What happens if a supernova explosion goes off right beside another star? The star swells up which scientists predict as a frequent occurrence in the Universe. Supernova explosions are the dramatic deaths of massive stars that are about 8 times heavier than our Sun.
Most of these massive stars are found in binary systems, where two stars closely orbit each other, so many supernovae occur in binaries. The presence of a companion star can also greatly influence how stars evolve and explode. For this reason, astronomers have long been searching for companion stars after supernovae-- a handful have been discovered over the past few decades and some were found to have unusually low temperatures.
When a star explodes in a binary system, the debris from the explosion violently strikes the companion star. Usually there’s not enough energy to damage the whole star, but it heats up the star’s surface instead. The heat then causes the star to swell up, like having a huge burn blister on your skin. This star blister can be 10 to 100 times larger than the star itself.
The swollen star appears very bright and cool, which might explain why some discovered companion stars had low temperatures. Its inflated state only lasts for an ‘astronomically’ short while--after a few years or decades, the blister can “heal” and the star shrinks back to its original form.
In their recently published study by a team of scientists led by OzGrav postdoctoral researcher Dr Ryosuke Hirai (Monash University), the team carried out hundreds of computer simulations to investigate how companion stars inflate, or swell up, depending on its interaction with a nearby supernova. It was found that the luminosity of inflated stars is only correlated to its mass and doesn’t depend on the strength of the interaction with supernova. The duration of the swelling is also longer when the two stars are closer in distance.
“We applied our results to a supernova called SN2006jc, which has a companion star with a low-temperature. If this is in fact an inflated star as we believe, we expect it should rapidly shrink in the next few years,” explains Hirai
The number of companion stars detected after supernovae are steadily growing over the years. If scientists can observe an inflated companion star and its contraction, these data correlations can measure the properties of the binary system before the explosion—these insights are extremely rare and important for understanding how massive stars evolve.
“We think it’s important to not only find companion stars after supernovae, but to monitor them for a few years to decades to see if it shrinks back,” says Hirai.
As featured on Phys.org.
Have you heard the joke about how many stars it takes to create a merging binary black hole? Hundreds of thousands in the real world… but only a few, if they’re OzStars and you’re using the latest COMPAS version with machine learning tools.
Five years on from the first discovery of gravitational waves, an international team of scientists, including from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), are continuing the hunt for new discoveries and insights into the Universe. Using the super-sensitive, kilometre-sized LIGO detectors in the United States, and the Virgo detector in Europe, the team have witnessed the explosive collisions of black holes and neutron stars. Recent studies, however, have been looking for something quite different: the elusive signal from a solitary, rapidly-spinning neutron star.
Take a star similar in size to the Sun, squash it down to a ball about twenty kilometres across ━ roughly the distance from Melbourne airport to the city centre ━ 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 about a bit, and you’ll hear a faint “humming” sound. Scientists call this a continuous gravitational wave.
So far, these humming neutron stars have proved elusive. As OzGrav postdoctoral researcher Karl Wette from the Australian National University explains: “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! 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.”
A recent collaborative study with OzGrav has taken a closer look at the remnants of exploded stars, called supernovae. OzGrav PhD student Lucy Strang from the University of Melbourne explains: “Our search targets fifteen young supernova remnants containing young neutron stars. 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.”
OzGrav Associate Investigator Lilli Sun from the Australian National University says: “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. In our endeavor to search for continuous waves from these young neutron stars in our third observing run, we take into consideration, for the first time, the possibilities that the interior configuration and structure of the star can result in signals emitted at two different harmonics. Although no signal has been detected in O3, we set interesting constraints on the neutron star properties. If such a signal can be detected in future observations when the detectors are more sensitive, it will shed light on the fascinating structure of a neutron star.”
OzGrav postdoctoral researcher Carl Blair from the University of Western Australia says: “Gravitational waves are being used to probe the most exotic objects in the Universe. Neutron stars ━ composed of matter collapsed in on itself like a giant atomic nuclei ━ have to be one of the most exotic. We don’t know that much about neutron stars because they’re so small and strange. 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.”
In addition, recent studies announced by the international research team ━ including the U.S./international LIGO Scientific Collaboration, European Virgo Collaboration and Japanese KAGRA Collaboration ━ have focussed on pulsars. These are neutron stars which act as cosmic lighthouses, beaming out copious energy in the form of radio waves. Pulsars are like giant spinning magnets, except they’re billions of times stronger than the ones stuck to your fridge. So strong, in fact, that the magnetic field distorts the shape of the neutron star, and may lead to a tell-tale hum of continuous gravitational waves. While the recent studies did not pick up anything, they found tight constraints on how loud the “hum” could be, which, in some cases, are starting to challenge theoretical predictions.
