The universe hums like an orchestra: its notes are gravitational waves — ripples in spacetime predicted by Albert Einstein. The LIGO observatories, made exquisitely sensitive thanks to Rainer Weiss, strive to catch these whispers. Most often, we hear thunderous crashes — the mergers of ordinary black holes born from stars. But astrophysicists tried to pick out quiet, barely-there notes — signals from primordial black holes that emerged right after the Big Bang.
These tiny specks (some lighter than an asteroid) might actually be dark matter — the invisible glue holding galaxies together. The idea of such ghost holes was fleshed out by Stephen Hawking. Scientists choreographed computer dances of pairs of such holes and compared them with real data. The result was stunning: the orchestra played not a single new note. Almost all the chords we heard belong to ordinary black holes.
If you squeezed Earth down to the size of a grape, it would become a black hole. Primordial black holes can be trillions of times lighter, yet their gravitational whisper is still detectable.
In science fiction, tiny black holes sometimes serve as fuel for starships — for instance, in Larry Niven’s Ringworld series.