New Gravitational Wave Detector Records Rare Events

The discovery of gravitational waves in the early 20^th century and their subsequent detection, first indirectly in 1974 from the orbital decay in a binary star system consisting of a neutron star and a pulsar, and then directly in 2015 by the LIGO and Virgo collaboration, has given astronomers a fresh new way to look at the Universe.

Until now, detections have been of gravitational waves caused by black holes and neutron stars, but astronomers have said that they might have recently caught mysterious signals that may have been created by primordial black holes or even clouds of dark matter. 

The possibility of the existence of gravitational waves was first discussed by physicist Oliver Heaviside in the 19^th century, but it was Einstein’s Special and General Theories of Relativity shortly thereafter that paved the way and formulated the mathematics needed to describe them. Einstein himself was sceptical of their existence, preferring to think of them as a mathematical artifact, possible in theory but not able to be measured.

Of course, direct detections by gravitational-wave interferometers at LIGO in the US and Virgo in Europe proved beyond doubt that we can build sensitive enough equipment to detect a wobble in the fabric of space-time less than 10,000 times smaller than the nucleus of an atom. But those detectors are tuned to detect a certain frequency of gravitational waves, the kind that is emitted as two massive objects spiral in towards each other in the moments before they merge.

What about high-frequency gravitational waves? So far, there has been much less funding and attention given to trying to detect these signals. While currently operating and planned detectors mostly focus on frequencies below 10 kHz, there are likely to be interesting physics to be discovered at all frequencies on the gravitational wave spectrum.

Exploring the limits of knowledge and branching into the unknown is what science is all about, and there are no known astrophysical sources that are small and dense enough to emit at frequencies in the MHz or GHz range. Any discoveries at such high frequencies would correspond to either exotic objects such as black holes or boson stars, or to interesting cosmological events in the early Universe.

LIGO and Virgo use these giant interferometers to search for gravitational waves, instruments that look at the interference patterns in laser beams to detect distortions in space-time. But there are other ways. One ingenious method is to use a sensitive device known as a bulk acoustic wave resonator, and astronomers at the University of Western Australia and the ARC Centre of Excellence for Dark Matter Particle Physics (CDM) have been running an experiment with one such device since November 2018.

In the 1960s physicist, Joseph Weber devised and constructed a gravitational-wave detector that became known as the Weber bar. It consisted of aluminium cylinders, 2-metres in length, that acted as antennae for detecting gravitational waves. The cylinders vibrated at a particular frequency and were designed to be set in motion by the then theorised phenomenon.

Weber claimed to have collected good evidence for the existence of gravitational waves, but his experiments were controversial, and his results could not be replicated. His ideas, though, work, at least in principle, and resonant mass detectors like Weber’s might just be able to detect high-frequency targets.

The bulk acoustic wave (BAW) resonator experiment running at the University of Western Australia uses a high-frequency gravitational wave detector that is based on the principles of the resonant mass detector. It consists of a quartz crystal disk that vibrates at high frequencies due to acoustic waves travelling through it. The waves induce an electric charge that is detected by placing conducting plates on the outer surfaces of the disk.

The BAW device is connected to an extremely sensitive amplifier capable of measuring the low voltage signals, and the whole assembly is shielded from electromagnetic fields and cryogenically cooled to just about 4 degrees above absolute zero.

No one said that detecting new physics was going to be easy. But at least these scientists didn’t need to wait long before they found something interesting within their research to look at.

In the first 153 days of operation, two events were detected that could, in principle, be high-frequency gravitational waves. Put another way, scientists are now confident that the device is sensitive enough to detect signals that have never been seen before.

“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 William Campbell, one of the team of researchers working with the resonator.

“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. 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.”

One possible explanation for the signals is that they were created by primordial black holes. These are hypothetical black holes that formed soon after the Big Bang, with masses potentially much less would normally be found. Merging low mass primordial black holes would not be able to be detected by LIGO or Virgo.

Another explanation is that the signals were caused by high-mass dark matter candidate particles. After years of hunting for dark matter, scientists are almost as clueless about what it is as they were when it was first hypothesised.

But more mundane explanations cannot yet be ruled out. Cosmic ray events have been detected by similar devices at similar frequencies in the past.

There is still a lot of work still to do before any of these claims as to the origin of the events can be made with certainty. But these results are a promising start and point to an exciting future for gravitational wave astronomy.