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A tabletop gravitational wave detector based around a chunk of ringing quartz recorded two mysterious signals in its first 153 days of operation.
It is not known exactly what these signals are; they could come 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 far too early to draw any conclusions, but the next iteration of the detector will be able to determine what made the quartz resonate.
“It’s exciting that this event has shown that the new detector is sensitive and is giving us results, but now we need to figure out 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 very sensitive gravitational wave detectors.”
The first revolutionary gravitational wave detection was carried out just six years ago. Since then, the LIGO and Virgo detectors have revealed that the Universe resonates with previously hidden gravitational waves, standing out from collisions between black holes and neutron stars.
These detectors are huge, with arms 4 kilometers (2.5 miles) long. The lasers along these arms are minutely disturbed by gravitational waves, producing interference patterns in the recombinant 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 that occur later are larger, so shorter high-frequency waves could reveal information about the Big Bang and the Universe in early times.
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. Astronomers would therefore be deeply interested in detecting these signals.
Tobar and his physicist colleague Maxim Goryachev at the University of Western Australia designed a tabletop detector for high-frequency gravitational waves in 2014. Today, together with an international team, they carried out observational trials.
The detector itself is a quartz crystal disc, called a Mass Acoustic Wave (BAW) resonator, with a slightly convex side. Theoretically, high frequency gravitational waves should generate stationary sound waves in the disc, which are trapped as phonons by the convex side.
The disk is cryogenically cooled to reduce thermal noise, and conductive plates placed at very small distances from the crystal pick up the minute piezoelectric signals generated by the acoustic modes vibrating within it. This signal is absolutely tiny, so a superconducting quantum interference device, or SQUID, is used to act as an extremely sensitive signal amplifier.
The entire detector is placed in a radiation protected vacuum chamber to avoid as much interference as possible. With this setup, the team performed two observation passes and performed detection during each pass – the first on May 12, 2019 and the second on November 27, 2019.
Now, there are a number of plausible possibilities here. The relaxation of mechanical stresses inside the quartz disc is one of them; an internal radioactive event caused by external ionizing radiation is another, although researchers are not aware of any external event that could have caused it.
Likewise, although a meteor shower could produce sound waves, the shielding should have protected the aircraft from them. The culprit could even have been cosmic rays.
The other options are more exciting – disturbances caused by topological defects in dark matter, or massive particles of dark matter, could theoretically have caused the signals.
Or, finally, there is the possibility of high frequency gravitational waves. This would require much more investigation, as the shape of the signal does not display the “chirp” characteristic of a cosmic fusion.
For the next iteration of the detector, researchers will add a second crystal, with its own SQUID and reading, as well as a muon detector to exclude cosmic rays. This should help determine the cause of the signals detected by the team.
“This experiment is one of only two currently active in the world looking for high frequency gravitational waves at these frequencies, and we plan to extend our reach to even higher frequencies, where no other experiment has been done. ‘has been done before, ”said Tobar.
“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 discover the presence of gravitational waves, it will be really exciting.”
The research was published in Physical examination letters.
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