Using data from the Sloan Digital Sky Survey, an international team of researchers has discovered a record-breaking cluster of quasars—young active galaxies—stretching four billion light-years across.
"This discovery was very much a surprise, since it does break the cosmological record as the largest structure in the known universe," said study leader Roger Clowes, an astronomer at University of Central Lancashire in England.
For comparison, our own galaxy, the Milky Way, is just a hundred thousand light-years across, while the local supercluster of galaxies in which it's located, the Virgo Cluster, is only a hundred million light-years wide.
Astronomers have known for years that quasars can form immense clusters that stretch to more than 700 million light-years across, said Clowes. But the epic size of this group of 73 quasars, sitting about 9 billion light-years away, has left them scratching their heads.
That's because current astrophysical models appear to show that the upper size limit for cosmic structures should be no more than 1.2 billion light-years.
"So this represents a challenge to our current understanding and now creates a mystery—rather than solves one," Clowes said.