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.
RESEARCH BRIEF: Simulating the complicated history of Eta Carinae - Ryosuke Hirai, Monash University
‘Eta Carinae’ is an extraordinary star that has fascinated mankind for decades. It’s surrounded by an expanding ‘Homunculus nebula’, shaped like an hourglass. This nebula was expelled in an energetic explosion called the ‘Great Eruption’ that occurred in the 1840s, when Eta Carinae became the second brightest star in the sky and was visible to the naked eye for over a decade.
There are other clusters of bullets outside the Homunculus nebula, that were shot out several centuries before the Great Eruption. Eta Carinae itself is extremely massive with a mass more than 100 times the Sun and is orbited by another star that has a mass about 30 times the Sun on a highly eccentric orbit.
With all these and many more peculiar features, scientists have been puzzled for a long time on how the star exploded and created the surrounding messy nebula.
Out of many other proposed models, our recently published study focused on one hypothesis that the star system used to be a triple system that eventually became unstable and caused a stellar merger. As more detailed observations are made, this scenario is becoming increasingly popular but has lacked detailed theoretical investigations so far.
In this work we performed the first comprehensive set of detailed theoretical calculations for this scenario. We first carried out three-body dynamical simulations to see how a triple system becomes unstable and eventually two of the stars collide. We started with a stable system in which one star is in a wide orbit around the other two stars which are in close orbit. Once the most massive star approaches the end of its life, it expands and starts transferring matter to its companion. This makes the system unstable and causes two of the stars to merge within a few thousand years. We found that before the final merger, the stars can wildly swap their positions and encounter each other at close distances, grazing each other’s surfaces.
We carried out additional N-body simulations to see how a star responds to these close encounters. Part of the surface material can be peeled off and sent away as narrow sprays. Combining the orbital dynamics and close-encounter simulations, we found that the multiple grazing encounters—that occur centuries before the merger—can reproduce the messy structure outside the Homunculus nebula.
We also carried out hydrodynamical simulations to see how the outflow from the stellar merger is shaped into the hourglass shape we see today. We proposed a new scenario that takes similar ideas for how the triple-ring nebula for the supernova SN1987A was formed. As the stars merge, a huge amount of energy is released inside the star, causing the Great Eruption. But unlike supernovae, a large fraction of the energy and mass remains in the star. This energy slowly leaks out over the following century as strong bipolar winds. The wind sweeps up the inner parts of the explosion ejecta and forms a hollow shell-like structure. Our simulations show that with this scenario, we can closely reproduce the shape and size of the Homunculus nebula.
Our combination of simulations successfully reproduces the main features of Eta Carinae’s surrounding nebula and provides strong support to the stellar-merger-in-a-triple scenario. This not only gives us insight into the origin of Eta Carinae, but also many other astronomical objects that can be created through mergers in triple systems. For example, some massive black holes found by LIGO (GW190521) are considered to have been created this way. Using the rich information from Eta Carinae, we can learn much more about the formation of exotica in our Universe.
Written by OzGrav researcher Ryosuke Hirai, Monash University
An artist’s impression of the Cygnus X-1 system. This system contains the most massive stellar-mass black hole ever detected without the use of gravitational waves, weighing in at 21 times the mass of the Sun. Credit: International Centre for Radio Astronomy Research. Credit: International Centre for Radio Astronomy Research.
New observations of the first black hole ever detected have led astronomers to question what they know about the Universe’s most mysterious objects. Published in the journal Science, the research shows the system known as Cygnus X-1 contains the most massive stellar-mass black hole ever detected without the use of gravitational waves.
Cygnus X-1 is one of the closest black holes to Earth. It was discovered in 1964 when a pair of Geiger counters were carried on board a sub-orbital rocket launched from New Mexico. The object was the focus of a famous scientific wager between physicists Stephen Hawking and Kip Thorne, with Hawking betting in 1974 that it was not a black hole. Hawking conceded the bet in 1990. In this latest work, an international team of astronomers used the Very Long Baseline Array—a continent-sized radio telescope made up of 10 dishes spread across the United States—together with a clever technique to measure distances in space.
