Tag Archives: Black holes

Computer simulation of gravitational wave emissions S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project, W. Benger (Airborne Hydro Mapping GmbH)

What are Gravitational Waves?

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.

Numerical simulation of two inspiralling black holes that merge to form a new black hole. Shown are the black hole horizons, the strong gravitational field surrounding the black holes, and the gravitational waves produced ( S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project, W. Benger (Airborne Hydro Mapping GmbH)).

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.

Theoretical Underpinnings

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.

Poincaré and Einstein both saw the possibility of gravitational waves propagating through space-time at the speed of light

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.

A simulation of gravitational waves emitted by a binary pulsar consisting of two neutron stars

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.”

An animation illustrating how gravitational waves are emitted by two neutron stars as they orbit each other eventually colliding (credit: NASA/Goddard Space Flight Center).

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).

Schematic showing how LIGO works. (Johan Jarnestad/The Royal Swedish Academy of Sciences)

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.

(SXS Collaboration)

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.

Max Planck Institute for Gravitational Physics
(Albert-Einstein-Institut)

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.

(R. Williams (STScI), the Hubble Deep Field Team, NASA)

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 the cosmic 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, [2019]

Maggiore. M., Gravitational Waves: Astrophysics and Cosmology, Oxford University Press, [2019]

Collins. H., Gravity’s Kiss:  The Detection of Gravitational Waves, MIT Press, [2017]

Look Deeper, LIGO, [https://www.ligo.caltech.edu/page/look-deeper]

What are Black Holes: The Journey From Theory to Reality

Black holes are cosmic bodies that pack an immense amount of mass into a surprisingly small space. Due to their extremely intense gravity, nothing can escape their grasp — not even light which defines the universe’s speed limit.

April 10th, 2019 marked a milestone in science history when the team at the Event Horizon Telescope revealed the first image of a supermassive black hole. As a result, these areas of space created when stars reach the end of their nuclear fuel burning and collapse creating massive gravitational wells, completed their transition from theory to reality.

This transition has been further solidified since with the revelation of a second, much clearer image of the supermassive black hole (SMBH) at the centre of the galaxy Messier 87 (M87). This second image revealing details such as the orientation of the magnetic fields that surround it and drive its powerful jets that extend for light-years.

(EHT Collaboration)

The study of black holes could teach us much more than about these spacetime events and the environments that home them, however. Because cosmologists believe that most galaxies have an SMBH sat at their centre, greedily consuming material like a fat spider lurking at the centre of a cosmic web, learning more about these spacetime events can also teach us how galaxies themselves evolve.

The origin of black holes is one that runs in reverse to that of most astronomical objects. We didn’t discover some mysterious object in the distant cosmos and then began to theorise about it whilst making further observations.

Rather, black holes entered the scientific lexicon in a way that is more reminiscent of newly theorised particles in particle physics; emerging first from the solutions to complex mathematics. In the case of black holes, the solutions to the field equations employed by Einstein in his most important and revolutionary theory.

Just as a physical black hole forms from the collapse of a star, the theory of black holes emerged from the metaphorical collapse of the field equations that govern the geometrical theory of gravity; better known as general relativity.

One of the most common misconceptions about black holes arises from their intrinsic uniqueness and the fact that there really isn’t anything else like them in the Universe.

That’s Warped: Black Holes and Their Effect on Spacetime

General relativity introduced the idea that mass has an effect on spacetime, a concept fundamental to the idea that space and time are not passive stages upon which the events of the universe play out. Instead, those events shape that stage. As John Wheeler brilliantly and simply told us; when it comes to general relativity:

“Matter tells space how to curve. Space tells matter how to move.”

John Wheeler

The most common analogy is for this warping of space is that of placing objects on a stretched rubber sheet. The larger the object the deeper the ‘dent’ and the more extreme the curvature it creates. In our analogy, a planet is a marble, a star an apple, and a black hole a cannonball.

Thus, considering this a black hole isn’t really ‘an object’ at all but, is actually better described as a spacetime event. When we say ‘black hole’ what we really mean is an area of space that is so ‘warped’ by a huge amount of mass condensed into a finite point that even light itself doesn’t have the necessary velocity to escape it.

This point at which light can no longer escape marks the first of two singularities that define black holes–points at which solutions of the equations of general relativity go to infinity.

The Event Horizon and the Central Singularity

The event horizon of a black hole is the point at which its escape velocity exceeds the speed of light in vacuum (c). This occurs at a radius called the Schwarzchild radius–named for astrophysicist Karl Schwarzschild, who developed a solution for Einstien’s field equations whilst serving on the Eastern Front in the First World War.

His solution to Einstein’s field equations–which would unsurprisingly become known as the Schwarzschild solution– described the spacetime geometry of an empty region of space. It had two interesting features — two singularities — one a coordinate singularity the other, a gravitational singularity. Both take on significance in the study of black holes.

Dealing with the coordinate singularity, or the Schwarzchild radius first.

The Schwarzchild radius (Rs) also takes on special meaning in cases where the radius of a body shrinks within this Schwarzschild radius (ie. Rs >r). When a body’s radius shrinks within this limit, it becomes a black hole.

All bodies have a Schwarzschild radius, but as you can see from the calculation below for a body like Earth, Rs falls well-within its radius.

That’s part of what makes black holes unique; their Schwartzchild radius is outside their physical radius because their mass is compressed into such a tiny space.

Because the outer edge of the event horizon is the last point at which light can escape it also marks the last point at which events can be seen by distant observers. Anything past this point can never be observed.

The reason the Schwarzschild radius is called a ‘coordinate singularity’ is that it can be removed with a clever choice of coordinate system. The second singularity can’t be dealt with in this way. This makes it the ‘true’ physical singularity of the black hole itself.

This is known as the gravitational singularity and is found at the centre of the black hole (r=0). This is the end-point for every particle that falls into a black hole. It’s also the point the Einstein field equations break down… maybe even all the laws of physics themselves.

The fact that the escape velocity of the event horizon exceeds the speed of light means that no physical signal could ever carry information from the central singularity to distant observers. We are forever sealed off from this aspect of black holes, which will therefore forever remain in the domain of theory.

How to Make a Black Hole

We’ve already seen that for a body with the mass of Earth to become a black hole, its diameter would have to shrink to less than 2cm. This is obviously something that just isn’t possible. In fact, not even our Sun has enough mass to end its life as a black hole. Only stars with around three times the mass of the Sun are massive enough to end their lives in this way.

But why is that the case?

It won’t surprise you to learn that for an astronomical body to become a black hole it must meet and exceed a series of limits. These limits are created by outward forces that are resisting against the inward force that leads to gravitational collapse.

For planets and other bodies with relatively small masses, the electromagnetic repulsion between atoms is strong enough to grant them stability against total gravitational collapse. For large stars the situation is different.

During the main life cycle of stars–the period of the fusion of hydrogen atoms to helium atoms–the primary protection against gravitational collapse is the outward thermal and radiation pressures that are generated by these nuclear processes. That means that the first wave of gravitational collapse occurs when a star’s hydrogen fuel is exhausted and inward pressure can no longer be resisted.

Should a star have enough mass, this collapse forces together atoms in the nucleus enough to reignite nuclear fusion— with helium atoms now fusing to create heavier elements. When this helium is exhausted, the process happens again, with the collapse again stalling if there is enough pressure to trigger the fusion of heavier elements still.

Stars like the Sun will eventually reach the point where their mass is no longer sufficient to kick start the nuclear burning of increasingly heavier elements. But if it isn’t nuclear fusion that is generating the outward forces that prevent complete collapse, what is preventing these lower-mass stars from becoming black holes?

Placing Limits on Gravitational Collapse


Lower-mass stars like the Sun will end their lives as white dwarf stars with a black hole form out of reach. The mechanism protecting these white dwarfs against complete collapse is a quantum mechanical phenomenon called degeneracy.

This ‘degeneracy pressure is a factor of the Pauli exclusion principle, which states that certain particles– known as fermions, which include electrons, protons, and neutrons– are forbidden from occupying the same ‘quantum states.’ This means that they resist being tightly crammed together.

This theory and the limitation it introduced led Indian-American astrophysicist Subrahmanyan Chandrasekhar to question if there was an upper cap at which this protection against gravitational collapse would fail.

Chandrasekhar –awarded the 1983 Nobel Prize in physics for his work concerning stellar evolution– proposed in 1931 that above 1.4 solar masses, a white dwarf would no longer be protected from gravitational collapse by degeneracy pressure. Past this limit — termed the Chandrasekhar limit — gravity overwhelms the Pauli exclusion principle and gravitational collapse can continue.

But there is another limit that prevents stars of even this greater mass from creating black holes.

Thanks to the 1932 discovery of neutrons— the neutral partner of protons in atomic nuclei — Russian theoretical physicist Lev Landau began to ponder the possible existence of neutron stars. The outer part of these stars would contain neutron-rich nuclei, whilst the inner sections would be formed from a ‘quantum fluid’ comprised of mostly neutrons

These neutron stars would also be protected against gravitational collapse by degeneracy pressure — this time provided by this neutron fluid. In addition to this, the greater mass of the neutron in comparison to the electron would allow neutron stars to reach a greater density before undergoing collapse.

By 1939, Robert Oppenheimer had calculated that the mass-limit for neutron stars would be roughly 3 times the mass of the Sun.

To put this into perspective, a white dwarf with the mass of the Sun would be expected to have a millionth of our star’s volume — giving it a radius of 5000km, roughly that of the Earth. A neutron star of a similar mass though would have a radius of about 20km — roughly the size of a city.

Above the Oppenheimer-Volkoff limit, gravitational collapse begins again. This time no limits exist between this collapse and the creation of the densest possible state in which matter can exist. The state found at the central singularity of a black hole.

We’ve covered the creation of black holes and the hurdles that stand in the way of the formation of such areas of spacetime, but theory isn’t quite ready to hand black holes over to practical observations just yet. The field equations of general relativity can also be useful in the categorisation of black holes.

The four types of black holes

Categorising black holes is actually fairly straight-forward thanks to the fact that they possess very few independent qualities. John Wheeler had a colourful way of describing this lack of characteristics. The physicist once commented that black holes ‘have no hair,’ meaning that outside a few characteristics they are essentially indistinguishable. This comment became immortalised as the no-hair theorem of black holes.

Black holes have only three independent measurable properties — mass, angular momentum and electric charge. All black holes must have mass, so this means there are only four different types of a black hole based on these qualities. Each is defined by the metric or the function used to describe it.

This means that black holes can be quite easily catagorised by the properties they possess as seen below.

This isn’t the most common or most suitable method of categorising black holes, however. As mass is the only property that is common to all black holes, the most straight-forward and natural way of listing them is by their mass. These mass categories are imperfectly defined and so far black holes in some of the categories–most notably intermediate black holes– remain undetected.

Cosmologists believe that the majority of black holes are rotating and non-charged Kerr black holes. And the study of these spacetime events reveals a phenomenon that perfectly exemplifies their power and influence on spacetime.

The Anatomy of a Kerr Black Hole

The mathematics of the Kerr metric used to describe non-charged rotating black holes reveals that as they rotate, the very fabric of spacetime that surrounds them is dragged along in the direction of the rotation.

The powerful phenomenon is known as ‘frame-dragging’ or the Lense-Thirring effect and leads to the violent churning environments that surround Kerr black holes. Recent research has revealed that this frame-dragging could be responsible for the breaking and reconnecting of magnetic field lines that in-turn, launch powerful astrophysical jets into the cosmos.

The static limit of a Kerr black hole also has an interesting physical significance. This is the point at which light–or any particle for that matter– is no-longer free to travel in any direction. Though not a light-trapping surface like the event horizon, the static limit pulls light in the direction of rotation of the black hole. Thus, light can still escape the static limit but only in a specific direction.

British theoretical physicist and 2020 Nobel Laureate Sir Roger Penrose also suggested that the static limit could be responsible for a process that could cause black holes to ‘leak’ energy into the surrounding Universe. Should a particle decay into a particle and its corresponding anti-particle at the edge of the static limit it would be possible for the latter to fall into the black hole, whilst its counterpart is launched into the surrounding Universe.

This has the net effect of reducing the black hole’s mass whilst increasing the mass content of the wider Universe.

We’ve seen what happens to light at the edge of a black hole and explored the fate of particles that fall within a Kerr black hole’s static limit, but what would happen to an astronaut that strayed too close to the edge of such a spacetime event?

Death by Spaghettification

Of course, any astronaut falling into a black hole would be completely crushed upon reaching its central gravitational singularity, but the journey may spell doom even before this point has been reached. This is thanks to the tidal forces generated by the black hole’s immense gravitational influence.

As the astronaut’s centre of mass falls towards the black hole, the object’s effect on spacetime around it causes their head and feet to arrive at significantly different times. The difference in the gravitational force at the astronaut’s head and feet gives rise to such a huge tidal force that means their body would be simultaneously compressed at the sides and stretched out.

Physicists refer to this process as spaghettification. A witty name for a pretty horrible way to die. Fortunately, we haven’t yet lost any astronauts to this bizarre demise, but astronomers have been able to watch stars meet the same fate.

For a stellar-mass black hole, spaghettification would occur not just before our astronaut reaches the central singularity, but also well before they even hit the event horizon. For a black hole 40 times the mass of our Sun — spaghettification would occur at about 1,000 km out from the event horizon, which is, itself, 120 km from the central gravitational singularity.

