Gravitational-waves are ripples in space-time created by distant astronomical objects and detected by large complex detectors (like LIGO, Virgo, and KAGRA). Finding gravitational-wave signals in detector data is a complicated task requiring advanced signal processing techniques and supercomputing resources. Due to this complexity, explaining gravitational-wave searches in the undergraduate laboratory is difficult, especially because live demonstration using a gravitational-wave detector or supercomputer is not possible. Through simplification and analogy, table-top demonstrations are effective in explaining these searches and techniques.
A team of OzGrav scientists, across multiple institutions and disciplines, have designed a table-top demonstration with data analysis examples to explain gravitational-wave searches and signal processing techniques. The demonstration can be used as a teaching tool in both physics and engineering undergraduate laboratories and is to be published in the American Journal of Physics. Link to preprint here.
Lead author of the project James Gardner (who was an OzGrav undergraduate student at the University of Melbourne during the project and now a postgraduate researcher at the Australian National University) explains: “This demonstration offers some charming insights into a live field of research that students like me should appreciate for its recency compared to the age of most ideas they encounter”.
Table-top gravitational-wave demonstrations
Gravitational wave detectors are very complicated and huge — laser light is sent down tubes kilometres long! But the workings of a gravitational-wave detector can be demonstrated using table-top equipment. Researchers at the University of Adelaide have developed AMIGO to do just that! Deeksha Beniwal, co-author of this study and an OzGrav PhD student at the University of Adelaide explains: “With AMIGO, the portable interferometer, we can easily share how LIGO uses the fundamental properties of light to detect ripples from the most distant reaches of the universe.”
This work expands on the portable interferometer demonstration with a selection of examples for students in both physics and electrical engineering. Changrong Liu, co-author of this study and an OzGrav PhD student in electrical engineering at the University of Melbourne, explains: “This project offers a great opportunity for electrical engineering students like me to put some of their knowledge into the real and exciting physical world”.
Explaining the hunt for continuous gravitational waves
To demonstrate searching for signals with the table-top set up, the team first needed to make some fake signals to find! This is where the analogy of sound comes in: audio signals are used to mimic gravitational waves interacting with the detector. The team focused on demonstrating the hunt for continuous gravitational waves, a type of gravitational wave that hasn’t been detected yet.
Hannah Middleton, co-author of the study and an OzGrav Associate Investigator (at the University of Birmingham), explains: “Continuous waves are long-lasting signals from spinning neutron stars. These signals should be present in the detector data all the time, but the challenge is to find them. This demonstration is directly inspired by the techniques developed by OzGrav physicists and electrical engineers in the hunt for continuous gravitational waves!“
A continuous wave signal can be slowly changing in frequency, so the audio signals used in this demonstration also change in frequency. ”We show, through using sound as an analogue to gravitational waves, what it takes to detect a wandering tone: a long signal that slowly changes pitch like whalesong,” explains Gardner.
Prof. Andrew Melatos, co-author of this study and leader of the OzGrav-Melbourne node explains: “We hope that undergraduate educators will emphasize the cross-disciplinary spirit of the project and use it as an opportunity to speak more broadly to students about careers at the intersection of physics and engineering. The future is very bright career-wise for students with experience in cross-disciplinary collaboration”
Written by OzGrav Assoc. Investigator Hannah Middleton (University of Birmingham) and OzGrav postgrad researcher James Gardner (ANU).
Scientists from the ARC Centre of Excellence for Gravitational Wave Discovery and the University of Cologne (Germany) have developed new simulations of stars’ complicated lives, boosting research on how new stars are born and how old stars die.
These stellar evolution simulations, called the BoOST project, can be used to predict how often gravitational waves should be detected—gravitational waves (ripples in space-time) are expected to happen when the death throes of two stars merge. The project can also help to study the birth of new stars out of dense clouds in space.
Not all stars are the same. Sure, they all look like tiny, shining points on the sky, but it's only because they are all so far away from us. We only see stars that are close and bright enough. The rest, we may see with telescopes.