OzGrav PhD student Deeksha Beniwal from the University of Adelaide says: “Gravitational-wave observation from O3 run of LIGO and Virgo detectors has allowed us to set realistic constraints on signals expected from young pulsars. O3 observations also provide an opportunity to test out different pipelines ━ such as different search methods for continuous wave signals ━ in realistic environments.”
OzGrav postdoctoral researcher Meg Millhouse from the University of Melbourne says: “Continuous gravitational waves from neutron stars are much smaller than the gravitational waves LIGO and Virgo have seen so far. 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.”
Scientists estimate that there are billions of neutron stars in the Milky Way with a faint murmur of continuous gravitational waves. Further studies have therefore taken an “ears wide open” approach, combing through the LIGO and Virgo data for any hint of a signal. The results so far suggest that these murmurings are extremely quiet and out of the detectors’ “ear” range. However, as detector technology becomes more advanced and sensitive, the first ever detection of continuous gravitational waves could soon become a reality.
Also featured on Cosmos Magazine, Space Australia, IFL Science and Sci Tech Daily
Gravitational-wave scientists propose new method to refine the Hubble Constant—the expansion and age of the Universe
A team of international scientists, led by the Galician Institute of High Energy Physics (IGFAE) and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), has proposed a simple and novel method to bring the accuracy of the Hubble constant measurements down to 2%, using a single observation of a pair of merging neutron stars.
The Universe is in continuous expansion. Because of this, distant objects such as galaxies move away from us. In fact, the further away they are, the faster they move. Scientists describe this expansion through a famous number known as the Hubble constant, which tells us how fast objects in the Universe recede from us depending on their distance to us. By measuring the Hubble constant in a precise way, we can also determine some of the most fundamental properties of the Universe, including its age.
For decades, scientists have measured Hubble’s constant with increasing accuracy, collecting electromagnetic signals emitted throughout the Universe but arriving at a challenging result: the two current best measurements give inconsistent results. Since 2015, scientists have tried to tackle this challenge with the science of gravitational waves: ripples in the fabric of space-time that travel at the speed of light. Gravitational waves are generated in the most violent cosmic events and provide a new channel of information about the Universe. They’re emitted during the collision of two neutron stars—the dense cores of collapsed stars–and can help scientists dig deeper into the Hubble constant mystery.
Unlike black holes, merging neutron stars produce both gravitational and electromagnetic waves, such as x-rays, radio waves and visible light. While gravitational waves can measure the distance between the neutron-star merger and Earth, electromagnetic waves can measure how fast its whole galaxy is moving away from Earth. This creates a new way to measure the Hubble constant. However, even with the help of gravitational waves, it’s still tricky to measure the distance to neutron-star mergers--that’s, in part, why current gravitational-wave based measurements of the Hubble constant have an uncertainty of ~16%, much larger than existing measurements using other traditional techniques.
In a recently published article in the prestigious journal The Astrophysical Journal Letters, a team of scientists led by ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and Monash University alumni Prof Juan Calderón Bustillo (now La Caixa Junior Leader and Marie Curie Fellow at the Galician institute of High Energy Physics of the University of Santiago de Compostela, Spain), has proposed a simple and novel method to bring the accuracy of these measurements down to 2% using a single observation of a pair of merging neutron stars.
According to Prof Calderón Bustillo, it’s difficult to interpret how far away these mergers occur because ‘currently, we can’t say if the binary is very far away and facing Earth, or if it’s much closer, with the Earth in its orbital plane’. To decide between these two scenarios, the team proposed to study secondary, much weaker components of the gravitational-wave signals emitted by neutron-star mergers, known as higher modes. ‘Just like an orchestra plays different instruments, neutron-star mergers emit gravitational waves through different modes,’ explains Prof Calderón Bustillo. ‘When the merging neutron stars are facing you, you will only hear the loudest instrument. However, if you are close to the merger’s orbital plane, you should also hear the secondary ones. This allows us to determine the inclination of the neutron-star merger, and better measure the distance’.
However, the method is not completely new: ‘We know this works well for the case of very massive black hole mergers because our current detectors can record the merger instant when the higher modes are most prominent. But in the case of neutron stars, the pitch of the merger signal is so high that our detectors can’t record it. We can only record the earlier orbits,’ says Prof Calderón Bustillo.