As well as developing the Oppenheimer-Volkoff limit, Oppenheimer also used general relativity to describe how a total gravitational collapse should appear to a distant observer. They would consider the collapse to take an infinitely long time, the process appearing to slow and freeze as the star’s surface shrinks towards the Schwarzschild radius.

An astronaut falling into a black hole would be immortalized in a similar way to a distant observer, though they themselves–could they have survived spaghettification– they would notice nothing. The passing of Rs would just seem a natural part of the fall to them despite it marking the point of no return.

Much More to Learn…

After emerging from the mathematics of general relativity at the earlier stages of the 20th Century, black holes have developed from a theoretical curiosity to the status of scientific reality. In the process, they have indelibly worked their way into our culture and lexicon.

The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of a black hole released in 2019, has today a new view of the massive object at the centre of the Messier 87 (M87) galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of a black hole. (EHT Collaboration)
The Event Horizon Telescope (EHT) collaboration, which produced the first-ever image of a black hole released in 2019, has today a new view of the massive object at the centre of the Messier 87 (M87) galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of a black hole. (EHT Collaboration)

Perhaps the most exciting thing about black holes is that there is so much we don’t yet know about them. As a striking example of that, almost all the information listed above resulted just from theory and the interrogation of the maths of Einstein’s field equations.

Unlocking the secrets held by black holes could, in turn, reveal how galaxies evolve and how the Universe itself has changed since its early epochs.

Sources and Further Reading

Relativity, Gravitation and Cosmology, Robert J. Lambourne, Cambridge Press, [2010].

Relativity, Gravitation and Cosmology: A basic introduction, Ta-Pei Cheng, Oxford University Press, [2005].

Extreme Environment Astrophysics, Ulrich Kolb, Cambridge Press, [2010].

Stellar Evolution and Nucleosynthesis, Sean G. Ryan, Andrew J. Norton, Cambridge Press, [2010].

Cosmology, Matts Roos, Wiley Publishing, [2003].

Baby universes branching off of our universe shortly after the Big Bang appear to us as black holes. (Kavli IPMU)

Primordial black holes could hide a multiverse of possibilities

Before the stars and galaxies even began to form in the early Universe, some researchers believe that the cosmos could have been occupied by a multitude of tiny primordial black holes. These purely hypothetical black holes would have formed in a radically different way than larger and more familiar black holes which physicists, cosmologists, and astronomers have confirmed to exist. 

Whereas larger black holes form as a result of the death of massive stars, primordial black holes would have been born immediately after the ‘Big Bang’ when areas of high density underwent gravitational collapse. Despite having a long history in theoretical physics, primordial black holes had moved out of favour, that is until recently.

Baby universes branching off of our universe shortly after the Big Bang appear to us as black holes. (Kavli IPMU)
Baby universes branching off of our universe shortly after the Big Bang appear to us as black holes. (Kavli IPMU)

Now researchers from the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) — including Kavli IPMU members Alexander Kusenko, Misao Sasaki, Sunao Sugiyama, Masahiro Takada and Volodymyr Takhistov —  are studying the possibility of such objects existing both in the early Universe and in our current epoch.

The team believes the discovery of primordial black holes could point to a potential multiverse, with other ‘baby universes’ born alongside our own. Meaning that behind the event horizon — the point at which not even light can escape — of these primordial black holes could lurk an entire universe, hidden from view.

The scientists’ findings are documented in a paper published in the journal Physical Review Letters.

Beyond the discovery of these early black holes themselves, such an investigation could answerquestions surrounding many lingering and mysterious aspects of physics. 

Primordial Black holes and Lingering Mysteries

The team believes that the existence of primordial black holes could account for a small amount of the gravitational waves detected at the LIGO/VIRGO interferometer. Until recently, this had been ruled out as primordial black holes existing binary pairs should result in more gravitational-wave signals than we currently detect. 

Recent research has begun to illustrate how primordial black holes could exist and still produce gravitational wave signals that conform to the number detected at LIGO. 

Such objects could even explain how some heavy elements are synthesised. Should primordial black holes exist, they could either collide with neutron stars — obliterating them — or infest the centres of such stellar remnants and ‘eat them’ from the inside out. Either of these processes would lead to the release of neutron-rich material would be released. 

the team searched the Andromeda galaxy with the HSC for clues indicating the prescence of primordial black holes (Kavli IPMU/HSC Collaboration)

The synthesis of heavy elements has puzzled astrophysicists for some time, as the processes behind it rely on the presence of large numbers of neutrons, meaning primordial black holes could play a key role in providing such neutron-rich conditions. 

Perhaps more exciting than this even; the team’s research could reveal if primordial black holes comprise the majority of dark matter — the mysterious substance which makes up between 80–90% of the Universe’s total matter content.

The idea that primordial black holes could account for dark matter — or at least some of it — isn’t a new idea. But, like the discussion of these objects themselves, theories connecting them to dark matter have also fallen out of favour over recent years. 

In order to discover primordial black holes, the Kalvi team used the Hyper Suprime-Cam (HSC) of the 8.2m Subaru Telescope, a gigantic digital camera at the summit of Mount Mauna Kea, Hawaii to study the early Universe for clues.

 Searching the Early Universe for Primordial Black Holes

Because the early Universe was so dense, it would take only a small density fluctuation of around 50% to create a black hole. This means, that whilst the gravitational perturbations that created galaxies were much smaller than this, there are a variety of events in the early cosmos that could have triggered the start of such a genesis event.

One such process would be the creation of a small ‘daughter universe’ branching off from our own universe during its initial period of rapid inflation. Should this baby universe collapse a vast amount of energy would be released within its small volume, thus giving rise to a tiny black hole. 

This idea of branching Universes gets even stranger, however, should one of these baby-universes reach and exceed some critical size. General relativity suggests that if this was to happen the universe in question would exist in a state that appears different from the inside than it does from the outside. 

Hyper Suprime-Cam (HSC) is a gigantic digital camera on the Subaru Telescope ideal fr spotting primordial black holes (HSC project / NAOJ)

An observer from with the baby universe would see it as an expanding universe, whilst an observer outside the event horizon would see the baby universe as a black hole. This means that in both cases, the event horizon of the primordial black hole hides its internal structure — and an entire universe. 

The team’s paper points to a scenario in which primordial black holes are created by this nucleation of what they term ‘false vacuum bubbles.’ 

The fact that primordial black holes have thus far escaped detection indicates it is going to take an extremely powerful instrument to see the Universe in such a way that these multiverse camouflaging objects can be spotted.

Fortunately the HSC fits the bill.

The Hyper Suprime-Cam sees the Big Picture

As the paper’s authors describe, thanks to its unique capability to picture the entire Andromeda galaxy every few minutes, the HSC could be the ideal instrument to capture primordial black holes. This imaging can be achieved with the aid of gravitational lensing, the curvature of light by an object of great mass.

The team used gravitational lensing, the curvature of light by objects with tremendous mass, to help identify primordial black holes. (Kavli IPMU/HSC Collaboration)
The team used gravitational lensing, the curvature of light by objects with tremendous mass, to help identify primordial black holes. (Kavli IPMU/HSC Collaboration)

As a primordial black hole passes the line of sight to a bright object such as a star, the curvature it causes in spacetime results in a momentary brightening of the object or an apparent shift in position. 

The greater the mass, the more extreme the curvature and thus the stronger the effect meaning that the astronomers can measure the mass of the lensing object. This effect only lasts an extremely brief time, however.

Because the HSC can see the entire galaxy, it can simultaneously observe up to one hundred million stars — giving astronomers a good chance of catching a transiting primordial black hole. 

The team have already identified a prime candidate for a ‘multiverse’ hiding primordial black hole in the first run of HSC observations. The object had a mass around that of the Moon and has inspired the team to conduct further observations, thus widening their search and possibly finding a solution to some of physics’ most pressing mysteries. 

Original Research

Kusenko. A., Sasaki. M., Sugiyama. S., et al, [2021], ‘Exploring Primordial Black Holes from the Multiverse with Optical Telescopes,’ Physical Review Letters, [https://doi.org/10.1103/PhysRevLett.125.181304]

2020: A Year in Space

It’s difficult to mention the year 2020 without referencing COVID-19, but as more human beings than ever before were wishing they could take a break from the surface of the planet, space research continued to push our knowledge of the stars. Whilst much of the scientific community was consumed with combating a pandemic, physicists, astronomers, cosmologists, and other researchers were further pushing our understanding of space and the objects which dwell there.

These are some of my personal favourite space-related breakthroughs and research that have come about this year. The list is by no means exhaustive. 

Black Holes go silent

In terms of black hole science, 2019 was always going to be a difficult year to top being the year that brought us the first direct image of a supermassive black hole (SMBH). That doesn’t mean that 2020 has been a slow year for black hole developments, however.

One of the most striking and memorable examples of black hole research announced this year was the discovery of a ‘silent’ black hole in our cosmic ‘back yard.’ An international team led by researchers from European Southern Observatory (ESO) including found the black hole in the system HR 6819, located within the Milky Way and just 1,000 light-years from the Earth.

A silent and thus invisible black hole discovered lurking in our ‘solar backyard’ could be an indicator of a much larger population. (ESO/L. Calçada) Background: This wide-field view shows the region of the sky, in the constellation of Telescopium, where HR 6819 can be found. (ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin)

The observation marks the closest to Earth a black hole has ever been discovered and Dietrich Baade, Emeritus Astronomer at ESO in Garching believes that it is just ‘the tip of the Iceberg’. 

“It’s remarkable because not only is it the first of its kind found, but it’s also so nearby,” said Baade. “Discovering a first only an astronomical stone’s throw away is the biggest surprise one can probably imagine.”

The black hole was described as ‘silent’ by the team because it is not current accreting material — the destructive process that creates powerful x-ray emissions and makes these light-trapping objects observable. 

Close-up screen capture image of the LB-1 which, like HR 6819, could also host a silent black hole (Hubble/Public Domain)

“If there is one, there ‘must’ be more,” Baade remarked in May. “If the Earth is not in a privileged position in the Universe — and all available evidence suggests without a doubt it is not — this means that there must be many more silent black holes.”

Baade also remarked that as current cosmological models suggest that the number of stellar-mass black holes is between 100,000,000 to 1,000,000,000 and we have observed nowhere near this many objects, more quiet black holes are “badly needed” to confirm current models. “HR 6819 is the tip of an iceberg, we do not yet know how big the iceberg is.”

Silent black holes weren’t the only examples of this hind of science making noise in 2020, however. Long-missing Intermediate Mass Black Holes were discovered. And just like a proverbial bus, you wait decades for one and then two turn up at once.

Intermediate mass black holes found and found again

Missing black holes were the subject of another piece of exciting space science in September 2020, when researchers from the VIRGO/LIGO collaboration discovered the tell-tale signal of an intermediate-mass black hole (IMBH) in gravitational-wave signals. To add to the excitement, the signals originated from the largest black hole merger ever observed.

An artistic interpretation of the binary black hole merger responsible for GW190521. The space-time, figured by a fabric on which a view of the cosmos is printed, is distorted by the GW190521 signal. The turquoise and orange mini-grids represent the dragging effects due to the individually rotating black holes. The estimated spin axes, or self-rotations, of the black holes, are indicated with the corresponding coloured arrows. The background suggests a star cluster, one of the possible environments where GW190521 could have occurred. Credits: Raúl Rubio / Virgo Valencia Group / The Virgo Collaboration.)

The merger — identified as gravitational wave event GW190521 —was detected in gravitational waves and is the first example of 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,” Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS) said when announcing the team’s observation. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”

The black hole birthed in the detected merger appears to have a mass of between 100–1000 times that of the Sun — most likely 142 solar masses — putting it in the mass range of an IMBH — a ‘missing link’ between stellar-mass black holes and much larger SMBHs. 

Earlier in 2020, another team had used the Hubble Space Telescope X-ray data collected in 2018 to identify what they believed to be an IMBH with a mass 50,000 times that of the Sun named 3XMM J215022.4−055108 (or J2150−0551 for short). 

This Hubble Space Telescope image identified the location of an intermediate-mass black hole (IMBH), weighing over 50 000 times the mass of our Sun (NASA, ESA, and D. Lin (University of New Hampshire))

Whether GW190521 or J2150−0551 will go down in history as the first discovered IMBH is currently a little muddy, but what is less questionable is that 2020 will go down as the year in which these ‘missing link’ black holes were first discovered, bringing with them exciting implications for the future investigation of black holes of all sizes. 

“Studying the origin and evolution of the intermediate-mass black holes will finally give an answer as to how the supermassive black holes that we find in the centres of massive galaxies came to exist,” said Natalie Webb of the Université de Toulouse in France, part of the team that found J2150−0551. And IMBHs weren’t the only missing element of the Universe that turned up in 2020.

Discovering the Universe’s missing mass

In May astronomers, including Professor J. Xavier Prochaska of UC Santa Cruz, announced that they had found the missing half of missing baryonic matter demanded by cosmological models. 

“The matter in this study is ‘ordinary’ matter — the material that makes up our bodies, the Earth, and the entirety of the periodic table. We refer to this matter as ‘baryonic’–matter made up of baryons like electron and protons,” Prochaska said when he spoke exclusively to ZME Science earlier this year. “Of particular interest to astronomers is to ascertain the fraction of the material that is tightly bound to galaxies versus the fraction that is out in the open Universe — what we refer to as the intergalactic medium or cosmic web.”

The matter the team discovered isn’t ‘dark matter’ — which accounts for roughly 85–90% of the Universe’s matter content — but rather ‘ordinary’ matter that has been predicted to exist by our models of universal evolution but has remained hidden.