If you use a telescope to measure the colour of a star, it turns out that some stars are rather red, some are blue, and some are in between. And if you measure their brightness, it turns out that some are brighter than others. This is because a star’s colour and brightness depend on its heaviness and age, among other things. It's a complex theory that has been developing since the age of the first computer simulations in the 1950's.
Today, we have computer simulations that can predict how a star lives its complicated life, from birth until death. This is called 'stellar evolution' and applies to the stars that are close enough for us to observe with telescopes.
But there are stars so far away that even the largest telescopes can’t view them clearly; there are stars hiding inside thick clouds (yes, such clouds exist in space); and there are dead and dying stars that used to exist once upon a time. Is there a way to study these unreachable stars to observe similarities and differences from those that we can actually see?
Stellar evolution simulations can help here because we can simulate any star—even the stars we can’t see. For example, stars that were born soon after the Big Bang used to have a different chemical composition than those stars that we see today. From computer simulations, we can figure out how these early stars looked like: their colour, brightness etc.
What's more, we can even predict what happens to them after they die. Some of them become black holes, for example, and we can tell the mass of this black hole based on how heavy the star had been before it exploded.
And this presents more opportunity for discovery! For example, it’s possible to predict how often two black holes merge. This gives us statistics about how many times we can expect to detect gravitational waves from various cosmic epochs. Or, when trying to understand how stars are born out of dense clouds, we can count the number of hot bright stars and the number of exploding stars around these cloudy regions. Both hot bright stars and explosions change the clouds' structure and influence the birth of new stars in delicate ways.
The BoOST project predicts how stars live their lives. These diagnostic diagrams show stellar evolution simulations of massive and very massive stars (colourful labels in solar mass units). These are stellar lives in the Milky Way (left), in the Small Magellanic Cloud (middle) and in a metal-poor dwarf galaxy (right). One line on these diagrams belongs to one star’s whole life from birth to death. Their brightness is shown to change on the vertical axis, and their apparent ‘colour’ (surface temperature, with lower values meaning red and higher, blue) on the horizontal axis. These simulations can give a boost to research on how new stars are born and how old stars die.
Lead scientist on the study Dorottya Szécsi from the University of Cologne says: ‘Much like the theory of stellar life got a boost in the 1950's from computerization, we hope our BoOST project will contribute to other research fields, because both the birth of new stars and the ultimate fate of old stars depend on how stars live their complicated and very interesting lives’.
“Given the importance of massive stars in astrophysics, from determining star formation rates to the production of compact remnants, it is essential that our theoretical models of stars keep pace with advancements in observations,” says OzGrav postdoctoral researcher and study co-author Poojan Agrawal.
Link to paper: https://www.aanda.org/articles/aa/full_html/2022/02/aa41536-21/aa41536-21.html
Pulsars, a class of neutron stars, are extremely predictable stars. They are formed from the hearts of massive stars that have since collapsed in on themselves, no longer able to burn enough fuel to fend off the crushing gravity the star possesses. If the conditions are right, the star will continue to collapse in on itself until what’s left is a remnant of what was there before, usually only about the size of the Melbourne CBD, but 1-2 times as heavy as our Sun, making these some of the densest objects in the Universe.
These stars don’t produce much visible light, but from their magnetic poles, they emit surprisingly bright beams of radio waves. If we’re lucky, as the star rotates, those beams will wash over the Earth and we observe ‘pulses’. While most pulsars spin around in about a second, there is a subclass of these stars that spin around in just a few thousandths of a second—they’re called ‘millisecond’ pulsars.
Observing the pulses from these millisecond pulsars gives physicists clues to many questions, including testing General Relativity and understanding the densest states of matter. But one of the main goals of observing these incredibly fast, dense stars is to detect ultra-long wavelength gravitational waves. And by long, we mean many light-years long. These gravitational waves distort space-time between us and the pulsars, causing the pulses to arrive earlier or later than expected. It’s likely that these gravitational waves come from a background produced by all the binary supermassive black holes in the Universe, which form from galaxies crashing into one another.
As part of OzGrav, we try and detect this gravitational wave background by looking at collections of the most predictable stars (called pulsar timing arrays) and measuring how they change over time. We did this by using the world’s most sensitive radio telescopes, including the Australian Murriyang telescope (also known as the Parkes telescope) and the ultra-sensitive MeerKAT array telescope in South Africa.
