To look through space is to look through time, and a five-year survey has pushed that to its limits, peering back some 13 billion years to a time just after the Big Bang. Astronomers on the international project have turned up 100 supermassive black holes from a time when they were thought to be rare, suggesting we might need to rewrite our understanding of their evolution.

Supermassive black holes are those with mass equal to hundreds of millions or even billions of Suns, and they're often found at the center of galaxies – including our own Milky Way. Although the black holes themselves are invisible, they ironically become some of the most easily-spotted objects in the night sky, because the gas falling into them heats up and shines brightly as a quasar.

Although their brightness means they can be spotted across huge distances, the mind-boggling vastness of the universe means that we can usually only see the absolute brightest quasars at long distances. One named SDSS J0100+2802, for example, can be seen from 12.8 billion light-years away because it shines with the light of 420 trillion Suns.

Because only the brightest could be seen in the early universe, astronomers couldn't get a good grasp on just how common they may have been back then. To take a more complete survey of fainter ones, an international team used the powerful Hyper Suprime-Cam (HSC) on the Subaru Telescope in Hawaii to look for new quasar candidates.

Over 300 nights across five years, the team came up with a long list of candidates, then investigated them closer using the Subaru Telescope as well as Gran Telescopio Canarias in Spain and the Gemini South Telescope in Chile.

Of those candidates, the astronomers were able to identify 83 new quasars at very long distances, roughly 13 billion light-years. Taken together with the 17 already-known quasars in that survey area, it appears that supermassive black holes are far more common in the early universe than we previously thought.

That find presents a bit of a problem to our current understanding of their formation. The universe exploded into existence about 13.8 billion years ago, which means these supermassive black holes must have grown to such huge sizes within 800 million years – far quicker than models suggest.

"It is remarkable that such massive dense objects were able to form so soon after the Big Bang," says Michael Strauss, one of the co-authors of the study. "Understanding how black holes can form in the early universe, and just how common they are, is a challenge for our cosmological models."

The most distant quasar discovered to date is about 13.2 billion light-years away, but now having 100 examples of them quickly growing supermassive shows this is more than just an anomaly. These quasars will no doubt be the focus of future studies, and inspire more surveys to find new ones.

"The quasars we discovered will be an interesting subject for further follow-up observations with current and future facilities," says Yoshiki Matsuoka, lead author of the study. "We will also learn about the formation and early evolution of supermassive black holes, by comparing the measured number density and luminosity distribution with predictions from theoretical models."

The research was published in a series of papers over the years, with the most recent findings appearing in The Astrophysical Journal and The Astrophysical Journal Letters.

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