Road noise is a great informant. Rare cars sound as individual signals; dense traffic merges into a steady hum. Just the same, mergers of black holes produce either single bursts or a continuous background of gravitational waves.

Astrophysicists used this similarity to traffic to figure out from the character of the noise which black holes are creating it. Many faint, overlapping signals — a sure sign of a swarm of primordial black holes. Rare powerful bursts point to other objects. It’s like telling a stream of trucks from a pack of motorcycles by the traffic hum. Applying this approach to realistic models, scientists showed that you can even refine the sizes of these holes and their abundance in the Universe.

The idea of primordial black holes was proposed by Stephen Hawking, and modern gravitational wave detectors owe much to Kip Thorne. This kind of "eavesdropping" on the cosmic road will reveal details of the universe’s infancy, as these holes carry memories of the Big Bang.