Research highlight: Optical observations of the BepiColombo spacecraft as a proxy for a potential threatening asteroid
BepiColombo is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) designed to study the planet Mercury. Launched in late 2018, its complex trajectory involved a fly-by past Earth on April 10, 2020. We took advantage of the event to organise a coordinated observing campaign. The main goal was to compute and compare the observed fly-by orbit properties with the values available from the Mission Control. The method we designed could then be improved for future observation campaigns targeting natural objects that may collide our planet.
The incoming trajectory of the probe limited the ground-based observability to only a few hours, around the time when it was closest to Earth. The network of telescopes we used has been developed by ESA’s NEO Coordination Centre (NEOCC) with capabilities to quickly observe imminent impactors, thus presenting similar orbits. Our team successfully acquired the target with various instruments such as the 6ROADS Chilean telescope, the 1.0 m Zadko telescope in Australia, the ISON network of telescopes, and the 1.2 m Kryoneri telescope in Corinthia, Greece.
The observations were difficult due to the object’s extremely fast angular motion in the sky. At one point, the telescopes saw the probe covering twice the size of the moon in the sky each minute. This challenged the tracking capabilities and timing accuracy of the telescopes. Each telescope was moving at the predicted instantaneous speed of the target while taking images, "tracking" the spacecraft. Field stars appeared as trails, while BepiColombo itself was a point source, but only if the observation started exactly at the right moment. Because the probe was moving so fast, any date errors of the telescope images translate into position errors of the probe. To reach a precise measurement of 0.1 metres, the date of the images needed to have a precision of 100 milliseconds.
The final results were condensed into two measurable quantities that could be directly compared with the Mission Control ones, the perigee distance, and the time of the probe’s closest approach to Earth. Both numbers were perfectly matched, proving our method a success: it calculated a more accurate prediction of BepiColombo’s orbit; it also provided valuable insights for future observations of objects colliding with Earth:
• A purely optical observing campaign can provide trajectory information during a fly-by at sub-kilometre and sub-second levels of precision.
• A similar campaign would lead to a sub-kilometre and sub-second precision for the time and location of the atmospheric entry of any colliding object.
• Timing accuracy below 100 milliseconds is crucial for the closest observations.
• It’s possible to organise astrometric campaigns with coverage from nearly every continent.
Link to research paper: https://doi.org/10.1016/j.actaastro.2021.04.022
Written by OzGrav researcher Dr Bruce Gendre, University of Western Australia.
From gravitational wave science to global technology company: Liquid Instruments is a Canberra start-up bringing NASA technology to the world.
Liquid Instruments (LI) Pty Ltd, a spin-off company from the Australian National University (ANU), is revolutionising the $17b test and measurement market. Test and Measurement devices are used by scientists and engineers to measure, generate and process the electronic signals that are fundamental to the photonics, semiconductor, aerospace and automotive industries. The LI team has raised more than $25M USD in Venture Capital investment, and now has more than 1000 users in 30 countries.
LI was founded by researchers from the gravitational wave group at ANU to commercialise advanced instrumentation technology derived from both ground and space-based gravity detectors. OzGrav Chief Investigator Daniel Shaddock (ANU), CEO of Liquid Instruments, began as an engineer at NASA’s Jet Propulsion Laboratory in 2002, working on the Laser Interferometer Space Antenna (LISA), a joint project between NASA and the European Space Agency. The work on LISA’s phasemeter was the genesis for forming Liquid Instruments.
LI’s software-enabled hardware employs advanced digital signal processing to replace multiple pieces of conventional equipment at a fraction of the cost and with a drastically improved user experience. Their first product Moku:Lab provides the functionality of 12 instruments in one simple integrated unit.
On 23 June, the company launched two new hardware devices, the Moku:Go—an engineering lab in a backpack for education, and the Moku:Pro – a multi-GHz device for professional scientists and engineers. Lik the Moku:Lab, this revolutionary new hardware includes a suite of instruments with robust hardware features giving a breakthrough combination of performance and versatility.
Daniel Shaddock says: “Moku:Pro takes software defined instrumentation to the next level with more than 10x improvements in many dimensions. Moku:Pro is a new weapon for scientists. Moku:Go takes all the great features of Moku:Lab but reduces the cost by 10x to make it more accessible than ever before. We hope it will help train the next generation of scientists and engineers in universities around the world.