But it’s not quite that simple. From our observations with MeerKAT we found that the most precisely timed (read: predictable) pulsar, J1909-3744, was misbehaving. We found that the pulses were changing shape, with bright pulses arriving earlier and narrower than faint ones. This lead to greater uncertainty in its predicted emission. Fortunately, we were able to establish a method to account for this change and time tag the pulsar more precisely than ever before. This method could be of use for other pulsars and will be important when more advanced telescopes are available in the future.
Written by OzGrav PhD student Matthew Miles, Swinburne University
MEDIA RELEASE: Searching for elusive continuous gravitational waves from the densest objects in the Universe
Caption: Artist’s impression of continuous gravitational waves generated by a spinning asymmetric neutron star (left) accreting matter from a companion star (right). Credit: Mark Myers, OzGrav-Swinburne University of Technology
Take a star similar in size to the Sun, squash it down to a ball about twenty kilometres across and you’d get a neutron star: the densest object in the known Universe. Now set your neutron star spinning at hundreds of revolutions per second and listen carefully. If your neutron star isn’t perfectly spherical, it will wobble a bit, causing it to continuously send out faint ripples in the fabric of space and time. These ripples are called continuous gravitational waves.
So far, these elusive continuous gravitational waves haven’t been detected; however, in a recent study, an international collaboration of scientists, led by Australian OzGrav researcher Julian Carlin (from the University of Melbourne), searched for them from a specific category of neutron star: accreting millisecond X-ray pulsars (AMXPs).
To break it down, AXMPs are:
As AMXPs accumulate matter from their companion star, they’re likely to send out stronger signals than a lone neutron star. This is because the strength of a neutron star’s signal is proportional to its asymmetry. Astronomers theorise that this build up of matter on the AMXPs could create small mountains on the surface as material is funnelled by the magnetic field onto the magnetic poles. This is illustrated by the artist's impression shown in Figure 1.
This search uses data from the third observing run of LIGO, Virgo, and KAGRA which lasted from April 2019 to March 2020. The team searched for continuous gravitational waves from 20 AMXPs - 14 of which hadn’t been searched before.
The search method used in this work is the result of a collaboration between physicists and engineers at the University of Melbourne. “The methods we are using to search for continuous gravitational waves from spinning neutron stars are similar to those used in speech recognition software!” said Hannah Middleton (an OzGrav postdoc at both the University of Melbourne and Swinburne University).
Unfortunately, continuous gravitational waves were not convincingly detected this time. However, as detector technology and data analysis algorithms keep improving, it’s possible that a detection will be made in the next observing run.
Julian Carlin said: “It may turn out that the weak candidates we’ve spotted here are the first signs of a real signal, and we just need a little bit more data to pull it out of the noise”.
“If a detection were made, it’d allow us to peer into the hearts of neutron stars ━ teaching us how matter behaves in extremely dense environments,” he continues. “Detecting continuous gravitational waves from neutron stars would give us great insights into how these fantastic astronomical clocks really tick.”
“The hunt for continuous gravitational waves is one of the top challenges in gravitational wave science”, said Andrew Melatos, an OzGrav Chief Investigator whose research group at the University of Melbourne has been chasing these tiny signals for more than a decade. “Pulsars are one of Nature’s most bountiful gifts. Their radio signals revolutionised astronomy, shedding new light on everything from the gas between the stars to Einstein’s theory of gravity and the strongest magnetic fields in the Universe. Who knows what surprises their gravitational wave murmurs will reveal?”
Dr. Karl Wette, an OzGrav research fellow at The Australian National University and co-chair of the LIGO continuous wave working group, said: "Gravitational waves are becoming an essential tool for fundamental physics and astronomy. We've now heard the echoes of nearly 100 pairs of black holes and neutron stars smashing into each other. We're keeping our ear to the ground, and hope to pick out the tell-tale hum of a rapidly-spinning neutron star in the coming years. Australia has a strong track record in this area of research, and it's particularly pleasing to see Australian students and junior researchers making important contributions.”
