The surfaces of neutron stars may feature mountains, albeit ones that are no more than millimetres tall, new research has revealed. The minuscule scale of neutron star mountains is a result of the intense gravity produced by these stellar remnants that are the second densest objects in the Universe after black holes.
Because neutron stars have the mass equivalent to a star like the Sun compressed into a diameter that is about the size of a city on Earth–about 10km– they have a gravitational pull at their surface that is as much as 40,000 billion times stronger than Earth’s.
This presses features on that surface flat, making for almost perfect spheres. Yet the new research, presented at the National Astronomy Meeting 2021 shows that these stellar remnants do feature some tiny topological deformations, analogous to mountains on a planet’s surface.
The finding was a result of complex computer modelling by a team of researchers led by the University of Southhampton’s Fabian Gittins. The Ph.D. student’s team simulated a realistic neutron star and then calculated the forces acting upon it. What the research really shows is how well neutron stars can support deviations from a perfect sphere without its crust being strained beyond breaking point.
This revealed how mountains could be created on such dense stellar remnants and demonstrated that such formations would be no taller than a fraction of a millimetre.
“For the past two decades, there has been much interest in understanding how large these mountains can be before the crust of the neutron star breaks, and the mountain can no longer be supported,” says Gittins. These results show how neutron stars truly are remarkably spherical objects. “Additionally, they suggest that observing gravitational waves from rotating neutron stars maybe even more challenging than previously thought”.”
Mountain formation has been formulated for neutron stars before, but these new findings suggest such features would be hundreds of times smaller than the mountains of a few centimetres previously predicted. This is because those older models took the crusts of neutron stars to the edge of breaking point at every single point; something the up-to-date research suggests is less than realistic.
Neutron stars form when massive stars run out of fuel to power nuclear fusion. This means that the toward force balancing against gravity’s inward pull is cancelled and leads to the gravitational collapse of the star. During the course of this collapse, the massive star ejects its outer material in supernova explosions and leaving behind a core of ultradense material. This stellar remnant is only protected from further collapse–and in turn, becoming a black hole–by the quantum mechanical properties of the neutron-rich material that composes it.
The finding may have implications that go beyond the modelling of neutron stars. Tiny deformations on the surface of rapidly spinning neutron stars called pulsars could launch gravitational waves–the tiny ripples in spacetime predicted by general relativity and detected here on Earth by the LIGO/Virgo collaboration.
Unfortunately, as precise and sensitive as the LIGO laser interferometer is, it is still not powerful enough to detect gravitational waves launched by these ant-hill like mountains. It is possible that future upgrades to these Earth-based detectors and advancements such as the space-based gravitational wave detector LISA could make observing the effect of these tiny bumps possible.
Whilst it may not have the snappiest name, the event GW150914 is pretty significant in terms of our understanding of the Universe. This event, with a name that includes ‘GW’ as a prefix which is an abbreviation of ‘Gravitational Wave’ and the date of observation–15/09/14– marked humanity’s first direct detection of gravitational waves.
This was groundbreaking on two fronts; firstly it successfully confirmed a prediction made by Albert Einstein’s theory of general relativity almost a century before. A prediction that stated events occurring in the Universe do not just warp spacetime, but in certain cases, can actually send ripples through this cosmic fabric.
The second significant aspect of this observation was the fact that it represented an entirely new way to ‘see’ the Universe, its events and objects. This new method of investigating the cosmos has given rise to an entirely new form of astronomy; multimessenger astronomy. This combines ‘traditional’ observations of the Universe in the electromagnetic spectrum with the detection of gravitational waves, thus allowing us to observe objects that were previously invisible to us.
Thus, the discovery of gravitational waves truly opened up an entirely new window on the cosmos, but what are gravitational waves, what do they reveal about the objects that create them, and how do we detect such tiny tremblings in reality itself?
Gravitational Waves: The Basics
Gravitational waves are ripples in the fabric of spacetime.
These ripples travel from their source at the speed of light.
The passage of gravitational waves squash and stretch space itself.
Gravitational waves can be detected by measuring these infinitesimally small changes in the distance between objects.
They are created when an object or an event that curves spacetime causes that curvature to change shape.
Amongst the causes of gravitational waves are colliding black holes and neutron stars, supernovae, and stars that are undergoing gravitational collapse.
Imagine sitting at the side of a lake, quietly observing the tranquil surface of the water undisturbed by nature, the wind, or even by the slightest breeze. Suddenly a small child runs past hurling a pebble into the lake. The tranquillity is momentarily shattered. But, even as peace returns, you watch ripples spread from the centre of the lake diminishing as they reach the banks, often splitting or reflecting back when they encounter an obstacle.
The surface of the lake is a loose 2D analogy for the fabric of spacetime, the pebble represents an event like the collision of two black holes, and our position on Earth is equivalent to a blade of grass on the bank barely feeling the ripple which has diminished tremendously in its journey to us.
