Gravitational waves are tiny disturbances of the fabric of space-time caused by some of the most cataclysmic events in the cosmos, like the collision of two neutron stars. To study them, observatories have to be big but there are some theoretical waves that could be detected by a smaller tabletop detector. Researchers in Australia have actually built one such detector – and it's detected its first mysterious signals.
Over the first 153 days of operation, the tabletop detector reported two peculiar events that appear to be genuine (as opposed to picking up "noise"). It's not clear yet what the signals are but in a new paper in Physical Review Letters, the researchers put forward that this is the first detection of high-frequency gravitational waves, which have never been detected before now.
Gravitational waves detectors like LIGO and Virgo are huge. LIGO is made up of our facilities across America, with arms 4 kilometers (2.5 miles) long. They appear to have a strong sensitivity for detecting low-frequency waves. Detecting high-frequency waves is much harder. Longer wavelengths indicate the events that caused the gravitational waves occurred later, so shorter, high-frequency waves may be able to tell us something about the early universe. A possible source of these waves is black holes that formed right after the Big Bang, known as primordial black holes.
The new device, built by researchers at the ARC Centre of Excellence for Dark Matter Particle Physics (CDM) and the University of Western Australia, is built around a quartz crystal bulk acoustic wave resonator (BAW). A quartz crystal disk is at the core of the device and if a high-frequency wave goes through it, it begins to vibrate. These vibrations can produce a small electric charge that can be measured by a superconducting quantum interference device (or SQUID). The disk is kept at extremely low temperatures and within multiple radiation shields to try and take potential confounding effects into account.
Currently, however, observations using the new detector are not good enough to confidently confirm this is a genuine detection of high-frequency gravitational waves. The device itself may even be at fault. There may be some mechanical tension or presence of charge within. Or the detector may have picked up a different physical phenomenon such as a meteor event, or even yet-to-be-proven physics like dark matter.
“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,” Professor Michael Tobar said in a statement. However, he added, "With this work, we have demonstrated for the first time that these devices can be used as highly sensitive gravitational wave detectors."
The team is now planning to build a second detector and a muon detector sensitive to particles coming from outer space. This will help the team assess if the signals are due to external factors or not, and if they are clarifying the source.