"With improved detectors in the fourth observation run, the number of detections is expected to increase manifold,” said OzGrav PhD student Chayan Chatterjee at the University of Western Australia. “So, it will be extremely exciting to watch out for more continuous gravitational wave candidates as well as other ground-breaking discoveries!"
Read the full scientific article https://journals.aps.org/prd/abstract/10.1103/PhysRevD.105.022002
Link to the LIGO Science Summary: https://www.ligo.org/science/Publication-O3LMXBsAMXPs/.
In our recently accepted paper, we examined the black hole-neutron star merger called GW200115, second observed by LIGO and Virgo in January 2020. Curiously, GW200115’s black hole could have been spinning rapidly, with its spin misaligned with respect to the orbital motion. This is strange because it implies that the system would have formed in pretty unexpected ways.
So, is there something we’re missing? In our paper we show that the puzzling black hole spin is probably due to something that was added to the LIGO-Virgo measurements instead. It has to do with things called ‘priors’ which encode assumptions about the population of black hole-neutron star binaries based on our current knowledge. We argue that a better explanation for the GW200115 merger is that the black hole was not spinning at all, and consequently, we place tighter constraints on the black hole and neutron star masses.
What is a prior?
Imagine you want to know the probability of having drawn an Ace from a deck of cards, given that the card is red. You’d need to know the separate probabilities of drawing an Ace and a red card. The probability of drawing an Ace, independent of the data (“the card is red”) is the ‘prior’ probability of drawing an Ace. Astronomy is similar to a game of cards: we can think of observed gravitational-wave signals as having been dealt to us randomly by the Universe from a cosmic deck of cards. The prior should express our current best knowledge of this deck before we make a measurement, because it‘s used to calculate the probability of each possible black hole spin. In the LIGO-Virgo analysis of GW200115, it was assumed that all black hole spins are equally likely. This is fine if we have no strong preference for any value, but we do: observation and theory tell us we shouldn’t expect a rapidly spinning black hole to be paired with a neutron star. This information is key to accurately measuring the properties of GW200115.
In our paper, we begin by demonstrating that if GW200115 originated from a black hole-neutron star binary with zero spin, the unrealistic LIGO-Virgo prior (which assumes the black hole can equally likely spin with any magnitude and direction) generates preference for a large misaligned black hole spin. We do this by simulating a gravitational-wave signal from a non-spinning binary, placing it into simulated (but realistic) LIGO-Virgo noise, and inferring its properties assuming any spin value is equally likely. Our simulated experiment yields a similar spin measurement to LIGO-Virgo’s and we’re able to explain analytically why signals from black hole-neutron star binaries with zero spin will generically yield such measurements when very broad spin priors are assumed. While this doesn’t prove that GW200115 is non-spinning, it suggests that the puzzling LIGO-Virgo spin measurement is probably due to their unrealistic priors.
Next, we look to astrophysics to figure out a more realistic prior. We use current theoretical modelling to suggest that there’s roughly a 95% probability that black hole-neutron star binaries do not spin at all, and only around 5% do spin. We use this astrophysical prior to update the LIGO-Virgo measurements of GW200115’s spins and masses. When we do this, we find that there is almost zero probability that the black hole had any spin at all. While this might seem circular at first glance—after all, we’re giving zero-spin almost 20 times more weight than non-zero spin—it’s also a reflection of the fact that the data don’t strongly support a rapidly spinning black hole. Additionally, we show that our prior reduces the uncertainty on the black hole and neutron star masses by a factor of 3. Reassuringly, the mass of the neutron star looks significantly more like those found in double neutron star systems in the Milky Way.
Written by Rory Smith and Ilya Mandel, Monash University
RESEARCH PATHS: Orbital path shapes of colliding dead stars may indicate origin of binary stellar systems
We measured the shapes of the orbits of dead stars by their *eccentricity*: higher eccentricity means the orbital shape is more squashed, while an eccentricity of 0 means that it is circular. The coloured shapes represent the probability of eccentricity for each event, with the widest point of the shape at the highest point of probability. There are two events with their highest point of probability above the detection threshold for eccentricity, which is indicated with a dotted line.
