World-first detector reports rare events

A ground-breaking detector that aims to capture high-frequency gravitational waves using quartz has been built by researchers at EQUS, the ARC Centre of Excellence for Dark Matter Particle Physics and the University of Western Australia.

In its first 153 days of operation, two events were detected that could, in principle, be high-frequency gravitational waves.

Although there are many proposed sources of high-frequency gravitational waves, including primordial black holes and clouds of dark matter particles, no such signals have previously been recorded.

Gravitational waves were originally predicted by Einstein, who theorised that the movement of astronomical objects could send waves of spacetime curvature rippling through the Universe, almost like the waves caused by stones dropped into a flat pond.

This prediction was proven in 2015 by the first detection of a gravitational-wave signal.

Since then, a new era of gravitational-wave research has begun, but the current generation of active detectors are sensitive to only low-frequency signals; such signals are believed to be caused by two black holes spinning and merging or a star disappearing into a black hole.

The team’s detector is built to detect high-frequency gravitational waves using a quartz-crystal bulk acoustic-wave resonator.

At the heart of this device is a quartz-crystal disk that vibrates at high frequencies as acoustic waves travel through its thickness.

These waves induce electric charge across the device, which is detected by conducting plates on the outer surfaces of the disk.

The device is connected to a superconducting quantum interference device (SQUID), which acts as an extremely sensitive amplifier for the low-voltage signal from the bulk acoustic-wave resonator.

The assembly is then placed in multiple radiation shields to protect it from stray electromagnetic fields and cooled to a low temperature, to allow low-energy acoustic vibrations of the quartz crystal to be detected as large voltages, with the help of the SQUID.

The team—Dr Maxim Goryachev, Professor Michael Tobar, William Campbell, Ik Siong Heng, Serge Galliou and Professor Eugene Ivanov—are now working to determine the nature of the signal, potentially confirming the detection of high-frequency gravitational waves.

Mr Campbell said gravitational waves are just one possible candidate for what was detected.

Other explanations include the presence of charge particles or build-up of mechanical stress, a meteor event, an internal atomic process or even very high-mass dark matter candidates interacting with the detector.

“It’s exciting that this event has shown that the new detector is sensitive and giving us results, but now we have to determine exactly what those results mean,” Mr Campbell said.

“With this work, we have demonstrated for the first time that these devices can be used as highly sensitive gravitational-wave detectors.

“This experiment is one of only two currently active in the world searching for gravitational waves at these high frequencies and we have plans to extend our reach to even higher frequencies, where no other experiments have looked before.

“The development of this technology could potentially provide the first detection of gravitational waves at these high frequencies, giving us new insight into this area of gravitational wave astronomy.

“The next generation of the experiment will involve building a clone of the detector and a muon detector sensitive to cosmic particles.

“If two detectors find the presence of gravitational waves, that will be really exciting.”

The results are published in Physical Review Letters (DOI: 10.1103/PhysRevLett.127.071102).