The team made the discovery using mysterious Fast Radio Bursts (FRBs) and the measurement of the redshift of the galaxy from which they originate as a detection method. FRBs can be used as a probe for baryonic matter because as they travel across the Universe, every atom they encounter slows them down by a tiny amount.

This means that they carry with them a trace of these encounters along with them in the spectral splitting as seen above. This allowed the team to infer the presence of clouds of ionised gas that are invisible to ‘ordinary’ astronomy because of how diffuse they are. 

Asteroid Samples Returned by Hayabusa2

Japan’s Hayabusa2 probe and its continued investigation of the asteroid Ryugu has been the gift that has just kept giving in 2020. Just this month the probe returned to Earth samples collected from an asteroid — which has an orbit that brings it between Earth and Mars — for the first time.

Though probes have landed on asteroids and collected samples before, these samples have been examined in situ. Thus this is the first time researchers have been able to get ‘up close and personal’ with matter from an asteroid.

Artist’s impression of the Hayabusa2 probe achieving touchdown on Ryugu. Image credit: JAXA

Hayabusa2 arrived at Ryugu in late June 2018, making its touch-down on the surface of the asteroid in February of the following year after months careful manoeuvring conducted by the Japan Aerospace Exploration Agency (JAXA) and the selection of an optimal region from which to collect samples. 

Ahead of the return of samples on December 5th, the probe sent back some stunning images of the asteroid’s surface. These images were more than purely aesthetic, however. Examination of dust grains on the surface of Ryugu gave the team, including Tomokatsu Morota, Nagoya University, Japan, indications of a period of rapid heating by the Sun. 

The surface of near-Earth carbonaceous asteroid 162173 Ryugu, as observed by the Hayabusa2 spacecraft just before its landing. This image was produced from images obtained by ONC-W1 at the bottom and ONC-W2 on the side of the spacecraft. The spacecraft’s solar ray paddle casts a shadow on Ryugu’s surface. Image credit: JAXA/U. Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Inst. Tech./Meiji U./U. Aizu/AIST

“Our results suggest that Ryugu underwent an orbital excursion near the
Sun,” said Morota in May. “This constrains the orbital transition processes of asteroids from the main belt to near-Earth orbit.”

Impressive though this achievement is, its the collection of samples from the asteroid and their subsequent safe return to earth that is the ‘main course’ of the Hayabusa2 mission. “The most important objective of the touchdown is sample collection from Ryugu’s surface,” Morota explained. 

Animation created from CAM-H and ONC-W1 data obtained during the 1st touchdown operation (Feb. 21, 2019). Image credit: JAXA/U. Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Inst. Tech./Meiji U./U. Aizu/AIST

It is hoped that access to these samples will help answer lingering questions about asteroid composition as well as assisting researchers to confirm Ryugu’s suspected age of 100 million years old — which actually makes it quite young in terms of other asteroids. 

Asteroids like Ryugu can act as a ‘snapshot’ of the system’s in which they form at the time of that formation. This is because whereas planets undergo a lot of interaction with other bodies, asteroids remain pretty much untouched. 

Whilst researchers will no doubt be elated by the return of the Ryugu samples and the continuing success of the Hayabusa2 mission, 2020 wasn’t all good news for fans of asteroid research. 

Goodbye to Arecibo

The iconic radio telescope at the Arecibo Observatory in Puerto Rico collapsed at the beginning of December, ahead of its planned demolition. The telescope which will be familiar to moviegoers as the setting of the climactic battle in Pierce Brosnan’s first outing as James Bond, 1995’s Goldeneye, had been in operation up until November, playing a role in the detection of near-Earth asteroids and monitoring if they present a threat to the planet.

An image of the radio telescope before its December 1st collapse (NSF)

The collapse of the radio telescope’s 900-tonne platform which was suspended above the telescope’s 305-metre-wide dish, on December 1st, followed the snapping of one of its main cables in November.

The US National Science Foundation (NSF), which operates the observatory had announced that same month that the telescope would be permanently closed citing ‘safety concerns’ after warnings from engineers that it could collapse at any point.

Following the collapse, the NSF release heart-wrenching footage of the radio telescope collapsing recorded by drones. The footage shows cables snapping at the top of one of the three towers from which the instrument platform was suspended. The platform then plummets downward impacting the side of the dish. 

Video shows the radio telescope’s instrument platform fall and collision into the side of the dish (NSF)

The observatory had played a role in several major space-science breakthroughs since its construction in 1963. Most notably, observations made by the instrument formed the basis of Russell A. Hulse and Joseph H. Talyor’s discovery of a new type of pulsar in 1974. The breakthrough would earn the duo the 1993 Nobel Prize in Physics. 

Some good could ultimately come out of the collapse of Arecibo. Questions had been asked about the maintenance of the radio telescope for some time and the fact that the cable which snapped in November dated back to the instrument’s construction 57 years ago has not escaped notice and comment.

As a result, various space agencies are being encouraged to make efforts to better maintain large-scale equipment and facilities so that losses like this can be avoided in the future.

This aerial view shows the damage at the Arecibo Observatory after one of the main cables holding the receiver broke in Arecibo, Puerto Rico, on December 1, 2020. – The radio telescope in Puerto Rico, which once starred in a James Bond film, collapsed Tuesday when its 900-ton receiver platform fell 450 feet (140 meters) and smashed onto the radio dish below. (Photo by Ricardo ARDUENGO / AFP) (Photo by RICARDO ARDUENGO/AFP via Getty Images)

For most of us, 2020 is going to be a year that we would rather forget. Whilst very few of us come honestly comment that we have had anything approaching a ‘good year’ space science has plowed ahead, albeit mildly hindered by the global pandemic.

Our knowledge and understanding of space science are better off at the end of 2020 than it was twelve months earlier, and that is at least something positive that has emerged from this painful year.

Sir Roger Penrose has been awarded the 2020 Nobel Prize in physics for his work revolutionising our theories regarding black holes and reshaping general relativity. (Robert Lea)

Singularity Minded: The Black Hole Science that Won a Nobel Prize

The 2020 Nobel prize in physics has been jointly awarded to Roger Penrose, Reinhart Genzel, and Andrea Ghez for their contributions to our understanding of black holes — the Universe’s most mysterious and compact objects. Whilst Genzel and Ghez claim their share of the most celebrated prize in physics for the discovery of a supermassive compact object at the centre of our galaxy — an object that we would later come to realize was a supermassive black hole which was later named Sagittarius A* (Sgr A*) — Penrose is awarded his share for an arguably more fundamental breakthrough. 

Sir Roger Penrose has been awarded the 2020 Nobel Prize in physics for his work revolutionising our theories regarding black holes and reshaping general relativity. (Robert Lea)
Sir Roger Penrose has been awarded the 2020 Nobel Prize in physics for his work revolutionising our theories regarding black holes and reshaping general relativity. (Robert Lea)

The Nobel is awarded to Penrose based on a 1965 paper in which he mathematically demonstrated that black holes arise as a direct consequence of the mathematics of Einstein’s theory of General Relativity. Not only this; but for a body of a certain mass, the collapse into a singularity wasn’t just possible, or even probable. If that collapse could not be halted, singularity formation, Penrose argued, is inevitable. 

“For the discovery, that black hole formation is a robust prediction of the general theory of relativity”

The Nobel Commitee awards the 2020 prize in physics to Sir Roger Penrose

The fact that Penrose showed that black holes mathematically emerge from general relativity may seem even more revolutionary when considering that the developer of general relativity — a geometric theory of gravity that suggests mass curves the fabric of spacetime — Albert Einstein did not even believe that black holes actually existed. 

How Black Holes connect to General Relativity (Robert Lea)

It was ten years after Einstein’s death in April 1955 when Penrose showed that singularities form as a result of the mathematics of general relativity and that these singularities act as the ‘heart’ of the black hole. At this central–or gravitational–singularity, Penrose argued, all laws of physics displayed in the outside Universe ceased to apply. 

The paper published in January 1965 — just eight years after Penrose earned his Ph.D. from The University of Cambridge — ‘Gravitational Collapse and Spacetime Singularities’ is still widely regarded as the second most important contribution to general relativity after that of Einstein himself. 

Yet, Penrose wasn’t the first physicist to mathematically unpick general relativity and discover a singularity. Despite this, his Penrose Singularity Theorem is still considered a watershed moment in the history of general relativity. 

Black Holes: A Tale of Two Singularities

“A black hole is to be expected when a large massive body reaches a stage where internal pressure forces are insufficient to hold the body apart against the relentless inward pull of its own gravitational influence.”

Roger Penrose, The Road to Reality

Black holes are generally regarded as possessing two singularities; a coordinate singularity and an ‘actual’ gravitational singularity. Penrose’s work concerned the actual singularity, so named because unlike the coordinate singularity, it could not be removed with a clever choice of coordinate measurement.

That doesn’t mean, however, that the coordinate singularity is unimportant or even easy to dismiss. In fact, you may already be very familiar with the coordinate singularity, albeit under a different name — the event horizon. This boundary marks the point where the region of space defined as a black hole begins, delineating the limit at which light can no longer escape. 

The discovery of the event horizon occurred shortly after the first publication of Einstein’s theory of general relativity in 1915. In 1916, whilst serving on the Eastern Front in the First World War astrophysicist Karl Schwarzschild developed the Schwarzschild solution, which described the spacetime geometry of an empty region of space. One of the interesting features of this solution — a coordinate singularity. 

The coordinate singularity — also often taking a third more official name as the Schwarzschild radius (Rs) — exists for all massive bodies at r =Rs = 2GM/c². This marks the point where the escape velocity of the body is such that not even light can escape its grasp. For most cosmic bodies the Schwarzschild radius falls well within its own radius (r). For example, the Sun’s Rs occurs at a radius of about 3km from the centre compared to an overall radius of 0.7 million km.

Thus, the Schwarzschild radius or event horizon marks the boundary of a light-trapping surface. A distant observer could see an event taking place at the edge of this surface, but should it pass beyond that boundary — no signal could ever reach our observer. An observer falling with the surface, though, would notice nothing about this boundary. 

The passing of Rs would just seem a natural part of the fall to them despite it marking the point of no return. To the distant observer… the surface would freeze and become redder and redder thanks to the phenomena of gravitational redshift — also the reason the event horizon is sometimes referred to as the surface of infinite redshift.

The very definition of a black hole is a massive body whose surface shrinks so much during the gravitational collapse that its surface lies within this boundary. But, what if this collapse continues? When does it reach a central singularity at the heart of the black hole–r= 0 for the mathematically inclined?

Birthing a Black Hole

“We see that the matter continues to collapse inwards through the surface called the event horizon, where the escape velocity indeed becomes the speed of light. Thereafter, no further information from the star itself can reach any outside observer, and a black hole is formed.”

Roger Penrose, The Road to Reality

Penrose and other researchers have found that the equations of general relativity open the possibility that a body may undergo a complete gravitational collapse — shrinking to a point of almost infinite density — and become a black hole.

In order for this to happen, however, a series of limits have to be reached and exceeded. For example, planets are unable to undergo this gravitational collapse as the mass they possess is insufficient to overcome the electromagnetic repulsion between their consistent atoms — thus granting them stability.

What would it take for Earth to become a black hole? (Robert Lea)
(Robert Lea)

Likewise, average-sized stars such as the Sun should also be resistant to gravitational collapse. The plasma found at the centre of stars in this solar-mass range is believed to be roughly ten times the density of lead protecting from complete collapse, whilst the thermal pressure arising from nuclear processes and radiation pressure alone would be sufficient to guarantee a star of low to intermediate-mass stability.

For older, more evolved stars in which nuclear reactions have ceased due to a lack of fuel. It’s a different story. Especially if they have a mass ten times greater than the Sun.

It was suggested as early as the 1920s that small, dense stars — white dwarf stars — were supported against collapse by phenomena arising from quantum mechanics called degeneracy.

This ‘degeneracy pressure’ arises from the Pauli exclusion principle, which states that fermions such as electrons are forbidden from occupying the same ‘quantum state’. This led a physicist called Subrahmanyan Chandrasekhar to question if there was an upper limit to this protection.

In 1931, Chandrasekhar proposed that above 1.4 times the mass of the Sun, a white dwarf would no longer be protected from gravitational collapse by degeneracy pressure. Beyond this boundary— unsurprisingly termed the Chandrasekhar limit — gravity overwhelms the Pauli exclusion principle and gravitational collapse continues unabated.

Cross Section of A Black Hole (©Johan Jarnestad/The Royal Swedish Academy of Sciences)

The discovery of neutrons — the neutral partner of protons in atomic nuclei — in 1932 led Russian theorist Lev Landau to speculate about the possibility of neutron stars. The outer part of these stars would contain neutron-rich nuclei, whilst the inner sections would be formed from a ‘quantum-fluid’ comprised of mostly neutrons.

Again, neutron stars would be protected against gravitational collapse by degeneracy pressure — this time provided by this neutron fluid. In addition to this, the greater mass of the neutron in comparison to the electron would allow neutron stars to reach a greater density before undergoing collapse.

To put this into perspective, a white dwarf with the mass of the Sun would be expected to have a millionth of our star’s volume — giving it a radius of 
5000 km roughly that of the Earth. A neutron star of a similar mass though, that would have a radius of about 20km — roughly the size of a city.

