A coin-sized detector might observe gravitational waves before the giant LIGO interferometers, according to two Australian physicists who have built the device. The detector is designed to register very high frequency gravitational waves via the exceptionally weak vibrations they would induce. Other scientists caution that the astrophysical objects thought to emit such radiation may do so very weakly or might not actually exist.
Predicted by Einstein’s general theory of relativity but yet to be directly observed, gravitational waves are ripples in space–time generated by accelerating massive objects. The tiny detector has been made by Maxim Goryachev and Michael Tobar of the University of Western Australia in Perth and is based on the decades-old technology of resonant-mass detection.
Pioneered by Joseph Weber of the University of Maryland in the US in the late 1950s, resonant-mass detectors have traditionally employed metal bars about a metre long and around a tonne in weight, which makes them sensitive to gravitational waves with frequencies up to about a few kilohertz. It turned out, however, that the tiny vibrations that would be induced by gravitational waves are extremely difficult to detect above the thermal noise in the bar – even when it was chilled to cryogenic temperatures.
Goryachev and Tobar overcame this problem by targeting gravitational radiation in the 1–1000 MHz range. Tobar initially thought that the kind of gram-scale detector suited to these frequencies would be far too light to produce any kind of measurable signal. But he then realized that they could achieve the necessary sensitivities by cooling down a quartz bulk-acoustic-wave (BAW) cavity and boosting its output using extremely low-noise “SQUID” amplifiers. “Our technology has actually been around for decades,” he says, “but at room temperatures.”
Their device consists of a quartz disc about 2.5 cm in diameter hinged to another piece of quartz and placed in a vacuum chamber. A passing high-frequency gravitational wave would cause the disc to vibrate, setting up standing waves of sound across the 2 mm thickness of the disc. The upper surface of the disc is slightly curved to trap sound quanta (phonons), which improves the signal-to-noise ratio. The piezoelectric nature of quartz allows the tiny vibrations to be converted into an electrical signal that is amplified by the SQUIDs.
The researchers are currently operating their device at 4 K, and hope to obtain the dedicated cryostat and sensitive SQUIDs needed to reach the design temperature of 10 mK within the next year. The device costs about $500,000 to make and the physicists say that its compactness and ease of manufacture lends it to being scaled up into arrays that would improve sensitivity and help filter out spurious events.
Having accounted for all known sources of noise, Goryachev and Tobar reckon that their detector would be sensitive to strains in space–time as low as 10–22, the figure that Advanced LIGO is set to achieve. Advanced LIGO is an upgrade of two existing LIGO detectors in the US, which are searching for gravitational waves using huge masses located at the ends of optical interferometers with arms that are 4 km long. The huge detectors are expected to detect signals between about 0.1–1 kHz, from sources such as binary neutron stars or colliding black holes by the end of 2018.
Cosmic strings and axions
Goryachev and Tobar say that their device should detect low-mass black holes encircled by dark matter, with the latter giving off gravitational waves just as bound electrons in an atom emit electromagnetic radiation. Other possible sources, they add, include plasma flows and exotic cosmological entities such as cosmic strings or clouds of axions. Tobar says that they could detect gravitational waves before Advanced LIGO, adding “We can at least put the first serious upper limits on these sources.”
Mike Cruise, an astrophysicist at the University of Birmingham in the UK, praises the “very sophisticated but believable” proposal, but cautions that many high-frequency sources “are very speculative and may well not exist” and may also be far weaker than those probed by interferometers. “The gravitational energy available is likely to go down by the cube of the wavelength,” he says, “which is very punishing when wavelengths decrease by factors of a thousand or a million.”
The detector is described in a preprint on arXiv.