Over time, matter pulls together into a web-like structure along a scaffolding of dark matter (Illustration: Center for Cosmological Physics/U Chicago)
The survey, which is still ongoing, has already covered an area of the sky around 300 times the size of the Full Moon. Astronomers led by Liping Fu of the Institute of Astrophysics in Paris, France, have analysed data gathered by the 340-megapixel MegaCam – the largest astronomical camera in the world – attached to the 3.6-metre Canada France Hawaii Telescope (CFHT) in Hawaii, US.
The observations reveal dark matter via a subtle effect called weak lensing, which studies galaxies' apparent shapes. The shapes can be distorted by the gravity of any intervening matter, which deflects the galaxies' light.
The pattern of those distortions points to a web of vast dark-matter filaments, some stretching at least 270 million light years across space.
That is just what is predicted by current theories of cosmic structure formation. Theorists think that after the big bang, an almost smooth distribution of dark matter gradually collapsed under its own gravity into blobs and filaments, which in turn attracted and compressed gas to form stars and galaxies.
Cosmic freeways"This is a very significant milestone in the subject," says Richard Ellis, a cosmologist at Caltech in Pasadena, US, who was not part of the team.
In its reach, the study dwarfs an earlier survey of dark matter by the Hubble Space Telescope, although it doesn't provide such fine detail as Hubble.
"The CFHT map is rather like a first glimpse of the interstate freeways across the US, whereas Hubble gave us a more detailed map of the roads as if in a single state," Ellis told New Scientist. He says that both are needed to give a full picture of how dark matter is structured in the universe.
The new result seems to resolve what had been a niggling discrepancy in cosmological measurements. Previous lensing surveys implied that dark matter was somewhat more abundant and more tightly clumped than suggested by studies of relic microwave radiation from the big bang.
Uncertain distanceThe team puts the previous discrepancy down to uncertain measurements of distance to the lensed galaxies, which affects calculations of how much dark matter is needed to cause the observed distortions.
The new findings now agree with the microwave results, concluding that dark matter forms almost exactly 25% of the total contents of the universe (the rest is made up of 71% dark energy and 4% ordinary luminous matter).
As the CFHT survey expands to cover more of the sky, the team hopes to be able to address some deep questions about dark matter – such as whether it is really an invisible substance, or actually a change in the strength of gravity at large scales.
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Journal reference: Astronomy & Astrophysics (vol 479, p 9)