The LIGO-Virgo-KAGRA Collaboration recently announced that the number of times we've seen dead stars crashing into each other on the other side of the Universe has grown to 90. It's clearly not uncommon for these dead stars—most of them black holes—to slam together in violent merger events. But one outstanding mystery pervades these detections: how do two compact stellar remnants find each other in the vast emptiness of space, and go on to merge together? In our recent paper, we found clues to solve this mystery from the orbital path shapes formed by the stellar objects before they collided.
Often, stars are born into binary systems containing two stars that orbit each other. If these binary stars undergo specific evolutionary mechanisms, they can remain close when they die, and their corpses—black holes and/or neutron stars—can collide with each other. This kind of binary should trace a circular orbital path before it merges. However, sometimes stellar remnants meet in more exciting environments, like the cores of star clusters. In this kind of environment, binary stellar remnants can trace orbital paths around each other that look like ‘squashed’ circles—more egg-shaped or sausage-shaped.
Dense clusters of stars can produce binaries in circular orbits; however, about 1 in 25 of the mergers that combine in a dense star cluster are expected to have orbital shapes that are visibly squashed. To map the paths taken by cosmic couples in their pre-merger moments, we studied the space-time ripples produced by the collisions of 36 binary black holes. Two of these collisions—one of them being the monster binary black hole GW190521—contained the distinctive signatures of elongated (squashed) orbits. This means that more than a quarter of the observed collisions may be occurring in dense star clusters, because every squashed-orbit system indicates that 24 more mergers may also have happened in this environment.
While this result is exciting, it’s not conclusive: other dense environments, like the centres of galaxies, can also produce merging stellar remnants with squashed orbital shapes. To distinguish the formation habitats of the observed population, we need to scrutinise the orbital shapes of more colliding stellar remnants. Luckily, the number of detected stellar-remnant collisions is growing quickly, so this merger mystery may be solved soon.
Written by OzGrav PhD student Isobel Romero-Shaw, Monash University
Multimessenger astronomy is an emerging field which aims to study astronomical objects using different ‘messengers’ or sources, like electromagnetic radiation (light), neutrinos and gravitational waves. This field gained enormous recognition after the joint detection of gravitational waves and gamma ray bursts in 2017. Gravitational waves can be used to identify the sky direction of an event in space and alert conventional telescopes to follow-up for other sources of radiation. However, following up on prompt emissions would require a rapid and accurate localisation of such events, which will be key for joint observations in the future.
The conventional method to accurately estimate the sky direction of gravitational waves is tedious—taking a few hours to days—while a faster online version needs only seconds. There is an emerging capacity from the LIGO-Virgo collaboration to detect gravitational waves from electromagnetic-bright binary coalescences, tens of seconds before their final merger, and provide alerts across the world. The goal is to coordinate prompt follow up observations with other telescopes around the globe to capture potential electromagnetic flashes within minutes from the mergers of two neutron stars, or a neutron star with a black hole—this was not possible before. The University of Western Australia’s SPIIR team is one of the world leaders in this area of research. Determining sky directions within seconds of a merger event is crucial,as most telescopes need to know where to point in the sky. In our recently accepted paper , led by three visiting students (undergraduate and Masters by research) at the OzGrav-UWA node, we applied analytical approximations to greatly reduce the computational time of the conventional localisation method while maintaining its accuracy. A similar semi-analytical approach has also been published in another recent study .
The results from this work show great potential and will be integrated into the SPIIR online pipeline going forward in the next observing run. We hope that this work complements other methods from the LIGO-Virgo collaboration and that it will be part of some exciting discoveries.
Written by OzGrav PhD student Manoj Kovalam, University of Western Australia.
This work is now accepted by PRD: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.104008
The first evidence of the existence of black holes was found in the 1960s, when strong X-rays were detected from a system called Cygnus X-1. In this system, the black hole is orbited by a massive star blowing an extremely strong wind, more than 10 million times stronger than the wind blowing from the Sun. Part of the gas in this wind is gravitationally attracted towards the black hole, creating an ‘accretion disc’, which emits the strong X-rays that we observe. These systems with a black hole and a massive star are called ‘high-mass X-ray binaries’ and have been very helpful in understanding the nature of black holes.
