A radio receiver catches waves by turning a tuning knob. Scientists built a similar device to catch the axion, a particle that may account for the invisible mass of the Universe. Inside a copper cylinder sit rods made of aluminum oxide. Turning one knob makes all the rods move together, like a single mechanism. The idea came from kirigami, the art of cutting and folding paper, where one cut sets the whole structure moving in concert.
An ordinary resonator searches for the axion at its fundamental frequency, but then it has to shrink in size, and the signal weakens. Higher frequencies solve that problem, yet the field inside starts to cancel itself out.
Dielectric rods redistribute the field so that it once again interacts efficiently with the magnetic field. An auxetic mechanism, in which turning a shaft moves the rods strictly along the radius, keeps the field shape intact during tuning.
The setup was cooled to 40 millikelvins, colder than interstellar space, and placed in a magnetic field of 9.8 tesla. Two new scans covered the range 4.98-5.07 gigahertz, and together with earlier runs that added up to almost 300 megahertz of continuous coverage. Sensitivity reached 1.8 of the KSVZ benchmark, the theoretical model that predicts how strongly the axion couples to the photon. No candidate has turned up, but higher modes have now become a practical tool for future searches.
Auxetic materials get thicker, not thinner, when stretched, unlike ordinary ones. The tuning mechanism uses this property in reverse: turning the shaft makes the rods move apart or together in sync, changing the resonator frequency the way you tune a radio.
In Liu Cixin's novel The Three-Body Problem, dark matter remains a mystery that humanity tries to crack with giant accelerators. Axion haloscopes like the one described here offer a quieter path: listening to the cosmos in the microwave range, hoping to catch the whisper of an invisible field.