Astronomers regularly observe gravitational waves (GW)—ripples in space and time—that are caused by pairs of black holes merging into one. Einstein’s theory of gravity predicts that GW, which squeeze and stretch space as they pass, will permanently distort space, leaving a ‘memory’ of the wave behind. However, this memory effect has not yet been detected as it’s extremely small, leaving the faintest traces.

Researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University have finally developed a method to search and detect GW memory. Led by OzGrav PhD student Moritz Huebner, the recently published paper explains the tricky conquest of searching for memory by analysing data from numerous different observations. Huebner will be presenting these results at the Australian National Institute for Theoretical Astrophysics (ANITA) in Canberra this Thursday 6 February 2020.
 
The scientific models expect memory to leave an extremely faint trace on the detectors which is far smaller than the waves from the black hole collision itself.  Therefore, data needs to be combined from many different gravitational wave events. To do this, the team used some of the most precise GW and memory models developed from the study of black hole mergers.
 
‘Our algorithms carefully comb through the data and measure the exact evidence for the existence of GW memory,’ said Huebner.
 
For each individual observation, this painstaking method can take hundreds of hours on a normal computer chip to explore all the possibilities of how a GW signal came about—this prompted the researchers to focus on fine-tuning the setting to reduce the amount of computing hours without compromising the search. So far, the results of the search applied to the first ten black-hole collisions—detected by LIGO and Virgo between 2015 and 2017—have proven inconclusive. LIGO and Virgo are not yet sensitive enough to make any statements about GW memory.


So, will we ever be able to detect memory?
 
‘Thankfully, we can now use data from the first ten black-hole collisions and have a decent idea of how many observable GW events there will be in the future. We can also calculate how much evidence of memory can be detected in each event,’ said Huebner.
 
Throughout the study, the researchers also discovered that their new search method must take data from approximately 2000 black hole mergers to detect memory. While this might sound implausible, the team expects to hit this number by the mid-2020s.
 
Plus, LIGO and Virgo are continuously being upgraded and have seen more than 40 mergers since April 2019, when the third observation run started. With further technological advances and the Japanese KAGRA observatory soon coming online, the team is confident that they’ll detect multiple binaries every day which will finally lead to revealing GW memory.
 
Link to journal: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.101.023011
​

Scientists from the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav) reveal the eccentricity of binary black holes: the shape of the orbit formed when two black holes fall into a dance as they spiral towards each other and eventually collide. While the most common orbit is thought to be circular, about one in 20 are in egg-shaped eccentric orbits, which can indicate completely different binary life histories.

Since the first detection of gravitational waves (GW) in September 2015, LIGO and its European counterpart Virgo have published the discovery of ten merging black-hole binaries. The latest run has already uncovered more than 30 new detections, with more forecast by April 2020.

OzGrav PhD student (and first author) Isobel Romero-Shaw recently published a study on the origins of GW 190425 – an event which was only announced this month (January 2020) by the LIGO/Virgo collaboration.

The GW signals provide a wealth of information about the pre-merger binaries; however, no one has yet deciphered how these black holes pair up in the first place.

New research,  published in the journal Monthly Notices of the Royal Astronomical Society, reveals an important clue to how these black hole binaries are formed, how long they’ve been ‘together’ and what happens when they finally collide.

The study, led by Romero-Shaw, OzGrav Chief Investigator Eric Thrane and Associate Investigator Paul Lasky—all from Monash University—looked at data from the first and second rounds of observation of LIGO and Virgo, in particular, the ten black hole collisions that these two observation runs confirmed. They found that the orbits of all ten of these systems were remarkably circular, which is consistent with the expectation that about one in 20 orbits are not.

The current LIGO/Virgo run has already detected more than 30 additional collision signals. According to Romero-Shaw, the large amount of data coming from the third observing run ‘will mean we are much more likely to see eccentric collisions of black holes, which will give us real insight into how these systems form’.

According to Thrane, the more common circular orbits come from black holes who have been together from when they were garden-variety stars before they exploded and became black holes. Thrane explains: ‘These binaries are like siblings if you like. They grew up together and their orbit is circular’.

