, respectively – a team of astronomers have found direct evidence for the existence of dark energy.

Density fluctuations in the early Universe are imprinted on the CMB, revealing clusters (superclusters) and voids (supervoids) of galaxies which cause faint glows and shadows (respectively) in microwaves that pass through them. Image: UH/Granett, Neyrinck, Szapudi /Millenium simulation.

The nature of dark energy is one of the biggest puzzles of modern science, but it is thought to work against the tendency of gravity to pull galaxies together, causing the Universe’s expansion to speed up. Impressively, astronomers from the University of Hawaii Institute for Astronomy were able to catch this elusive dark energy in action as it stretches out the largest known structures in the Universe: supervoids and superclusters, vast regions of space half a billion light years across, containing either a deficit or surplus of galaxies, brought about by density fluctuations in the early Universe.

The key to the team’s success was to measure the subtle imprints that superclusters and supervoids leave in microwaves that pass through them. But this signal is extremely difficult to detect since ripples in the primordial cosmic microwave background radiation (CMB) – the faint hiss of microwaves left over from the big bang – are larger than the imprints of individual superclusters and supervoids. Therefore, to extract a signal, the team compared an existing database of galaxies with a map of the CMB and averaged together local regions around the 50 largest supervoids and the 50 largest superclusters from a collection of bright galaxies drawn from the Sloan Digital Sky Survey. As expected, the microwaves were slightly stronger if they had passed through a supercluster, and marginally weaker if they had passed through a supervoid.

Astronomers from the University of Hawaii 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 colouring) and supervoids with directions where the microwaves are unusually weak (blue colouring). Image: B. Granett, M. Neyrinck, I. Szapudi.

“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.”

Essentially, the dark energy is giving the microwaves a memory of where they’ve been. “With this method, for the first time we can actually see what supervoids and superclusters do to microwaves passing through them,” says Benjamin Granett, first author on the paper describing the results, which will appear in a forthcoming issue of the Astrophysical Journal Letters.

“We plan to follow up with one of the coldest regions of the CMB, the ‘Cold Spot’, to determine whether it is due to a large void as hypothesised recently,” reveals Szapudi. The so-called cold spot is in fact only a few millionths of a degree colder than its surrounds, but some scientists think that it may be caused by a huge hole devoid of nearly all matter, perhaps as large as a thousand light million years in size.