The Search for Continuous Gravitational Waves

You wouldn’t know it, but right now, as you read this article, your body is being squeezed and stretched. You don’t feel it because the amplitude of this flexing is only about 1/10,000^th the diameter of a proton, but it is happening.

What’s causing it is not anything local, nor anything to do with your body itself. In fact, the entire Earth is being squeezed and stretched in the same manner. Instead, the source origin of these forces acting on you is located millions or even billions of light-years away.

Whilst we can be thankful that we don’t experience these effects on the macro scale, we are reminded that there are extreme forces at play in the Universe – powerful enough to distort the very rigid fabric of space-time itself.

The cause of this effect is gravitational waves, and the sources that are generating them, are massive compact objects in binary systems – like neutron stars or black holes – in inspiral orbits, drawing closer and closer together and emitting gravitational radiation in the form of these waves.

These waves were a predicted outcome of Einstein’s General Theory of Relativity and for the most part of 100 years, they were just that, theory. Then in the ’70s, indirect evidence was published that showcased a binary pulsar-neutron star system whose orbit was decaying with the exact predicted value that Einstein had determined – and the only explanation was that the system was emitting gravitational radiation. This exciting discovery went on to earn a Nobel Prize.

Then in 2015, it finally happened. With technology and instruments advancing to the threshold needed to make a discovery, two kilometre-sized laser interferometers based in the US (designed as a giant version of the Michelson-Morely experiment) confirmed the first-ever chirp from a system of massive compact objects as they merged.

Gravitational waves were finally observed (and also earned another Nobel Prize as an outcome).

Since the 2015 discovery, over 50 confirmed observations have taken place which includes systems that contain a range of masses of black holes, as well as two neutron star merger events. And since then, a lot more interferometers have been established around the world.

To date so far, however, the discoveries have been limited to just these types of events – binary black hole and binary neutron star mergers. This of course is because the interferometers that make the detections are tuned to a certain band of frequencies that gravitational waves radiate in, linked to the masses of objects (and in these cases, the detectors are good at picking up signals of binary compact inspiral merger events).

But there are lots of different extreme objects and phenomena in the Universe, and if it involves large enough masses that are accelerating (especially in an asymmetrical manner), then there should be lots of gravitational waves out there.

Now, researchers from OzGrav have participated in several studies to try and quantify a special type of continuous gravitational-wave – one that is generated not by a binary system of extreme masses, but rather single, high-mass rotating compact objects – neutron stars and pulsars.

Neutron stars (and pulsars) are born in the violent death of massive stars when a supernova explosion blasts the star apart and sends the core of the progenitor collapsing in on itself. Protons and electrons are squeezed together in this rapid collapse, forming neutrons that are all pushed as close together as they can – leaving an extremely dense, city-sized (roughly 20 km across) compact star. So dense is the material that makes up this object, that a single teaspoon would weigh as much as a cube of Earth, with sides measuring 800 metres.

As part of the conservation of angular momentum, the collapsing progenitor causes the remnant object to rapidly increase its rotational velocity – much like an ice skater spinning on the spot as they pull their arms inwards.

All that mass contained in such a small space, also produces enormous gravity – so much so, that the surface of neutron stars are relatively smooth. However, this smoothness might not always occur at the time the neutron star is born – but rather take time to smooth out. In essence, newborn neutron stars can have mountains – but not in the Earth sense you are thinking of – rather tiny features that can range from a few millimetres to about 10 cm high.

This might not seem relevant, but it does create asymmetry on the rapidly rotating object, which in turn can induce a wobble – like a spinning top that processes as it rotates. It’s also predicted that if any neutron star has asymmetrical features, like these tiny mounts, then they should also emit gravitational waves, but at different frequencies from the types that have already been detected using the laser interferometers.

“Imagine you’re out in the Australian bush listening to the wildlife. The gravitational waves from black hole and neutron star collisions we’ve observed so far are like squawking cockatoos ━ loud and boisterous, they’re pretty easy to spot,” said OzGrav postdoctoral researcher Karl Wette from the Australian National University.