Eccentric orbits occur when black holes fall under each other’s gravitational influence by chance as they are zipping around galaxies. ‘These are more like adults who meet later in life and pair up.  Their orbital relationship is more interesting -- much like in life,’ he added.

Importantly, when these two objects collide, the shape of their orbit means their gravitational-wave signal looks different. These detected explosions can now be used to retrospectively study the objects that collided.

Lasky said that the current LIGO and Virgo observing run is detecting ‘large numbers of these binaries and by April 2020—when the run finishes—we will have a far greater insight into what these events mean’.
 
VIDEO EXPLAINER: https://www.youtube.com/watch?v=LmW4Hd4sJvg
Credit: Johan Samsing

​More than a century ago, Albert Einstein published his iconic theory of General Relativity – that the force of gravity arises from the curvature of space and time and that objects, such as the Sun and the Earth, change this geometry. Advances in instrumentation have led to a flood of recent (Nobel prize-winning) science from phenomena further afield linked to General Relativity. The discovery of gravitational waves was announced in 2016; the first image of a black hole shadow and stars orbiting the supermassive black hole at the centre of our own galaxy was published just last year.
 
Almost twenty years ago, a team led by Swinburne University of Technology’s Professor Bailes—director of the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav)—started observing two stars rotating around each other at astonishing speeds with the CSIRO Parkes 64-metre radio telescope. One is a white dwarf, the size of the Earth but 300,000 times its density; the other is a neutron star which, while only 20 kilometres in diameter, is about 100 billion times the density of the Earth. The system, which was discovered at Parkes, is a relativistic-wonder system that goes by the name ‘PSR J1141-6545’.
 
Before the star blew up (becoming a neutron star), a million or so years ago, it began to swell up discarding its outer core which fell onto the white dwarf nearby. This falling debris made the white dwarf spin faster and faster, until its day was only measured in terms of minutes.

In 1918 (three years after Einstein published his Theory), Austrian mathematicians Josef Lense and Hans Thirring realised that if Einstein was right all rotating bodies should ‘drag’ the very fabric of space time around with them. In everyday life, the effect is miniscule and almost undetectable. Earlier this century, the first experimental evidence for this effect was seen in gyroscopes orbiting the Earth, whose orientation was dragged in the direction of the Earth’s spin. A rapidly spinning white dwarf, like the one in PSR J1141-6545, drags space-time 100 million times as strongly! 
 
A pulsar in orbit around such a white dwarf presents a unique opportunity to explore Einstein’s theory in a new ultra-relativistic regime.
 
Lead author of the current study, Dr Vivek Venkatraman Krishnan (from Max Planck Institute for Radio Astronomy - MPIfR) was given the unenviable task of untangling all of the competing relativistic effects at play in the system as part of his PhD at Swinburne University of Technology. He noticed that unless he allowed for a gradual change in the orientation of the plane of the orbit, General Relativity made no sense.
 
MPIfR’s Dr Paulo Friere realised that frame-dragging of the entire orbit could explain their tilting orbit and the team presents compelling evidence in support of this in today’s journal article—it shows that General Relativity is alive and well, exhibiting yet another of its many predictions.
 
The result is especially pleasing for team members Bailes, Willem van Straten (Auckland University of Tech) and Ramesh Bhat (ICRAR-Curtin) who have been trekking out to the Parkes 64m telescope since the early 2000s, patiently mapping the orbit with the ultimate aim of studying Einstein’s Universe. ‘This makes all the late nights and early mornings worthwhile’, said Bhat.

Expert commentary:
Lead author Vivek Venkatraman Krishnan, Max Planck Institute for Radio Astronomy (MPIfR):
‘At first, the stellar pair appeared to exhibit many of the classic effects that Einstein’s theory predicted. We then noticed a gradual change in the orientation of the plane of the orbit.’

‘Pulsars are cosmic clocks. Their high rotational stability means that any deviations to the expected arrival time of its pulses is probably due to the pulsar’s motion or due to the electrons and magnetic fields that the pulses encounter.’

