Using a Galactic-Scale Detector to narrow in on Gravitational Waves

One of the most exciting frontiers of astrophysics to emerge in the last few years is gravitational wave astronomy. Predicted in theory in Einstein’s relativity over 100 years ago, and finally confirmed in 2015 with direct observations - this new window into the Universe grants us a way to explore a whole new dimension of science, allowing us to learn more about the cosmos than ever before. 

But so far, we’ve only directly observed gravitational wave events generated by the merger of massive compact objects, like stellar-mass black holes, or neutron stars, as they spiral towards each other as part of their fateful orbital dance. 

When a gravitational wave signal reaches Earth, it is feeble - akin to the movement of a length measured equal to 1/10,000th the width of a proton. This is because the realm we live in - space-time - is extremely rigid, not to mention these events occur at billions of light-year distances, and look-back time. 

But stellar-mass black holes and neutron stars are not the only objects that can cause gravitational waves. Any accelerating mass theoretically does - even you could if you stood there and did some squats right now. Admittingly, your mass is feeble relative to that of the Earth, the Sun and other objects - so too your gravitational waves are going to register as practically zero (unless you had some extraordinary detection equipment). 

But what if we consider the opposite end of the mass spectrum - supermassive black holes, which measure in at millions to billions of times the mass of our Sun - what happens when these objects start spiralling towards each other and merging?

That’s a key question that several astrophysics teams around the world are working towards - trying to detect the gravitational waves that are generated from these events. And what we’re looking for isn’t related to a single set of binary supermassive black holes merging, but instead, a sea of humming that comes from the entire merger history of all galaxies as they’ve collided and coalesced in the Universe’s past. This is known as the gravitational wave background (GWB).

Recently, some results have started emerging by several teams across the world, hunting for these types of gravitational waves, indicating that we could potentially be close to a new discovery. But as with all good science mysteries, it is too early to call it - and more data is needed. 

Now, a new paper (published in The Astrophysical Journal Letters) led by Dr Boris Goncharov and Professor Ryan Shannon, from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) has indicated that we could be on the right track and getting a little closer to making the first detections of the gravitational waves. 

“Although we do not know exactly how loud the astrophysical background is compared to our current measurement precision, most theorists agree that the detection may be just around the corner,” said lead author Dr Goncharov, who is currently with the Gran Sasso Science Institute in Italy.  

“The most exciting result is that the experimental noise background starts to show some of the properties of the gravitational-wave signal from supermassive black hole binaries.”  

Whilst the results from the recent paper go towards the greater discussion on detecting gravitational waves from supermassive black hole binaries, they don’t exactly give us the smoking gun as yet - so they are interpreted with caution, an approach most scientists err on the side of. 

Though, there is one thing that is to be excited about with absolute certainty - that at the heart of this exciting science adventure is one of Australia’s most iconic instruments - the CSIRO Parkes radio telescope, and an array of our scientists from several institutions across the country working towards resolving these questions.

Like their electromagnetic counterparts, gravitational waves also come in a spectrum - with different mass objects generating different frequencies. The confirmed gravitational wave detections made thus far are at higher frequencies and are generated by the stellar-mass objects. 

To catch these signals, giant L-shaped interferometers are used, utilising lasers bouncing back and forth along their kilometre-scale arms, to produce an interference pattern - which when disturbed, might be indicative of a passing gravitational wave.  

But the gravitational waves generated by supermassive black holes have a much lower frequency - with the wave period between crests taking years and decades. Terrestrial-based instruments are unable to cope with this scale, and so an ingenious method has been devised - build a detector as big as the galaxy itself. 

“Terrestrial interferometers are tuned to detect the high-frequency chirps of stellar-mass remnants.” said co-author Prof. Ryan Shannon from Swinburne University of Technology. “To detect lower frequency signals requires a much larger detector, impossible geographically to fit on Earth.”   

To do this, astronomers utilise a unique kind of stellar object, known as a pulsar, to help constitute the arms of our galactic interferometer. Pulsars are rapidly rotating remnant cores of once former massive stars, which have since undergone a violent supernova event.