Future gravitational-wave detectors, like the proposed Australian project NEMO, will be able to access the actual merger stage of neutron stars. ‘When two neutron stars merge, the nuclear physics governing their matter can cause very rich signals that, if detected, could allow us to know exactly where the Earth sits with respect to the orbital plane of the merger,’ says co-author and OzGrav Chief Investigator Dr Paul Lasky, from Monash University. Dr Lasky is also one of the leads on the NEMO project. ‘A detector like NEMO could detect these rich signals,’ he adds.
In their study, the team performed computer simulations of neutron-star mergers that can reveal the effect of the nuclear physics of the stars on the gravitational waves. Studying these simulations, the team determined that a detector like NEMO could measure Hubble’s constant with a precision of 2%.
Co-author of the study Prof Tim Dietrich, from the University of Potsdam, says: ‘We found that fine details describing the way neutrons behave inside the star produce subtle signatures in the gravitational waves that can greatly help to determine the expansion rate of the Universe. It is fascinating to see how effects at the tiniest nuclear scale can infer what happens at the largest possible cosmological one’.
Samson Leong, undergraduate student at The Chinese University of Hong Kong and co-author of the study points out “one of the most exciting things about our result is that we obtained such a great improvement while considering a rather conservative scenario. While NEMO will indeed be sensitive to the emission of neutron-star mergers, more evolved detectors like Einstein Telescope or Cosmic Explorer will be even more sensitive, therefore allowing us to measure the expansion of the Universe with even better accuracy!”.
One of the most outstanding implications of this study is that it could determine if the Universe is expanding uniformly in space as currently hypothesised. 'Previous methods to achieve this level of accuracy rely on combining many observations, assuming that the Hubble constant is the same in all directions and throughout the history of the Universe,’ says Calderón Bustillo. ‘In our case, each individual event would yield a very accurate estimate of “its own Hubble constant”, allowing us to test if this is actually a constant or if it varies throughout space and time.’
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-kilometre-sized detectors using powerful lasers—haven’t heard anything yet.
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 recently published study, 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.
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 hypothesised 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.”
Scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) have described a way to determine the birth population of double neutron stars--some of the densest objects in the Universe formed in collapsing massive stars. The recently published study observed different life stages of these neutron star systems.
Scientists can observe the merging of double neutron star systems using gravitational waves--ripples in the fabric of space and time. By studying neutron star populations, scientists can learn more about how they formed and evolved. So far, there have been only two double neutron star systems detected by gravitational-wave detectors; however, many of them have been observed in radio astronomy.
One of the double neutron stars observed in gravitational wave signals, called GW190425, is far more massive than the ones in typical Galactic populations observed in radio astronomy, with a combined mass of 3.4 times that of our Sun. This raises the question: why is there a lack of these massive double neutron stars in radio astronomy? To find an answer, OzGrav PhD student Shanika Galaudage, from Monash University, investigated how to combine radio and gravitational-wave observations.
The birth, mid-life and death of double neutron stars
Radio and gravitational-wave astronomy enables scientists to study double neutron stars at different stages of their evolution. Radio observations probe the lives of double neutron stars, while gravitational waves study their final moments of life. To achieve a better understanding of these systems, from formation to merger, scientists need to study the connection between radio and gravitational wave populations: their birth populations.
Shanika and her team determined the birth mass distribution of double neutron stars using radio and gravitational-wave observations. “Both populations evolve from the birth populations of these systems, so if we look back in time when considering the radio and gravitational-wave populations we see today, we should be able to extract the birth distribution,” says Shanika Galaudage.
The key is to understand the delay-time distribution of double neutron stars: the time between the formation and merger of these systems. The researchers hypothesised that heavier double neutron star systems may be fast-merging systems, meaning that they’re merging too fast to be visible in radio observations and could only be seen in gravitational-waves.
GW190425 and the fast-merging channel
The study found mild support for a fast-merging channel, indicating that heavy double neutron star systems may not need a fast-merging scenario to explain the lack of observations in radio populations. “We find that GW190425 is not an outlier when compared to the broader population of double neutron stars,” says study co-author Christian Adamcewicz, from Monash University. “So, these systems may be rare, but they‘re not necessarily indicative of a separate fast-merging population.”
In future gravitational wave detections, researchers can expect to observe more double neutron star mergers. “If future detections reveal a stronger discrepancy between the radio and gravitational-wave populations, our model provides a natural explanation for why such massive double neutron stars are not common in radio populations,” adds co-author Dr Simon Stevenson, an OzGrav postdoctoral researcher at Swinburne University of Technology.
There is a growing interest in introducing quantum physics at an early age in schools because of its applications in emerging technologies, such as quantum computers. To make it accessible to school students, our recently published study presents a novel way of exploring basic quantum mechanical phenomena, such as matter-wave interference, diffraction, and reflection.