‘Type Ia’ supernovae involve an exploding white dwarf close to its Chandrasekhar mass. For this reason, type Ia supernova explosions have almost universal properties and are an excellent tool to estimate the distance to the explosion, like a cosmic distance ladder. Collapsing massive stars will form a different kind of supernova (type II) with more variable properties, but with comparable peak luminosities.
To date, the most luminous events occur in core-collapse supernovae in a gaseous environment, when the circumstellar medium near the explosion transforms the kinetic energy into radiation and thus increases the luminosity. The origin of the circumstellar material is usually the stellar wind from the massive star’s outer layers as they’re expelled prior to the explosion.
A natural question is how will type Ia supernovae look like in a dense gaseous environment? And what is the origin of the circumstellar medium in this case? Will they also be more luminous than their standard siblings? To address this question, OzGrav researchers Evgeni Grishin, Ryosuke Hirai, and Ilya Mandel, together with an international team of scientists, studied explosions in dense accretion discs around the central regions of active galactic nuclei. They constructed an analytical model which yields the peak luminosity and lightcurve for various initial conditions, such as the accretion disc properties, the mass of the supermassive black hole, and the location and internal properties of the explosion (e.g. initial energy, ejecta mass). The model also used suites of state-of-the-art radiation hydrodynamical simulations.
The explosion generates a shock wave within the circumstellar medium, which gradually propagates outward. Eventually, the shock wave reaches a shell that is optically thin enough, such that the photons can ‘breakout’. The location of this breakout shell and the duration of the photon diffusion determine the lightcurve properties.
If the amount of the circumstellar medium is much smaller than the ejecta mass, the lightcurves look very similar to type Ia supernovae. Conversely, a very massive circumstellar mass can choke the explosion and it will not be seen. The sweet spot lies somewhere in between, where the ejecta mass is roughly comparable to the amount of circumstellar material. In the latter case, the peak luminosity 100 times bigger than the standard type Ia Supernovae, which makes it one of the brightest supernova events to date.
The research paper describing this work (Grishin et al., “Supernova explosions in active galactic nuclear discs”) was recently published in Monthly Notices of the Royal Astronomical Society. The luminous explosions may be observed in accretion discs of accretion rate, or in galaxies with smaller supermassive black hole masses where background active galactic nucleus activity will not hinder observations with advanced instruments.
The underlying physical processes of photon diffusion and shock breakout can be creatively explained with poetry:
All of a sudden, the heat is intense.
We must cool down, but the path is opaque.
Every direction around is so dense,
Which one should the photons take?
They have to break out, for God's sake...
At first, they are stuck, no matter the way,
They sway side to side, they randomly walk.
The leader in front leads them astray,
How hogtied is this radiant flock...
But wait, do you also gaze at the shock?
The ominous furnace is starting to snap,
Its violent grip getting frail.
The path is now clear, the direction is "up!"
We're sitting on the shock front's tail.
We're seizing the shock, we'll prevail!
The shock front behind us, but we're still out of place,
We propel with incredible might.
We keep on ascending, increasing the pace,
Any particle is now out of sight,
In this vacuum, we're free from inside,
And can travel as fast
as the light.
Written by OzGrav researcher Evgeni Grishin, Monash University
RESEARCH HIGHLIGHT: Extraction of binary black hole gravitational wave signals from detector data using deep learning
One of the major challenges involved in gravitational wave data analysis is accurately predicting properties of the progenitor black hole and neutron star systems from data recorded by LIGO and Virgo. The faint gravitational wave signals are obscured against the instrumental and terrestrial noise.
LIGO and Virgo use data analysis techniques that aim to minimise this noise with software that can ‘gate’ the data – removing parts of the data which are corrupted by sharp noise features, called ‘glitches’. They also use methods that extract the pure gravitational-wave signal from noise altogether. However, these techniques are usually slow and computationally intensive; they’re also potentially detrimental to multi-messenger astronomy efforts, since observation of electromagnetic counterparts of binary neutron star mergers—like short-gamma ray bursts—relies heavily on fast and accurate predictions of the sky direction and masses of the sources.
In our recent study, we’ve developed a deep learning model that can extract pure gravitational wave signals from detector data at faster speeds, with similar accuracy to the best conventional techniques. As opposed to traditional programming, which uses a set of instructions (or code) to perform, deep learning algorithms generate predictions by identifying patterns in data. These algorithms are realised by ‘neural networks’ – models inspired by the neurons in our brain and are ‘trained’ to generate almost accurate predictions on data almost instantly.