After nearly 60 years since the first discovery, only a handful of similar high-mass X-ray binaries have been detected. Many more of them were expected to exist, especially given that many binary black holes (the future states of high-mass X-ray binaries) have been discovered with gravitational waves in the past few years. There are also many binaries found in our Galaxy that are expected to eventually become a high-mass X-ray binary. So, we see plenty of both the predecessors and descendants, but where are all the high-mass X-ray binaries themselves hiding?
One explanation states that even if a black hole is orbited by a massive star blowing a strong wind, it does not always emit X-rays. To emit X-rays, the black hole needs to create an accretion disc, where the gas swirls around and becomes hot before falling in. To create an accretion disc, the falling gas needs ‘angular momentum’, so that all the gas particles can rotate around the black hole in the same direction. However, we find it is generally difficult to have enough angular momentum falling onto the black hole in high-mass X-ray binaries. This is because the wind is usually considered to be blowing symmetrically, so there is almost the same amount of gas flowing past the black hole both clockwise and counter-clockwise. As a result, the gas can fall into the black hole directly without creating an accretion disc, so the black hole is almost invisible.
But if this is true, why do we see any X-ray binaries at all? In our paper, we solved the equations of motion for stellar winds and we found that the wind does not blow symmetrically when the black hole is close enough to the star. The wind blows with a slower speed in the direction towards and away from the black hole, due to the tidal forces. Because of this break of symmetry in the wind, the gas can now have a large amount of angular momentum, enough to form an accretion disc around the black hole and shine in X-rays. The necessary conditions for this asymmetry are rather strict, so only a small fraction of black hole + massive star binaries will be able to be observed.
The model in our study explains why there are only a small number of detected high-mass X-ray binaries, but this is only the first step in understanding asymmetric stellar winds. By investigating this model further, we might be able to solve many other mysteries of high-mass X-ray binaries.
Written by OzGrav Postdoc Ryosuke Hirai, Monash University
The authors Debatri Chattopadhyay (Swinburne University) and Isobel Romero-Shaw (Monash University)—who are both completing their PhDs in astrophysics with OzGrav—are determined to educate children and young people about the pivotal scientific discoveries and contributions made by women scientists. They also want to encourage more girls, women, and minorities to take up careers in Science, Technology, Engineering, Mathematics and Medicine (STEMM), which is a male-dominated field.
Debatri, who is originally from India, came to Australia pursuing her PhD at Swinburne University in 2017. She was acutely aware of the lack of women in STEMM fields, as both of her parents worked in biological sciences. “My father is a scientist, so I was aware that this was a field I could go into, and he would talk about amazing biologists like Barbara McClintock, but there was almost no representation of female scientists on TV or in newspapers,” she recalls.
“This colouring book will help children learn about the colourful lives and brilliant minds of these amazing women scientists. As a colouring book, it encourages creative minds to think about scientific problems - which is very much needed for problem solving”, says Isobel, who designed the book and illustrated each of the featured scientists. “These women, who made absolutely pioneering discoveries, used their creativity to advance the world as we know it.”
“I did intense research for the biographies of the women featured in the book and at every nook and crevice was amazed at the perseverance they showed. It is for them and countless others, unfortunately undocumented, that we can do what we do today,” says Debatri. “With Christmas approaching, this book is a perfect gift for young children who have a hunger for science. It’s both fun and educational!” she added.
Last year, both Isobel and Debatri were also selected to participate in Homeward Bound, a global program designed to provide cutting-edge leadership training to 1000 women in STEMM over 10 years. To raise awareness of climate change, this journey will also take Isobel and Debatri all the way to Earth’s frozen desert, Antarctica.
The initiative aims to heighten the influence and impact of women with a science background in order to influence policy and decision making as it shapes our planet. In 2017-18, OzGrav Chief Investigator Distinguished Prof Susan Scott was also selected to participate in this program and embark on the journey to Antarctica.
“The saying that ‘you can’t be what you can’t see’ is addressed in this new colouring book,” says Distinguished Prof Scott. “The important women scientists depicted in the book come to life as role models as they are coloured in. Women are underrepresented in physics education and work in Australia. Educating children about women scientists throughout history is an important step in encouraging more girls and women to take up STEMM careers and boost diversity.”
