Gravitational waves are ripples in the very fabric of spacetime, akin to vibrations of a giant cosmic string. Conventional detectors like LIGO and Virgo pick up low-frequency "notes"—disturbances from mergers of massive black holes of stellar masses. However, there also exist high-frequency gravitational waves — ultra-fast jitters with frequencies in the megahertz and gigahertz range. Their presumed sources are microscopic primordial black holes, hypothetical objects born in the extreme conditions right after the big bang.

The mechanism converting these gravitational disturbances into an observable signal relies on an effect predicted within general relativity: when a gravitational wave passes through a region with a magnetic field, part of its energy converts into electromagnetic radiation — radio waves. You can think of it as an acoustic guitar plugged into an amplifier: the mechanical vibrations of the string (the analog of the gravitational wave) are transformed by the pickup into an electrical signal (the analog of the radio wave). This analogy simplifies the essence, skipping the complex tensor nature of the gravitational field.

Next-generation radio telescopes, such as Canada's CHIME (Canadian Hydrogen Intensity Mapping Experiment) and China's FAST (Five-hundred-meter Aperture Spherical Telescope), are capable of registering this extremely faint radio signal. Their wide field of view and high sensitivity allow them to "hear" what was previously inaccessible to direct gravitational-wave observatories.

An unexpected consequence of this technique is the possibility of searching for particles beyond the Standard Model. The same high-frequency gravitational waves could be generated not only by primordial black holes but also by clouds of hypothetical ultralight bosons swarming around rotating black holes. If such particles exist, they should form a Bose-Einstein condensate around the black hole, which, when annihilating, emits a monochromatic "ring"—an almost continuous, narrowband gravitational-wave signal. Detecting such a monochromatic signal with radio telescopes would be the first direct evidence for certain models of dark matter — the mysterious substance that does not participate in electromagnetic interaction but reveals itself through the gravitational binding of galaxies and clusters.