A mysterious, gigantic arc of light residing behind a massive cluster of galaxies has been discovered by astronomers.
It resides about 10bn light years away and was spotted by Nasa’s Spitzer Space Telescope, with Hubble confirming the distance.
It was observed when the universe was roughly a quarter of its current age of 13.7bn years.
The giant arc is the stretched shape of a more distant galaxy whose light is distorted by the monster cluster's powerful gravity, an effect called gravitational lensing.
The thing is, say researchers – it shouldn’t exist.
"When I first saw it, I kept staring at it, thinking it would go away," said study leader Anthony Gonzalez of the University of Florida in Gainesville.
"According to a statistical analysis, arcs should be extremely rare at that distance. At that early epoch, the expectation is that there are not enough galaxies behind the cluster bright enough to be seen, even if they were 'lensed' or distorted by the cluster.
“The other problem is that galaxy clusters become less massive the farther back in time you go. So it's more difficult to find a cluster with enough mass to be a good lens for gravitationally bending the light from a distant galaxy."
Galaxy clusters are collections of hundreds to thousands of galaxies bound together by gravity.
They are the most massive structures in our universe.
Our own galaxy, the Milky Way, is part of a cluster known as The Local Group, which spans around three-million light years and contains nearly three-dozen galaxies.
You may think that’s fairly sizeable, but it’s an absolute minnow compared to ‘superclusters’.
The biggest of these is The Great Wall - a supercluster of mind-blowing proportions that is, as far as we know, the single biggest object in the universe.
It measures 500m light-years long, 200m light-years wide, 15m light-years deep and contains tens of thousands of galaxies.
Astronomers frequently study galaxy clusters to look for faraway, magnified galaxies behind them that would otherwise be too dim to see with telescopes. Many such gravitationally lensed galaxies have been found behind galaxy clusters closer to Earth.
The surprise in this observation is spotting a galaxy lensed by an extremely distant cluster.
Dubbed IDCS J1426.5+3508, the cluster is the most massive found at that epoch, weighing as much as 500trn suns.
"The chance of finding such a gigantic cluster so early in the universe was less than one percent in the small area we surveyed," said team member Mark Brodwin of the University of Missouri-Kansas City.
"It shares an evolutionary path with some of the most massive clusters we see today, including the Coma Cluster and the recently discovered El Gordo Cluster."
An analysis of the arc revealed that the lensed object is a star-forming galaxy that existed 10bn to 13bn years ago. The team hopes eventually to obtain a more accurate distance to the lensed galaxy.
Gonzalez has considered several possible explanations for the arc.
One explanation is that distant galaxy clusters, unlike nearby clusters, have denser concentrations of galaxies at their cores, making them better magnifying glasses.
However, even if the distant cores were denser, the added bulk still should not provide enough gravitational muscle to produce the giant arc seen in Gonzalez's observations, according to a statistical analysis.
Another possibility is that the initial microscopic fluctuations in matter made right after the big bang were different from those predicted by standard cosmological simulations, and therefore produced more massive clusters than expected.
"I'm not yet convinced by any of these explanations," Gonzalez said. "After all, we have found only one example. We really need to study more extremely massive galaxy clusters that existed between eight billion and ten billion years ago to see how many more gravitationally lensed objects we can find."
The results of the study will appear in The Astrophysical Journal.