OzGrav Chief Investigator and study co-author Prof Ilya Mandel, from Monash University, says the black hole is so massive it’s actually challenging how astronomers thought they formed. ‘Stars lose mass to their surrounding environment through stellar winds that blow away from their surface. But to make a black hole this heavy, we need to dial down the amount of mass that bright stars lose during their lifetimes,’ says Prof Mandel. ‘The black hole in the Cygnus X-1 system began life as a star approximately 60 times the mass of the Sun and collapsed tens of thousands of years ago,’ he says. ‘Incredibly, it’s orbiting its companion star—a supergiant—every five and a half days at just one-fifth of the distance between the Earth and the Sun. These new observations tell us the black hole is more than 20 times the mass of our Sun—a 50 per cent increase on previous estimates.’
Second study author Dr Arash Bahramian from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) says this was an exciting discovery, resulting from a collaboration between astronomers focused on different observational and theoretical aspects of black holes, coming together for a new extensive and rigorous look at a known but previously elusive black hole. ‘It is exciting that we can measure so precisely so many aspects of the system, like its distance from us, its motion and speed through the Galaxy, and the binary motion of the black hole and the star around each other,’ says Dr Bahramian. ‘Our new distance estimate caused an interesting domino effect, leading us to new measurements for the mass and spin of the black hole, which in turn led to fascinating new insights about how stars evolve and how black holes form.’
Lead researcher James Miller-Jones also from ICRAR says over six days the researchers observed a full orbit of the black hole and used observations taken of the same system with the same telescope array in 2011. ‘This method and our new measurements show the system is further away than previously thought, with a black hole that’s significantly more massive,’ says Prof Miller-Jones.
In a separate but related development University of Birmingham PhD candidate Coenraad Neijssel, affiliated with OzGrav and Monash, led a companion paper to this work simultaneously published in the Astrophysical Journal. ‘Using the updated measurements of the system properties, we were able to unwind the previous history of the binary as well as predict its future,’ says Coenraad. ‘Precise observations like this are critical for improving our understanding of the evolution of massive stars.
This article is an edited of the original media release written by Silvia Dropulich at Monash University Media Office. Also featured in the New York Times and The Daily Mail.
A new technology that can improve gravitational-wave detectors, one of the most sensitive instruments used by scientific researchers, has been pioneered by physicists at The University of Western Australia in collaboration with an international team of researchers.
The new technology allows the world’s existing gravitational wave detectors to achieve a sensitivity that was previously thought only to be achievable by building much bigger detectors.
The paper, published today in Communications Physics, was led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at UWA, in collaboration with the ARC Centre of Excellence for Engineered Quantum Systems, the Niels Bohr Institute in Copenhagen and the California Institute of Technology in Pasadena.
Emeritus Professor David Blair, from UWA’s Department of Physics, said the technology merged quantum particles of sound vibration called phonons with photons of laser light, to create a new type of amplification in which the merged particles cycled back and forth billions of times without being lost.
“More than a hundred years ago Einstein proved that light comes as little energy packets, which we now call photons,” Emeritus Professor Blair said.
One of the most sophisticated applications of photons are gravitational-wave detectors, which allow physicists to observe ripples in space and time caused by cosmic collisions.
“Two years after Einstein's prediction of photons, he proposed that heat and sound also come in energy packets, which we now call phonons,” Emeritus Professor Blair said.
“Phonons are much trickier to harness individually in their quantum form because they’re usually swamped by vast numbers of random phonons called thermal background.”
Emeritus Professor Blair was awarded the prestigious Prime Minister’s Prize for Science in 2020 for his contribution to the first detection of gravitational waves.
Lead author Dr Michael Page said the trick was to combine phonons and photons together in such a way that a broad range of gravitational wave frequencies could be amplified simultaneously.
“The new breakthrough will let physicists observe the most extreme and concentrated matter in the known universe as it collapses into a black hole, which happens when two neutron stars collide,” Dr Page said.
Emeritus Professor David Blair said the waveforms sounded like a brief scream that was pitched too high for current detectors to hear.
“Our technology will make those waveforms audible, and will also reveal whether the neutrons in neutron stars get split up into their constituents called quarks when they are in this extreme state” Emeritus Professor Blair said.
