NASA scientists working with the Hubble Space Telescope have discovered an arc of light behind an extremely massive cluster of galaxies about 10 billion light-years away. Given that the clusters formed over 13.7 billion years ago at a time when the Universe was very young, there shouldn't really be anything "behind" it. But there is.


The discovery suggests that our understanding of the composition of the early Universe may not be entirely accurate.


Galaxy clusters are collections of anywhere from hundreds to even thousands of galaxies that are bound together by gravity. They are the most massive structures in the Universe. Astronomers like to study them on account of an effect called gravitational lensing which allows them to see magnified galaxies behind them that would otherwise be too dim.

But when looking at this faraway cluster, astronomers saw a giant arc – the stretched shape of a more distant galaxy whose light is distorted by the massive cluster's powerful gravity.

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Speaking through a NASA release, study leader Anthony Gonzalez had this to say:

When I first saw it, I kept staring at it, thinking it would go away. 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.


Further analysis of the arc revealed that it is a star-forming galaxy that existed 10 to 13 billion years ago. The discovery could shed some light on how, during the first moments after the big bang, conditions were set up for the growth of hefty clusters.

As for the galaxy cluster itself, it is the most massive one found at that epoch, consisting of nearly 500 trillion suns. It is five to 10 times larger than other clusters found at that stage in the Universe's development.


Gonzalez has come up with a couple of theories to explain this pair of anomalies.

He speculates that distant galaxy clusters, unlike the ones nearby, have denser concentrations of galaxies at their cores, making them better magnifying glasses. This would explain why the arc could be seen so easily. Another possibility is that the initial microscopic fluctuations in matter immediately after the Big Bang were different than what we observe today – something that would throw off the simulations. This could explain why the clusters are more massive 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 8 billion and 10 billion years ago to see how many more gravitationally lensed objects we can find."

Image via NASA. Inset diagram of gravitational lensing via Berkeley.