Weird black holes may hold secrets of the early universe

Growing evidence suggests that rogue black holes and other eccentric behemoths exist

By Ashley Yeager

Our galaxy’s heart is a gluttonous monster. Like the mythical Kammapa of the Sotho people of southern Africa, the Milky Way’s central, supermassive black hole has swallowed nearly everything around it, growing heftier and heftier the more it eats. And it’s not alone. Black holes weighing as much as thousands, millions or even billions of suns sit at the center of nearly all known massive galaxies.

For decades, scientists thought that was the only place they’d find such behemoths, because only massive galaxies had enough material to feed the monsters’ excessive appetites. But beginning about two decades ago, computer simulations of the earliest black holes started turning up oddities — big black holes that weren’t smack-dab where they were expected. These misfits must be nothing more than flukes, many scientists reasoned at the time, dismissing the results without a second thought.

But others weren’t so certain the oddballs should be cast off. If observations show that these unusual black holes exist in the nearby universe, these astrophysicists speculated, they could be untapped clues to the universe’s infancy and adolescence.

“We can, weirdly, [learn about] the super-beginning of the universe by looking at things really close to us,” says theoretical astrophysicist Jillian Bellovary of Queensborough Community College in New York City.

The notion remained just an idea for years. But now, the existence of these misfits isn’t so easy to ignore. Astronomers have turned up signs of a number of unexpectedly massive black holes in the universe’s tiniest galaxies, and surprisingly, some of those black holes don’t appear to sit at their galaxies’ centers. Even more intriguing, astronomers have spotted evidence of black holes wandering at their galaxies’ edges, and in rare cases, being kicked from their homes into intergalactic space.

Perhaps these black holes aren’t merely cosmic nonconformists but instead big players in the story of our universe. If so, they are a tool for probing one of the greatest mysteries in all of astrophysics — how the cosmic Kammapas we see today came to be.

“Without understanding what black holes are doing, you cannot understand galaxy evolution,” says Xiaohui Fan, a cosmologist at the University of Arizona in Tucson, making it impossible to explain the landscape of the universe.

Earlier this year, astronomers spotted a black hole weighing millions of suns that they believe had been ejected from its host galaxy, leaving a trail of stars (as seen in this illustration). But some researchers aren’t convinced.

Little galaxies can have big black holes

Our current cosmological understanding of how black holes got so big goes something like this: As galaxies grow, collide and merge over cosmic time, they take on gobs of new stars, gas and dust. The black holes at the galaxies’ centers grow in lockstep, ballooning as they merge with one another and feed on the newly acquired material. A rough estimate puts a supermassive black hole’s heft at somewhere around a thousandth of the mass of its home galaxy.

In this scenario, the universe’s littlest galaxies, called dwarf galaxies, probably didn’t go through many mergers in the past. Tipping the scales at only about a trillionth the mass of the Milky Way, they should have relatively runty black holes, or none at all.

But in the late 2000s, astrophysicist Marta Volonteri of Institut d’Astrophysique de Paris at Sorbonne University helped run computer simulations that tracked the evolution of massive black holes from birth to today. In those efforts, almost as soon as they popped into existence, even the smallest galaxies could have surprisingly large black holes. As time passed, some of those galaxies never grew or merged with others, leaving them unmarred after billions of years of cosmic evolution.