“The most exciting thing about seeing nuclear matter turn into a black hole is that the process is like the reverse of the Big Bang that created the universe. Observing this happen will be like watching a movie of the Big Bang played backwards.”
Emeritus Professor Blair said while the technology did not represent an instant solution to improving gravitational-wave detectors it offers a low-cost route to improvement.
As featured on the UWA news website.
Astronomers have for the first time used distant galaxies as ‘scintillating pins’ to locate and identify a piece of the Milky Way’s missing matter.
For decades, scientists have been puzzled as to why they couldn’t account for all the matter in the universe as predicted by theory. While most of the universe’s mass is thought to be mysterious dark matter and dark energy, 5 percent is ‘normal matter’ that makes up stars, planets, asteroids, peanut butter and butterflies. This is known as baryonic matter.
However, direct measurement has only accounted for about half the expected baryonic matter.
Yuanming Wang, a doctoral candidate in the School of Physics at the University of Sydney, has developed an ingenious method to help track down the missing matter. She has applied her technique to pinpoint a hitherto undetected stream of cold gas in the Milky Way about 10 light years from Earth. The cloud is about a trillion kilometres long and 10 billion kilometres wide but only weighing about the mass of our Moon.
The results, published in the Monthly Notices of the Royal Astronomical Society, offer a promising way for scientists to track down the Milky Way’s missing matter.
“We suspect that much of the ‘missing’ baryonic matter is in the form of cold gas clouds either in galaxies or between galaxies,” said Ms Wang, who is pursuing her PhD at the Sydney Institute for Astronomy.
“This gas is undetectable using conventional methods, as it emits no visible light of its own and is just too cold for detection via radio astronomy,” she said.
What the astronomers did is look for radio sources in the distant background to see how they ‘shimmered’.
“We found five twinkling radio sources on a giant line in the sky. Their signals show their light must have passed through the same cold clump of gas,” Ms Wang said.
Just as visible light is distorted as it passes through our atmosphere to give stars their twinkle, when radio waves pass through matter, it also affects their brightness. It was this ‘scintillation’ that Ms Wang and her colleagues detected.
Dr Artem Tuntsov, a co-author from Manly Astrophysics, said: “We aren’t quite sure what the strange cloud is, but one possibility is that it could be a hydrogen ‘snow cloud’ disrupted by a nearby star to form a long, thin clump of gas.”
Hydrogen freezes at about minus 260 degrees and theorists have proposed that some of the universe’s missing baryonic matter could be locked up in these hydrogen ‘snow clouds’. They are almost impossible to detect directly.
“However, we have now developed a method to identify such clumps of ‘invisible’ cold gas using background galaxies as pins,” Ms Wang said.
Ms Wang’s supervisor, Professor Tara Murphy, said: “This is a brilliant result for a young astronomer. We hope the methods trailblazed by Yuanming will allow us to detect more missing matter.”
The data to find the gas cloud was taken using the CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope in Western Australia.
Dr Keith Bannister, Principal Research Engineer at CSIRO, said: “It is ASKAP’s wide field of view, seeing tens of thousands of galaxies in a single observation that allowed us to measure the shape of the gas cloud.”
Professor Murphy said: “This is the first time that multiple ‘scintillators’ have been detected behind the same cloud of cold gas. In the next few years, we should be able to use similar methods with ASKAP to detect a large number of such gas structures in our galaxy.”
The research was done in collaboration with CSIRO, Manly Astrophysics, the University of Wisconsin-Milwaukee and the ARC Centre of Excellence for Gravitational Wave Discovery, OzGrav.
Media release written and edited by The University of Sydney media office.
Congratulations to OzGrav researchers Adam Deller, Ryan Shannon, Cherie Day, Stefan Oslowski, Chris Flynn, Wael Farah, on receiving the Newcomb Cleveland prize from the AAAS. The award is for the best paper published in Science Magazine in the last year and was awarded for their paper that presented the discovery of the first localised one-off FRB: 'A single fast radio burst localized to a massive galaxy at cosmological distance'.