Gravitational waves were first predicted by Henri Poincare in 1905 as disturbances in the fabric of spacetime that propagate at the speed of light, but it would take another ten years for the concept to really be seized upon by physicists. This happened when Albert Einstein predicted the same phenomenon as part of his revolutionary 1916 geometric theory of gravity, better known as general relativity.
Whilst this theory is most well-known for suggesting that objects with mass would cause warping of spacetime, it also went a step further positing that an accelerating object should change this curvature and cause a ripple to echo through spacetime. Such disturbances in spacetime would not have been permissible in the Newtonian view of gravity which saw the fabric of space and time as separate entities upon which the events of the Universe simply play out.
But upon Einstein’s dynamic and changing stage of united spacetime, such ripples were permissible.
Gravitational waves arose from the possibility of finding a wave-like solution to the tensor equations at the heart of general relativity. Einstein believed that gravitational waves should be generated en masse by the interaction of massive bodies such as binary systems of super-dense neutron stars and merging black holes.
The truth is that such ripples in spacetime should be generated by any accelerating objects but Earth-bound accelerating objects cause perturbations that are far too small to detect. Hence why our investigations must turn to areas of space where nature provides us with objects that are far more massive.
As these ripples radiate outwards from their source in all directions and at the speed of light, they carry information about the event or object that created them. Not only this, but gravitational waves can tell us a great deal about the nature of spacetime itself.
Where do Gravitational Waves Come From?
There are a number of events that can launch gravitational waves powerful enough for us to detect with incredibly precise equipment here on Earth. These events are some of the most powerful and violent occurrences that the Universe has to offer. For instance, the strongest undulations in spacetime are probably caused by the collision of black holes.
Other collision events are associated with the production of strong gravitational waves; for example the merger between a black hole and a neutron star, or two neutron stars colliding with each other.
But, a cosmic body doesn’t always need a partner to make waves. Stellar collapse through supernova explosion–the process that leaves behind stellar remnants like black holes and neutron stars– also causes the production of gravitational waves.
To understand how gravitational waves are produced, it is useful to look to pulsars–binary systems of two neutron stars that emit regular pulses of electromagnetic radiation in the radio region of the spectrum.
Einstein’s theory suggests that a system such as this should be losing energy by the emission of gravitational waves. This would mean that the system’s orbital period should be decreasing in a very predictable way.
The stars draw together as there is less energy in the system to resist their mutual gravitational attraction, and as a result, their orbit increases in speed, and thus the pulses of radio waves are emitted at shorter intervals. This would mean that the time it takes for the radio wave to be directly facing our line of sight would be reduced; something we can measure.
This is exactly what was observed in the Hulse-Taylor system (PSR B1913±16), discovered in 1974, which is comprised of two rapidly rotating neutron stars. This observation earned Russell A. Hulse and Joseph H. Taylor, Jr, both of Princeton University, the 1993 Nobel Prize in Physics. The reason given by the Nobel Committee was: “for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation.”
Though inarguably an impressive and important scientific achievement, this was still only indirect evidence of gravitational waves. Whilst the effect Einstein predicted of shortening of the pulsar’s spin was definitely present, this wasn’t an actual direct detection.
In fact, though not alive to witness this momentous achievement, Einstein had predicted that this would be the only way we could ever garner any hint of gravitational waves. The great physicist believed those spacetime ripples would be so faint that they would remain impossible to detect by any technological means imaginable at that time.
Fortunately, Einstein was wrong.
How do we Detect Gravitational Waves?
It should come as no surprise that actually detecting a gravitational wave requires a piece of equipment of tremendous sensitivity. Whilst the effect of gravitational waves–the squashing and stretching space itself–sounds like something that should pre-eminently visible, the degree by which this disturbance occurs is so tiny it is totally imperceptible.
Fortunately, there is a branch of physics that is pretty adept at deal with the tiny. To spot gravitational waves, researchers would use an effect called interference, something demonstrated in the most famous quantum physics experiment of all time; the double-slit experiment.
Physicists realised that a laser interferometer could be used to measure the tiny squashing and stretching of space as it would cause the arms of the equipment to shrink by a minute amount. This means when splitting a laser and sending it through the arms of an interferometer the squeezing of space caused by the passage of a gravitational wave would cause one laser to arrive slightly ahead of the other–meaning they are out of phase and causing destructive interference. Thus, this difference in arrival times causes interference that gives an indication that gravitational waves have rippled across one of the arms.
But, not just any laser interferometer would do. Physicists would need an interferometer so large that it constituents a legitimate feat in engineering. Enter the Laser Interferometer Gravitational-wave Observatory (LIGO).
The LIGO detector uses two laser emitters based at the Hanford and Livingstone observatories, separated by thousands of kilometres apart to form an incredibly sensitive interferometer. From these emitters, lasers are sent down the ‘arms’ of the interferometer which are actually 4km long vacuum chambers.
This results in a system that is so sensitive it can measure a deviation in spacetime that is as small as 1/10,000 the size of an atomic nucleus. To put this into an astronomical context; it is equivalent to spotting a star at a distance of 4.2 light-years and pinpointing its location to within the width of a human hair! This constitutes the smallest measurement ever practically attempted in any science experiment.
