The Mystery of Dark Matter Clarified—a Little

What is dark matter? Well, we know what it's probably not: black holes.

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Astronomers got their first hints that the universe is filled with some invisible, mysterious, massive substance back in the 1930’s—something that must be there and holding things together gravitationally, otherwise the rotation of galaxies would cause them to spin apart. Even now, nobody knows for sure what the mystery stuff is. The leading candidate for the past decade or two has been some sort of exotic elementary particle, forged in the Big Bang—but so far, despite plenty of searching, such a particle has never actually been found.

That means there’s still hope for a dark horse in the dark matter sweepstakes: black holes are certainly dark, and there could be lots and lots of them floating around that we haven’t noticed. But the hope that they’re the answer to the riddle has faded recently with a couple of new papers—one based on what is effectively a thought experiment, the other on an ingenious set of observations with the Kepler space telescope, which was launched in 2009 to search for exoplanets, or worlds orbiting other stars.

One kind of particularly small black hole was already off the table. Such things could have been created in the violent turbulence of very early universe, but would have long since evaporated (some people feared one might be created by the Large Hadron Collider when it switched on in 2008, and go on to swallow the Earth, but since you’re reading this, it didn’t).

(MORE: Fuggedaboudit: Has Dark Matter Disappeared?)

Smallish black holes, however, from about a third the mass of the moon to a third the mass of the sun, were still a possibility. So Harvard astrophysicist Avi Loeb and several colleagues decided to test the idea. Way back in 1974, physicists Bill Press and Saul Teukolsky thought about the fact that a black hole could actually reflect light rather than swallowing it if it were spinning fast and the light came in at an angle. If you could somehow surround the hole with mirrors, the light would bounce back and forth from black hole to mirrors and back again, the energy getting amplified with every bounce. “It would be similar to a laser,” says Loeb, and once the energy got high enough, the whole thing would explode. “They called it a ‘black hole bomb,’” he says.

Good thing for the black holes then that there are no free-floating mirrors in space, meaning that plenty of the smallish bodies should have survived and might still be around today to act as dark matter. But until about 400,000 years after the Big Bang, Loeb realized, the universe was a sea of hot, free-flying particles, including zillions of electrons, and that a dense cloud of electrons can reflect light, just like a mirror. That would spell trouble. If smallish black holes did form back then, they’d almost certainly have been spinning rapidly, and thanks to the electrons, black hole bombs should have been going off right and left.

Some could have survived to make up the dark matter, says Loeb, but the energy released by the ones that exploded would have distorted the cosmic micowave background radiation — the light flash left over from the Big Bang — in ways that should easily be seen today. There’s no such distortion, meaning that no significant numbers of black holes in this size range ever existed in the first place.

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That’s the pure-thought basis for the new findings: the direct-observation version comes from Kim Griest, of the University of California, San Diego, and two co-authors. They figured that if smallish black holes were around in any significant numbers today, they should have drifted across the Kepler telescope’s field of view. Kepler’s has done its exoplanet-hunting by searching for the silhouettes of distant planets as they passed in front of their parent stars, slightly blocking the light they emit.

But while planets marginally dim the light of a star, a black hole moving in front of it should have the opposite effect. As Einstein realized in the 1930’s, the gravity of a massive object warps the space around it, bending any light rays that pass by. If something is shining in the background, it will appear to be distorted or magnified. Gravitational lensing has been seen in galaxies and quasars, among other places, and if a black hole wandered in front of any background star, it would  brighten it in the same way (actually, just a part of the star’s surface, since a black hole is so physically small).

This sort of brightening should be evident in Kepler’s observations—if the black holes are there. But it’s not. So they aren’t.

That still doesn’t rule out black holes entirely, according to Loeb. “If they’re somewhere between the mass of an asteroid and a third the mass of the moon,” he says, “they’re still allowed.” If you’re a black hole though, and you aspire to be the source of dark matter, things are getting awfully tough.

(MORE: Look Close. Something’s Strange in the Photo of the Universe)