The deep learning architecture we designed, called a ‘denoising autoencoder’, consists of two separate neural networks: the Encoder and the Decoder. The Encoder reduces the size of the noisy input signals and generates a compressed representation, encapsulating essential features of the pure signal. The Decoder ‘learns’ to reconstruct the pure signal from the compressed feature representation. A schematic diagram of a denoising autoencoder model is shown in Figure 1.
For the Encoder network, we’ve included a Convolutional Neural Network (CNN) which is widely used for image classification and computer vision tasks, so it’s efficient at extracting distinctive features from data. For the Decoder network, we used a Long Short-Term Memory (LSTM) network —it learns to make future predictions from past time-series data.
Our CNN-LSTM model architecture successfully extracts pure gravitational wave signals from detector data for all ten binary-black hole gravitational wave signals detected by LIGO-Virgo during the first and second observation runs. It’s the first deep learning-based model to obtain > 97% match between extracted signals and ‘ground truth’ signal ‘templates’ for all these detected events. Proven to be much faster than current techniques, our model can accurately extract a single gravitational wave signal from noise in less than a milli-second (compared to a few seconds by other methods).
The data analysis group of OzGrav-UWA is now using our CNN-LSTM model with other deep learning models to predict important gravitational wave source parameters, like the sky direction and ‘chirp mass’. We’re also working on generalising the model to accurately extract single signals from low-mass black hole binaries and neutron star binaries.
Submitted, not yet accepted nor published.
Written by OzGrav researcher Chayan Chatterjee, UWA
AUSTRALIAN SCIENTISTS AT HELM OF THE EXTRAORDINARY DISCOVERY OF TWO NEUTRON STAR-BLACK HOLE COLLISIONS WITNESSED FOR THE FIRST TIME
A newly discovered astronomical phenomenon was revealed in a globally coordinated announcement of not one, but two events witnessed last year: the death spiral and merger of two of the densest objects in the Universe—a neutron star and a black hole.
The discovery of these remarkable events, which occurred before the time of the dinosaurs but only just reached Earth, will now allow researchers to further understand the nature of the space-time continuum and the building blocks of matter.
The discoveries were made by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US, and the Virgo gravitational-wave observatory in Italy, with significant involvement from Australian researchers.
According to Dr Rory Smith—from the ARC Centre of Excellence for Gravitational-Wave Discovery (OzGrav) and Monash University, and the international co-lead on the paper published in the journal Astrophysical Journal Letters—the discoveries are a milestone for gravitational-wave astronomy. “Witnessing these events opens up new possibilities to study the fundamental nature of space-time, and matter at its most extreme,” Dr Smith said.
The first observation of a neutron star-black hole system was made on Jan 5th, 2020 when gravitational waves—tiny ripples in the fabric of space and time—were detected from the merger of the neutron star with the black hole by LIGO and Virgo.
Detailed analysis of the gravitational waves reveal that the neutron star was around twice as massive as the Sun, while the black hole was around nine times as massive as the Sun. The merger itself happened around a billion years ago, before the first dinosaurs appeared on Earth.
Remarkably, on January 15th 2020, another merger of a neutron star with a black hole was observed by LIGO and Virgo using gravitational waves. This merger also took place around a billion years ago, but the system was slightly less massive: the neutron star was around one and a half times as massive as the Sun, while the black hole was around five and a half times as massive.
Dr. Smith explains the significance: “Astronomers have been searching for neutron stars paired with black holes for decades because they’re such a great laboratory to test fundamental physics. Mergers of neutron stars with black holes dramatically warp space-time—the fabric of the Universe—outputting more power than all the stars in the observable Universe put together. The new discoveries give us a glimpse of the Universe at its most brilliant and extreme. We will learn a great deal about the fundamental nature of space-time and black holes, how matter behaves at the highest possible pressures and densities, how stars are born, live, and die, and how the Universe has evolved throughout cosmic time”.
The discovery involved an international team of thousands of scientists, with Australia playing a leading role. “From the design and operation of the detectors to the analysis of the data, Australian scientists are working at the frontiers of astronomy,” Dr Smith added.