These objects, only the size of a small city, are spinning at high angular velocity and have powerful magnetic fields - and as a result, are able to generate a beam of radio waves from their magnetic poles. If the alignment is right, these beams sweep past the Earth and we register a pulse of radio waves, much like a lighthouse beam seen by a ship far out at sea. 

Pulsars, and in particular an older sub-set population known as millisecond pulsars, are extremely stable timekeepers, with each pulse representing a cycle in the rotation of the object. So stable, they’re comparable to some of the best terrestrial atomic clocks, and an accurate clock in space is a great way for astrophysicists to perform timing experiments. 

“Normal pulsars are formed after a supernova explosion and have spin periods of around a second. They have strong magnetic fields (compare to fridge magnet) and relatively wide pulses,” said Shannon. 

“Millisecond pulsars are normal pulsars that have been spun up (recycled) by a companion star.  This recycling process causes the MSPs to have smaller magnetic fields and, by virtue of spinning faster, have narrower pulses. The lower magnetic fields mean result in them being more stable rotators. The narrower pulses mean we can keep track of the pulsar’s rotation very accurately," he added.     

When pulsars were discovered, scientists started to learn that not only are they extremely accurate clocks, but also that they seem to be spinning down as energy leaves these systems. This spin-down rate was found to be measurable out to very high precision - from 15 to 18 decimal places. 

This led astrophysicists to realise that they could build a model and experiments based on the prediction of what a pulsar’s spin rate would be in the future. For example, if the pulsar is spinning down by the value X per cycle, then 2 years from now, it should have a rotation period of exactly Y. A simple relationship that offers so much opportunity.  

As observations were undertaken, both the predicted model and the actual signal could be compared to note any discrepancies - which would be indicative of an event occurring with the pulsar along the line of sight. So what kind of event could occur that could throw the observed signal into conflict with the predicted signal? 

“Despite the interstellar medium may seem empty, it is filled with diffused ionised gas. Radio pulses travelling through this gas get dispersed, which causes low-energy pulses to arrive later than the high-energy ones from the same source,” said Goncharov.  

“The effect is similar to the emergence of a rainbow when the sunlight is dispersed on water drops in the air.” 

“Sudden and continuous changes in the density of the interstellar medium is something that we have to take into account in our data analyses. Additionally, sudden or continuous changes in pulsar magnetic fields or interior dynamics can yield rotational irregularities. By measuring pulse arrival times at different radio frequencies we disentangle these effects,” he said.  

There is, however, one type of notable event that might cause these timing residuals. It’s when the space-time between the Earth and the pulsar, changes - ever so slightly increasing or decreasing the distance between two. This event is indicative of a passing gravitational wave. 

Effectively, in this scenario, the pulsar and its line of sight beam back to the Earth, become the arm of a giant interferometer - very much like the powerful instruments used in the discovery of gravitational waves generated by stellar-mass binary compact objects.

Now, take a bunch of pulsars all spread across the galaxy in different directions and repeat this thinking - and you have a galactic scale interferometer, that is highly sensitive to tiny variations in space-time as a result of gravitational waves. This is known as a pulsar timing array (PTA). 

When an anomaly from a single pulsar in a PTA is detected, this could be for several reasons - sometimes intrinsic to the pulsar (like a glitch or a jitter), or sometimes associated with an occurrence along the line of sight (like if the signal passes through a dense cloud of interstellar gas).

But if a correlated signal is observed across many different pulsars that are spread across the sky, then that could be the tell-tale signature of the gravitational wave background, generated by supermassive black holes. 

There are now several PTA teams across the world, monitoring an array of pulsars over very long periods (some data sets span as far back over two decades) - all hoping to catch this correlated signal from these low-frequency gravitational waves. 

Collectively, the working group is known as the International Pulsar Timing Array project, but four major individual bodies make up the collective, along with a handful of smaller contributors. Amongst them all, however, some of the most iconic radio telescopes in the world are employed to keep a watchful eye on pulsar signals over the years. 