By 1939, Robert Oppenheimer had calculated that the mass-limit for neutron stars would be roughly 3 times the mass of the Sun. Above that limit — again, gravitational collapse wins. Oppenheimer also used general relativity to describe how this collapse appears to a distant observer. They would consider the collapse to take an infinitely long time, the process appearing to slow and freeze as the star’s surface shrinks towards the Schwarzschild radius.

Straight to the Heart: The Inevitability of the cental singularity

“So long as Einstein’s picture of classical spacetime can be maintained, acting in accordance with Einstein’s equation then a singularity will be encountered within a black hole. The expectation is that Einstein’s equation will tell us that this singularity cannot be avoided by any matter in the hole…”

Roger Penrose, The Road to Reality

For Penrose, the mathematical proof of a physical singularity at the heart of a black hole arising from this complete collapse was not enough. He wanted to demonstrate the singularity and the effects on a spacetime that would arise there. He did so with the use of ‘light cones’ travelling down a geodesic — an unerringly straight line. In the process, he unveiled the anatomy of the black hole. 

Lightcones: A Physicist’s Favorite Tool (Robert Lea)

A light cone is most simply described as the path that a flash of light created by a single event and travelling in all directions would take through spacetime. Light cones can be especially useful when it comes to physicists calculating which events can be causally linked. If a line can’t be drawn between the two events that fits in the light cone, one cannot have caused the other.

We call a line emerging from a lightcone a ‘world-line’–these move from the central event out through the top of the cone–the future part of the diagram. The worldline shows the possible path of a particle or signal created by the event at the origin of the lightcone. Throwing a light cone at a black hole demonstrates why passing the event horizon means a merger with the central singularity is inevitable.

Penrose considered what would happen to a light cone as it approached and passed the event horizon of what is known as a ‘Kerr black hole.’ This is a black hole that is non-charged and rotating. Its angular momentum drags spacetime along with it in an effect researchers call frame dragging.

Far from the black hole, light is free to travel with equal ease in any direction. The lightcones here have a traditionally symmetrical appearance which represents this.

Using Lightcones to Probe Black Holes ((©Johan Jarnestad/The Royal Swedish Academy of Sciences))

However, towards the static limit — the point at which the black hole starts to drag spacetime around with it — the lightcones begin to tip towards the singularity and in the direction of rotation and narrow. Thus the static limit represents the point at which light is no longer free to travel in any direction. It must move in a direction that doesn’t oppose the rotation of the black hole. Particles at this limit can no longer sit still — hence the name static limit.

Yet, despite the fact the dragging effect is so strong, here that not even light can resist it, signals can escape this region — it isn’t the event horizon — but they can only do so by travelling in the direction of the rotation.

Interestingly, Penrose suggests that particles entering the static limit and decaying to two separate particles may result in energy leaching from the black hole in what is known as the Penrose process, but that’s a discussion for another time.

Probing Black Holes with Lightcones (Robert Lambourne/ Robert Lea)
Probing Black Holes with Lightcones (Robert Lambourne/ Robert Lea)

So as our light cone moves toward the event horizon, it begins to narrow and tip. But, something extraordinary happens when it passes this boundary. As long as one is using so-called Swartzchild coordinates, once ‘inside the black hole’ proper, the lightcone flips on its side, with the ‘future end’ of the cone pointed towards the singularity.

This can mean only one thing for the worldline of that event, it points to the central singularity signalling that an encounter with that singularity is evitable.

The Anatomy of a Black Hole

“It is generally believed that the spacetime singularities of gravitational collapse will necessarily always lie within an event horizon, to that whatever happens to be the extraordinary physical effects at such a singularity, these will be hidden from the view of any external observer.”

Roger Penrose, The Road to Reality

Black holes aren’t particularly complex in construction and posses only three properties –mass, electric charge, and angular momentum–but physicists working with light cones were able to determine the layers of their anatomy–and crucially, the bounded surfaces that exist within them.
This is what was revolutionary about Penrose’s concepts, they introduced the concept of bounded surfaces to black holes. 

The structure of a Kerr (rotating) Black Hole. (Robert Lambourne/ Robert Lea)

Looking back on this from an era in which a black hole has been imaged for the first time and gravitational waves are beginning to be routinely measured from the mergers of such objects, it’s important to not underestimate the importance of Penrose’s findings.

Before any practical developments surround black holes could even be dreamed of, Roger Penrose provided the mathematical basis to not just suggest the existence of black holes, but also laying the groundwork for their anatomy, and the effect they have on their immediate environment.

Thus, what Penrose’s Nobel award can really be seen as a recognition of moving these objects — or more accurately, spacetime events — from the realm of speculation to scientific theory.

The first-ever image of a black hole was released 2019 came decades after Roger Penrose demonstrated such spacetime events are inevitable in the ungoverned collapse of a star with enough mass. (Event Horizon Telescope collaboration et al)
The first-ever image of a black hole was released 2019 came decades after Roger Penrose demonstrated such spacetime events are inevitable in the ungoverned collapse of a star with enough mass. (Event Horizon Telescope collaboration et al)

Original research and further reading

Penrose. R., ‘Gravitational Collapse and Space-Time Singularities,’  Physical Review Letters, vol. 14, Issue 3, pp. 57-59, [1965]

Penrose. R., ‘The Road to Reality,’ Random House, 2004

Senovilla. J. M. M., Garfinkle. G., ‘The 1965 Penrose Singularity Theorem,’ Classical and Quantum Gravity, [2015].

Relativity, Gravitation and Cosmology, Robert J. Lambourne, Cambridge Press, 2010.

Relativity, Gravitation and Cosmology: A basic introduction, Ta-Pei Cheng, Oxford University Press, 2005.

Extreme Environment Astrophysics, Ulrich Kolb, Cambridge Press, 2010.

Stellar Evolution and Nucleosynthesis, Sean G. Ryan, Andrew J. Norton, Cambridge Press, 2010.

Cosmology, Matts Roos, Wiley Publishing, 2003.

Artist’s impression of star being tidally disrupted by a supermassive black hole

Death By Spaghettification! Astronomers Spot a Star Being Consumed by a Black Hole

An international team of researchers has used telescopes from around the world — including instruments operated by the European Southern Observatory (ESO) — to glimpse a blast of light emitted by a star as it is torn apart by the tidal forces of a supermassive black hole

The event — technically known as a ‘tidal disruption event’ (TDE) — occurred 215-million light-years from Earth, but despite this intimidating sounding distance, this is the closest to our planet such a flare has ever been captured. This, and the fact the astronomers spotted the event early, means the team was able to study the phenomena in unprecedented detail, in turn uncovering some surprises in this violent and powerful process. 

Artist’s impression of star being tidally disrupted by a supermassive black hole
This illustration depicts a star (in the foreground) experiencing spaghettification as it’s sucked in by a supermassive black hole (in the background) during a ‘tidal disruption event’. In a new study, done with the help of ESO’s Very Large Telescope and ESO’s New Technology Telescope, a team of astronomers found that when a black hole devours a star, it can launch a powerful blast of material outwards. (ESO/M. Kornmesser)

The astronomers directed the ESO’s Very Large Telescope (VLT), based in the Atacama desert, Chile, and other instruments at a blast of light that first occurred last year. They studied the flare, located in AT2019qiz in a spiral galaxy in the constellation of Eridanus, for six months as it grew in luminosity and then faded. Their findings are published today in the Monthly Notices of the Royal Astronomical Society.

“My research focuses on close encounters between stars and supermassive black holes in the centres of galaxies. Gravity very close to a black hole is so strong that a star cannot survive, and instead gets ripped apart into thin streams of gas,” Thomas Wevers, co-author of the study and an ESO Fellow in Santiago, Chile, tells ZME Science. “This process is called a tidal disruption event, or sometimes ‘spaghettification’. 

“If not for such tidal disruption events, we would not be able to see these black holes. Hence, they provide a unique opportunity to study the properties of these ‘hidden’ black holes in detail.”

Thomas Wevers, ESO Fellow

Catching the Start of the Movie

Wevers, who was part of the Institute of Astronomy, University of Cambridge, UK, as the study was being conducted, explains that it can take several weeks — or even months — to identify these spaghettification events with any certainty. Such an identification also takes all the telescopes and observational power that can be mustered. This can often cause a delay that results in astronomers missing the early stages of the process.

This image shows the sky around the location of AT2019qiz, at the very centre of the frame. This picture was created from images in the Digitized Sky Survey 2. (ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin)

“It’s like watching a movie but starting 30 minutes in, a lot of information is lost if you can’t watch from the very beginning, and while you might be able to reconstruct roughly what has happened, you can never be completely sure,” the researcher explains. But, that wasn’t the case with this new event.

To stick to the analogy; this time the team had their popcorn and drink and were in their seats before the trailers started rolling. 

“In this new event, we were lucky enough to identify and hence observe it very quickly, which has allowed us to see and understand what happens in the early phases in great detail.”

Thomas Wevers, ESO Fellow

Spotting spaghettification events is not just difficult due to timing issues, though. Such events are fairly rare, with only 100 candidates identified thus far, and are often obscured by a curtain of dust and debris. When a black hole devours a star, a jet of material is launched outwards that can further obscure the view of astronomers. The prompt viewing of this event allowed that jet to be seen as it progressed. 

“The difficulty comes first from picking out these rare events in among all the more common things changing in the night sky: variable stars and supernova explosions,” Matt Nicholl, a lecturer and Royal Astronomical Society research fellow at the University of Birmingham, UK, and the lead author of the study tells ZME Science. “A second difficulty comes from the events themselves: they were predicted to look about 100 times hotter than the flare that we observed. Our data show that this is because of all the outflowing debris launched from the black hole: this absorbs the heat and cools down as it expands.”

Spaghettification: Delicious and Dangerous

The spaghettification process is one of the most fascinating aspects of black hole physics. It arises from the massive change in gravitational forces experienced by a body as it approaches a black hole. 

“A star is essentially a giant ball of hot, self-gravitating gas, which is why it is roughly spherical in shape. When the star approaches the black hole, gravity acts in a preferential direction, so the star gets squeezed in one direction but stretched in the perpendicular direction,” Wevers says. “You can compare it to a balloon: when you squeeze it between your hands, it elongates in the direction parallel to your hands. Because the gravity is so extreme, the result is that the star essentially gets squeezed into a very long and thin spaghetti strand — hence the name spaghettification.”


Death by Spagettification! (ESO/M. Kornmesser/ Robert Lea)

Nicholl continues, explaining what happens next to this stellar spaghetti strand: “Eventually, it wraps all the way around and collides with itself, and that’s when we start to see the light show as the material heats up before either falling into the black hole or being flung back into space.

“The distance at which the star encountered the supermassive black hole was around the same distance between the Earth and Sun — this shows how incredibly strong the gravitational pull of the black hole must be to be able to tear the star apart from that distance.”

“If you picture the Sun being torn into a thin stream and rushing towards us, that’s roughly what the black hole saw!”

Matt Nicholl, Royal Astronomical Society research fellow.

Suprises and Future Developments

The observations made by the astronomers have allowed them to study the dynamics of a star undergoing the spaghettification process in detail, something that hasn’t been possible before. And as is to be expected with such a first, the study yielded some surprises for the team. 

“The biggest surprise with this event was how rapidly the light brightened and faded,” Nicholl tells ZME. “It took about a month from the encounter for the flare to reach its peak brightness, which is one of the fastest we’ve ever seen.”

The researcher continues to explain that faster events are harder to find, so it suggests that there might be a whole population of short-lived flares that have been escaping astronomers’ attention. “Our research may have solved a major and long-standing mystery of why these events are 100 times colder than expected — in this event, it was the outflowing gas that allowed it to cool down.”

Confirming this idea means that the team must now seek scarce telescope time to investigate more of these events to see if this characteristic is unique to the AT2019qiz flare, or if it is a common feature of such events. “Because we studied only one event, it is still unclear whether our results apply universally to all such tidal disruption events. So we need to repeat our experiment multiple times,” Wevers says. “Unfortunately, we are at the whims of nature and our ability to spot new TDEs. When we do, we will need to confirm the picture we have put forward or perhaps adapt it if we find different behaviour.”

The ESO’s Very Large Telescope (VLT) will play an important role in the identification and study of future ‘spaghettification’ events. (ESO Photo Ambassador Serge Brunier.)

Wevers concludes by highlighting the unique position he, Nicholl, and their team find themselves in by studying such rare and difficult to observe events and the objects that lie behind them. “We aren’t yet in the phase where we think we have mapped all the behaviour that occurs following these cataclysmic events, so while each new TDE helps us to answer outstanding questions, at the same time it also raises new questions.

“We find ourselves continually in a catch-22 like situation, which in this case is a good thing as it propels our research forward!” exclaims Wevers.  “I find it pretty amazing that we can study gargantuan black holes, weighing millions or even billions of times the mass of our sun, and which are located hundreds of millions of light-years away, in such detail with our telescopes.”

Original research: Nicholl. M., Wevers. T., Oates. S. R., et al, ‘An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz,’ Monthly Notices of the Royal Astronomical Society, [2020].

With the help of ESO’s Very Large Telescope (VLT), astronomers have found six galaxies lying around a supermassive black hole, the first time such a close grouping has been seen within the first billion years of the Universe. This artist’s impression shows the central black hole and the galaxies trapped in its gas web. The black hole, which together with the disc around it is known as quasar SDSS J103027.09+052455.0, shines brightly as it engulfs matter around it. (ESO/L. Calçada)

‘Cosmic Web’ of a Supermassive Black Hole Ensnares Six Galaxies

Astronomers have discovered a tremendous cosmic web in the early Universe. Trapped within its threads are six galaxies feeding gas to a central supermassive black hole. 