Black holes and neutron stars are two of the most extreme objects ever observed in the Universe—they are born from exploding massive stars at the end of their lives. Typical neutron stars have a mass of one and a half times the mass of the Sun, but all of that mass is contained in an extremely dense star, about the size of a city. The star is so dense that atoms cannot sustain their structure as we normally perceive them on Earth.
Black holes are even more dense objects than neutron stars: they have a lot of mass, normally at least three times the mass of our Sun, in a tiny amount of space. Black holes contain an “event horizon” at their surface: a point of no return that not even light can escape.
“Black holes are a kind of cosmic enigma,” explained Dr Smith. “The laws of physics as we understand them break down when we try to understand what is at the heart of a black hole. We hope that by observing gravitational waves from black holes merging with neutron stars, or other black holes, we will begin to unravel the mystery of these objects.”
When LIGO and Virgo observe neutron stars merging with black holes, they are orbiting each other at around half the speed of light before they collide. “This puts the neutron star under extraordinary strain, causing it to stretch and deform as it nears the black hole. The amount of stretching that the neutron star can undergo depends on the unknown form of matter that they’re made of. Remarkably, we can measure how much the star stretches before it disappears into the black hole, which gives us a totally unique way to learn about the building blocks of protons and neutrons,” Dr Smith said.
"Using one of the most powerful Australian supercomputers and the most accurate solutions to Einstein's famous field equations known to date, we were able to measure the properties of these collisions, such as how heavy the neutron stars and black holes are, and how far away these events were," he said.
Publication in Astrophysical Journal (ApJL)
Also featured in The Australian , The Financial Review , The ABC , ABC radio , ABC Breakfast radio ,
Cosmos magazine , Channel 10 news , The Conversation and more.
Gamma-ray bursts are enormous cosmic explosions and are one of the brightest and most energetic events in the Universe. Their brightness changes over time, illuminating deep space like a flashlight shining into a dark room. Intense radiation emitted from most observed gamma-ray bursts is predicted to be released during a supernova as a star implodes to form a neutron star or a black hole.
In the recently observed gamma-ray burst event called GRB 160203A, remains of the explosion started glowing much brighter than expected, according to standard scientific models, even several hours after the initial flash. We now believe that this “rebrightening” was caused by the main body of the burst crashing through shells of material ejected by the source star, or interstellar “knots”. Both theories suggest that the standard gamma-ray burst model needs to be re-examined, and perhaps the surrounding space isn’t as smooth and uniform as originally predicted.
In our study, we began collecting reports from all over the world that observed the gamma-ray burst event, including the archives of the Zadko research telescope. By carefully calibrating the data from different sources and comparing the different brightness over time, we unpacked the surrounding galaxy and defined key characteristics of the burst: the temporal index (how quickly it fades over time), the spectral index (the overall colour of the burst), and the extinction (how much light is absorbed by the matter between here, on Earth, and the burst). One surprising finding was that the density of the burst’s host galaxy is unusually dense – about the same as our own galaxy, the Milky Way.
The next step was to see how and when the data moved away from the model. With further calculations, we identified three interesting time periods that indicated significant brightness differences compared to the model’s prediction. Although the third period was probably a coincidence, the first and second periods were too large to ignore. Normally, rebrightening is caused by something happening to the host galaxy(?), such as suddenly collapsing into a black hole; however, these kinds of events normally happen within the first few minutes of a gamma-ray burst – in this event, the first rebrightening didn’t start until three hours after the initial explosion.
As a result, we decided to expand the conventional model of gamma-ray bursts to explain this unusual event. One of the properties of such events is the relationship between the density of the medium and the intensity of radiation emitted from the explosion. What’s particularly convincing about this explanation is its applicability to many contexts. As stars prepare to explode into supernovas and gamma-ray bursts, they eject their outer shells into the surrounding space. For bursts that don’t come from supernovas, these changes in brightness could be the result of turbulence in the interstellar medium. In either case, the change in brightness gives us a new tool to probe the structure of distant space, and we are now eagerly anticipating another burst with similar features to put our new model to the test.
Written by OzGrav PhD student Hayden Crisp, University of Western Australia
What happens if a supernova explosion goes off right beside another star? The star swells up which scientists predict as a frequent occurrence in the Universe. Supernova explosions are the dramatic deaths of massive stars that are about 8 times heavier than our Sun.
