Two mysterious signals were recorded by a tabletop gravitational-wave detector built around a piece ringing quartz in its first 153 days.
These signals could be from many phenomena, but it's not clear what they are. One of these phenomena is high-frequency gravitational wave detection, which is something that has never been done before.
Although it's too early to draw any conclusions, the next iteration will be able pinpoint the cause of the quartz's resonance.
"It's thrilling that this event has demonstrated that the new detector can be sensitive and giving us results. But now we need to figure out exactly what those results mean," stated Michael Tobar, University of Western Australia physicist.
"With this work we have shown for the first time that these devices are highly sensitive gravitational-wave detectors."
Six years ago was the first time that a gravitational wave detector had been made. The LIGO and Virgo detectors discovered that the Universe was ringing with previously undiscovered gravitational waves. These are rippling from collisions between neutron stars and black holes.
These detectors have huge arms that measure 4 km (2.5 miles). These arms have lasers that are disrupted minutely by gravitational waves. This creates interference patterns in the recombined sunlight that can be used to analyze the cause of the waves. The technology has been optimized to operate in the low-frequency range.
Although high-frequency gravitational waves can be difficult to detect, they are definitely worth looking into. The wavelength of gravitational wave is proportional to how large the Universe is. Those occurring later are therefore shorter. These high-frequency waves could provide information about the Big Bang and the Universe at its beginning.
In the recent past, high-frequency gravitational waves sources could have been created by hypothetical objects like boson stars or primordial black holes. These waves could be even produced by dark matter clouds. These signals would interest astronomers greatly.
Maxim Goryachev, a University of Western Australia physicist, and Tobar designed a tabletop detector to detect high-frequency gravitational wave in 2014. They have now conducted observation runs with an international team.
The detector is made of quartz crystal and has one convex side. Theoretically high-frequency gravitational wave should produce standing sound waves in the disk. These are trapped by the convex side as phonons.
To reduce thermal noise, the disk is cryogenically chilled. Conducting plates placed very close to the crystal pick up tiny piezoelectric signals from the acoustic vibrations. The signal is extremely small, so the superconducting quantum interference devices, or SQUIDs, are used to amplify it.
To prevent interference, the entire detector is placed inside a radiation-shielded chamber. The team set up this system to conduct two observation runs. They made two detections during each run, one on 12 May 2019 and the other on 27 November 2019.
There are many possibilities. One possibility is the relaxation of mechanical stress within the quartz disk. Another is an internal radioactive event due to external ionizing radiation. However, researchers are not aware of any external events that could have led to this.
Also, a meteor shower could produce acoustic wave, but shielding should have protected it from them. Even cosmic rays could have been the culprit.
Other possibilities include more interesting disturbances, such as topological defects in dark material or massive dark matter particles. These could theoretically have been the cause of the signals.
The possibility of high-frequency gravitational wave could be another option. Because the signal's shape doesn't resemble a cosmic merger, this would require more research.
The researchers will add a second crystal to the detector's next iteration. It will have its own SQUID, readout, and a muon detector in order to rule out cosmic radiations. This will help to narrow down the causes of the signals that the team identified.
Tobar stated that the experiment was one of two currently operating in the world looking for high-frequency gravitational wave at these frequencies. He also said that plans are in place to expand our reach to higher frequencies where no other tests have looked before.
"The technology's development could lead to the detection of gravitational waves at high frequencies for the first time, providing new insights into this area in gravitational waveastronomy.
"The next generation will include building a clone detector and a muon detector sensitive for cosmic particles. It will be very exciting if two detectors detect gravitational waves.
The research was published in Physical Review Letters.