This includes the North American Nanohertz Observatory for Gravitational Waves group (NANOGrav), which uses telescopes like the Green Bank Telescope, and previously the Arecibo Observatory. The European Pulsar Timing Array (EPTA) utilises a number of telescopes across the EU continent, including the Sardinia radio telescope, the Lovell Telescope, and Effelsberg radio telescope. And the Indian Pulsar Timing Array group (IPTA) which uses the upgraded Giant Metrewave Radio Telescope. 

Additionally, teams from China using the Five-Hundred-Metre Aperture Spherical Telescope (FAST), and South Africa, who use the MeerKAT instrument also contribute towards the international pulsar timing array project. 

In Australia, the group is known as the Parkes Pulsar Timing Array project (PPTA) employing the use of one of Australia’s most loved iconic pieces of scientific infrastructure - the Parkes radio telescope (owned and operated by Australia’s national science agency, CSIRO, and also known by its traditional Wiradjuri name, Murriyang).

In January 2021, the team from NANOGrav released a paper that yielded some interesting results, in the context of the detection of these low-frequency gravitational waves. What the NANOGrav team found was ‘common noise’ across all the pulsars they were observing in a 12.5-year data set - and whilst the signal was different per pulsar, it was representative of something that might be appearing across the whole suite of pulsars being observed. 

Could this have been a precursor detection to the GWB? If so, then other pulsars (and other data sets) should also show the same thing, because the phenomena would not be biased to the NANOGrav data set alone. 

And here is where the Goncharov et al. paper steps in - the team from the PPTA analysed the data set collated using the Parkes radio telescope and found that the common noise process was present, but concluded that the evidence did not significantly support nor deny that the correlated signal was related to the GWB. 

“The signal needs to match three criteria to be attributed to gravitational waves: it has to be common among pulsars, it has to have a certain spectrum of timing irregularities, and it has to be correlated in pulsars across the sky in a certain way,” said Goncharov. 

“Only the latter is considered a smoking gun. Although we have not detected spatial correlations, I was personally very excited to find the properties of the common noise process in the PPTA data match the description of a gravitational-wave signal even more closely than in the NANOGrav data.” 

“This seemed too good to be true, which is why we seriously considered other possibilities. In particular, we realized that the methodology that PPTA and NANOGrav rely on does not make a distinction between a “common” noise process and a “similar” noise process.” he said. 

“The latter may also result in the observed noise process while being originated by similar rotational irregularities in some of the pulsars. This will not be a problem in the future, but it highlights the importance of detecting spatial correlations.” 

So what could it be? Everyone agrees it might be a glimpse at the GWB signal, though it is very likely too early to tell at this stage, and the collection of more data from all PTA teams will be required. 

Or it could be a bunch of other reasons that we are yet to have full confidence in - for example, there could be slight variations in the Solar system ephemeris used to make these determinations in the first place, with historical studies indicating that different ephemeris models used have yielded different results. 

Or maybe it is that we don’t yet have a complete picture of all the different aspects of ‘noise’ that are intrinsic to pulsars, nor if we can apply this modelling across all pulsars in the data sets, assuming that they all exhibit the exact same behaviours (which in itself, would be odd). 

In all these scenarios, the answers to the questions that remain lie within the collection of longer time-spanned data sets on the one hand, whilst improving our understanding of these exotic objects in the other.

“Eventually, time will tell whether the observed common noise process is part of the gravitational-wave background from supermassive binary black holes. For now, recent findings strengthened collaborations within the International Pulsar Timing Array, and we all look forward to the upcoming data sets which will hopefully clarify the nature of the common noise,” said Goncharov. 

“The scientific motivation is strong and new state-of-the-art instruments are now joining the observations. So, even the most pessimistic scenarios about the strength of the gravitational-wave signal suggest a detection in the near future.”  

We acknowledge the Wiradjuri people as the traditional owners of the Parkes radio telescope observatory.

Disclaimer notice: Rami Mandow is one of the authors of the Goncharov et al. 2021 paper featured in this article.