Most of these massive stars are found in binary systems, where two stars closely orbit each other, so many supernovae occur in binaries. The presence of a companion star can also greatly influence how stars evolve and explode. For this reason, astronomers have long been searching for companion stars after supernovae-- a handful have been discovered over the past few decades and some were found to have unusually low temperatures.
When a star explodes in a binary system, the debris from the explosion violently strikes the companion star. Usually there’s not enough energy to damage the whole star, but it heats up the star’s surface instead. The heat then causes the star to swell up, like having a huge burn blister on your skin. This star blister can be 10 to 100 times larger than the star itself.
The swollen star appears very bright and cool, which might explain why some discovered companion stars had low temperatures. Its inflated state only lasts for an ‘astronomically’ short while--after a few years or decades, the blister can “heal” and the star shrinks back to its original form.
In their recently published study by a team of scientists led by OzGrav postdoctoral researcher Dr Ryosuke Hirai (Monash University), the team carried out hundreds of computer simulations to investigate how companion stars inflate, or swell up, depending on its interaction with a nearby supernova. It was found that the luminosity of inflated stars is only correlated to its mass and doesn’t depend on the strength of the interaction with supernova. The duration of the swelling is also longer when the two stars are closer in distance.
“We applied our results to a supernova called SN2006jc, which has a companion star with a low-temperature. If this is in fact an inflated star as we believe, we expect it should rapidly shrink in the next few years,” explains Hirai
The number of companion stars detected after supernovae are steadily growing over the years. If scientists can observe an inflated companion star and its contraction, these data correlations can measure the properties of the binary system before the explosion—these insights are extremely rare and important for understanding how massive stars evolve.
“We think it’s important to not only find companion stars after supernovae, but to monitor them for a few years to decades to see if it shrinks back,” says Hirai.
As featured on Phys.org.
Have you heard the joke about how many stars it takes to create a merging binary black hole? Hundreds of thousands in the real world… but only a few, if they’re OzStars and you’re using the latest COMPAS version with machine learning tools.
Five years on from the first discovery of gravitational waves, an international team of scientists, including from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), are continuing the hunt for new discoveries and insights into the Universe. Using the super-sensitive, kilometre-sized LIGO detectors in the United States, and the Virgo detector in Europe, the team have witnessed the explosive collisions of black holes and neutron stars. Recent studies, however, have been looking for something quite different: the elusive signal from a solitary, rapidly-spinning neutron star.
Take a star similar in size to the Sun, squash it down to a ball about twenty kilometres across ━ roughly the distance from Melbourne airport to the city centre ━ and you’d get a neutron star: the densest object in the known Universe. Now set your neutron star spinning at hundreds of revolutions per second and listen carefully. If your neutron star isn’t perfectly spherical, it will wobble about a bit, and you’ll hear a faint “humming” sound. Scientists call this a continuous gravitational wave.
So far, these humming neutron stars have proved elusive. As OzGrav postdoctoral researcher Karl Wette from the Australian National University explains: “Imagine you’re out in the Australian bush listening to the wildlife. The gravitational waves from black hole and neutron star collisions we’ve observed so far are like squawking cockatoos ━ loud and boisterous, they’re pretty easy to spot! A continuous gravitational wave, however, is like the faint, constant buzz of a faraway bee, which is much more difficult to detect. So we’ve got to use a few different strategies. Sometimes we hone in on a particular direction ━ for example, a flowering bush where bees are likely to congregate. Other times, we close our eyes and listen keenly to all the sounds we can hear, and try to pick out any buzzing sounds in the background. So far, we haven’t had any luck, but we’ll keep trying! Once we do hear a continuous gravitational wave, we’ll be able to peer deep into the heart of a neutron star and unravel its mysteries, which is an exciting prospect.”
A recent collaborative study with OzGrav has taken a closer look at the remnants of exploded stars, called supernovae. OzGrav PhD student Lucy Strang from the University of Melbourne explains: “Our search targets fifteen young supernova remnants containing young neutron stars. We use three different pipelines: one optimized for sensitivity, one that can handle a rapidly evolving signal, and one optimized for one likely astrophysical scenario. This is the first LIGO study covering all three of these scenarios, maximising our chance of a continuous wave detection. Continuous gravitational waves are proving very difficult to detect, but the same properties that make them elusive make them appealing targets. The exact form of the signal (i.e. its frequency, how rapidly the frequency changes, how loud it is, etc.) is dependent on what neutron stars are made of. So far, the structure of neutron stars is an open question that draws in all kinds of physicists. Even without a detection, a search allows us to peek behind the curtain at the unknown physics of neutron stars. When we do detect continuous waves, we'll open the curtain and shine a spotlight on new physics. Until then, we can use the information we do have to refine our understanding and improve our search methods.”
