According to the "cosmic web" theory of the universe, clumps of so-called dark matter--the invisible stuff accounting for a lion's share of the universe's mass--are linked by slender filaments made of the same stuff. Now, researchers say they've found the first firm evidence of such filaments by mapping the dark matter in a small part of the sky.
In cosmology, what you see isn't what you get: Perhaps 80% of the universe's mass doesn't shine and so is invisible to astronomers' telescopes. Although nobody knows what this dark matter is, its gravitational tug is important in molding the universe. The same pull also reveals dark matter's presence when it distorts light from luminous stars and galaxies beyond. By carefully analyzing the images of many distant galaxies beyond a dark matter-containing region, astronomers can use the pattern of tiny distortions to draw up a plan of where the dark matter is hiding.
Cosmologist Andrew Taylor of the Institute for Astronomy in Edinburgh, U.K., and his colleagues have used this trick, called gravitational lensing, to map the dark matter in a vast supercluster known as Abell 901. An astonishing 10 million light-years across, the supercluster embraces several cluster of galaxies and is one of the largest known structures in the universe. That gargantuan size maximized the team's chances of capturing filaments in what is one of the sharpest dark matter snapshots yet.
The team scrutinized an area of sky the size of a full moon, analyzing images of more than 50,000 galaxies beyond the supercluster. "There do seem to be filamentary structures of matter connecting the clusters in that object," Taylor says. Evidence for the first filament was published in the 20 March of The Astrophysical Journal, but further analysis have yielded clues to the presence of several more such filaments, Taylor said on 10 April at a meeting of the U.K. National Astronomy Meeting in Bristol.
The term "cosmic web" was coined in 1996 by the University of Toronto's Richard Bond to describe a tangled structure of clumps and filaments naturally formed by dark matter left to experience the pull of gravity. The new findings support the theory, says Bond: "I think they have seen the filaments between the clusters." We are lucky to be around to see these filaments, says Taylor, since eventually they will be absorbed into the clusters they link and will vanish.