Deep Space Listening: 6000 Days of Research to Hear Continuous Gravitational Waves

Scientists’ focused search for neutron star gravitational “hums” turned up empty, but advanced methods sharpen future hunts.

Remember the days before working from home? It’s Monday morning, you’re running late to beat the traffic, and you can’t find your car keys. What do you do? You might try moving from room to room, casting your eye over every flat surface, in the hope of spotting the missing keys. Of course, this assumes that they are somewhere in plain sight; if they’re hidden under a newspaper, or fallen behind the sofa, you’ll never spot them. Or you might be so convinced that you last saw the keys in the kitchen and search for them there: inside every cupboard, the microwave, dishwasher, back of the fridge, etc. Of course, if you left them on your bedside table, upending the kitchen is doomed to failure. So, which is the best strategy?
 
Scientists face a similar conundrum in the hunt for gravitational waves—ripples in the fabric of space and time—from rapidly spinning neutron stars. These stars are the densest objects in the Universe and, provided they’re not perfectly spherical, emit a very faint “hum” of continuous gravitational waves. Hearing this “hum” would allow scientists to peer deep inside a neutron star and discover its secrets, yielding new insights into the most extreme states of matter. However, our very sensitive “ears”—4-kilometer-sized detectors using powerful lasers—haven’t heard anything yet.

Exploring Search Strategies for Gravitational Waves

Part of the challenge is that, like the missing keys, scientists aren’t sure of the best search strategy. Most previous studies have taken the “room-to-room” approach, trying to find continuous gravitational waves in as many different places as possible. But this means you can only spend a limited amount of time listening for the tell-tale “hum” in any one location—in the same way that you can only spend so long staring at your coffee table, trying to discern a key-shaped object. And since the “hum” is very quiet, there’s a good chance you won’t even hear it.
 
In a study recently published in Physical Review D, a team of scientists, led by postdoctoral researcher Karl Wette from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at the Australian National University, tried the “where else could they be but the kitchen?” approach.

Educated Guesses

Wette explains: “We took an educated guess at a specific location where continuous gravitational waves might be, based in part on what we already know about pulsars—they’re like neutron stars but send out radio waves instead of continuous gravitational waves. We hypothesized that there would be continuous gravitational waves detected near pulsar radio waves.” Just like guessing that your missing keys will probably be close to your handbag or wallet.
 
Using existing observational data, the team spent a lot of time searching in this location (nearly 6000 days of computer time!) listening carefully for that faint “hum.” They also used graphic processing units—specialist electronics normally used for computer games—making their algorithms run super-fast.

“Our search was significantly more sensitive than any previous search for this location,” says Wette. “Unfortunately, we didn’t hear anything, so our guess was wrong this time. It’s back to the drawing board for now, but we’ll keep listening.”

Reference: “Deep exploration for continuous gravitational waves at 171–172 Hz in LIGO second observing run data” by Karl Wette, Liam Dunn, Patrick Clearwater and Andrew Melatos, 23 March 2021, Physical Review D.
DOI: 10.1103/PhysRevD.103.083020

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