OzGrav Associate Investigator Lilli Sun from the Australian National University says: “Young neutron stars in supernova remnants are promising targets to look for those tiny continuous gravitational waves, because they haven't spent a long enough time to relax and smooth out the asymmetries introduced at their birth. In our endeavor to search for continuous waves from these young neutron stars in our third observing run, we take into consideration, for the first time, the possibilities that the interior configuration and structure of the star can result in signals emitted at two different harmonics. Although no signal has been detected in O3, we set interesting constraints on the neutron star properties. If such a signal can be detected in future observations when the detectors are more sensitive, it will shed light on the fascinating structure of a neutron star.”
OzGrav postdoctoral researcher Carl Blair from the University of Western Australia says: “Gravitational waves are being used to probe the most exotic objects in the Universe. Neutron stars ━ composed of matter collapsed in on itself like a giant atomic nuclei ━ have to be one of the most exotic. We don’t know that much about neutron stars because they’re so small and strange. Are they hard or soft? And when they spin fast as they collapse, do they wobble away that energy in the form of gravitational waves? While there is no evidence yet for continuous gravitational waves from neutron stars, limits have been placed on how wobbly a neutron star is from the fact that we haven’t measured gravitational waves from them yet.”
In addition, recent studies announced by the international research team ━ including the U.S./international LIGO Scientific Collaboration, European Virgo Collaboration and Japanese KAGRA Collaboration ━ have focussed on pulsars. These are neutron stars which act as cosmic lighthouses, beaming out copious energy in the form of radio waves. Pulsars are like giant spinning magnets, except they’re billions of times stronger than the ones stuck to your fridge. So strong, in fact, that the magnetic field distorts the shape of the neutron star, and may lead to a tell-tale hum of continuous gravitational waves. While the recent studies did not pick up anything, they found tight constraints on how loud the “hum” could be, which, in some cases, are starting to challenge theoretical predictions.
OzGrav PhD student Deeksha Beniwal from the University of Adelaide says: “Gravitational-wave observation from O3 run of LIGO and Virgo detectors has allowed us to set realistic constraints on signals expected from young pulsars. O3 observations also provide an opportunity to test out different pipelines ━ such as different search methods for continuous wave signals ━ in realistic environments.”
OzGrav postdoctoral researcher Meg Millhouse from the University of Melbourne says: “Continuous gravitational waves from neutron stars are much smaller than the gravitational waves LIGO and Virgo have seen so far. This means we need different techniques to detect them. And, because these are long lasting signals, we need to look at lots of data which can be very difficult computationally. The recent LIGO-Virgo papers published showcase a wide range of these clever approaches to detect continuous gravitational waves. Even though there were no detections in the most recent data analysed, we’re in a good position to keep searching and possibly make a detection when LIGO collects more data.”
Scientists estimate that there are billions of neutron stars in the Milky Way with a faint murmur of continuous gravitational waves. Further studies have therefore taken an “ears wide open” approach, combing through the LIGO and Virgo data for any hint of a signal. The results so far suggest that these murmurings are extremely quiet and out of the detectors’ “ear” range. However, as detector technology becomes more advanced and sensitive, the first ever detection of continuous gravitational waves could soon become a reality.
Also featured on Cosmos Magazine, Space Australia, IFL Science and Sci Tech Daily
Gravitational-wave scientists propose new method to refine the Hubble Constant—the expansion and age of the Universe
A team of international scientists, led by the Galician Institute of High Energy Physics (IGFAE) and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), has proposed a simple and novel method to bring the accuracy of the Hubble constant measurements down to 2%, using a single observation of a pair of merging neutron stars.
The Universe is in continuous expansion. Because of this, distant objects such as galaxies move away from us. In fact, the further away they are, the faster they move. Scientists describe this expansion through a famous number known as the Hubble constant, which tells us how fast objects in the Universe recede from us depending on their distance to us. By measuring the Hubble constant in a precise way, we can also determine some of the most fundamental properties of the Universe, including its age.