‘Pulsar timing is a powerful technique where we use atomic clocks at radio telescopes to estimate the arrival time of the pulses from the pulsar to very high precision. The motion of the pulsar in its orbit modulates the arrival time, thereby enabling its measurement.’

Dr Paulo Freire:
‘We postulated that this might be, at least in-part, due to the so-called “frame-dragging” that all matter is subject to in the presence of a rotating body as predicted by the Austrian mathematicians Lense and Thirring in 1918.’

Professor Thomas Tauris, Aarhus University:  
‘In a stellar pair, the first star to collapse is often rapidly rotating due to subsequent mass transfer from its companion. Tauris’s simulations helped quantify the magnitude of the white dwarf’s spin. In this system the entire orbit is being dragged around by the white dwarf’s spin, which is misaligned with the orbit.’

Dr Norbert Wex, Max Planck Institute for Radio Astronomy (MPIfR):
‘One of the first confirmations of frame-dragging used four gyroscopes in a satellite in orbit around the Earth, but in our system the effects are 100 million times stronger.’

Evan Keane (SKA Organisation):
‘Pulsars are super clocks in space. Super clocks in strong gravitational fields are Einstein’s dream laboratories. We have been studying one of the most unusual of these in this binary star system. Treating the periodic pulses of light from the pulsar like the ticks of a clock we can see and disentangle many gravitational effects as they change the orbital configuration, and the arrival time of the clock-tick pulses. In this case we have seen Lens-Thirring precession, a prediction of General Relativity, for the first time in any stellar system.’

From Willem van Straten (AUT):
‘After ruling out a range of potential experimental errors, we started to suspect that the interaction between the white dwarf and neutron star was not as simple as had been assumed to date.’

ANIMATION:
Click this link to see an animation depicting a neutron star orbiting a rapidly-spinning white dwarf. The white dwarf's spin drags the very fabric of space-time around with it, causing the orbit to tumble in space. Credit: Mark Myers, OzGrav ARC Centre of Excellence. 

LINK TO SCIENCE PAPER:
 https://science.sciencemag.org/cgi/doi/10.1126/science.aax7007  

Following the recent overwhelming success of deep learning and artificial intelligence in several fields of research, industry and medicine, researchers from the ARC Centre of Excellence of Gravitational Wave Discovery (OzGrav) and the University of Western Australia (UWA), including PhD student and the paper’s first author Chayan Chatterjee, have built a deep learning model using an Artificial Neural Network to pinpoint where in the sky gravitational wave signals have come from. The model can localise the source of gravitational waves produced by colliding pairs of black holes potentially as much as a thousand times faster than any other technique.
 
Professor Amitava Datta, a scientist from UWA who contributed to the study, says: ‘This work is a very interesting example of learning patterns from simulated data for predicting the outcome of real events, in this case the location of gravitational wave sources. Perhaps this approach using deep learning will be more and more useful in basic sciences in general.’
 
Data intensive astronomy expert Kevin Vinsen from the International Centre for Radio Astronomy Research says: ‘This project is an excellent example of how a multi-disciplinary approach can solve the problem’.
 
The basic structure of an Artificial Neural Network. The circles represent the neurons or nodes and the arrows represent connections between one neuron to another. Credit: Chayan Chatterjee
Gravitational waves are small ripples in the space-time continuum caused by colossal stellar events such as colliding black holes. In September 2015, following recent advances in detector sensitivity, the LIGO Scientific Collaboration detected gravitational waves for the first time. This was a landmark achievement in human discovery and heralded the birth of the new field of gravitational wave astronomy.  
The need for speed and accuracy is particularly important in the context of gravitational wave localisation—scientists need to tell a global network of telescopes where to point on the sky as quickly as possible, so they can see any electromagnetic light that may also have come from the gravitational wave event. The current algorithm used to locate gravitational wave sources in real time takes a few seconds to process. More accurate methods usually take several hours to compute. The light generated by gravitational wave events can be very short-lived at certain wavelengths, like short gamma ray bursts, which last a mere 2-3 seconds, so scientists need methods that can rapidly process huge data as fast and accurately as possible.
The idea behind deep learning is simple: it’s an algorithm designed to mimic the functioning of neurons in our brain to carry out tasks, like categorising observed stimuli. This is done by making the network learn the correlations between a labelled input dataset and the output it is trying to predict. Just like electric signals or synapses flow through neurons in our brain, the input information provided to an Artificial Neural Network travel through layers of nodes (usually several layers deep), with each layer introducing some non-linearity to the input. This non-linearity helps the network learn complex features of the data. The ‘learning’ happens through a rigorous ‘training’ of the network. During the training, the predictions of the network are compared with the true values, and the parameters of the network are adjusted to minimise any erroneous gaps.