And in 2015, this painstaking operation paid off.
On 14th September 2015, the LIGO and Virgo collaboration spotted a gravitational wave signal emanating from the spiralling in and eventual merger of two black holes, one 29 times the mass of the Sun, the other 36 times our star’s mass. From changes in the signal received the scientists were also able to observe the resultant single black hole.
The signal, named GW150914, represented not just the first observation of gravitational waves, but also the first time humanity had ‘seen’ a binary stellar-mass black hole system, proving that such mergers could exist in the Universe’s current epoch.
Different Kinds of Gravitational Waves
Since the initial detection of gravitational waves, researchers have made a series of important and revelatory detections. These have allowed scientists to classify different types of gravitational waves and the objects that may produce them.
Continuous Gravitational Waves
A single spinning massive object like a neutron star is believed to cause a continuous gravitational wave signal as a result of imperfections in the spherical shape of this star. if the rate of spin remains constant, so too are the gravitational waves it emits–it is continuously the same frequency and amplitude much like a singer holding a single note. Researchers have created simulations of what an arriving continuous gravitational wave would sound like if the signal LIGO detected was converted into a sound.
The sound of a continuous gravitational wave of the kind produced by a neutron star can be heard below.
Compact Binary Inspiral Gravitational Waves
All of the signals detected by LIGO thus far fit into this category as gravitational waves created by pairs of massive orbiting objects like black holes or neutron stars.
The sources fit into three distinct sub-categories:
Binary Black Hole (BBH)
Binary Neutron Star (BNS)
Neutron Star-Black Hole Binary (NSBH)
Each of these types of binary pairing creates its own unique pattern of gravitational waves but shares the same overall mechanism of wave-generation–inspiral generation. This process occurs over millions of years with gravitational waves carrying away energy from the system and causing the objects to spiral closer and closer until they meet. This also results in the objects moving more quickly and thus creating gravitational waves of increasing strength.
The ‘chirp’ of an eventual merger between neutron stars has been translated to sound waves and can be heard below.
Stochastic Gravitational Waves
Small gravitational waves that even LIGO is unable to precisely pinpoint could be passing over Earth from all directions at all times. These are known as stochastic gravitational waves due to their random nature. At least part of this stochastic signal is likely to have originated in the Big Bang.
Should we eventually be able to detect this signal it would allow us to ‘see’ further back into the history of the Universe than any electromagnetic signal could, back to the epoch before photons could freely travel through space.
The simulated sound of this stochastic signal can be heard below.
It is extremely likely given the variety of objects and events in the Universe that other types of gravitational wave signals exist. This means that the quest to detect such signals is really an exploration of the unknown. Fortunately, our capacity to explore the cosmos has been boosted tremendously by our ability to detect gravitational waves.
A New Age of Astronomy
GW150914 conformed precisely to the predictions of general relativity, confirming Einstein’s most revolutionary theory almost exactly six decades after his death in 1955. That doesn’t mean that gravitational waves are done teaching us about the Universe. In fact, these ripples in spacetime have given us a whole new way to view the cosmos.
Before the discovery of gravitational waves, astronomers were restricted to a view of the Universe painted in electromagnetic radiation and therefore our observations have been confined to that particular spectrum.
Using the electromagnetic spectrum alone, astronomers have been able to discover astronomical bodies and even thecosmic microwave background (CMB) radiation, a ‘relic’ of one of the very first events in the early universe, the recombination epoch when electrons joined with protons thus allowing photons to begin travelling rather than endlessly scattering. Therefore, the CMB is a marker of the point the universe began to be transparent to light.
Yet despite the strides traditional astronomy has allowed us to make in our understanding of the cosmos, the use of electromagnetic radiation is severely limited. It does not allow us to directly ‘see’ black holes, from which light cannot escape. Nor does it allow us to see non-baryonic, non-luminous dark matter, the predominant form of matter in galaxies–accounting for around 85% of the universe’s total mass. As the term ‘non-luminous’ suggests dark matter does not interact with the electromagnetic spectrum, it neither absorbs nor emits light. This means that observations in the electromagnetic spectrum alone will never allow us to see the majority of the matter in the universe.
Clearly, this is a problem. But one that can be avoided by using the gravitational wave spectrum as both black holes and dark matter do have considerable gravitational effects.
Gravitational waves also have another significant advantage over electromagnetic radiation.
This new form of astronomy measures the amplitude of the travelling wave, whilst electromagnetic wave astronomy measures the energy of the wave, which is proportional to the amplitude of the wave squared.
Therefore the brightness of an object in traditional astronomy is given by 1/distance² whilst ‘gravitational brightness’ falls off by just 1/distance. This means that the visibility of stars persists in gravitational waves for a much greater distance than the same factor persists in the electromagnetic spectrum.
