​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 

​‘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

​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.