Astronomers have made a startling discovery in the early Universe: six galaxies suspended in the cosmic web of a supermassive black hole. The finding represents the first time such a grouping has been found when the Universe was young —  just under a billion years after the ‘Big Bang.’

The cosmic web with its suspended galaxies seems to conform to the theory that supermassive black holes grew to monstrous sizes thanks to the fact that they sat at the centre of web-like structures with gas and dust to feed them. 

“The title of our article  —  ‘Web of the Giant’ — may suggest the idea of the supermassive black hole as a giant black spider at the centre of the web, with that web providing both the trap and the path to carry the material that feeds the giant at the centre,” Marco Mignoli, an astronomer at the National Institute for Astrophysics (INAF) in Bologna, Italy, tells ZME Science. “The importance of our work is that we are the first group to discover the galaxies that inhabit the web.”

With the help of ESO’s Very Large Telescope (VLT), astronomers have found six galaxies lying around a supermassive black hole, the first time such a close grouping has been seen within the first billion years of the Universe. This artist’s impression shows the central black hole and the galaxies trapped in its cosmic web. The black hole, which together with the disc around it is known as quasar SDSS J103027.09+052455.0, shines brightly as it engulfs matter around it. (ESO/L. Calçada)

The new observation of these six faint galaxies trapped in a web of filament ‘threads’ comprised of hot gas, stars, and galaxies surrounding a supermassive black hole, was made with the aid of the ESO’s Very Large Telescope (VLT) and is described in a paper published in the journal Astronomy and Astrophysics.

As is only fitting for a supermassive black hole ‘spider’, the web in which it sits is of tremendous size —  300 times that of the Milky Way. “From this filamentous structure, the giant black hole is probably accumulating material that has allowed it to grow extremely fast, reaching one billion solar masses in less than a billion years,” Mignoli, author of the paper, adds. “The galaxies stand and grow where the filaments cross, and streams of gas —  available to fuel both the galaxies and the central supermassive black hole —  can flow along the filaments.”

Supermassive Black Hole Feeding in the Early Universe

The light from this cosmic web has traveled to us from a time when the Universe was just 0.9 billion years old. This represents not just a time at which the first generation of black holes have formed from collapsing stars, but also the point where the faster-growing of these black holes have grown to truly monstrous sizes.

https://youtu.be/BHRYrINUigE

The question of how supermassive black holes managed to grow so rapidly has puzzled scientists for decades, with researchers unable to detect exactly how these black holes could obtain so much ‘black hole fuel’ so quickly. The findings seem to provide an answer, suggesting that the cosmic web and the galaxies within it contain enough gas to quickly grow the central black hole into a supermassive giant. 

“[The study] provides confirmation of several theories, that these primordial supermassive black holes are found at the center of immense filamentous structures composed by hot gas and by galaxies that are actively forming stars,” Mignoli says. “Such structures — ‘cosmic webs’ —  can provide the necessary material for the central black hole to grow extremely fast.”

Whilst this isn’t the first time astronomers have spotted a ‘cosmic web’, it is the first time its been occupied by a supermassive black hole ‘spider’ at its heart. 

 “Similar, early large scale structures have already been found, but none with a supermassive black hole at their centre,” Roberto Gilli, an astronomer at INAF in Bologna and co-author of the study, tells ZME Science. “Our work has placed an important piece in the largely incomplete puzzle that is the formation and growth of such extreme, yet relatively abundant objects so quickly after the Big Bang.”

What the researchers can’t be so sure of is how these black holes initially formed, the process by which they are ‘fed’, or how the cosmic web itself developed. “We have no observational evidence of from which seeds these giant black holes are grown,” Mignoli explains. “The structures are too far away, the gas flows too faint to be detected. And also from a theoretical point of view, there are problems that are too difficult to solve.”

One possibility is that cosmic webs such as that discovered by the team formed as a result of the gravitational influence of dark matter haloes. These bunches of mysterious substance — which makes up 90% of the matter in the known Universe — could have drawn together tremendous amounts of gas in the early Universe. From there, the gas and dark matter may have formed the matrix of a cosmic web. 

Mignoli explains that one of the most intriguing lingering questions is what process allows material to be transported from an intergalactic scale to the size of a black hole’s accretion disc — in the order of parsecs. 

Gilli offers some suggestions regarding this feeding process, albeit ones that are currently unsupported by observations: “According to theory, dense environments are a necessary but not sufficient condition,” Gilli explains. “[The feeding mechanism] could be related to gas availability in these dense regions: large reservoirs mean that there is enough gas to fuel the BH and grow fast. Some theories propose that direct gas streams through the web can fall directly into the black holes and grow them.”

Gilli also adds that another way by which supermassive black holes could gather tremendous mass is via galactic mergers. And, the researcher adds, the cosmic web could play a role in this process too. “Another way this web can aid black hole growth is through galaxy mergers: within these dense, filamentary environments, mergers of gas-rich galaxies are more frequent, and mergers normally destabilize gas within galaxies and allow it to fall within the black holes at their centres.”

Spider Hunting: Searching the Universe for more ‘Occupied’ Cosmic Webs

The galaxies observed by the team are some of the faintest ever studied by astronomers, and required employing the tremendous power of the VLT — located at the ESO’s Paranal Observatory in the Atacama Desert, Chile — for several hours. Thus, with the aid of the VLT’s MUSE and FORS2 instruments, the team was able to confirm the six galaxies were linked to a central supermassive black hole.

“Early supermassive black holes are among the most challenging systems in extragalactic astrophysics,” Gilli explains. “We designed this experiment more than 8 years ago in the hope of confirming theory expectations. Observations of such systems are painful and only by cumulating several years of effort we could finally confirm the existence of such a structure.”#

This image shows the sky around SDSS J103027.09+052455.0, a quasar powered by a supermassive black hole surrounded by at least six galaxies. This picture was created from images in the Digitized Sky Survey 2. (ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin)

The aim now is to find more examples of such structures, which Gilli believes should be fairly common in the early Universe. Perhaps then we can start to answer the questions that surround the formation and evolution of such events. 

“We’d like to confirm more structure like this and also to discover populations of SMBHs in the early Universe that should exist but are still missing from our census,” Gilli says. “There are billion-solar-mass black holes at galaxies’ centres and we still do not know where they come from. And there is even more mystery surrounding such systems in the early Universe.”

Mignoli. M., Gilli. R., Decarli. R., et al, [2020], ‘Web of the giant: Spectroscopic confirmation of a large-scale structure around the z=6.31 quasar SDSS J1030+0524,’ Astronomy and Astrophysics. 

Do Black Holes Merge? (NASA/Public Domain)

Double Trouble! Hunting for Supermassive Black Hole Mergers

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.

Do Black Holes Merge? (NASA/Public Domain)
Artist’s impression of a violent merger between two supermassive black holes (NASA/ Pubic Domain)

“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, [2020]. 

Astronomers witness light produced by the merger of two black holes for first time

An artist’s impression of a supermassive black hole. Credit: R Hurt (IPAC)/Caltech.

It doesn’t get any blacker than a black hole, the densest object in the universe. Their gravitational pull is so strong that nothing can escape its clutches, including light.

However, it’s an ironic twist of fate that when two black holes merge in a cataclysmic event, they can also produce a flare of light as powerful as a trillion suns. Astronomers have now confirmed this phenomenon for the first time in a new study.

On May 21, 2019, scientists affiliated with the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detected the gravitational waves generated by the merger of two black holes, in an event dubbed S190521g.

Gravitational waves are essentially ripples in the fabric of spacetime which are generated by interactions between very massive accelerating cosmic objects, such as neutron stars or black holes. Physicists liken gravity waves to the waves generated by a stone thrown into a pond.

Although gravitational waves were predicted by Einstein’s Theory of Relativity, their existence was confirmed very recently in 2016 by LIGO, whose founders were awarded the much deserved Nobel Prize in Physics one year later.

Since then, scientists have found many sequences of gravitational waves, with much more to follow once more sensible detectors come online.

This story isn’t about gravitaional waves, though. While studying S190521g, physicists at Caltech’s Zwicky Transient Facility (ZTF) also spotted a flare of light emanting from the pair of merging black holes.

“This supermassive black hole was burbling along for years before this more abrupt flare,” Matthew Graham, a research professor of astronomy at Caltech and the project scientist for ZTF, said in a statement. “The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event.”

Light-emitting black holes mergers aren’t exactly a new idea. They’ve been theorized before by physicists whose models suggested that merging black holes can plow into the hot gas, dust, and all the other jumbled mess of matter hovering around the black hole, waiting to be gobbled up.

The huge momentum and sudden release of kinetic energy of the merged black hole can cause gas to react, generating a bright flare.

Now, the theory has been shown to also work in practice. The light from S190521g was visible for days, before it slowly faded into oblivion about a month later.

However, the researchers say they will keep an eye on this newly birthed supermassive black hole. They hope to catch another flare within a couple of years as it is expected to ram into the surrounding disk of gas once more.

“Supermassive black holes like this one have flares all the time. They are not quiet objects, but the timing, size, and location of this flare was spectacular,” Mansi Kasliwal, an assistant professor of astronomy at Caltech, and co-author of the study, said in a statement.

“The reason looking for flares like this is so important is that it helps enormously with astrophysics and cosmology questions. If we can do this again and detect light from the mergers of other black holes, then we can nail down the homes of these black holes and learn more about their origins.”

The findings appeared in the journal Physical Review Letters.

Astrophysicists destroy virtual stars to simulate the birth of black holes

Artist impression of a supernova. Credit: Pixabay.

By employing the resources of one of the fastest supercomputers in the world, astrophysicists in Australia have simulated the last days of very large stars with masses many times that of the Sun. Their simulation provides new valuable insights into how massive stars end with a bang as they explode in supernovae events and how black holes and neutron stars rise out of the ashes.

Cosmic chaos inside a computer chip

The state-of-the-art OzSTAR supercomputer at the Swinburne University of Technology crunched the numbers for various simulations that modeled the core-collapse of three stars. These virtual stars are 39, 20, and 18 times more massive than the sun, respectively.

When such massive stars reach the end of their life cycles, they typically experience a core-collapse supernova. When this happens, they turn into some of the brightest objects in the universe. And, in the aftermath, they are ready to become neutron stars or black holes.

This extremely dramatic stellar death also generates gravitational waves, whose signature can inform astrophysicists about how both black holes and neutron stars are birthed — this was the main aim of this simulation.

For instance, in 2017, astronomers detected a cosmic cataclysmic event: The merger of two neutron stars from 130 million years ago. The force of the collision was so strong that it literally shook the fabric of space-time, generating gravitational waves that eventually reached Earth, where they were detected. The two neutron stars either merged into a huge single neutron star or collapsed into a black hole.

A 3D-volume render of a core-collapse supernova. Credit: Bernhard Mueller, Monash University.

But in order to detect various core-collapse supernovae from gravitational waves, scientists need to know what such signals will look like.

The new simulation modeled complicated physics, informing scientists what kind of signals they should expect to see in their detectors when a star explodes.

“Our models are 39 times, 20 times, and 18 times more massive than our sun. The 39-solar mass model is important because it’s rotating very rapidly, and most previous long-duration core-collapse supernova simulations do not include the effects of rotation,” said Jade Powell, a postdoctoral researcher at OzGrav.

According to the results, which were described in the Monthly Notices of the Royal Astronomical Society, the two most massive virtual stars generated explosions powered by neutrinos, while the smallest virtual star didn’t explode at all.

Such stars that don’t go fully supernova emit lower amplitude gravitational waves, but their frequency is still within detectable ranges of current detectors in use.

The findings also suggest that exploding stars producing large gravitational-wave amplitudes could be detected by the next generation of detectors, such as the upcoming Einstein Telescope.

“For the first time, we showed that rotation changes the relationship between the gravitational-wave frequency and the properties of the newly-forming neutron star,” explains Powell.

Astrophysicists find more evidence of ‘wandering’ black holes

Artist’s conception of a dwarf galaxy, its shape distorted, most likely by a past interaction with another galaxy, and a massive black hole in its outskirts (pullout). The black hole is drawing in material that forms a rotating accretion disc and generates jets of material propelled outward. Image credit: Sophia Dagnello, NRAO/AUI/NSF

Dwarf galaxies have traditionally been considered too small to host massive black holes, but new research emerging from Montanna State University (MSU) has revealed dozens of examples. The research, published in the Astrophysical Journal has delivered another surprise, these black holes aren’t located where scientists usually expect to find them.

“All of the black holes I had found before were in the centres of galaxies,” says Amy Reines, an assistant professor in the Department of Physics in the College of Letters and Science. “These were roaming around the outskirts. I was blown away when I saw this.”

Reines and her team searched 111 dwarf galaxies within a radius of a billion-light-years of Earth using the National Science Foundation’s Karl G. Jansky Very Large Array at the National Radio Astronomy Observatory, Albuquerque, New Mexico. During the course of their search, they identified 13 galaxies that very probably host black holes, the majority of which were not centralised. 

Reines is also a researcher in the MSU’s eXtreme Gravity Institute, which unites astronomers and physicists in order to study phenomena in which the gravitational influence is so powerful that it blurs the separation of space and time. This includes events and objects such as neutron stars, black holes, mergers and collisions between the two and even, the initial extreme period of rapid expansion of the universe — the big bang. 

