Two teams of bridge builders worked on opposite banks of a chasm. One team built from the side of light objects, where signals last for hours. The other worked from the side of heavy ones, where everything happens within seconds. Between them lay a gap: objects with masses from 0.01 to 0.1 solar masses. Their signals are too fast for some methods and too slow for others. In this work, a bridge has been thrown across the chasm.
The bridge was built from LIGO detector data spanning two observing runs. Instead of sifting through billions of possibilities, the scientists used a method that looks for bright spots on a time-frequency map. It's like looking for tracks in the sand: you don't need to check every centimeter, just notice the depressions. This approach saved hundreds of hours of computation.
No travelers turned up on the bridge. The scientists checked every suspicious signal, and all of them turned out to be instrument noise rather than genuine gravitational waves. But a null result is still a result. We now know that such mergers happen no more often than a certain limit. These are the first constraints on how many primordial black holes can hide in dark matter within this mass range.
The bridge stands. Now it can be walked in both directions: to test theories about the origin of black holes and to prepare future searches with more sensitive detectors.
Chirp mass is a combination of the masses of the two components of a binary system that determines the rate of change of the gravitational wave's frequency. For a system of two objects of 0.05 solar masses each, the signal in the LIGO detector would last about 20 minutes, changing frequency from 169 to 71 Hz.
The idea of primordial black holes as dark matter is explored in Greg Egan's novel Schild's Ladder, where exotic compact objects affect the structure of space. The search for invisible subsolar objects echoes the plot of the film Interstellar, where black holes play a key role in the fate of humanity.