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Supermassive black holes—objects containing hundreds of millions to billions of times the mass of a star—are one of the deepest mysteries of modern astrophysics. They lurk at the hearts of most large galaxies, including our own Milky Way. Given their ubiquity, these black holes may play a vital part in the formation and evolution of the universe. But how they grew so massive has long puzzled theorists worldwide.

The most sensible suggestion—that these monstrosities could only have grown so great by swallowing enormous quantities of gas over billions of years—is now known to be wrong. Recent observations have revealed the existence of black holes billions of times more massive than the sun just 800 million years after the big bang. And so, the riddle goes: How did they get there so quickly? Most astrophysicists agree supermassive black holes must stem from smaller “seed” black holes. They just don’t agree on how humble such a seed must be. One school of thought holds that the seed black holes should be big—thousands to several tens of thousands of times the sun’s mass; the other posits the seeds could be small—no heavier than a hundred solar masses.

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