Kate Becker The Visible Universe
Kate Becker The Visible Universe

They grow up too fast.

If you're a parent, you've probably heard this one before. Maybe it was from the kindly stranger making doe-eyes at you while your baby earnestly licked her very tiny, very dirty shoes. Maybe you said it yourself at your child's first step, first word, or first day of school.

Or maybe it was as you looked out at the far reaches of the universe and saw baby black holes, a mere billion years old, yet already grown to billions of times the mass of our sun. These young black holes are bigger than astronomers' models can explain.

They grew up too fast!

Black holes fall into two main categories: stellar mass black holes, born when very massive stars die and collapse, and supermassive black holes, which are millions or billions of times more massive. Our galaxy probably contains some 100 million stellar mass black holes, but just one supermassive black hole, a black bulls-eye in the center of the galaxy that contains about as much mass as four million suns.

Astronomers think that almost every galaxy in the universe has a supermassive at its center, and they now believe that these objects play a critical role in how galaxies form and evolve.

But where do they come from, and how did they get so big?


According to one hypothesis, supermassives started out as ordinary, stellar-mass black holes, the remains of the universe's first generation of stars. These baby black holes ate and ate and ate, guzzling down their surrounding gas so quickly that they ballooned up readily.

But there's a hitch.

Contrary to the popular image of black holes as devouring monsters, there's a limit to how rapidly they can swallow their meals. That's because stuff doesn't slide right down a black hole's gullet. Instead, material is drawn into orbit around the black hole, where it jams up into what astronomers call an accretion disk. Dust and gas on its way into the black hole must wait its turn as it circles around in the accretion disk. Plus, radiation from the hot accretion disk generates pressure that drives incoming stuff away from the black hole.

Black holes, it turns out, have a far more modest appetite than their beastly image suggests.

Now, astronomers Tal Alexander and Priyamvada Natarajan have come up with a potential solution to the baby black hole problem. Setting a model young black hole in a crowded star cluster, they showed that cold gas streaming into the center of the cluster could trap the radiation pushing out from the black hole. They also show that, in this environment, the black hole could richocet around like a bouncy ball, gathering up matter faster than the traditional limit, unimpeded by the accretion disk traffic-jam.

The work is still just a model, and the authors don't claim that it has settled the question once and for all.

Astronomers are investigating other hypotheses, too: It may be that supermassive black holes don't start out as stellar mass black holes; perhaps enormous reservoirs of gas collapse directly into black holes, or maybe they start out as behemoth stars unlike anything in the modern universe.

But the new model gives us one way of understanding how baby black holes could grow up so fast. Because, whether they're black holes or babies, no matter how fast they seem to be growing, they must be right on time.

Kate Becker is a science writer living in Boston. Contact her at spacecrafty.com, or connect via Facebook, facebook.com/katembecker, or twitter.com/kmbecker.