Of course, none of this is to suggest that gravitational wave astronomy will ‘replace’ traditional electromagnetic spectrum astronomy. In fact the two are most powerful when they are unified in an exciting new discipline–multimessenger astronomy
Sources and Further Reading
Maggiore. M., Gravitational Waves: Theory and Experiments, Oxford University Press, 
Maggiore. M., Gravitational Waves: Astrophysics and Cosmology, Oxford University Press, 
Collins. H., Gravity’s Kiss: The Detection of Gravitational Waves, MIT Press, 
Gravitational waves have been detected from what appears to be the largest black hole merger ever observed. The powerful and previously unobserved hierarchical merger resulted in an intermediate-mass black hole, an object never before detected.
A massive burst of gravitational waves equivalent to the energy output of eight Suns has been detected by the LIGO laser interferometer. Researchers at LIGO and its sister project VIRGO believe that the waves originate from a merger between two black holes. But, this isn’t your average black hole merger (if there is such a thing). The merger — identified as gravitational wave event GW190521 — is not only the largest ever detected in gravitational waves — but it is also the first recorded example of what astrophysicists term a ‘hierarchical merger’ occurring between two black holes of different sizes, one of which was born from a previous merger.
“This doesn’t look much like a chirp, which is what we typically detect,” says Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS), comparing the signal to LIGO’s first detection of gravitational waves in 2015. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”
Even more excitingly, it seems that black hole birthed in the event has a mass of between 100–1000 times that of the Sun, putting it in the mass range of an intermediate-mass black hole (IMBH). Something that researchers have theorised about for decades, but up until now, have failed to detect.
The gravitational wave signal–spotted by LIGO on 21st May 2019–appears to the untrained eye as little more than four short squiggles that lasted little more than one-tenth of a second, but for Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration, whose group focuses on the development of highly-accurate waveform models, it holds a wealth of information. “It’s amazing, but from about four gravitational-wave cycles, we could extract unique information about the astrophysical source,” she tells ZME Science.
“The waves are fingerprints of the source that has produced them.”
Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration.
As well as containing vital information about black holes and a staggering merger event, as the signal originated 17 billion light-years from Earth and at a time when the Universe was half its current age, it is also one of the most distant gravitational wave sources ever observed. The incredible distance the signal has travelled may initially seem at odds with the fact that the Universe is only 14.8 billion years old, but the disparity arises from the fact our universe is not static but is expanding.
Details of the international team’s important findings are featured in a series of papers publishing in journals such as Physical Review Letters, and The Astrophysical Journal Letters, today.
Missing Intermediete-Mass Black Holes
Thus far, the black holes discovered by astronomers have either been those with a mass inline with that of larger stars–so-called stellar-mass black holes, or supermassive black holes, with masses far exceeding this. Black holes that exist between these masses have remained, frustratingly hidden. Until now.
“The LIGO and Virgo collaborations detected a gravitational wave corresponding to a very interesting black hole merger. This was named GW190521 and corresponds to two large black holes during the final orbit and merger,” Pedro Marronetti, program director for gravitational physics at the National Science Foundation (NSF) responsible for the oversight of LIGO, tells ZME Science.
“What makes GW190521 extraordinary in comparison to other gravitational wave events is the mass of the black holes involved, the product of the merger is a 142 solar mass black hole and the first object of its kind with mass above 100 solar masses but below a million solar masses to be discovered.”
Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)
Thus, the resultant black hole of 142 solar masses exists in that crucial, thus far undetected, mass range indicating an intermediate-mass black hole (IMBH).
“These black holes, heavier than 100 solar masses but much lighter than the supermassive black holes at the centre of galaxies — which can be millions and billions of solar masses — have eluded detection until now,” Marronetti says. “Additionally, the heavier of the original black holes with 85 solar masses also presents an enigma.”
Pair Instability and Hierarchical Black Hole Mergers
The enigma that Marronetti refers to is the fact that heavier of the two black holes that entered the merger, is of a size that suggests it too must have been created by a merger event between two, even smaller, black holes. “The most common channel of formation of black holes involves heavy stars that end their lives in supernova explosions,” the NSF program director points out. “However, this formation channel prevents the creation of black holes heavier than 65 solar masses but lighter than 130 solar masses due to a phenomenon called ‘pair-instability’.”
As nuclear fusion ceases, there is no longer enough outward radiation pressure to prevent gravitational collapse. “The star suddenly starts producing photons that are energetic enough to create electron-positron pairs,” Marronetti explains. “These photons, in turn, create an outward pressure that is not strong enough to stop the star from collapsing violently due to its self-gravitational pull.”
This results in a difference in gravitational pressure between the star’s core and its outer layers. As a massive shock travels through these ‘puffed out’ outer layers they are blown away in a massive explosion. With smaller stars, this leaves behind an exposed core that becomes a stellar remnant such as a white dwarf, neutron star or black hole. But if the star is a range above 130 solar masses, but below 200 solar masses, the result is more disastrous.
“The resulting supernova explosion completely obliterates the star, leaving nothing behind– no black hole or neutron star is produced,” Marronetti says. “It will take stars heavier that 200 solar masses to collapse into black hole fast enough to avoid this complete disintegration.”