The researcher explains that whilst stellar-mass black holes — those with a mass of up to 10 times that of our Sun — form as large stars undergo gravitational collapse, we are, thus far, uncertain how supermassive black holes form. This class of black hole which can have masses of up to billions of times that of the Sun is most commonly found in the centre of galaxies. 

This is certainly the case with our galaxy, the Milky Way, which hosts the supermassive black hole Sagittarius A* (SgrA*) at its centre. Dwarf galaxies are smaller than spiral galaxies like the Milky Way, containing a few billion stars rather than 100–400 billion as spiral galaxies tend to.

The results collected by Reines confirm computer simulations generated by Jillian Bellovary, assistant professor at Queensborough Community College, New York and Research Associate at the American Museum of Natural History. 

How black holes get lost

Bellovary’s computer simulations suggested that black holes could be disturbed from the centre of dwarf galaxies by interactions they undergo as they travel through space. This result coupled with Reines’ study have the potential to change the way we look for black holes in dwarf galaxies going forward. This change in thinking could also impact theories of how both dwarf galaxies and supermassive black holes form. 

“We need to expand searches to target the whole galaxy, not just the nuclei where we previously expected black holes to be,” Reines adds.

No stranger for the search for black holes, Reines has been hunting these events for a decade, ever since she was a graduate student at the University of Virginia. Whilst she initially focused on star formation in dwarf galaxies, her research led her to something else that captured her interest: a massive black hole “in a little dwarf galaxy where it wasn’t supposed to be.”

Henize 2–10: a dwarf galaxy that hides a massive secret ( Reines et al. (2011))
Henize 2–10: a dwarf galaxy that hides a massive secret ( Reines et al. (2011))

The little dwarf galaxy she refers to is Heinze 2–10, located 30-million-light-years from Earth, which had previously been believed too small to host a massive black hole. “Conventional wisdom told us that all massive galaxies with a spheroidal component have a massive black hole and little dwarf galaxies didn’t,” Reines explains, adding that when she discovered such a relationship it was a “eureka” moment. After publishing these findings in the journal Nature she continued searching for further black holes in dwarf galaxies. “Once I started looking for these things on purpose, I started finding a whole bunch,” Reines says.

Visible-light images of galaxies that VLA observations showed to have massive black holes. Center illustration is artist’s conception of the rotating disk of material falling into such a black hole, and the jets of material propelled outward. Image credit: Sophia Dagnello, NRAO/AUI/NSF; DECaLS survey; CTIO
Visible-light images of galaxies that VLA observations showed to have massive black holes. Center illustration is artist’s conception of the rotating disk of material falling into such a black hole, and the jets of material propelled outward. Image credit: Sophia Dagnello, NRAO/AUI/NSF; DECaLS survey; CTIO

Changing her tactics by shifting from visual data from radio signals, Reines uncovered over 100 possible black holes in her first search of a sample that included 40,000 dwarf galaxies. In current search, as described in the latest paper, Reines returned to radio searches, hunting for radio signatures with that sample. This, she says, should allow her to find massive black holes in star-forming dwarf galaxies, even though she has only found one thus far. 

“When new discoveries break our current understanding of the way things work, we find even more questions than we had before,” comments Yves Idzerda, head of the Department of Physics at MSU.

As for Reines, the search continues. 

“There are lots of opportunities to make new discoveries because studying black holes in dwarf galaxies is a new field,” she said. “People are definitely captivated by black holes. They’re mysterious and fascinating objects.”

Original research: https://iopscience.iop.org/article/10.3847/1538-4357/ab4999

The rapidly spinning neutron star embedded in the center of the Crab nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, the crushed ultra-dense core of the exploded star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second.

How Black Holes and Neutron Stars Shine

The rapidly spinning neutron star embedded in the center of the Crab nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, the crushed ultra-dense core of the exploded star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second.
The rapidly spinning neutron star embedded in the center of the Crab nebula is the dynamo powering the nebula’s eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, the crushed ultra-dense core of the exploded star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second. NASA, ESA, J. Hester (Arizona State University)

Without question, black holes and neutron stars are the most mysterious objects in the known universe. One of the mysteries that has surrounded these dense objects that arise at the conclusion of a star’s lifetime is the source of the electromagnetic radiation emitted from regions of space that host them.

Astrophysicists have suspected for decades that this high-energy radiation is a product of electrons moving at near light speeds around black holes and neutron stars.

The missing piece of the puzzle, however, is exactly what process causes these electrons to be accelerated to such relativistic speeds.

Now, in a study published in the Astrophysical Journal, astrophysicists Luca Comisso and Lorenzo Sironi discuss massive supercomputer simulations that they have employed to uncover this mystery mechanism behind this acceleration.

The conclusion reached by the duo from Columbia University is that this velocity is granted by an interaction between chaotic motion and a phenomenon called ‘reconnection’ found in tremendously powerful magnetic fields.

“Turbulence and magnetic reconnection–a process in which magnetic field lines tear and rapidly reconnect–conspire together to accelerate particles, boosting them to velocities that approach the speed of light,” says Comisso, a postdoctoral research scientist at Columbia and first author on the study.

The researcher goes on to explain that regions that host black holes and neutron stars are permeated by a sea of extremely hot gas particles. As these gas particles move chaotically they drag magnetic field lines with them. This drives vigorous magnetic reconnection.

“It is thanks to the electric field induced by reconnection and turbulence that particles are accelerated to the most extreme energies, much higher than in the most powerful accelerators on Earth, like the Large Hadron Collider at CERN,” Comisso continues.

Calculating chaos

The difficulty in studying such turbulent gas is that chaotic motion is impossible to predict precisely. In fact, the problem of dealing with the mathematics of turbulence is so well-known that it is one of the seven problems that make up the Millennium Prize challenge in mathematics.

Here, a massive super-computer simulation shows the strong particle density fluctuations that occur in the extreme turbulent environments that host black holes and neutron stars. Dark blue regions are low particle density regions, while yellow regions are strongly over-dense regions. Particles are accelerated to extremely high speeds due to the interactions with strongly turbulence fluctuations in this environment.
Here, a massive super-computer simulation shows the strong particle density fluctuations that occur in the extreme turbulent environments that host black holes and neutron stars. Dark blue regions are low particle density regions, while yellow regions are strongly over-dense regions. Particles are accelerated to extremely high speeds due to the interactions with strongly turbulence fluctuations in this environment. (Luca Comisso and Lorenzo Sironi)

Comisso and Sironi tackled this issue from an astrophysical standpoint by designing and employing extensive supercomputer simulations to solve equations which describe turbulence of charged particles in a gas. These simulations are some of the largest ever created in this research area.

“We used the most precise technique–the particle-in-cell method–for calculating the trajectories of hundreds of billions of charged particles that self-consistently dictate the electromagnetic fields. And it is this electromagnetic field that tells them how to move,” adds Sironi, assistant professor of astronomy at Columbia and the study’s principal investigator.

The researchers point out that the most critical element of the study was the identification of the role played by magnetic reconnection in this turbulent environment.

Their simulations suggest that reconnection ‘selects’ particles that will be accelerated to magnetic fields to near relativistic speeds. It simulations also show that particles in this environment gained most of their energy by bouncing off the turbulent fluctuations at random.

The stronger the magnetic field involved in this process, the more rapid the acceleration of the electrons. These strong fields also cause the particles to travel in a curved trajectory, with such acceleration causing the emission of electromagnetic radiation.

“This is indeed the radiation emitted around black holes and neutron stars that make them shine, a phenomenon we can observe on Earth,” Sironi says.

Understanding the extreme environments around Black Holes and Neutron Stars

The duo of researchers point out that the ultimate aim of their study is to better understand the extreme environments surrounding both black holes and neutron stars and the events that occur in them.

This could, in turn, shine additional light on both fundamental physics and our understanding of how the universe functions.

But there is still much work to be done.

The team plan to compare the predictions revealed by their simulations to observations of electromagnetic radiation emitted by the Crab Nebula. By analysing the electromagnetic spectrum of this bright supernova debris leftover from the violent explosion of a star they can connect their work with actual observations.

“We figured out an important connection between turbulence and magnetic reconnection for accelerating particles, but there is still so much work to be done,” Comisso concedes. “Advances in this field of research are rarely the contribution of a handful of scientists, but they are the result of a large collaborative effort.”


Original research: https://iopscience.iop.org/article/10.3847/1538-4357/ab4c33

This is an artist's impression of planets orbiting a supermassive black hole. (Kagoshima University)

Planets could orbit Supermassive Black Holes

This is an artist's impression of planets orbiting a supermassive black hole. (Kagoshima University)
This is an artist’s impression of planets orbiting a supermassive black hole. (Kagoshima University)

The idea of stars orbiting the supermassive black holes that researchers believe lurk at the centre of most galaxies has been long established as a matter of fact in science. In ‘active galactic nuclei’ or AGNs, these black holes are surrounded by haloes of gas and dust in a violent churning environment. Such clouds of gas and dust have the potential to birth not only stars but planets as well. Yet, the question of whether planets can also orbit these spacetime events has yet to be established. 

Enter Keiichi Wada, a professor at Kagoshima University, and Eiichiro Kokubo, a professor at the National Astronomical Observatory of Japan. These scientists from the distinct fields of active galactic nuclei research and planet formation research respectively have calculated that as a result of gas disc growth, an entirely new class of planets may form around supermassive black holes. 

“With the right conditions, planets could be formed even in harsh environments, such as around a black hole,” Wada points out. 

In their research published in the Astrophysical Journal, the duo of theoreticians propose that protoplanetary discs that surround young stars may not be the only potential site for planet formation. The researchers instead focused calculations and mathematical models on the denser dust discs found around supermassive black holes in AGNs, thus arriving at a surprising conclusion. 

“Our calculations show that tens of thousands of planets with 10 times the mass of the Earth could be formed [at a distance of] around 10 light-years from a black hole,” says Eiichiro Kokubo. 

“Around black holes, there might exist planetary systems of astonishing scale.”

One of the hindrances to the formation of planets in such discs of dust has previously been the amount of energy generated in AGNs, Researchers had believed that this energy output would prevent the coagulation of ‘fluffy ice dust’ that can help the growth of dust grains that can lead to planet formation in protoplanetary discs.

But, what Wada and Kokubo discovered was that the huge density of dust discs around supermassive black holes in AGNs —potentially containing as much as a hundred thousand times the mass of the Sun worth of dust, which is a billion times more massive than a typical protoplanetary disc — helps protect the outer layers from bombardment from high-energy radiation such as gamma rays. 

 A schematic picture of the Active Galactic Nucleus (AGN) and the circumnuclear disc. (Wada, Kokubo, 2019)
A schematic picture of the Active Galactic Nucleus (AGN) and the circumnuclear disc. (Wada, Kokubo, 2019)

This helps form a low-temperature region similar to that found in protoplanetary discs, and thus, in turn, increases the likelihood of fluffy deposits building.

The process would lead to the formation of planets within a period of several hundred million years, according to the pair, and also result in much denser and more populated collections of planets. 

Unfortunately, the limits of current methods of identifying exoplanets would make identifying planets around a supermassive black hole challenging to say the least. 

“ Doppler spectroscopy, transit photometry, gravitational micro-lensing, or direct imaging are hopeless,” warn the duo in their paper. They go on to suggest that a method called photometry with an x-ray interferometer located in space could be a possible solution — if a way of distinguishing the effect caused by such planets from the natural variability of the AGN can be developed. 

For now, researchers will have to look to mathematical models alone to theorise about the potential for planets in orbit around black holes. 


Original research: https://arxiv.org/pdf/1909.06748.pdf

NGC1569 is a star-forming galaxy. Galaxies such as this could see their star formation rates affected by strong winds emanating from a central black hole. (HST/NASA/ESA)

Black Holes could stunt the growth of dwarf galaxies

NGC1569 is a star-forming galaxy. Galaxies such as this could see their star formation rates affected by strong winds emanating from a central black hole. (HST/NASA/ESA)
NGC1569 is a star-forming galaxy. Galaxies such as this could see their star formation rates affected by strong winds emanating from a central black hole. (HST/NASA/ESA)

Black holes at the centre of small dwarf galaxies could slow or even halt the formation of stars via the powerful winds they produce, researchers from University of California, Riverside, have discovered. This suppression of star-formation could have a marked influence on the evolution of such galaxies.

The result seems to confirm the long-held suspicion that supermassive black holes at the centre of galaxies can influence that galaxy’s evolution — including how they grow and the way that they age. But, the research also delivers a surprise; the winds that the astronomers measured coming from the black hole were more powerful than the team reckoned for. This means that models of star formation in dwarf galaxies may require a rethink.

“We expected we would need observations with much higher resolution and sensitivity, and we had planned on obtaining these as a follow-up to our initial observations,” said Gabriela Canalizo, a professor of physics and astronomy at UC Riverside who led the research team. “But we could see the signs strongly and clearly in the initial observations.

“The winds were stronger than we had anticipated.”

Gabriela Canalizo

Thus meaning that black holes don’t just influence the development of larger galaxies, but also play a role in the evolution of smaller dwarf galaxies — galaxies containing anywhere from a few thousand to a few billion stars.

Canalizo continues: “Our findings now indicate that their effect can be just as dramatic, if not more dramatic, in dwarf galaxies in the universe.”

The study — the results of which are discussed in the Astrophysical Journal — used data collected in the Sloan Digital Sky Survey (SDSS), a project which maps 35% of the sky above Earth. In doing so, the survey has been able to identify 50 dwarf galaxies — 29 of which demonstrated clear characteristics of possessing black holes at their centres. A further six of these showed evidence of high-velocity outflows of ionised gas — the powerful winds in question.