“A continuous gravitational wave, however, is like the faint, constant buzz of a faraway bee, which is much more difficult to detect.”

“So, we’ve got to use a few different strategies. Sometimes we hone in on a particular direction ━ for example, a flowering bush where bees are likely to congregate. Other times, we close our eyes and listen keenly to all the sounds we can hear and try to pick out any buzzing sounds in the background.”

“So far, we haven’t had any luck, but we’ll keep trying! Once we do hear a continuous gravitational wave, we’ll be able to peer deep into the heart of a neutron star and unravel its mysteries, which is an exciting prospect.”

Making up the OzGrav team, researchers from the University of Western Australia, Australian National University, University of Melbourne and the University of Adelaide all participated in the studies, which have now been released as a series of five publications that cover searching young supernovae remnants for continuous gravitational wave sources, all-sky searches in gravitational wave data sets produced by the interferometers in recent observing runs for any continuous gravitational-wave signals from neutron stars, and gravitational waves produced by pulsars.

Gravitational waves are generated by accelerating masses. Effectively, if you were doing squats next to the giant LIGO interferometers, your mass (which is accelerated during the squat) is producing gravitational waves. But these are so tiny that they basically occur on the quantum scale.

However, as you scale up the mass value – say to that of a black hole or a neutron star – then you start to see these distortions in space-time become more significant. The detections made by the LIGO/Virgo Science Collaboration so far are still significantly small (measuring on the sub-atomic scale), but this is due to the strength of the wave reducing in proportion to its distance from us – and lots of these events are occurring in faraway places in the Universe (thankfully, otherwise, we’d have much bigger problems).

We know that different masses produce different frequencies and wave-periods of gravitational waves and if we start to catalogue the sources generating them by their frequency, and the amplitude of the gravitational wave we start to see a spectrum.

The gravitational wave spectrum is analogous to the familiar electromagnetic spectrum and features different frequencies and the objects/phenomena that cause them. And much like the EM spectrum that requires different sets of antennas, telescopes and instruments to detect different bands – the gravitational wave spectrum requires different detectors to also zone in on different frequencies.

The different sources producing these ripples in space-time can take different forms – some are transient events, like the sudden merger of two black holes, whilst others are more of a continuous static noise in the background, like the rumbling hum of colliding supermassive black hole binaries over the history of the Universe. Some can be ancient and primordial, like the quantum fluctuations that occurred in the first few seconds after the Big Bang, and others can be more recent bursts like a relatively nearby supernova that collapses in an asymmetrical manner.

So far, we’ve only detected the transient events that come from massive compact inspiral mergers, but there are existing experiments (like pulsar timing arrays and cosmic microwave background polarization mapping) and future proposed experiments (like the orbiting interferometer known as LISA) that are all working towards confirming detections along this spectrum.

On the higher frequency end of the gravitational wave spectrum lies the continuous gravitational waves – produced by rotating massive compact objects that may exhibit asymmetrical features or be experiencing internal dynamics that create varying moments of inertia. So basically, neutron stars and pulsars that are not perfect spheres or are still settling into their final, more stable and firmer states.

Now, several new research papers (developed through the international collaboration of the U.S./international LIGO Scientific Collaboration, European Virgo Collaboration and Japanese KAGRA working groups) have been published which outline a number of different methods and studies in the field of continuous gravitational waves from neutron stars and pulsar sources.

One of the papers looked at 15 young supernova remnants in three wide-band (10 Hz – 2 Hz) directed searches in the data generated in the third observing run (O3) by the LIGO/Virgo interferometers, looking for the tell-tale signature of continuous gravitational waves generated by rotating clumpy neutron stars.

“Our search targets fifteen young supernova remnants containing young neutron stars,” said OzGrav PhD student Lucy Strang from the University of Melbourne. “We use three different pipelines: one optimized for sensitivity, one that can handle a rapidly evolving signal, and one optimized for one likely astrophysical scenario. This is the first LIGO study covering all three of these scenarios, maximising our chance of a continuous wave detection.”

