Not under threat (Image: Mehau Kulyk/SPL/Getty)

We can all rest easy. Small black holes that may be roaming space undetected would leave Earth unscathed if they hit us.

Various models suggest matter may have collapsed into black holes soon after the big bang. The smallest of these so-called primordial black holes would have evaporated through a process called Hawking radiation long ago.

But those weighing a billion tonnes or more could still be around, and many of these black holes would be hard to detect – unless they hit us, says Katherine Mack of the University of Cambridge. “We’d see that.”

In fact, over-eager physicists have already flagged two false alarms. In 1908, a mysterious explosion flattened more than 2000 square kilometres of forest near the Tunguska river in Siberia. In 1973 physicists proposed that a colliding black hole was to blame – a suggestion that was later proved wrong.


Then in 2003, another group suggested that an unexplained set of seismographic data could be from a dense object called a “quark nugget” smacking into the planet. It turned out to be an earthquake.

End to false alarms

Shravan Hanasoge at Princeton University and colleagues wondered if there was a way to avoid future false alarms. They ran detailed simulations of what would happen if a billion-tonne black hole struck Earth.

It would probably hit at a good clip, moving at a relative speed of a few hundred kilometres per second. But it would be smaller than an atomic nucleus, so it would only make a small, needle-like tunnel through the Earth.

Despite its small size, we would still know it had hit. That’s because when the black hole first reaches and then exits the Earth’s outer core, the outer core would vibrate, creating spherically symmetric shock waves. These would trigger every seismic detector on Earth at the same time – unlike regular earthquakes, which are more localised. “This distinguishes the signals from anything we would typically see,” says Hanasoge.

Happily, the effect would be minor, like a global magnitude-4 quake. “There would be no widespread destruction,” Hanasoge says. “It would be almost unnoticeable.”

Unfortunately for black-hole hunters such events would be extremely rare. Even if, as some predict, primordial black holes made up all of the dark matter in the galaxy, they would collide with the Earth just once every 10 million years.

Cosmic strings

But since we have the equipment that could detect them we might as well look, Hanasoge says. “It doesn’t take much. With the existing apparatus and existing methods, we’d know if this happens.”

Although the odds of success are low, the pay-off could be high. Primordial black holes may have arisen from objects such as cosmic strings, defects in the fabric of space-time that may have formed after the big bang.

“It gives us insight into what happened in the early universe that is hard to get at otherwise,” Mack says. Finding something left over from the earliest moments of the universe “is like a message in a bottle from the big bang straight to us. It gives us a very direct picture of what happened at that time. And that’s extremely exciting.”

Journal reference: arxiv.org/abs/1203.3806; the work will appear in The Astrophysical Journal