In their recently published paper, Chatterjee and the team from UWA successfully trained the Artificial Neural Network to learn the input data for source localisation. The data was pre-processed to extract the important physical Physics parameters from simulated gravitational wave signals, injected into ‘random noise’. The network classified these signals into several classes and accurately identified the source direction of the gravitational waves. The model localised the test samples much faster than other methods and at a low computational cost. The researchers plan to extend this work for pairs of merging neutron stars and neutron star-black hole systems.
 
 
Chatterjee says: ‘Hopefully the methods we introduce can also be translated to other areas of research and industry and help further untap the seemingly limitless potential of deep learning and artificial intelligence’.       
 
OzGrav’s Chief Investigator Professor Linqing Wen who led the study says: ‘The future is wide open for gravitational wave discovery using the machine learning technique’.
 
This paper is an outcome of a multi-disciplinary collaboration of UWA’s Gravitational Wave Astronomy Group led by OzGrav’s Chief Investigator Professor Linqing Wen, data intensive astronomy expert Kevin Vinsen from International Centre for Radio Astronomy Research (ICRAR), and Professor Amitava Datta of UWA’s Computer Science and Software Engineering.
                             
Link to publication: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.100.10302

​A new collaborative study with the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) reveals a possible collision of two neutron stars earlier in 2019—only the second time this type of cosmic event had ever been detected.   The gravitational-wave observatory network, that includes the National Science Foundation's LIGO and the European Virgo detectors, picked up what appeared to be gravitational ripples from a collision of two neutron stars back on 25 April 2019.  
 
Gravitational waves and light were first witnessed in the same event in 2017. This second event in 2019, called GW190425, did not result in any light being detected; however, researchers have learned that the collision resulted in a merged object with an unusually high mass.
 
OzGrav postdoctoral researcher Simon Stevenson says: ‘This event is a perfect example of how gravitational-wave astronomy is a completely new and unique way of looking at the Universe. Binaries with similar masses to this event may not exist in the Milky Way or may be completely invisible to conventional radio telescopes’.
 
Neutron stars are the remnants of dead stars that exploded. When two neutron stars spiral together, they undergo a violent merger that sends gravitational waves shuddering through the fabric of space and time.
 
The gravitational waves first detected in 2015 were generated by the fierce collision of two black holes. Since then, scientists have registered dozens of new candidate black hole mergers. The first detection of a neutron star merger took place two years later, in 2017.
 
OzGrav Postdoctoral Researcher Vaishali Adya says: ‘This detection manifests the importance of continued improvement of the already amazingly sensitive gravitational wave detectors, as this event would not have been observable prior to the latest upgrades. OzGrav played a vital role in these upgrades, one of which involved reducing the quantum noise in the detectors’.
 
OzGrav Postdoctoral Researcher Xingjiang Zhu says: ‘The combined mass of the merging objects is surprisingly high, much greater than any previously known double neutron star binaries including the one detected in 2017. This provokes us to think about the nature of this event and how the source might have been formed’.
 
The combined mass of the merged bodies in this event is about 3.4 times that of the mass of the Sun. Typically, neutron star collisions are only known to happen between pairs of neutrons stars with a total mass up to 2.9 times that of the Sun. One possibility for the unusually high mass is that the collision took place not between two neutron stars, but a neutron star and a black hole, since black holes are heavier than neutron stars. But if this were the case, the black hole would have to be exceptionally small for its class. Instead, the scientists believe it is more likely that the event was a shattering of two neutron stars and that their merger resulted in a newly formed black hole.
 
