A collection of galaxies that is a whopping four billion light years long is the biggest cosmic structure ever seen. The group is roughly one-twentieth the diameter of the observable universe ? big enough to challenge a principle dating back to Einstein, that, on large scales, the universe looks the same in every direction.
Roger Clowes of the University of Central Lancashire in Preston, UK, and colleagues discovered the structure using data from the Sloan Digital Sky Survey, the most comprehensive 3D map of the universe. They identified a cluster of 73 quasars, the brightly glowing cores found at the centre of some galaxies, far larger than any similar structure seen before.
Since 1982 astronomers have known that quasars tend to clump together in large quasar groups, or LQGs. "We look for quasars that have a certain separation from the next nearest quasar," says Clowes.
The newly discovered, and appropriately named, Huge-LQG (see black circles in image) happens to be in the same region of the sky as one of the earliest known quasar clusters, which Clowes helped find in 1991. That group contains 34 quasars and measures roughly one billion light years across (red crosses), so it is dwarfed by Huge-LQG.
Basic assumption
The discovery of Huge-LQG joins a collection of observations that seem to challenge the cosmological status quo. When Albert Einstein first applied his theory of general relativity to the universe as a whole, to make the calculations workable, he was forced to assume that one large part looks much like any other large part. This became known as the cosmological principle.
Still, a question remained: how large is a large part?
"As time went on, people did more and more surveys," says Clowes. "Each time they found structures the size of the new survey, and you began to wonder when it would all stop."
Previous calculations gave a value of one billion light years as the maximum possible size of a cluster. The 1991 LQG is at this supposed limit, but Huge-LQG smashes right through it. The researchers say this could undermine the cosmological principle, although it may simply mean that we need to revise upwards the size limit on large structures.
Dark flow
But other evidence, such as a controversial "stream" of galaxies that seem to be moving in the same direction, dubbed dark flow, is also poking holes in the uniformity of the universe.
The search for such large structures is key to furthering our understanding of the universe and creating new and improved cosmological models, says Subir Sarkar of the University of Oxford. "All of this suggests there is structure on scales at which the universe is supposed to be boring," he says.
But the cosmological principle is so ingrained that it is hard for researchers to shake. "People are maybe understandably reluctant to give up the thing, because it will make cosmology too bloody complicated," says Sarkar.
Journal reference: Monthly Notices of the Royal Astronomical Society, DOI: 10.1093/mnras/sts497
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