Our graphical approach, based on Feynman path integrals, offers insights into the quantum world in which observations represent quantum probability density. We combine tactile tools called phasor-wheels with real-life analogies and videos of single-quanta interference and employ elementary mathematics to teach these concepts.
Our approach uses practical, hands-on tools for teaching, making it appealing to students from high school (years 9 and 10) and above. The engaging material encouraged active participation and students found it easy to understand these abstract scientific concepts.
Written by By Rahul Choudhary – PhD student at UWA
General relativity, Einstein’s theory of gravity, is best tested at its most extreme--close to the event horizon of a black hole. This regime is accessible through observations of shadows of supermassive black holes and gravitational waves--ripples in the fabric of our Universe from colliding stellar-mass black holes. For the first time, scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), the Event Horizon Telescope (EHT) and the LIGO Scientific Collaboration, have outlined a consistent approach to exploring deviations from Einstein’s general theory of relativity in these two different observations. This research, published in Physical Review D, confirms that Einstein’s theory accurately describes current observations of black holes, from the smallest to the largest.
One of the hallmark predictions from general relativity is the existence of black holes.The theory provides a specific description of a black hole’s effect on the fabric of space-time: a four-dimensional mesh which encodes how objects move through space and time. Known as the Kerr metric, this prediction can be related to the bending of light around a black hole, or the orbital motion of binary black holes. In this study, the deviations from the Kerr metric were linked to features in these black hole observations.
In 2019, the Event Horizon Telescope generated silhouette images of the black hole at the centre of the galaxy M87, with a mass several billion times that of our Sun. The angular size of the shadow is related to the mass of the black hole, its distance from Earth and possible deviations from general relativity’s prediction. These deviations can be calculated from the scientific data, including previous measurements of the black hole’s mass and distance.
Meanwhile, since 2015 the LIGO and Virgo gravitational-wave observatories have been detecting gravitational waves from merging stellar mass black holes. By measuring the gravitational waves from the colliding black holes, scientists can explore the mysterious nature and metrics of the black holes. This study focussed on deviations from general relativity that appear as slight changes to the pitch and intensity of the gravitational waves, before the two black holes collide and merge.
Combining the measurements of the shadow of the supermassive black hole in M87 and gravitational waves from a couple of binary black hole detections, called GW170608 and GW190924, the researchers found no evidence for deviations from general relativity. Co-author of the study and OzGrav research assistant Ethan Payne (Australian National University) explained that the two measurements provided similar, consistent constraints. “Different sizes of black holes may help break the complementary behaviour seen here between EHT and LIGO/Virgo observations,” said Payne. “This study lays the groundwork for future measurements of deviations from the Kerr metric.”
Written by OzGrav research assistant Ethan Payne, the Australian National University.
A new black hole breaks the record––not for being the smallest or the biggest––but for being right in the middle.
The recently discovered ‘Goldilocks’ black hole is part of a missing link between two populations of black holes: small black holes made from stars and supermassive giants in the nucleus of most galaxies.
In a joint effort, researchers from the University of Melbourne and Monash University––including OzGrav Chief Investigator Eric Thrane––have uncovered a black hole approximately 55,000 times the mass of the sun, a fabled “intermediate-mass” black hole.
The discovery was published today in the paper Evidence for an intermediate mass black hole from a gravitationally lensed gamma-ray burst in the journal Nature Astronomy.
Lead author and University of Melbourne PhD student, James Paynter, said the latest discovery sheds new light on how supermassive black holes form. “While we know that these supermassive black holes lurk in the cores of most, if not all galaxies, we don’t understand how these behemoths are able to grow so large within the age of the Universe,” he said.
The new black hole was found through the detection of a gravitationally lensed gamma-ray burst.
The gamma-ray burst, a half-second flash of high-energy light emitted by a pair of merging stars, was observed to have a tell-tale ‘echo’. This echo is caused by the intervening intermediate-mass black hole, which bends the path of the light on its way to Earth, so that astronomers see the same flash twice.
Powerful software developed to detect black holes from gravitational waves was adapted to establish that the two flashes are images of the same object.
“This newly discovered black hole could be an ancient relic––a primordial black hole––created in the early Universe before the first stars and galaxies formed,” said study co-author Eric Thrane.
“These early black holes may be the seeds of the supermassive black holes that live in the hearts of galaxies today.”
The researchers estimate that some 46,000 intermediate mass black holes are in the vicinity of our Milky Way galaxy.
This article is an edited version of the original media release produced by Lito Vilisoni Wilson at the University of Melbourne.