Neutron star pairs are thought to form either early in life—when companion massive stars successively die one by one—or when they come together later in life within dense, busy environments. The data from the 2019 event do not indicate which of these scenarios is more likely—more data and new models are needed to explain the unexpectedly high mass. The discovery suggests that we may have detected an entirely new population of binary neutron star systems.
 
OzGrav Associate Investigator Greg Ashton says: ‘This event was really interesting. The chirp-like signal was seen by two of the three detectors for about 128 seconds before the final merger. Unfortunately, one of the detectors was not observing at the time, which meant that the sky localization was poor. Perhaps because of this, and because it was so far away, no electromagnetic light was observed from this event. Nevertheless, we saw it very clearly in the gravitational wave data and could use that to calculate the masses, spins, and orientations of the objects’.
 
‘Additional exciting and unexpected discoveries can be expected as the sensitivity of the LIGO detectors improves. OzGrav is working closely with LIGO to improve their sensitivity, developing new instrumentation and analysis techniques’, says Professor Peter Veitch, University of Adelaide OzGrav Node Leader
 
The results were announced today at the American Astronomical Society meeting in Honolulu, Hawaii.
 
The full scientific article will be available here post-embargo: https://dcc.ligo.org/P190425/public
​

​
​AS FEATURED IN THE AGE

Local scientists have begun the groundwork for a campaign to potentially make Australia the site of one of the biggest science instruments ever built.
Nicknamed Cosmic Explorer South, the 40-kilometre long, $1.5 billion instrument is designed to detect ripples in space time throughout the entire universe.

The global astronomy community is drawing up plans for three such detectors based around the world, which are slated for construction in the 2030s.
Australia is the perfect place to build one, local scientists believe.

"There is no doubt in my mind that Australia is the best location for this," says Professor Matthew Bailes, who directs the Australian Research Council Centre of Excellence for Gravitational Wave Discovery and is spearheading Australia’s bid, along with Professor David McClelland at the Australian National University.

"We don’t have a preferred site yet. But we have been placing little L-shaped things on maps to see where they might fit."

Explorer South is a next-generation gravitational wave detector. It would be able to sense gravitational waves from the very first stars born in the universe.

It will consist of two, 40-kilometre long vacuum tubes placed at right angles, like a big L.
Down the middle scientists would fire a "James Bond-strength" laser, says Professor Bailes.
A device of that size would dwarf another piece of big science, the Large Hadron Collider, which is 27 kilometres in diameter and straddles the Franco-Swiss border.

A white paper for the gravitational wave project was quietly submitted to the Australian Academy of Science earlier this year, proposing an initial $5 million investment to begin exploratory work.
The technique has already been proved by the American-based LIGO, which first detected a gravitational wave in 2015, using two arms, each four kilometres long.

How it works
When black holes collide, so much energy is released it causes ripples in space time.
These ripples spread out through the universe, like ripples on a pond. When they reach Earth, they slightly stretch and then shrink our space-time (yes, you are being very slightly stretched and shrunk too). As a gravitational wave passes through Earth, LIGO's arms stretch a little; a laser beam shone down the middle of the arms allows scientists to detect that tiny change in distance.

Scientists now want to go further – using those ripples to get information about the formation of the universe. To do that, they need to go bigger.

International astronomers are now planning the next generation of gravitational wave detectors. One wave detector, the Einstein Telescope, is planned in Europe. Another, Cosmic Explorer, is being planned for America.

A third detector in the southern hemisphere – Cosmic Explorer South – would allow scientists to triangulate the source of any waves they detected. Senior experts from the National Science Foundation, the American government’s science funding agency, are aware of Australia's interest.

Any location needs to be extremely flat and geologically stable, which makes Australia’s vast interior perfect. The arms of the detector need to be perfectly straight. But if you build something 40 kilometres long, you need to take into account the Earth’s curve.