Full media release by CSIRO here: https://www.csiro.au/en/News/News-releases/2021/In-the-blink-of-an-eye-astronomers-win-prestigious-American-science-prize
A recent study by an international team of scientists—led by the Galician Institute of High Energy Physics, the University of Aveiro, and including OzGrav researchers—shows that the “heaviest black hole collision” ever observed might be something even more mysterious—dark matter.
Gravitational waves are ripples in the fabric of space-time that travel at the speed of light. Predicted in Einstein’s General Theory of Relativity, they originate in the most violent events of our Universe, carrying information about their sources. Since 2015, humankind can observe and interpret gravitational waves thanks to the two Advanced LIGO detectors (Livingston and Hanford, USA) and the Advanced Virgo detector (Cascina, Italy). To date, these detectors have already observed around 50 gravitational-wave signals which originated in the coalescence and merger of two of the most mysterious entities in the Universe—black holes and neutron stars—deepening our knowledge of the Universe.
Gravitational wave astronomy could eventually provide us with evidence for previously unobserved or unexpected objects and shed light on current open issues, like the nature of dark matter—a discovery that may have already happened.
In September 2020, the LIGO and Virgo collaborations (LVC) announced the gravitational-wave signal called GW190521. The signal was consistent with the collision of two black holes of 85 and 66 times the mass of the Sun, which produced a final 142 solar mass black hole—this was the first-ever detected intermediate-mass black hole. This discovery was extremely important as intermediate black holes were long considered the missing link between two well-known black-hole families: the stellar-mass black holes, that form from the collapse of stars, and the supermassive black holes, that hide in the centre of almost every galaxy.
Despite its significance, the observation of GW190521 posed an enormous challenge to scientists’ understanding of stellar evolution: the life and death of stars is significantly more massive than our Sun. If this is correct, the heaviest of the two colliding black holes shouldn’t have occurred as the end-result of the gravitational collapse of a massive star.
In an article recently published in Physical Review Letters, a team of scientists lead by OzGrav alumnus Dr Juan Calderón Bustillo, (now “La Caixa Junior Leader - Marie Curie Fellow”, at the Galician Institute of High Energy Physics) and Dr Nicolás Sanchis-Gual, a postdoctoral researcher at the University of Aveiro and at the Instituto Superior Técnico (University of Lisbon), together with OzGrav researchers from Monash University Dr Rory Smith and Avi Vajpeyi, and collaborators from the University of Valencia and The Chinese University of Hong Kong, has proposed an alternative explanation for the origin of the signal GW190521: the collision of two exotic compact objects known as boson stars. Such hypothetical stars are among the simplest exotic compact objects proposed, and present as well-founded dark matter candidates. Within this interpretation, the team estimated the mass of a new particle constituent of these stars: an ultra-light boson with a mass billionths of times smaller than that of the electron.
Dr Nicolás Sanchis-Gual, explains: “Boson stars are objects almost as compact as black holes, but they don’t have a ‘no-return’ surface, or event horizon. When they collide, they form a boson star that can become unstable, eventually collapsing to a black hole, and producing a signal consistent with what LIGO and Virgo observed. Unlike regular stars, which are made of what we commonly know as matter, boson stars are made up of what we know as ultralight bosons. These bosons are one of the most appealing candidates for constituting dark matter, which forms ~27% of the Universe.”
The team compared the GW190521 signal to computer simulations of boson-star mergers and found that these explain the data slightly better than the analysis conducted by LIGO and Virgo. The result implies that the source would have different properties than stated earlier. Dr Calderón Bustillo explains: “First, we wouldn’t be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole. 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”.
The team found that even though the analysis tends to favour “by design” the merging black-holes hypothesis, a boson star merger is actually preferred by the data, although in a non-conclusive way. Professor José A. Font from the University of Valencia says: “Our results show that the two scenarios are almost indistinguishable given the data, although the exotic boson-star one is slightly preferred. This is very exciting since the computational framework of our current boson-star simulations is still fairly limited and subject to major improvements. A more evolved model might lead to even larger evidence for the boson-star scenario and would also allow us to study similar gravitational-wave observations under the boson-star merger assumption”.