Also featured in CNet , Cosmos magazine, New Scientist, SciTech Daily, The Independent, Sky News and Space.com
There are some stars that just don’t look at all like stars. Rather than being composed nearly entirely of hydrogen and helium like other stars, they consist of matter that has no electromagnetic signature. These stars are hypothetical because we’re still refining the techniques to find them.
Boson stars are just one in this family of objects, known as exotic stars, and they’re composed almost entirely of bosons. And what is a boson? It’s one of the two types of fundamental particles, the one that carries forces. The other, the fermion, is what makes up ‘normal’ stars, and all the other matter that we see.
There are a variety of boson stars and sometimes they’re categorised to reflect the type of boson that they are made of. For example, Proca stars are vector boson stars, meaning that their constituent bosons have a spin of one. They’re also unique amongst boson stars because the stars themselves can spin without being disrupted.
A boson star would most likely be shaped like an enormous donut because of the centrifugal forces acting on the bosonic matter, and, bizarrely, they’d be transparent; any matter absorbed by them would be visible at their centres. If boson stars do exist, they might provide the evidence we need for a long sought-after dark matter particle. That’s because the said particle, the axion, is a boson. And we’ve been searching (unsuccessfully) for axions in numerous experiments on Earth for decades.
After the initial excitement of the first-ever observation of an intermediate-mass black hole—gravitational-wave event GW190521—it was quickly realised that the very existence of such an object was not consistent with any of our stellar models. Perhaps it was itself a product of previous, smaller, black hole collisions, or maybe there was something else at play.
Thus, the challenge for scientists was to come up with a theory that could explain the presence of the intermediate-mass black hole progenitor of GW190521, while still being consistent with the original signal. And by assuming that it was caused by merging boson stars, rather than black holes, an international team of scientists, led by OzGrav alumnus Dr Juan Calderón Bustillo at the University of Santiago de Compostela and Dr Nicolás Sanchis-Gual at the University of Lisbon, might have been able to do just that.
Apart from the problems associated with the pair-instability mass gap, any potential hypothesis needed to explain something a bit unusual about the GW190521 signal. Normally gravitational waves that originate in merging binary systems oscillate at higher and higher frequencies as the two progenitors spiral in towards each other. But for GW190521, the inspiral signal before the merger was barely detectable. An extremely abbreviated inspiral could perhaps be explained if two black holes collided head-on rather than by circling into each other, and so that is the first thing that Dr Bustillo and Dr Sanchis-Gual’s team looked at. What they found didn’t help much.
‘We first tried to fit the data to head-on collisions of black holes, but these happen to produce a final black hole whose spin is too low to reproduce the GW190521 signal. The reason is that the lack of an inspiral diminishes a lot of the spin of the final black hole, and the individual spins of the black holes, which also contribute to the spin of the final one, are bounded by a limit called the Kerr limit,’ says Dr Bustillo.
That’s when the team started looking at boson stars, or Proca stars to be exact. They compared the GW190521 signal to computer simulations of Proca star mergers and found that statistically they were a considerably better fit to the data than when it was assumed that the progenitors were black holes.
‘First, we would not be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole,’ explains Dr Bustillo. ‘Second, because boson star mergers are much weaker, we infer a much closer distance than the one estimated by LIGO and Virgo. This leads to a much larger mass for the final black hole, of about 250 solar masses, so the fact that we have witnessed the formation of an intermediate-mass black hole remains true.’
This is an exciting result as the final black hole formed by the merger in this case would have to be about 62% larger than previously thought. And rather than the signal originating from a point that is now some 17-billion light-years from us, it would have been just over 1.8-billion light-years away.
‘Of course, there are potentially many ways in which this event may be explained, as this is an event for which we have very little information about what produced the final black hole we observe. The best we can say right now is that the data tells us that a collision of Proca stars is approximately 8 times more likely than the black hole collision scenario.’
And what of the implications of discovering the first boson stars?
‘That would be dramatic,’ says Dr Bustillo. ‘Boson stars and their building blocks—the ultralight bosons—are one of the most solid candidates for forming what we know as dark matter. If our result is further confirmed by future observations, it would represent the first actual measurement of the particle responsible for dark matter.’
‘Gravitational-wave astronomy is still very much in its infancy,’ says Dr Rory Smith—an OzGrav researcher from Monash University and one of the collaborators in this research. ‘However, the fact that we’re already starting to draw connections between gravitational-wave observations and fundamental particle physics is a remarkable sign of how powerful this new field is. Even if future observations rule out boson stars as real astronomical objects, we should expect many new and exciting discoveries in the future.’
This is an edited extract from the original article featured in Space Times, written by Dan Lambeth.