“Continuous gravitational waves are proving very difficult to detect, but the same properties that make them elusive make them appealing targets. The exact form of the signal (i.e. its frequency, how rapidly the frequency changes, how loud it is, etc.) is dependent on what neutron stars are made of.”

“So far, the structure of neutron stars is an open question that draws in all kinds of physicists. Even without a detection, a search allows us to peek behind the curtain at the unknown physics of neutron stars. When we do detect continuous waves, we'll open the curtain and shine a spotlight on new physics. Until then, we can use the information we do have to refine our understanding and improve our search methods,” she said.

“Young neutron stars in supernova remnants are promising targets to look for those tiny continuous gravitational waves because they haven't spent a long enough time to relax and smooth out the asymmetries introduced at their birth,” said OzGrav Associate Investigator Lilli Sun from the Australian National University.

“We don’t know that much about neutron stars because they’re so small and strange,” added OzGrav postdoctoral researcher Carl Blair from the University of Western Australia.

“Are they hard or soft?  And when they spin fast as they collapse, do they wobble away that energy in the form of gravitational waves? While there is no evidence yet for continuous gravitational waves from neutron stars, limits have been placed on how wobbly a neutron star is from the fact that we haven’t measured gravitational waves from them yet.”

Additionally, some of the papers focused on pulsars as the generator of continuous gravitational waves, which can produce them through glitches that occur in their spin, as well as their spin down (pulsars are slowing down as they emit energy through radio beams, pulsar wind, high-energy radiation, etc.). The powerful magnetic fields of pulsars also distort their shape creating the non-symmetrical structures which generate continuous gravitational waves as they radiate.

“Gravitational-wave observation from O3 run of LIGO and Virgo detectors has allowed us to set realistic constraints on signals expected from young pulsars,” said OzGrav PhD student Deeksha Beniwal from the University of Adelaide.

“O3 observations also provide an opportunity to test out different pipelines ━ such as different search methods for continuous wave signals ━ in realistic environments.”

All of these publications will now help place constraints and limits on the types of signals that scientists should expect when searching for continuous gravitational waves, either from neutron stars and pulsars, which in turn will also help drive the next generation of instruments to be built, algorithms to be developed, and processing pipelines to be established.

“Continuous gravitational waves from neutron stars are much smaller than the gravitational waves LIGO and Virgo have seen so far,” said OzGrav postdoctoral researcher Meg Millhouse from the University of Melbourne.

“This means we need different techniques to detect them. And, because these are long-lasting signals, we need to look at lots of data which can be very difficult computationally. The recent LIGO-Virgo papers published showcase a wide range of these clever approaches to detect continuous gravitational waves.”

“Even though there were no detections in the most recent data analysed, we’re in a good position to keep searching and possibly make a detection when LIGO collects more data,” said Meg.

With the expected number of neutron stars and pulsars to be in the billions in the Milky Way galaxy alone, the background hum of these continuous gravitational waves should be all around us. Studies directing their searches are opting for the option to listen to all background sounds, and hopefully, be able to find the particular frequency that is linked to these continuous gravitational waves associated with rotating compact massive objects.

When we finally do catch it, it’s going to really change the way we look at how stars evolve, how they die, and how matter behaves under such extreme conditions.

All five papers have now been published and can be accessed on the below links:

Searches for continuous gravitational waves from young supernova remnants in the early third observing run of Advanced LIGO and Virgo – read the paper on arXiv.org

Gravitational-wave constraints on the equatorial ellipticity of millisecond pulsars – read the paper in the journal, The Astrophysical Journal Letters

Diving below the spin-down limit: Constraints on gravitational waves from the energetic young pulsar PSR J0537-6910 – read the paper on arXiv.org

Constraints from LIGO O3 data on gravitational wave emission due to r-modes in the glitching pulsar PSR J0537-6910 – read the paper on arXiv.org

All-sky search in early O3 LIGO data for continuous gravitational-wave signals from unknown neutron stars in binary systems – read the paper on arXiv.org

Video credit: Mark Myers/OzGrav-Swinburne University.