London: Astronomers have discovered the largest known structure in the universe - a cluster of galaxies that spans an astonishing four billion light years.
The sprawling structure is known as a large quasar group (LQG), in which quasars powered by super-massive black holes clump together.
Quasars are the nuclei of galaxies from the early days of the universe that undergo brief periods of extremely high brightness that make them visible across huge distances.
These periods are 'brief' in astrophysics terms but actually last 10-100 million years, the journal 'Monthly Notices' of the Royal Astronomical Society reported.
Researchers said since 1982 it has been known that quasars tend to group together in clumps or 'structures' of surprisingly large sizes, forming large quasar groups or LQGs.
The research team, led by Dr Roger Clowes from the University of Central Lancashire (UCLan), identified the LQG which is so significant in size that it also challenges the Cosmological Principle: That the universe, when viewed at a sufficiently large scale, looks the same no matter where you are observing it from.
The modern theory of cosmology is based on the work of Albert Einstein, and depends on the assumption of the Cosmological Principle. The Principle is assumed but has never been demonstrated observationally 'beyond reasonable doubt'.
The Milky Way, for instance, is separated from its nearest neighbour, the Andromeda Galaxy, by about by about 0.75 Megaparsecs (Mpc) or 2.5 million light-years.
Whole clusters of galaxies can be 2-3 Mpc across but LQGs can be 200 Mpc or more across. Based on the Cosmological Principle and the modern theory of cosmology, calculations suggest that astrophysicists should not be able to find a structure larger than 370 Mpc.
Clowes' newly discovered LQG however has a typical dimension of 500 Mpc. But because it is elongated, its longest dimension is 1200 Mpc or 4 billion light years - some 1600 times larger than the distance from Milky Way to Andromeda.
"While it is difficult to fathom the scale of this LQG, we can say quite definitely it is the largest structure ever seen in the entire universe. This is hugely exciting not least because it runs counter to our current understanding of the scale of the universe," Clowes said.
"Even travelling at the speed of light, it would take 4 billion years to cross. This is significant not just because of its size but also because it challenges the Cosmological Principle, which has been widely accepted since Einstein," said Clowes.