A tabletop gravitational wave detector based around a piece of ringing quartz has recorded two mysterious signals in its first 153 days of operation.
It's unclear exactly what these signals are; they could be from a number of phenomena. But one of those phenomena is exactly what the detector is designed to pick up – high-frequency gravitational waves, which have never been recorded before.
It's way too soon to come to any conclusions, but the next iteration of the detector will be able to narrow down what caused the quartz to resonate.
"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," said physicist Michael Tobar of the University of Western Australia.
"With this work, we have demonstrated for the first time that these devices can be used as highly sensitive gravitational wave detectors."
The first groundbreaking gravitational wave detection was made just six years ago. Since then, the LIGO and Virgo detectors have gone on to reveal that the Universe is ringing with previously hidden gravitational waves, rippling out from collisions between black holes and neutron stars.
These detectors are huge, with arms 4 kilometers (2.5 miles) long. Lasers along these arms are minutely disrupted by gravitational waves, producing interference patterns in the recombined light that can be analyzed to reveal the nature of the event that caused the waves. So far, the technology has been optimized for the low-frequency regime.
High-frequency gravitational waves are much harder to detect, but definitely worth pursuing. The wavelength of gravitational waves is proportional to the size of the Universe; those occurring later are larger, so shorter, high-frequency waves could reveal information about the Big Bang, and the Universe at the beginning of time.
High-frequency gravitational wave sources in the more recent past could include hypothetical objects such as boson stars and primordial black holes. These waves could even be produced by clouds of dark matter. So astronomers would be deeply interested in detecting these signals.
Tobar and his colleague physicist Maxim Goryachev of the University of Western Australia designed a tabletop detector for high-frequency gravitational waves in 2014. Now, along with an international team, they have conducted observing runs.
The detector itself is a disk of quartz crystal, called a bulk acoustic wave (BAW) resonator, with one side slightly convex. Theoretically, high-frequency gravitational waves should generate standing sound waves in the disk, which are trapped as phonons by the convex side.
The disk is cryogenically cooled to reduce thermal noise, and conducting plates placed at very small distances from the crystal pick up minute piezoelectric signals generated by the acoustic modes vibrating therein. This signal is absolutely tiny, so a superconducting quantum interference device, or SQUID, is employed to act as an extremely sensitive signal amplifier.
The whole detector is placed in a radiation-shielded vacuum chamber to prevent as much interference as possible. With this setup, the team conducted two observing runs, and made a detection during each run – the first on 12 May 2019, and the second on 27 November 2019.
Now, there are a number of plausible possibilities here. The relaxation of mechanical stress inside the quartz disk is one; an internal radioactive event caused by external ionizing radiation is another, although the researchers know of no external event that could have caused this.
Likewise, although a meteor shower can produce acoustic waves, the shielding should have protected the device from these. The culprit could even have been cosmic rays.
The other options are more exciting – disturbances caused by topological defects in dark matter, or massive dark matter particles, could theoretically have caused the signals.
Or, finally, there's the possibility of high-frequency gravitational waves. This would require a lot more investigation, since the shape of the signal doesn't display the 'chirp' characteristic of a cosmic merger.
For the next iteration of the detector, the researchers will be adding a second crystal, with its own SQUID and readout, along with a muon detector to rule out cosmic rays. This should help narrow down what caused the signals the team detected.
"This experiment is one of only two currently active in the world searching for high-frequency gravitational waves at these frequencies, and we have plans to extend our reach to even higher frequencies, where no other experiments have looked before," Tobar said.
"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 research has been published in Physical Review Letters.