As Marronetti points out; this means that the 85-solar mass back hole could only be formed by the merger of two smaller black holes, as at these masses, collapsing stars can’t form black holes. “This is a quite unusual event that can only occur in regions of dense black hole population such as globular clusters,” the researcher adds. “GW190521 is the first detection that is likely to be due to this ‘hierarchical merger’ of black holes.”
Marronetti continues by explaining that a hierarchical merger consists of one or more black holes that were produced by a previous black hole merger. This hierarchy of mergers allows for the formation of progressively heavier and heavier black holes from an original population of small ones.
“We don’t really know how common these hierarchical mergers are since this is the first time we have direct evidence of one. We can only say that they are not very common.”
Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)
LIGO Delivering Discoveries and Surprises
The team uncovered the unusual nature of this particular merger by assessing the gravitational wave signal with a powerful state of the art computational models. Not only did this reveal that GW190251 originates from the most massive black hole merger ever observed and that this was no ordinary merger but a hierarchical merger, but also crucial information about the black holes involved in the event.
“The signal carries information about the masses and spins of the original back holes as well as their final product,” Marronetti adds, alluding to the fact that the LIGO -VIRGO team were able to measure that spin and determine that as the black holes circled together, they were also spinning around their own axes. The angles of these axes appeared to have been out of alignment with the axes of their orbit. This misaligned spin caused the black holes to ‘wobble’ as they moved together.
“Our waveform models were used to detect GW190521 and also to interpret its nature, extracting the properties of the source, such as masses, spins, sky location, and distance from Earth. For the first time, the waveform models included new physical effects, notably the precession of the spins of the black holes and higher harmonics,” Buonanno says. “What we mean when we say higher harmonics is like the difference in sound between a musical duet with musicians playing the same instrument versus different instruments.
“The more substructure and complexity the binary has — for example, black holes with different masses or spins—the richer is the spectrum of the radiation emitted.”
Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration.
Unanswered Questions and Future Investigations
Even with the staggering amount of information the team has been able to collect about the merger that gave rise to the signal GW190251, there are still some unanswered questions and details that must be confirmed.
The LIGO-VIRGO detectors use two very distinct methods to search the Universe for gravitational waves, an algorithm to pick out a specific wave pattern most commonly produced by compact binary mergers, and more general ‘burst’ searches. The latter searches for any signal ‘out of the ordinary’ and it’s the mechanism via which the researchers found GW190215.
Morronetti expresses some surprise that the methods used by the team were able to unlock these secrets, believing that this result demonstrates the versatility of LIGO. “My main surprise was that this event was detected using a search algorithm that was not specifically created to find merger signals,” says the NSF director. “This is the first detection of its kind and shows the capability of LIGO to detect phenomena beyond compact mergers.”
“This is of tremendous importance since it showcases the instrument’s ability to detect signals from completely unforeseen astrophysical events. LIGO shows that it can also observe the unexpected.”
Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)
This leaves open the small chance that the signal was created by something other than a hierarchical merger. Perhaps something entirely new. The authors hint at the tantalising prospect of some new phenomena, hitherto unknown, in their paper, but Marronetti is cautious: “By far, the most likely cause is the merger of two black holes, as explained above. However, this is not as certain as with past LIGO/Virgo detections.
“There is still the small chance that the signal was caused by a different phenomenon such as a supernova explosion or an event during the Big Bang. These scenarios are possible but highly unlikely.”
Confirming the nature of the event that gave rise to the GW190251 signal is something that the LIGO team will be focusing on in the future as the interferometer also searches for similar events via the gravitational waves they emit. “
With GW190521, we have seen the tip of the iceberg of a new population of black holes,” Buonanno says, adding that LIGO’s next operating run (O4) will explore a volume of space 3 times larger than the current run (O3). “Having access to a larger number of events, which were too weak to be observed during O3, will allow us to shed light on the formation scenario of binary black holes like GW150921.”
Supermassive black holes sat at the centre of active galaxies could have company. Binary pairs of these titanic cosmic objects could merge to form an even more monstrous black hole. Observational methods are finally becoming sensitive enough to spot such an event.
The image of a supermassive black hole sat monolithic and alone at the centre of its galaxy, mercilessly swallowing any matter unfortunately enough to cross its path could be seriously challenged over the coming years. Theories of how galaxies grow and evolve and the role supermassive black holes play in these processes have long suggested that these objects may not dwell alone. In fact, not only may such spacetime events live in pairs, but after being brought together, they may merge in what could be the most powerful single event in the Universe, profoundly affecting its evolution.
“Astrophysical black holes are among the most fascinating objects in the Universe: they are ideal laboratories to study the fundamental laws of physics and one of the main drivers of the evolution of the Universe,” explains Alessandra De Rosa, a research astrophysicist at the National Institute of Astrophysics, Roma, Italy. “Understanding how they work and interact with their close environment, and unveil the physical conditions of the medium around them is one of the major challenges of 21st-century Astrophysics.”
Understanding the relationship between black holes and the galaxies that host them is key to building a model that satisfactorily describes the evolution of both. But, thus far evidence of this process is sparse. So, why are supermassive black hole mergers so hard to spot?