"So you want to find a valley, where the shape of the valley is exactly compensated by the curve of the Earth," says Professor Bailes.

If you were to place it in Melbourne’s CBD, one 40 kilometre arm would stretch nearly to Healesville and the other well past Sunbury. The extreme size makes Cosmic Explorer South 100 times more powerful than LIGO, says Professor Bailes. It could measure distances down to one ten-thousandth of the width of a proton, and be sensitive enough to detect the gravity of a passing cloud.

Professor Bailes would not reveal any possible locations, because of the risk of land speculators buying them. But it would need to be near an airport or regional hub, he said. The government needs long lead times on science mega-projects, and there is a lot of due diligence to do before asking for government funding, Professor Bailes says.

If Explorer South were built here, Australia would likely have to spend about $300 million on the project, with the rest kicked in by governments around the world.

AS FEATURED IN THE AGE. 

Neutron stars are among the densest objects in the Universe, and they rotate extremely fast and regularly.
 
Until they don’t.
 
Occasionally, these neutron stars start to spin even faster, caused by portions of the inside of the star moving outwards. It’s called a ’glitch’, and it’s a rare glimpse into what lies within these mysterious objects.
 
In a recent paper published in Nature Astronomy, a team from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University; McGill University, in Canada; and the University of Tasmania, studied the Vela Pulsar: a neutron star in the southern sky, 1,000 light years away from Earth.
 
According to the paper’s first author Dr Greg Ashton, from OzGrav-Monash, only 5% of pulsars are known to glitch and Vela ‘glitches’ about once every three years. This makes Vela a famous prized jewel among ‘glitch hunters’ like Ashton and his colleague, Dr Paul Lasky, also from OzGrav-Monash.
 
By reanalysing data from observations of the Vela glitch in 2016, taken by co-author Jim Palfreyman from the University of Tasmania, Ashton and his team found that during the glitch the star started spinning even faster, before relaxing down to a final state. 
 
According to Dr Lasky, this observation (done at the Mount Pleasant Observatory in Tasmania) is particularly important because, for the first time, scientists got invaluable insights into the interior of the star, revealing that the inside of the star actually has three different components. 
 
“One of these components, a soup of superfluid neutrons in the core, moves outwards first and hits the rigid crust of the star causing it to spin up.  But then, a second soup of superfluid that moves in the crust catches up to the first causing the spin of the star to slow back down.  This overshoot has been predicted a couple of times in the literature, but this is the first real time it’s been identified in observations,” he said.
 
One such prediction of the overshoot came from the study’s co-author Vanessa Graber, from McGill University, who visited the Monash team as an OzGrav international visitor earlier this year. 
 
Another observation, according to Dr Ashton, defies explanation: “Immediately before the glitch, we noticed that the star seems to slow down its rotation rate before spinning back up.  We actually have no idea why this is, and it’s the first time it’s ever been seen!  We speculate it’s related to the cause of the glitch, but we’re honestly not sure,” he said. Ashton suspects this paper will spur some new theories on neutron stars and glitches

OzGrav Associate Investigator Adam Deller and the Distributed FX Correlator (DiFX) team was recently awarded  the Peter McGregor Prize by the Astronomical Society of Australia (ASA). This is a newly created and prestigious award to celebrate major achievements in the development of astronomical instrumentation and technology, both hardware and software.

The Distributed FX Correlator (DiFX) is a software package that contains tools necessary to turn an array of radio telescope signals into a functioning radio interferometer. The nominations and letters of support made a very strong case arguing the innovative nature of the DiFX and its impact.

The DiFX transformed long-baseline interferometry. It has contributed significantly to reducing the barrier to entry and play a major role in radio astronomy research internationally. The system has enabled a wide range of science, as testified by the very high number of references to the key technical papers. The open-access nature of the software has put a new tool in the hands of astronomers, with demonstrated positive results. Its scalability and adaptability continues to enable researchers to tailor its behaviour and pursue what would otherwise be difficult science goals. DiFX was also used to image the black hole event horizon in 2018, so it’s evidently powerful.