This result would not only involve the first observation of boson stars, but also that of their building block, a new particle known as ultra-light boson. Such ultra-light bosons have been proposed as the constituents of what we know as dark matter, which makes up around 27% of the observable Universe. Professor Carlos Herdeiro, from University of Aveiro says that “one of the most fascinating results is that we can actually measure the mass of this putative new dark-matter particle, and that a value of zero is discarded with high confidence. If confirmed by subsequent analysis of this and other gravitational-wave observations, our result would provide the first observational evidence for a long-sought dark matter candidate”.
OzGrav researcher Dr Rory Smith adds: “Gravitational-wave astronomy is still very much in its infancy. However, the fact that we are already able to start drawing 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”.
Written by Dr Juan Calderón Bustillo. Also featured on Space Australia.
In the moments immediately following the Big Bang, the very first gravitational waves rang out. The product of quantum fluctuations in the new soup of primordial matter, these earliest ripples through the fabric of space-time were quickly amplified by inflationary processes that drove the universe to explosively expand.
Primordial gravitational waves, produced nearly 13.8 billion years ago, still echo through the Universe today. But they are drowned out by the crackle of gravitational waves produced by more recent events, such as colliding black holes and neutron stars.
Now a team of international scientists, including reasearchers from the Massachusetts Institute of Technology and OzGrav, has developed a method to tease out the very faint signals of primordial ripples from gravitational-wave data. Their results were published in Physical Review Letters.
Gravitational waves are being detected on an almost daily basis by LIGO and other gravitational-wave detectors, but primordial gravitational signals are several orders of magnitude fainter than what these detectors can register. It’s expected that the next generation of detectors will be sensitive enough to pick up these earliest ripples.
In the next decade, as more sensitive instruments come online, the new method could be applied to dig up hidden signals of the Universe’s first gravitational waves. The pattern and properties of these primordial waves could then reveal clues about the early universe, such as the conditions that drove inflation.
‘If the strength of the primordial signal is within the range of what next-generation detectors can detect, which it might be, then it would be a matter of more or less just turning the crank on the data, using this method we’ve developed,’ says Sylvia Biscoveanu—MIT graduate student and the study’s lead author. ‘These primordial gravitational waves can then tell us about processes in the early Universe that are otherwise impossible to probe.’ OzGrav researchers Colm Talbot, Eric Thrane and Rory Smith were also co-authors of the study.
The hunt for primordial gravitational waves has concentrated mainly on the cosmic microwave background, or CMB, which is thought to be radiation that is leftover from the Big Bang. Scientists believe that when primordial gravitational waves rippled out, they left an imprint on the CMB, in the form of B-modes, a type of subtle polarization pattern.Physicists have looked for signs of B-modes, most famously with the BICEP Array, a series of experiments including BICEP2, which in 2014 scientists believed had detected B-modes; however, the signal turned out to be due to galactic dust.
As scientists continue to look for primordial gravitational waves in the CMB, others are hunting the ripples directly in gravitational-wave data. The general idea has been to try and subtract away the ‘astrophysical foreground’—any gravitational-wave signal that arises from an astrophysical source, such as colliding black holes, neutron stars, and exploding supernovae. Only after subtracting this astrophysical foreground can physicists get an estimate of the quieter, nonastrophysical signals that may contain primordial waves.
The problem with these methods, Biscoveanu says, is that the astrophysical foreground contains weaker signals that are too faint to discern and difficult to estimate in the final subtraction.
In their study, the researchers used a predictive model to describe the more obvious ‘conversations’ of the astrophysical foreground. The team used this more accurate model to create simulated data of gravitational wave patterns and then characterize every astrophysical signal. Once they identified distinct, nonrandom patterns in gravitational-wave data, they were left with more random primordial gravitational-wave signals and instrumental noise specific to each detector. Applying their new methods, the team was able to fit both the foreground and the background at the same time, so the background signal didn’t get contaminated by the residual foreground.
Biscoveanu says she hopes that once more sensitive, next-generation detectors come online, the new method can be used to cross-correlate and analyse data from two different detectors, to sift out the primordial signal. Then, scientists may have a useful thread they can trace back to the conditions of the early Universe.
This article is an edited extract from the original article featured on MIT’s news website written by Jennifer Chu.