The next step for the researchers was to use the Keck telescopes — based in Hawaii — to both detect and measure the properties of these winds, marking the first time this has been achieved.

Discussing what her team found, Canalizo adds: “We found some evidence that these winds may be changing the rate at which the galaxies are able to form stars.”

Studying dwarf galaxies could be the key to understanding how galaxies in general evolve

The study of these smaller galaxies could help scientists answer lingering about galactic evolution in general.

“Larger galaxies often form when dwarf galaxies merge together,” explains Christina Manzano-King, a doctoral student in Canalizo’s lab and the first author of the paper. As a consequence of this, she continues, dwarf galaxies are particularly useful in understanding how galaxies evolve.

Dwarf galaxies hosting active galactic nuclei that have spatially extended outflows. (SDSS)
Dwarf galaxies hosting active galactic nuclei that have spatially extended outflows. (SDSS)

“Dwarf galaxies are small because after they formed, they somehow avoided merging with other galaxies,” she adds. “Thus, they serve as fossils by revealing what the environment of the early universe was like.

“Dwarf galaxies are the smallest galaxies in which we are directly seeing winds — gas flows up to 1,000 kilometres per second — for the first time.”

Christina Manzano-King

Explaining what causes these powerful winds, Manzano-KIng points to material being fed into the black hole. This material — usually gas and dust — forms an accretion disc around the black hole. In this disc — which gradually feeds the black hole — conditions are so violent that friction and tremendous tidal forces heats the material. This releases radiative energy which shoves gas out of the galaxy’s centre and into intergalactic space.

This negatively affects the amount of gas available for star formation.

Manzano-King continues: “What’s interesting is that these winds are being pushed out by active black holes in the six dwarf galaxies rather than by stellar processes such as supernovae.

“Typically, winds driven by stellar processes are common in dwarf galaxies and constitute the dominant process for regulating the amount of gas available in dwarf galaxies for forming stars.”

Astronomers believe that winds emanating from black holes can compress gas and thus aid the gravitational collapse of gas clouds, kick-starting star-formation. But, if the wind is too strong and thus expels gas from the galaxy’s centre, rather than aiding the star formation process, gas becomes unavailable and hinders the process.

This is exactly what appears to be happening in the six galaxies that the team’s research highlighted. In these cases, the wind has had a clear detrimental impact on star formation rates.

Rethinking the relationship between black holes and star formation rates

This research may result in a rethinking of models of star formation and the evolution of galaxies. Current models do not take into account the impact of black holes in dwarf galaxies.

From left to right: Laura Sales, Christina Manzano-King, and Gabriela Canalizo. The team’s research could force a rethinking of star formation rates in dwarf galaxies ( Stan Lim, UC Riverside)
From left to right: Laura Sales, Christina Manzano-King, and Gabriela Canalizo. The team’s research could force a rethinking of star formation rates in dwarf galaxies ( Stan Lim, UC Riverside)

“Our findings show that galaxy formation models must include black holes as important, if not dominant, regulators of star formation in dwarf galaxies,” points out Laura V. Sales, assistant professor of physics and astronomy at UC Riverside.

As for the future of this research, the team next plans to investigate characteristics of gas outflows such as mass and momentum.

“This would better inform theorists who rely on such data to build models,” concludes Manzano-King. “These models, in turn, teach observational astronomers just how the winds affect dwarf galaxies.

“We also plan to do a systematic search in a larger sample of the Sloan Digital Sky Survey to identify dwarf galaxies with outflows originating in active black holes.”


Original research: ‘AGN-Driven Outflows in Dwarf Galaxies’ Christina M. Manzano-King, Gabriela Canalizo, and Laura V. Sales.

Get ready for black hole videos, researchers say

The already famous black hole image. Image credits: Event Horizon Telescope Group.

It was once thought impossible to take photos of black hole. After all, here is an object that’s so massive light itself can’t escape it — how could you take a photo? Against all odds, a team of 347 researchers managed to overcome that obstacle, using a creative approach that linking multiple radio telescopes.

Previously, astronomers were able to detect light swallowed by the black holes, but couldn’t have the sharpness in the images to really “see” that event. In the late 2000s, researchers obtained approval to use three telescopes to establish a proof of concept. In 2008, they had the first real black hole measurements. By 2017, they had 8 telescopes to work with, and ultimately, in 2018, they managed to take the first real photo of a black hole.

“I’ve been working on this for 20 years. So my wife was finally convinced that what I was doing was worth it a little bit,” joked Shep Doeleman, the project leader and an astronomer at the Harvard-Smithsonian Center for Astrophysics.

The image took the world by storm — not just because it was showing something once thought impossible, but because it was a stunning achievement in its own right. However, Doeleman says, that’s just the start of it. By 2020, we will have the first video of a black hole, he estimates — albeit, it will probably resemble more of a low-quality GIF than a 4K video. But things will move quickly, according to Doeleman.

“What I predict is that by the end of the next decade we will be making high quality real-time movies of black holes that reveal not just how they look, but how they act on the cosmic stage,” Shep Doeleman, the project’s director, told AFP in an interview.

“It could be that maybe we will make the first crude movie” by 2020, he added. Ideally, scientists would need more telescopes, both on Earth and in orbit, to improve the resolution further more.

There is evidence that most (if not all) galaxies have massive black holes at their core. The black hole captured in the iconic image lies at the center of the Messier 87 galaxy, and it lies 53.5 million light-years away from Earth. The black hole is relatively calm and perfectly suited for such observations. Researchers would also like to image the one at the center of our own Milky Way: Sagittarius-A*. However, this might prove more problematic as Sagittarius-A is much more turbulent.

“Orbits of matter around M87 take about a month to circulate. Whereas orbits around Sagittarius-A* can take only half an hour, during one night of observing Sagittarius-A* can change before your eyes,” explained Doeleman.

Nevertheless, Doeleman is optimistic.

“We do see ourselves as explorers, we’ve taken a journey in our minds. And we are instruments at the edge of a black hole. And now we’re coming back to report what we found.”

The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.

Supermassive black hole pairs imaged in merging galactic cores

For the first time, astronomers have observed pairs of galaxies in the final stages of merging into a single, larger galactic body. The findings suggest that such events are more common than astronomers used to think.

The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.

The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.

Astronomers suspect that supermassive black holes lurk at the heart of every sizable galaxy, holding the galactic fiber together. For instance, at the galactic core of the Milky Way, there should be a supermassive black hole called Sagittarius A*, a staggering 4.5 million solar masses in size.

Black holes likely reach this sort of dizzying size through the merger of galaxies. However, evidence has been conflicting due to a lack of direct imaging of the process, which is obscured by swirling clouds of gas and dust. What’s more, simulations show that the more these galaxies progress in their merger, the greater the concealment effect.

To peer through all the matter that obscures supermassive black holes, scientists combed through a huge catalog of 10 years’ worth of X-ray measurements taken by the Burst Alert Telescope (BAT) aboard NASA’s Neil Gehrels Swift Observatory.

“The advantage to using Swift’s BAT is that it observes high-energy, ‘hard’ X-rays,” said study co-author Richard Mushotzky, a professor of astronomy at the University of Maryland. “These X-rays penetrate through the thick clouds of dust and gas that surround active galaxies, allowing the BAT to see things that are literally invisible in other wavelengths.”

Next, the research team analyzed another catalog of galaxies from NASA’s Hubble Space Telescope and the Keck Observatory in Hawaii whose X-ray signatures matched the Swift readings.

Various colliding galaxies along with closeup views of their coalescing nuclei in the bright cores. The images were taken in near-infrared light by the Keck Observatory in Hawaii. Credit: Keck images: W. M. Keck Observatory and M. Koss.

Various colliding galaxies along with closeup views of their coalescing nuclei in the bright cores. The images were taken in near-infrared light by the Keck Observatory in Hawaii. Credit: Keck images: W. M. Keck Observatory and M. Koss.

The breakthrough lied with the Keck Observatory’s adaptive optics technology whose deformable mirrors controlled by a computer enable a phenomenal increase in resolution. Using this tech, researchers were able to produce extremely sharp, near-infrared images of X-ray-producing black holes not found in the Hubble archive.

“People had conducted studies to look for these close interacting black holes before, but what really enabled this particular study were the X-rays that can break through the cocoon of dust,” explained Koss. “We also looked a bit farther in the universe so that we could survey a larger volume of space, giving us a greater chance of finding more luminous, rapidly-growing black holes.”

In total, 96 galaxies were observed with the Keck telescope and 385 galaxies from the Hubble archive. According to the results, 17 percent of these galaxies host a pair of black holes at their center, which are locked in the late stages of a galactic merger. This was surprising to learn since previously simulations suggested that black hole pairs spend very little time in this phase.

“Seeing the pairs of merging galaxy nuclei associated with these huge black holes so close together was pretty amazing,” said Michael Koss, co-author of the new study published in the journal Nature. “In our study, we see two galaxy nuclei right when the images were taken. You can’t argue with it; it’s a very ‘clean’ result, which doesn’t rely on interpretation.”

The findings suggest that galactic mergers may indeed be a key process by which black holes grow to stupendous masses. And the stages that the astronomers have mapped out in this study likely foretell the fate of our very own galaxy. In about 6 billion years, scientists estimate that the Milky Way will merge with the Andromeda galaxy into one big galaxy.

There is still much to learn about black hole mergers, though. Right now, hardware is a huge limitation, but, once the James Webb Space Telescope is deployed in 2021, scientists will be able to measure masses, growth rates, and other physical parameters of black hole pairs.

Supermassive black holes eventually stop star formation

Researchers analyzed the correlation between the mass of supermassive black hole and the history of star formation in its galaxy. They found that the bigger the black hole is, the harder it is for the galaxy to generate new stars.

Scientists have been debating this theory for a while, but until now, they lacked enough observational data to prove or disprove it.

Via Pixabay/12019

Researchers from the University of Santa Cruz, California used data from previous studies measuring supermassive black hole mass. They then used spectroscopy to determine how stars formed in galaxies featuring such gargantuan black holes and correlate the two.

Spectroscopy is a technique that relies on measuring the wavelength of light emerging from objects — stars, in this case. The paper’s lead author Ignacio Martín-Navarro used computational analysis to determine how the black holes affected star formation — in a way, he tried to solve a light puzzle.

“It tells you how much light is coming from stellar populations of different ages,” he said in a press release.

Via Pixabay / imonedesign.

Next, the research team plotted the size of supermassive black holes and compared them to a history of star formation in that galaxy. They found that as the black holes grew more and more, star formation was significantly slowed down. Other characteristics of the galaxies, such as shape or size, were found irrelevant to the study.

“For galaxies with the same mass of stars but different black hole mass in the center, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes. So star formation lasted longer in those galaxies with smaller central black holes,” Martín-Navarro said.

Star gas from Carina Nebula, source: Pixabay/skeeze

Scientists still trying to determine why this happens. One theory suggests that the lack of cold gas is the main culprit for reduced star formation. The supermassive black holes suck in the nearby gas, creating high-energy jets in the process. These jets ultimately expel cold gas from the galaxy. Without enough cold gas, there is no new star formation, so the galaxy becomes practically sterile.

In the press release, co-author Aaron Romanowsky concluded:

“There are different ways a black hole can put energy out into the galaxy, and theorists have all kinds of ideas about how quenching happens, but there’s more work to be done to fit these new observations into the models.”

The paper was published in Nature on the 1st of January 2018.

Black-holes-little-book

Book review: ‘The Little Book of Black Holes’

Black-holes-little-book

The Little Book of Black Holes
By Steven S. Gubser and Frans Pretorius
Princeton University Press, 200pp | Buy on Amazon

On September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Livingston, Louisiana, detected gravitational waves produced by the merger of two black holes. This was the culmination of decades-long efforts, allowing scientists to finally pick up the faint whispers murmured by accelerating massive objects, causing ripples in space-time much like a stone thrown in a pond generates oscillations on the water’s surface.

This milestone achievement was confirmed recently on August 17, 2017, by the third detection of gravitational waves produced by merging neutron stars whose signal was also detected with conventional methods in ground-based telescopes.

Remarkably, these revolutionizing discoveries were predicted more than a hundred years ago by Einstein’s Theory of General Relativity, which describes how matter distorts the fabric of space-time based on its mass — more massive objects have a greater effect. It was another German physicist by the name of Karl Schwartzchild who found a rigorous solution to the field equations in Albert Einstein’s theory of general relativity. He did this while serving on the Russian front during World War I. His work became one of the pillars of modern relativistic studies, eventually leading to the conceptualization of, perhaps, the most mysterious objects in the universe: black holes. 

Black holes are strange. They’re the last stage in the evolution of some massive stars (at least 10 times more massive than the Sun) which collapse in a region in space where the pulling force of gravity is so strong that not even light is able to escape. This means that we can’t directly observe black holes. No one has ‘seen’ a black hole so far, but after decades of research, scientists are confident they exist because nothing other than a black hole can explain the physics around us.

Black holes are also notoriously difficult to grasp. Despite this, Steven Gubser and Frans Pretorius, two young professors of physics at Princeton, do an excellent job with their “Little Book of Black Holes”. The brief overview provides a great rundown of the physics and thought system required to get to the bottom of a black hole (spoiler: it’s not pleasant once you cross inside).

This lovely book is a rollercoaster ride through time and space, taking the reader right through the ins and outs of peculiar objects like black holes, white holes, and even wormholes, with bouts of ‘real-life’ illustrations to keep the experience (somewhat) grounded. All of this and much more in less than 200 pages, which speaks volumes about the authors’ ability to condense an eminently complex subject into a relatable form.