Hidden in Plain Sight. How Supermassive Black Hole Binaries and Mergers evade Observation
Despite the potential power of such a merger event, we haven’t as of yet managed to distinguish individual binary supermassive black holes or much evidence that such collisions occur. This is because these pairings and the mergers that may eventually arise from them lurk in what is known as the Active Galactic Nuclei (AGN) — compact regions at the centre of galaxies where the electromagnetic emissions dwarf that of the entire galaxy which surrounds it.
Because this emission — which occurs from the radio wave to the gamma-ray regions of the electromagnetic spectrum — is so powerful, astronomers believe that it does not arise as a result of stellar activity. Rather, they theorise that the powerful electromagnetic radiation emitted by the AGN is the result of at least one supermassive black hole accreting matter — a violent process in which dust, gas, and even stars are ripped apart in a violent and tremendously hot accretion disc surrounding a central supermassive black hole before falling onto what can roughly be described as its ‘surface.’
Here’s the problem; that electromagnetic emission is so overwhelmingly powerful and the AGN is so small in comparion to its host galaxy that there is no way that traditional astronomy — which relies on electromagnetic signals — alone, can distinguish the finer detail of this region. Finer detail that could reveal occupation by two, rather than just one, supermassive black holes.
“Currently, observational evidence for these pairs is almost non-existent,” De Rosa laments. “This can be explained if they quickly shrink to small separations and become impossible to be resolved with telescopes as pairs. So, we must rely on indirect signatures.”
Fortunately, supermassive black hole mergers, if they occur, would not just be prodigious producers of electromagnetic radiation. They should also produce intense gravitational wave signals.
De Rosa is the lead author of a review paper published in the journal New Astronomy Reviews that looks both the history of our search for supermassive black hole binaries and puts forward a road map for future discovery of such events. The researcher emphasises the importance of ‘multimessenger astronomy’ — which combines traditional electromagnetic observations with the detection of gravitational waves, allowing astronomers to view the Universe in an entirely new way, thus making events and objects are previously hidden to them — events like black hole mergers — accessible.
But before examing mergers, it’s worth considering the truly epic processes that bring supermassive black hole pairings together in the first place.
Cosmic Matchmaking: Bringing Together Supermassive Black Holes
It may not be too surprising to find supermassive black holes hanging-out together in pairs, as our observations of the Universe thus far, show that stellar objects seem to prefer to hang out in pairs. These binary systems are far more common than single star systems such as our solar system, and three-star systems — the latter of which prove to be far too unstable.
“A binary supermassive black hole is made up of two supermassive black holes that are orbiting around each other,” says Julie Comerford, an Associate Professor in the Department of Astrophysical and Planetary Sciences at the University of Colorado, Boulder who specializes in the study of AGNs. “Such binary systems are common in the universe — around half of all stars are in binary systems, where two stars are orbiting around each other.”
As black holes evolve from such stellar objects, and these objects enjoy the company, it would seem intuitive to believe that black hole binaries should be fairly common. There’s a problem with that thinking though.
Only the most massive stars end their lives as black holes, and supermassive black holes are even rarer. Couple that with the fact that most binary systems contain a massive star coupled with a much smaller counterpart. Thus, It’s quite unlikely that two stars in the same binary system would both end up as supermassive black holes. In fact, after the transformation of the first star, it’s likely its partner will be stripped of material and left as a neutron star, a much smaller white dwarf, or destroyed entirely– possibly consumed by its counterpart.
So, if supermassive binaries aren’t likely to grow together, this means that some event must create this union– the merger of two galaxies.
“Each massive galaxy has a supermassive black hole at its centre, so the way you make a supermassive black hole binary is by merging two galaxies together,” Comerford tells ZME Science. “Each galaxy brings its own supermassive black hole to the merger, and as the galaxies combine the supermassive black holes begin their own dance of orbiting around each other.”
This means that spotting such a supermassive black hole binary would provide good evidence that the galaxy it occupies is the result of a merger, or even, that such a merger is still ongoing. It would also give us a hint at what is to come for our own galaxy. “This will one day happen to our Milky Way Galaxy — when it merges with the Andromeda Galaxy in about 4 billion years,” Comerford continues. “Our supermassive black hole and Andromeda’s supermassive black hole will form a binary!”
Mathematical modelling of these galaxy mergers seems to show that the process causes major gas inflow towards the central supermassive black hole — or black holes, as the case may be — this powers accretion and various nuclear processes activating the galactic nucleus. This inflow of gas, dust and other material could also result in the growth of the supermassive black hole.
“Astronomers believe that galaxies merge one or more times during their cosmological life,” says Alessandra De Rosa, who is is a research astrophysicist at the National Institute of Astrophysics, Roma, Italy. “These gigantic collisions are likely to be the primary process by which supermassive black holes are activated.”
Thus, galactic mergers aren’t just responsible for bringing supermassive black holes together, they also could kick start the feeding frenzy that makes an AGN the source of incredibly powerful radiation.