The DiFX collaboration includes the following contributors:
Adam Deller (leader), Walter Alef, James Anderson, Matthias Bark, Matthew Bailes, Walter Brisken, Roger Cappallo, Geoff Crew, Richard Dodson, Dave Gordon, Zheng Meyer-Zhao, John Morgan, Chris Phillips, Cormac Reynolds, Jon Romney, Helge Rottman, John Spitzak, Matteo Stagni, Steven Tingay, Jan Wagner, Mark Wainright, Randall Wayth."

Congratulations to everyone involved!

The Hubble constant is one of the most fundamental pieces of information that describes the state of the Universe in the past, present, and future. It tells us how fast the Universe is expanding – a valuable piece of information in science’s search for answers.
 
The two best ways of estimating the Hubble constant are based on: the background hiss of the Universe left over from the big bang (the ‘Planck’ observations of the cosmic microwave background radiation), and on massive stars blowing themselves to pieces in the distant Universe (‘type 1a supernovae’ observations). According to the measurements of the exploding stars, the Universe is expanding a bit faster than the measurements of the background hiss would indicate, and the difference is now very significant. So, either one of them is incorrect or something is missing in our understanding of physics and cosmology. We’d like to know what is really happening in the Universe, so we need a third, independent check.
 
This is where the merger of two neutron stars can shed some light. Neutron star mergers are phenomenally energetic events – two stars, each more massive than the Earth’s Sun, whip around each other hundreds of times per second before colliding and producing an enormous blast of material, light and gravitational waves. In 2017, gravitational waves and light were first detected from a neutron star merger that had occurred 130 million years ago, in an event scientists refer to as GW170817.
 
Scientists realised that a burst of gravitational waves can be used as a ‘standard siren’: based on the shape of the gravitational wave signal, we can tell how ‘bright’ the event should have been in gravitational waves. We can then measure the actual brightness of the event and work out what the distance must have been. However, this only works well if we know how the merging stars were oriented on the sky (edge on, face on, or somewhere in between).
 
The gravitational wave data itself can’t accurately tell whether a merger was nearby and edge on, or distant and face on. To answer that question, a team including OzGrav Associate Investigator Adam Deller used radio telescopes to take a super-high-resolution movie of a narrow but powerful jet of material left behind after two neutron stars merged in the GW170817 event. The resolution of the radio images was so high, if it was an optical camera, it would see individual hairs on someone’s head 10 km away. By examining the miniscule changes in this radio-emitting bullet of gas (compared against models developed by supercomputers), the angle of the jet and the orientation of the merging neutron stars was found.
 
Using this information, Adam and the team could tell how far away the merging neutron stars were and, by comparing this with how fast their host galaxy is rushing away from the Earth, they could finally work out the prized Hubble constant. Despite this incredible result, which is published in Nature Astronomy today, the current measurement is still not good enough to distinguish between ‘Planck’ vs ‘Type 1a supernovae’. Further observations of merging neutron stars will soon lead to a more accurate Hubble constant. 

Inside a small room in the Melbourne suburb of Hawthorn a team of astrophysicists, with a little help from work experience students and a 12-year-old boy, are leading a global hunt for one of the universe’s most mysterious and powerful phenomena.

For one week this is the control centre for about 64 telescopes dotted around – and above – the Earth.
The team is on the world’s biggest chase for fast radio bursts, mysterious and super-powerful blasts of radio-wave energy visible clear across the universe. But the Deeper Wider Faster team, as it is known, has even bigger ambitions.
​
Over the past few years, astronomers have spotted a number of these bursts. The CSIRO announced on Friday that it had spotted one and traced its location to a galaxy 3.6 billion light years away.

But the scientists in the Hawthorn room not only want to spot one; they hope to find out what causes the bursts.

Fast radio bursts – astronomers refer to them as FRBs – are extremely short and powerful bursts of radio waves originating from somewhere outside the Milky Way. Some contain more energy within a moment than our sun produces in decades.

