Filling in a cosmic blind spot, researchers have unveiled a new technique for detecting gravitational waves, advancing the precision limits of current instrumentation.
By observing the milli-Hertz frequency range, also known as the “mid-band,” the approach will make certain astrophysical and cosmological phenomena observable for the first time, the researchers reveal in a new paper to be published in Classical and Quantum Gravity. This range has long eluded the high-frequency collection of ground-based interferometers and the low-frequency observations of pulsar timing arrays, falling in a gap between the two.
Gravitational Waves
Over a century ago, Albert Einstein first predicted the ripples in spacetime called gravitational waves, but it wasn’t until 2015 that they were observed directly. Now, researchers from the University of Birmingham and the University of Sussex have devised a combination of atomic clock and optical cavity technologies to detect these hard-to-spot mid-frequency gravitational waves.
At the core of the new method is optical resonator technology, repurposed from its original application in atomic clocks. The resonator works by measuring small phase shifts that occur as laser light passes through gravitational waves. With a much smaller form factor than large-scale interferometers like LIGO, the devices are not as susceptible to seismic and other forms of “noise” creeping into the data.
“By using technology matured in the context of optical atomic clocks, we can extend the reach of gravitational wave detection into a completely new frequency range with instruments that fit on a laboratory table,” said co-author Dr Vera Guarrera of the University of Birmingham.
Expanding Coverage
The team’s research offers additional hope not only of investigating the long-neglected mid-band but also of extending it to a broader range. Integrating clock networks into the detectors may further increase their capabilities to lower frequencies. This would allow the new devices to serve as a catch-all for signals below the range of observations, such as LIGO, which are exclusively focused on high frequencies.
Currently, the design contains two orthogonal ultrastable optical cavities and an atomic frequency reference. Working together, these devices enable the detection of multichannel gravitational wave signals. Notably, the device not only detects the signal itself but can also identify its wave polarization and source direction.
“This opens the exciting possibility of building a global network of such detectors and searching for signals that would otherwise remain hidden for at least another decade,” Dr Guarrera said.
The Future of Mid-band
Signals in the band originate from events including black hole mergers and compact binaries of white dwarves. Other technologies aimed at finally observing this band are planned for space platforms like LISA, but those projects will not launch until the next decade.
“This detector allows us to test astrophysical models of binary systems in our galaxy, explore the mergers of massive black holes, and even search for stochastic backgrounds from the early universe. With this method, we have the tools to start probing these signals from the ground, opening the path for future space missions,” said co-author Professor Xavier Calmet, of the University of Sussex.
The primary benefit of the new process is that it can be employed now by repurposing available technologies. However, the LISA project will offer much greater sensitivity. This leaves researchers with a trade-off between timeliness and precision in their investigations of the milli-Hz band.
Fundamentally, the team hopes that their innovation will serve to plant the seeds for future studies that will investigate the mid-band, as even more advanced space-based technologies and applications continue to be developed over time.
The paper, “Detecting Milli-Hz Gravitational Waves with Optical Resonators,” is available in prepublication form from Classical and Quantum Gravity.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.