Prepare for a lot of weirdness but if you can make it to the end, you might feel a little shiver after grazing the last chapter. One can only imagine what Albert Einstein, the man who started it all and who never acknowledged the existence of black holes, would say were he alive today to see where modern cosmology is at. The authors made this thoughtful leap in the last chapter of the book where they write a candid letter to Einstein bringing him up to speed with quasars, dark energy, LIGO, and, of course, black holes.

“A lot of Princeton professors don’t wear ties to work anymore, but most of us do wear socks. Lake Carnegie is as beautiful as ever. We don’t see many sailors out there, but there’s been an eagle nesting right on the edge of the lake. We haven’t figured out a unified field theory yet, but we’re still trying. The best is yet to come.”

Yours truly

Steve and Frans

 

Some galaxy types are brighter due to ‘hungrier’ black holes, and this could fundamentally change how we study galaxies

According to the unified model for the evolution of galaxies and quasars, Type I and Type II galaxies have the same fundamental structure and energetic profile. The two types of galaxies look different to us, however, because they point toward Earth at different angles, this theoretical framework suggests. Astronomers now report that not only do Type I and Type Ii galaxies look different, they are, in fact, different from each other.

Artist's illustration of galaxy with jets from a supermassive black hole. Credit: Wikimedia Creative Commons.

Artist’s illustration of a galaxy with jets from a supermassive black hole. Credit: Wikimedia Creative Commons.

The team comprised of more than 40 scientists used NASA’s Swift Burst Alert Telescope to examine 836 active galaxies with high-energy or ‘hard’ X-rays, the same radiation involved in imaging human skeletons. The results were then compared to data from 12 different ground-based telescopes in order to assess the mass and growth rate of the galaxies’ active nuclei. For a galaxy, this nucleus coincides with a supermassive black hole whose massive gravity keeps millions of stars in check around its center.

Black hole behavior defines galaxies

The differences in X-ray spectra between the type I and Type II galaxies unambiguously suggest that the two are not the same at all, structurally or energy-wise. Specifically, the black holes at the center of Type I galaxies consume matter and energy significantly faster than those that lie at the heart of Type II galaxies. The results are backed by previous observations that showed Type II galaxies have a lot more dust in the vicinity of the black hole, dust which pushes against the gas that’s funneled into the black hole by gravity.

“The unified model has been the prevailing wisdom for years. However, this idea does not fully explain the differences we observe in galaxies’ spectral fingerprints, and many have searched for an additional parameter that fills in the gaps,” Richard Mushotzky, a professor of astronomy at UMD and a co-author of the study, said in a statement.

“Our new analysis of X-ray data from NASA’s Swift Burst Alert Telescope suggests that Type I galaxies are much more efficient at emitting energy.”

For decades, astronomers have been studying Type II galaxies in a disproportionate amount because the high-intensity brightness of Type I galaxies made them more difficult to study. Since the unified model suggested the two types of galaxies were very similar, if not the same, many conclusions that were appended to Type I galaxies from Type II observations might require revising. In the future, more such work will help scientists get a better grasp of how black holes influence the evolution of their host galaxies.

As an interesting sidenote, this project first began as a doctoral thesis of one of the researchers involved with the team. Realizing the significance, other researchers jumped on board into had since become a very lucrative collaboration in science.

“This project began in 2009, as part of my doctoral work at UMD, and has radically grown with the help of more than 40 researchers across the globe,” said Michael Koss (M.S. ’07, Ph.D. ’11, astronomy), a research scientist at Eureka Scientific, Inc. and a co-author of the paper. “When I started out, I spent a month of lonely nights by myself at the Kitt Peak National Observatory observing a few dozen galaxies. I never dreamed we would eventually expand to such a large sample, enabling us to answer many amazing scientific questions for the first time.”

Scientific reference: The close environments of accreting massive black holes are shaped by radiative feedback, Nature(2017). DOI: 10.1038/nature23906

Picture showing the shadow of a black hole found at the center of galaxy M87. Credit: EHT Collaboration.

12 Physics-bending Facts about Black Holes

Bugs black hole

Credit: Giphy

Black holes are among the most mysterious objects in the universe. Thanks to the painstaking efforts of scientists who have dedicated their lives to studying black holes, we now know a lot about these infinitely dense abysses. Chances are you won’t come across anything weirder than a black hole, and here are some mind-blowing facts to prove it.

1. A black hole is practically invisible

black hole

Credit: Giphy

Nothing can escape the massive gravitational pull of a black hole, not even light. Hence, there are no photons that are reflected which can then be used to image the black hole as an object — it’s called ‘black hole’ for a reason, after all.

That’s not to say that we can’t detect them. Astrophysicists use proxies like the stream of matter and energy pulled from a ripped-apart star, for instance. The gas molecules start swirling around the black hole in the form of a disk and emit powerful X-rays. Black holes can also be detected by watching for motions of stars near the black hole. This brings us to our next slide…

2. This is what a black hole looks like

Picture showing the shadow of a black hole found at the center of galaxy M87. Credit: EHT Collaboration.

Picture showing the shadow of a black hole found at the center of galaxy M87. Credit: EHT Collaboration.

In April 2019, a huge collaboration of international scientists made a historic announcement: they have captured the first picture of a black hole. To be more precise, the researchers imaged the black hole’s event horizon (the point of no return). The image was constructed from data gathered by observatories all over the globe, which were combined to create a virtual telescope as big as the Earth.

“We have seen what we thought was unseeable,” said Sheperd Doeleman, an astronomer at Harvard University and the Harvard-Smithsonian Center for Astrophysics, who directed the project behind the black hole image. “We’ve exposed a part of our universe we’ve never seen before,” he added during a press conference at the National Press Club in Washington, D.C.

3. Black holes are anything but empty

Black hole

Credit: Giphy

Don’t let the name fool you. Rather, think of something immensely massive packed inside a very small volume. For instance, a star thousands of times more massive than the Sun squeezed into a sphere approximately the diameter of New York City. The result is a very strong gravitational field from which nothing can escape — only a more massive black hole. But even then the latter would ‘eat’ the former.

4. Black holes can reach gargantuan proportions (but could also be as small as a photon)

Black Hole gif

Credit: Giphy

Black holes are classed into three types based on their mass, as follows:

  • Stellar black holes. When a large star runs out of fuel it may collapse into a black hole. All black holes start out small then grow in size as they consume matter from around the galaxy.
  • Supermassive black holes. Ranging from hundreds of thousands to billions of solar masses, supermassive black holes can form in a number of ways. For one, a small black hole can be fortunate enough to come across a large gas cloud. Hundreds of thousands of tiny black holes may also merge to form one mammoth hole. Finally, a stellar cluster, which is a group of many stars, can all collapse together to form a supermassive black hole.
  • Intermediate black holes. It’s only recently that astrophysicists from NASA found that there are black holes with masses in between stellar and supermassive varieties. These mid-mass black holes were first discovered in 2014 and contain the mass of a few hundred to a few hundred thousand suns.
  • BONUS: Primordial black holes. These hypothetical black holes could have formed shortly after the Big Bang following energy particle interactions. As such they imply a completely new mode of formation, different from the collapse of stars that birth the black holes we all know. NASA says these black holes act as dark matter. 

Hypothetically speaking, the smallest black hole might the size of the Planck length — the smallest anything can be. The LHC at CERN is trying to produce quantum black holes at energies around 13 TeV (approximately 10-20 grams). 

5. There’s a supermassive black hole at the heart of the Milky Way — it’s four million times more massive than the sun

The the supermassive black hole Sagittarius A* (Sgr A*) can be seen in the middle of this image. Credit: NASA.

The supermassive black hole Sagittarius A* (Sgr A*) can be seen in the middle of this image. Credit: NASA.

Virtually every galaxy has a supermassive black hole — a black hole typically millions of times more massive than the sun — at their center. Our local black hole is located in a region known as Sagittarius A* and lies around 26 000 light-years away from the Solar System. Thankfully, that’s comfortably far away. It’s about four million times the mass of the Sun, which in turn is about one million times more massive than the Earth.

According to ESA’s Herschel space observatory, up to one light-year away from the Milky Way’s center molecular gas is heated to around 1,000ºC. That’s much hotter than typical interstellar clouds, which are usually only a few tens of degrees above the –273ºC of absolute zero.

6. The idea of a black hole-like object has been around for centuries

Karl Schwarzschild. Credit: AIP.

Karl Schwarzschild. Credit: AIP.

It wasn’t until 1967 that Princeton physicist John Wheeler coined the term ‘black hole’, but scientists have been describing massive objects crammed inside small volumes and theorizing what would happen for long before that. A common confusion is that Albert Einstein was the first to discover black holes, but that’s not true at all. While Einstein was responsible for developing the theory of general relativity in 1915 that predicts black holes, it was Karl Schwarzschild who used these equations to prove black holes do, indeed, exist.

After Schwarzschild solved the equations, he came up with the Schwarzschild Radius — a minimum threshold for a mass to collapse into a black hole. Any mass can theoretically become a black hole if this condition is met. Per this theory, if you were to squeeze the Earth in a small sphere with a radius of 8.9 mm, it should become a black hole. For the sun, the Schwarzschild Radius is three kilometers. 

But even long before Einstein or Schwarzschild, in the 1790s, John Michell of England and Pierre-Simon Laplace of France independently suggested the existence of an “invisible star.” They calculated the mass and size – which is now called the “event horizon” – that an object needs in order to have an escape velocity greater than the speed of light. Their results were off, however, because they used Newton’s Laws instead of Einstein’s theory of general relativity.

7. Objects falling through a black hole become ‘spaghettified’, before disappearing into oblivion

Black Hole.

Credit: Youtube.

At the very center of a black hole is the gravitational singularity. It’s a one-dimensional point where a huge mass is squeezed into an infinitely small space. Density and gravity become infinite and the space-time curves infinitely. That’s a lot of infinity, enough to make your head go dizzy. In fact, not even the laws of physics can stand that much infinity, apparently.  As American physicist Kip Thorne puts it, [the singularity] is “the point where all laws of physics break down”.

Equations crunched so far suggest that an object falling into a black hole becomes ‘spaghettified’ or stretched out as it nears the singularity. If it was you who was falling inside the black hole, you’d be able to see distorted images as light bends around your point of view. Eventually, the ripped apart object loses dimensionality completely and disappears into the singularity. Poof!

8. Black holes don’t suck

Artist's impression of a star torn apart by the gravity of a black hole. Credit: Chandra X-Ray Observatory.

Artist’s impression of a star torn apart by the gravity of a black hole. Credit: Chandra X-Ray Observatory.

In fact, they’re oddly awesome! Smug replies aside, there’s this common misconception that black holes act as a huge vacuum cleaner of sorts. While it’s true black holes can pull matter in at incredible rates, it’s all due to gravity — not some mysterious fan. If you were to replace the sun with a black hole of equal mass, theoretically speaking, everything in the solar system should orbit as usual.

9. Black holes can stop time

black hole

Credit: Giphy

Were you to wear a spacesuit that would render you immune to the gravitational pull of a black hole, you would see objects passing through the black hole’s event horizon appear to slow down then freeze in time. In other words, it’s as if they had never passed through the event horizon (the boundary after which nothing can escape the clutches of a black hole). That’s because the space-time is distorted and the light takes longer to reach your telescope’s glass. In fact, it takes an infinitely long time.  As time elapses, the light subsequently becomes red-shifted and dimmer as its wavelength becomes longer, eventually disappearing from the sight of the observer as it becomes infrared radiation, then radio waves.

10. Black holes might be able to grow infinitely large

Credit: Wikimedia Commons.

Credit: Wikimedia Commons.

Black Holes can keep growing because anything (gas, liquid or solid matter) that enters the event horizon gets sucked in. Theoretically, a black hole can grow indefinitely. However, there’s evidence that suggests that ultra-massive black holes found in the cores of some galaxies never seem to exceed 10 billion solar masses. Recent studies suggest that black hole may not physically grow beyond this mass because they would then begin to disrupt the accretion discs that feed them.

11. Black holes might be wormholes

black hole facts

Credit: Giphy

A wormhole or a Lorentzian wormhole is a sort of theoretical ‘tunnel’ through space-time, often used as the preferred mode of interstellar travel in movies like Star Trek. The opening is a shortcut through intervening space to another location in the Universe. That seems to be in stark contrast to a black hole which is less of a funnel and more of a meat grinder. However, some physicists claim that there are many characteristics which both black holes and wormholes share. Perhaps, a wormhole is a black hole and vice-versa.

Wait, it gets weirder…

12. Black holes might spawn new universes

big bang

Credit: Giphy

Our very own universe that we hold so dear might have actually been birthed by a black hole and the billions of black holes in the universe might, at their own turn, each spawn their own versions of the universe. As mentioned earlier, in the singularity the laws of physics break down. Some physicists speculate, however, that they actually are ‘reborn again, in a baby universe.’

“A star that collapses into a black hole very quickly squeezes down to infinite density and time stops — that’s according to general relativity. And basically that moment when time stops is deferred by quantum mechanics, by quantum uncertainty, and rather than collapsing to infinite density, the star collapses to a certain extreme density, and then bounces back and begins to expand again. And that expanding star becomes the birth of a new universe. The point where time ends inside a black hole becomes joined to the point where time begins in a Big Bang in a new universe,” says pioneering physicist Lee Smolin.

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