“It’s a millisecond burst that goes off once, unpredictably. And you have no idea when. Trying to catch that, it’s hard,” says Jeff Cooke, chief investigator at the ARC Centre for Excellence in Gravitational Wave Discovery. He has spent the past half-decade pulling all this together.

Last week's telescope global hook-up finished on Saturday. The team hasn't spotted anything thus far, but there were problems with one of the radio telescopes and data analysis will continue to see if anything was missed.

No one knows what the bursts are. Most scientists agree they are not linked to aliens.
Each astrophysicist in the control room last week at Swinburne University’s Hawthorn campus has their own favourite theory.

Professor Cooke hopes they come from magnetars, a kind of neutron star with a super-powerful magnetic field. An incredibly intense "star-quake" on its surface might produce an FRB.
Sara Webb hopes the answer is a blitzar, a pulsar that explodes then collapses into a black hole.
And Igor Andreoni hopes FRBs come from colliding neutron stars, which generate explosions so powerful they warp the space-time fabric.

To find out, the three have spent five years wrangling telescopes around the world to work together.
The global coalition ranged from the high-tech – like IceCube, a sensor embedded deep under the Antarctic ice, and the Hard X-ray Modulation Telescope, flying in  a low orbit of Earth – to decidedly low-tech: a telescope set up in a Pakenham backyard by an amateur astronomer.

The telescopes have been looking at the same patch of sky. But – and here’s the trick – they  all look with different eyes. The X-ray Modulation Telescope, for example, sees in X-rays, while IceCube’s one-kilometre-wide sensor is designed to pick up neutrino signatures.

Different signals can travel through space at different speeds. Radio waves typically are the slowest. This means by the time a radio telescope detects a fast radio burst, all the other signals – things that could tell us what the FRB actually is – may have already gone racing past the Earth.

The hope for last week's hook-up was that by looking with about 64 sets of different eyes, when an FRB fired off the other telescopes would spot something – a burst of light, or X-rays, or anything at all– that gave a clue as to what the FRB really is.

The researchers have done six runs at this in the past four years, each time harnessing more and more telescopes. So far, nothing.

Data flowed last week from the telescopes around the world to a supercomputer able to pick out anything that could possibly be a signal – like a space explosion – that could generate an FRB.
The probabilities told the scientists their radio telescopes would spot one or two FRBs last week. The supercomputer, however, could identify tens of thousands of possible signals every day.

The aim was for humans to check any signals within minutes of their happening in order to send a quick command to giant telescopes in Chile and South Africa that stood ready to try to get a good image of the FRB.

However, the team will now need to go back over some of the results due to problems with a telescope not providing needed data.

Many scientists don’t even believe an FRB will come with another signal. “So that makes it even harder,” Dr Andreoni says.

In the control room one day early last week, in charge of assessing the thousands of possible FRBs is that crack team of astrophysicists – and a few work experience students. There are six of the latter, all here for the day from school. They are joined by Blake Iscaro, a 12-year-old with a deep interest in space who matter-of-factly says he is using scavenged parts to build a radio-telescope in his backyard.
The supercomputer sprays the data up on the wall. Then it's up to the team to spot something. Fast.

“Computers find things you know pretty well. They don’t find things you have not ever seen before,” Professor Cooke says. “If you’re going to trigger these gigantic telescopes, which cost a ridiculous amount of money, you don’t want them to point at something that wasn’t real.”

Two stations down from Professor Cooke, year 10 student Max Petschack leans closer to his screen, face screwed up in concentration, staring at what looks like a yellow blob.

The light curve shows the blob became dramatically brighter between measurements. “This one’s interesting,” he says.

Michelle Ko leans over to look. “Wow,” she says.

Max points at the light curve. “That’s a, what, point four difference?”

“Yeah, that’s a huge difference,” says Michelle, before calling Dr Andreoni over. He leans over the children’s shoulders, checking their work.

Is this the moment?

Sadly, he does not think so. There is a lot of turbulence in the atmosphere messing with their brightness measurements. And the dot Max found is too round. “That means it’s probably a star,” Dr Andreoni says.