In their “day jobs”, Isobel tries to figure out how the collapsed remains of supergiant stars—black holes and neutron stars—meet up and crash together. She does this by studying the vibrations that these collisions send rippling through space-time—these are called gravitational waves. She also recently published an illustrated book, available on Amazon, called Planetymology: Why Uranus is not called George and other facts about space and words. Planetymology explains the ties between ancient history, astronomy, and language, and introduces the reader to the harsh realities of conditions on other planets.
Debatri is involved in doing simulations in supercomputers of dead stars in binaries or in massive collections of other stellar systems - called globular clusters. Her detailed theoretical calculations help us to understand the astrophysics behind the observations of gravitational waves and radio pulsars, as well as predict what surprising observations might be made in the future. Debatri is also a trained Indian classical dancer and was a voluntary crew member of the Melbourne-based tall ship `Enterprize’. She has recently submitted her thesis and joined as a postdoctoral fellow at the Gravity Exploration Institute, Cardiff University.
GIVEAWAY: To celebrate the launch of Women in Physics, OzGrav is giving away free copies of the colouring book to three lucky winners. Simply share the book via Twitter (re-tweet) and tag @ARC_OzGrav to be in the draw. Winners will be announced Monday 20 December!
Gravitational waves are cosmic ripples in the fabric of space and time that emanate from catastrophics events in space, like collisions of black holes and neutron stars--the collapsed cores of massive supergiant stars. Extremely sensitive gravitational-wave detectors on Earth, like the Advanced LIGO and Virgo detectors, have successfully observed dozens of gravitational-wave signals, and they’ve also been used to search for dark matter: a hypothetical form of matter thought to account for approximately 85% of all matter in the Universe. Dark matter may be composed of particles that do not absorb, reflect, or emit light, so they cannot be detected by observing electromagnetic radiation. Dark matter is material that cannot be seen directly, but we know that dark matter exists because of the effect it has on objects that we can observe directly.
Ultralight boson particles are a new type of subatomic particle that scientists have put forward as compelling dark matter candidates. However, these ultralight particles are difficult to detect because they have extremely small mass and rarely interact with other matter -- which is one of the key properties that dark matter seems to have.
The detection of gravitational waves provides a new approach to detecting these extremely light boson particles using gravity. Scientists theorise that if there are certain ultralight boson particles near a rapidly spinning black hole, the extreme gravity field causes the particles to be trapped around the black hole, creating a cloud around the black hole. This phenomenon can generate gravitational waves over a very long lifetime. By searching for these gravitational-wave signals, scientists can finally discover these elusive boson particles, if they do exist, and possibly crack the code of dark matter or rule out the existence of some types of the proposed particles.
In a recent international study in the LIGO-Virgo-KAGRA collaboration, with OzGrav Associate Investigator Dr Lilli Sun from the Australian National University being one of the leading researchers, a team of scientists carried out the very first all-sky search tailored for these predicted gravitational wave signals from boson clouds around rapidly spinning black holes.
“Gravitational-wave science opened a completely new window to study fundamental physics. It provides not only direct information about mysterious compact objects in the Universe, like black holes and neutron stars, but also allows us to look for new particles and dark matter,” says Dr Sun.
Although a signal was not detected, the team of researchers were able to draw valuable conclusions about the possible presence of these clouds in our Galaxy. In the analysis, they also took into consideration that the strength of a gravitational wave signal depends on the age of the boson cloud: the boson cloud shrinks as it loses energy by sending out gravitational waves, so the strength of the gravitational wave signal would decrease as the cloud ages.
“We learnt that a particular type of boson clouds younger than 1000 years is not likely to exist anywhere in our Galaxy, while such clouds that are up to 10 million years old are not likely to exist within about 3260 light-years from Earth,” says Dr Sun.
“Future gravitational wave detectors will certainly open more possibilities. We will be able to reach deeper into the Universe and discover more insights about these particles”.
Also featured on Cosmos Magazine and Sci Tech Daily.