Two teams of bridge builders were working on opposite banks of a chasm. One was throwing a crossing from the light-mass side, where signals stretch on for hours like a slow hum. The other was building from the far edge, where heavy pairs plunge into merger within seconds, like a landslide. Between them yawned a void: chirp masses from 0.01 to 0.1 solar masses. There the signal lasts from minutes to hours, and the frequency changes too fast to catch with the old methods. In this work, a bridge has been thrown across the chasm.
The bridge was built from LIGO data from the O4a and O4b runs. Instead of sifting through billions of templates, as matched filtering does, the authors used the BinaryGFH-v2 method. It works with peak maps: time-frequency representations where local power maxima are picked out. For each hypothesis about mass and initial frequency, the method turns a smeared trace into a single line. This approach requires hundreds of times less computation than earlier semicoherent searches, and tens of thousands of times less than matched filtering.
No traveler was found on the bridge. After coincidences between the Hanford and Livingston detectors were matched, about 158,870 events remained in O4a and 139,260 in O4b. After a significance threshold, one candidate survived in O4b, and with a slightly looser selection, three. All of them were rejected: their time-frequency tracks crossed known instrumental lines and combs, that is, the detectors' own noise rather than cosmic signals.
The absence of events became a result in itself. We now have upper limits on the merger rate in this mass range at 90% confidence. These limits connect the constraints from long-duration planetary-mass searches and from matched filtering of higher masses. Assuming that primordial black holes form binaries in the early Universe without suppression of mergers, the first gravitational-wave constraints on their fraction of dark matter have been obtained: fPBH < 1 for equal masses and for asymmetric systems with a secondary mass from 0.0003 to 0.01 solar masses when the primary mass is 2.5 solar masses.
The bridge stands. Now it can be walked in both directions: to test models of primordial black hole formation and to plan future searches with more sensitive detectors. The chasm no longer separates the two banks.
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.