Never-before-detected gravitational waves hint at dark matter
A new type of gravitational wave detector running in Western Australia has recorded two rare events that might be signals of dark matter or primordial black holes. These high-frequency gravitational waves are beyond the range of most detectors and have never been recorded before.
Gravitational waves are ripples in the very fabric of spacetime, first predicted by Einstein over a century ago but not directly detected until 2015. In the years since, dozens of detections have been made, mostly by facilities like LIGO, which can detect waves with frequencies between 7 kHz and 30 Hz. That’s in the range for waves produced by cataclysmic events like black holes and neutron stars colliding.
But gravitational waves are also expected to fall outside that range. Two experiments are currently searching for very high frequency waves, which could represent other cosmic events or objects. And now the first batch of data has been returned for one of these experiments, including two detections of particular interest.
The project is run by the ARC Center of Excellence for Dark Matter Particle Physics (CDM) and the University of Western Australia, and it’s based on a unique type of gravitational wave detector, known as a bulk acoustic wave (BAW) resonator.
The experiment uses a disk of quartz crystal, which vibrates at high frequencies as acoustic waves pass through it. This generates an electric charge, which can be picked up by two conducting plates pressed against the quartz disk. This signal then passes through a superconducting quantum interference device (SQUID), which amplifies the signal so that it can be picked up by the detector.
The whole thing is then encased inside several radiation shields to prevent interference from outside electromagnetic radiation, and supercooled to almost absolute zero to reduce noise in the signal. This allows the device to detect frequencies in the MHz range.
The BAW experiment was conducted for 153 days over two separate runs in 2019, and in that time scientists observed two rare events, the first on May 12 and the other on November 27. Both of these were at high frequencies of around 5 MHz.
Exactly what the signals could be remains unknown, but there are a few exciting possibilities. Primordial black holes are hypothesized to have been created within milliseconds of the Big Bang, and could have become the seeds of the supermassive black holes at the centers of galaxies. The high frequency gravitational waves could have been emitted by one of these primordial black holes, which are yet to be conclusively shown to exist.
Another possibility is that the signal came from a cloud of dark matter particles, passing through the instrument. This strange stuff is thought to pervade the universe and only interact with regular matter through its powerful gravitational influence.
Confirmation of either scenario would be a huge breakthrough for physics, but it’s worth keeping a cool head about it for now. The team says that the signal could also be far less exciting, such as interference by charge particles, mechanical stress, a random atomic process or a meteor event.
“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,” says William Campbell, an author of the study.
Along with figuring that out, the team says that future versions of the technology will search for gravitational waves at even higher frequencies, and compare the results from multiple detectors to eliminate interference.
“The next generation of the experiment will involve building a clone of the detector and a muon detector sensitive to cosmic particles,” says Campbell. “If two detectors find the presence of gravitational waves, that will be really exciting.”
This isn't the first detection outside the regular range. Other teams have picked up signals with extremely low frequencies, which could be caused by a different type of dark matter, or the background "hum" of the universe.
The research was published in the journal Physical Review Letters.