For decades, scientists have measured Hubble’s constant with increasing accuracy, collecting electromagnetic signals emitted throughout the Universe but arriving at a challenging result: the two current best measurements give inconsistent results. Since 2015, scientists have tried to tackle this challenge with the science of gravitational waves: ripples in the fabric of space-time that travel at the speed of light. Gravitational waves are generated in the most violent cosmic events and provide a new channel of information about the Universe. They’re emitted during the collision of two neutron stars—the dense cores of collapsed stars–and can help scientists dig deeper into the Hubble constant mystery.
Unlike black holes, merging neutron stars produce both gravitational and electromagnetic waves, such as x-rays, radio waves and visible light. While gravitational waves can measure the distance between the neutron-star merger and Earth, electromagnetic waves can measure how fast its whole galaxy is moving away from Earth. This creates a new way to measure the Hubble constant. However, even with the help of gravitational waves, it’s still tricky to measure the distance to neutron-star mergers--that’s, in part, why current gravitational-wave based measurements of the Hubble constant have an uncertainty of ~16%, much larger than existing measurements using other traditional techniques.
In a recently published article in the prestigious journal The Astrophysical Journal Letters, a team of scientists led by ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and Monash University alumni Prof Juan Calderón Bustillo (now La Caixa Junior Leader and Marie Curie Fellow at the Galician institute of High Energy Physics of the University of Santiago de Compostela, Spain), has proposed a simple and novel method to bring the accuracy of these measurements down to 2% using a single observation of a pair of merging neutron stars.
According to Prof Calderón Bustillo, it’s difficult to interpret how far away these mergers occur because ‘currently, we can’t say if the binary is very far away and facing Earth, or if it’s much closer, with the Earth in its orbital plane’. To decide between these two scenarios, the team proposed to study secondary, much weaker components of the gravitational-wave signals emitted by neutron-star mergers, known as higher modes. ‘Just like an orchestra plays different instruments, neutron-star mergers emit gravitational waves through different modes,’ explains Prof Calderón Bustillo. ‘When the merging neutron stars are facing you, you will only hear the loudest instrument. However, if you are close to the merger’s orbital plane, you should also hear the secondary ones. This allows us to determine the inclination of the neutron-star merger, and better measure the distance’.
However, the method is not completely new: ‘We know this works well for the case of very massive black hole mergers because our current detectors can record the merger instant when the higher modes are most prominent. But in the case of neutron stars, the pitch of the merger signal is so high that our detectors can’t record it. We can only record the earlier orbits,’ says Prof Calderón Bustillo.
Future gravitational-wave detectors, like the proposed Australian project NEMO, will be able to access the actual merger stage of neutron stars. ‘When two neutron stars merge, the nuclear physics governing their matter can cause very rich signals that, if detected, could allow us to know exactly where the Earth sits with respect to the orbital plane of the merger,’ says co-author and OzGrav Chief Investigator Dr Paul Lasky, from Monash University. Dr Lasky is also one of the leads on the NEMO project. ‘A detector like NEMO could detect these rich signals,’ he adds.
In their study, the team performed computer simulations of neutron-star mergers that can reveal the effect of the nuclear physics of the stars on the gravitational waves. Studying these simulations, the team determined that a detector like NEMO could measure Hubble’s constant with a precision of 2%.
Co-author of the study Prof Tim Dietrich, from the University of Potsdam, says: ‘We found that fine details describing the way neutrons behave inside the star produce subtle signatures in the gravitational waves that can greatly help to determine the expansion rate of the Universe. It is fascinating to see how effects at the tiniest nuclear scale can infer what happens at the largest possible cosmological one’.
Samson Leong, undergraduate student at The Chinese University of Hong Kong and co-author of the study points out “one of the most exciting things about our result is that we obtained such a great improvement while considering a rather conservative scenario. While NEMO will indeed be sensitive to the emission of neutron-star mergers, more evolved detectors like Einstein Telescope or Cosmic Explorer will be even more sensitive, therefore allowing us to measure the expansion of the Universe with even better accuracy!”.
One of the most outstanding implications of this study is that it could determine if the Universe is expanding uniformly in space as currently hypothesised. 'Previous methods to achieve this level of accuracy rely on combining many observations, assuming that the Hubble constant is the same in all directions and throughout the history of the Universe,’ says Calderón Bustillo. ‘In our case, each individual event would yield a very accurate estimate of “its own Hubble constant”, allowing us to test if this is actually a constant or if it varies throughout space and time.’