But what happens when these binary pairs of supermassive black holes form? Do they remain in a binary, or do they combine to form an even larger supermassive black hole? The merger of supermassive black holes to form larger objects would explain certainly one lingering cosmological question; how did these objects grow to such tremendous sizes in such a short period of time?
Despite the convenience of this phenomenon to tie up some loose cosmic-ends, we still don’t really know if it’s happening or not.
Using Gravitational Waves to Shed Light on Black Hole Binaries
After being brought together by a galaxy merger, when the supermassive black holes are very small separations, the gravitational waves that they emit carry away energy and enable the black holes to merge.
Thus, supermassive black holes at the centre of each galaxy are dragged close to each other, and eventually, form what is known as a dual active galactic nucleus. Theoretically, the final stage of this coming together — particularly if the black holes are gravitationally bound — will be the coalescence of these monsters in a merger that results in an even larger supermassive black hole. This merger would be accompanied by the emission of a gravitational-wave signal. Signals that thanks to the Laser Interferometer Gravitational-Wave Observatory LIGO, and its upcoming space-based counterpart Laser Interferometer Space Antenna (LISA), we can now theoretically detect.
“We think that binary supermassive black holes ultimately merge with each other and produce very energetic gravitational waves. In fact, supermassive black hole mergers are second only to the Big Bang as the most energetic phenomena in the Universe,” Comerford explains. The problem is, that even LIGO — responsible for the first detection of gravitational waves from colliding stellar-mass black holes — isn’t yet capable of detecting gravitational waves from merging supermassive black hole.
“These gravitational waves are too high frequency to be detected by LIGO, so they have not yet been detected,” Comerford adds. “But, we expect that pulsar timing arrays will detect gravitational waves from supermassive black hole mergers for the first time in just a few years.”
De Rosa concurs with the possible breakthrough in detecting gravitational waves from supermassive black hole mergers, highlighting not just the future contribution of pulsar timing arrays, but also, that of LISA — a space-based laser interferometer set to launch in 2034. “In the next decades, space-borne gravitational wave observatories, such as the next large mission of the European Space Agency, LISA, and experiments such as the Pulsar Timing Arrays, will provide first direct evidence of binary and merging SMBHs in the Universe,” she explains.
For Comerford, the breakthrough new gravitational wave detection methods and multi-messenger astronomy stand poised to answer fundamental questions that have influenced her entire career. “When I was a graduate student, my group found some intriguing galaxy spectra that we thought might be produced by supermassive black hole pairs. I wondered if these unusual spectra could be the key to finding supermassive black hole binaries. I’ve been working on new and better ways to find supermassive black hole pairs ever since,” the researcher concludes.
“I think the shocking thing is that we don’t actually know if supermassive black hole binaries merge! It could be that they just circle around each other and are not able to get close enough to each other where the gravitational waves can take over and make them merge.
“When we detect gravitational waves from supermassive black holes, that will be the first time that we actually know that supermassive black hole binaries do merge.”
Sources and Further Reading
De Rosa. A, Vignali. C, Bogdanovic. T, et al, ‘The Quest for Dual and Binary Supermassive Black Holes: A Multi-messenger View,” New Astronomy Reviews, .
Generally, such waves are caused by the collision of immensely massive objects such as two black holes or two neutron stars — this is what happened in 2017 and again in April 2019.
However, these collisions generally last longer, whereas the new signals are short and they appear to come in a series from a very localized portion of the universe.
LIGO picked up the signals coming from the constellation Orion, which has some believing that an explosion of the red supergiant Betelgeuse might be forthcoming. Since October, the star — seen as the shoulder on the left side of Orion — has dimmed by a factor of two, something that has never been documented prior. This has some scientists believing that it could occur soonish (sometime between tomorrow or a 100,000 years from now). If it does occur, the star could leave us in spectacular fashion in the form of a supernova, where the glow could be as bright as the moon.
However, some don’t believe that to be the case. The burst “seems a little too short for what we expect from the collapse of a massive star,” Andy Howell, a scientist at Los Cumbres Observatory Global Telescope Network and an adjunct faculty member in physics at the University of California, Santa Barbara, told Live Science. Howell said that another reason he doesn’t believe this to be the case is that there were no neutrinos detected. Neutrinos are small subatomic particles supernovas are known to release which do not carry a charge.
Another possibility could be noise from LIGO itself, however, the fact that the burst was found by all three LIGO detectors (in Hanford, Washington; Livington, Louisiana; and Piso, Italy) essentially rules this out as well.
So that leaves astronomers scratching their heads as to what the latest burst could be. At least for now.
“The universe always surprises us,” Howell says. “There could be totally new astronomical events out there that produce gravitational waves that we haven’t really thought about.”
We were expecting gravitational waves to answer questions about the nature of the universe. That they did — but they are also posing pressing questions, for which there seems to be no answer yet.
Recent developments in science such as the detection of gravitation waves by way of the minute displacement of mirrors at LIGO and the development of atomic and magnetic force microscopes to reveal atomic structure and spins of single atoms have pushed the boundaries of what can be defined as measurable.
