And those scientists who are seeking out ancient black holes, including Kashlinsky, think they're pretty heavy — perhaps between 20 and 100 times the mass of the sun. That idea even got a boost after the recent and groundbreaking discovery of gravitational waves, which two black holes of unusual size (30 solar masses) triggered when they collided.
Yet an unpublished research on "primordial" black holes — those formed in the hot particle soup of the Big Bang, not by collapsing stars — suggests ones that are very small in diameter could exist in droves.
If these mini-black holes are real, Kashlinsky says the heaviest of them would weigh less than the moon, yet would be shrunken down to about 0.25 millimeter in diameter, or about the width of a human hair.
Timothy Brandt, an astrophysicist at the Institute for Advanced Study, said the very lightest, asteroid-size holes would have an apparent size of less than an atom.
The reason is because black holes are so dense. In fact, beyond a certain point, pretty much any bit of matter in the universe squeezed tightly enough will collapse beyond a gravitational point-of-no-return.
That boundary is spherical and called an event horizon, and beyond it not even photons of light — the fastest things in the universe — can escape if they fall in.
Any black holes smaller than an asteroid probably evaporated long ago due to Hawking radiation, a fantastical consequence of the laws of nature that Stephen Hawking deduced in 1974.
So what if tiny black holes are out there — how often would they swing by, and what might they do?