Yet, as scientists push the limits of mechanical measurements the spectre of Heisenberg’s Uncertainty principle remains to remind that no matter how accurate their equipment and procedures become, nature has an intrinsic, in-built limit to what they can ‘know’.
One of the main results of early investigations in quantum physics, the uncertainty principle says that even as the sensitivity of our measuring equipment improves — these conventional measures are limited by a “measurement backaction”. The most common and easiest to explain example of the uncertainty principle is the idea that knowledge of a particle’s exact location immediately destroys knowledge of its momentum — and by extension, the ability to predict its location in the future.
Sense and sensitivity in laser interferometers
Despite this seeming hinderance, researchers are hard at work developing potential methods to help them ‘sidestep’ Heisenberg’s uncertainty principle. Thes techniques hinge on the careful collection of only certain information about a system, whilst intentionally omitting other aspects.
So, for example, waves and wavefunctions are of vital importance in quantum mechanics. Using this selective method researchers would attempt to take the measurement of the wave’s amplitude, whilst simultaneously ignoring its phase.
These methods could, in principle at least, have unlimited sensitivity with the drawback of only being able to gauge half of the information about a system. That is the aim of Tobias Kippenberg at Ecole Polytechnique Federale De Lausanne (EPFL). In conjunction with scientists at the University of Cambridge and IBM Research, Zurich, Kippenberg has uncovered new dynamics that place further unexpected constraints on such systems and just what levels of sensitivity are achievable.
The team’s work shows particular interest to the interferometers that are used to measure gravitational waves. The sensitivity of these instruments is of vital importance as gravitational waves are incredibly difficult to detect. As these pieces of equipment use disturbances in laser beams shined down their massive, kilometre-scale arms, improving their sensitivity means trying to avoid backaction in electromagnetic waves.
The team’s study — published in the journal Physical Review X — demonstrates that small deviations optical frequency, coupled with deviations in mechanical frequency can lead to mechanical oscillations being amplified out of control. This mimics the physics displayed in a state physics refer to as “degenerate parametric oscillator”.
This behaviour was found by Kippenberg and his team in two radically different systems — one operating with optical radiation, the other operating with microwave radiation. This is a fairly disastrous discovery as it implies that the dynamics are not unique to any particular system, but rather, are common across many such systems.
The researchers from EPFL investigated these dynamics further — tuning the frequencies and demonstrating a perfect match with pre-existing theories. EPFL scientist Itay Shomroni, the paper’s first author, explains: “Other dynamical instabilities have been known for decades and shown to plague gravitational wave sensors.
“Now, these new results will have to be taken into account in the design of future quantum sensors and in related applications such as backaction-free quantum amplification.”
Original research: Shomroni, A. Youssefi, N. Sauerwein, L.Qiu, P. Seidler, D. Malz, A. Nunnenkamp, T. J. Kippenberg. Two-tone optomechanical instability and its fundamental implications for backaction-evading measurements. Physical Review X 9, 041022; 30 October 2019. DOI:10.1103/PhysRevX.9.041022
The discovery of gravitational waves took the scientific world by storm, confirming a theory first proposed by Einstein. Now, researchers representing LIGO, Virgo, and some 70 observatories are set to announce more details about this intriguing phenomenon.
It was no surprise that this year’s Nobel Prize in Physics was awarded for contributions in detecting gravitational waves. The prize was awarded to three LIGO physicists (Laser Interferometer Gravitational-Wave Observatory), but LIGO itself is a project involving more than a thousand people. The Virgo interferometer also includes over 300 scientists and technicians, and tens of smaller observatories are additionally offering valuable contributions. All in all, many people are working on gravitational waves, and we’re starting to see results.
The first detection of gravitational waves, made Sept. 14, 2015, and announced Feb. 11, 2016. Great claims require great evidence, and physicists wanted to be sure they’ve got these solid claims. Finding these waves is no easy feat — we can only detect them from interactions between massive objects such as neutron stars or black holes, and even these interactions produce incredibly small effects. The interferometers are basically mirrors placed 4 kilometers apart, and they barely distort by 10−18 m, which is less than one-thousandth the charge diameter of a proton.
Binary systems made up of two massive objects orbiting each other are an important source for gravitational-wave astronomy, but even these create incredibly small distortions. Credit: NASA/Dana Berry, Sky Works Digital.
Since 2015, gravitational waves have been detected three more times, the last occasion being a joint report from both LIGO and Virgo. We can say, with a confidence level of over 99.99999%, that we’ve spotted gravitational waves — but this is in no way the end of the story. If anything, it’s the beginning of a new field of science. Gravitational-wave astronomy is witnessing its baby years. The solid theoretical foundation has been there for a century, but the practical part is just beginning — and there’s really no telling how far this field can take us. This is why this announcement is so exciting. Monday, Oct. 16, at 10 a.m. EDT at the National Press Club in Washington, D.C., scientists representing almost everybody who’s working in gravitational wave detection will hold a press conference.
We have really no idea what they are about to announce, but since it’s such a massive participation, it could be something big. Since they were first published, the papers on gravitational waves have been cited more than 1,700 times total, so the scientific world will be listening, and so will we.