Black holes are the most iconic things in our universe. Their mind-bending properties were
debated for generations, and were pondered by some of the greatest minds of our time. Today,
we know that black holes are real, and as a result, we can begin asking the next big
questions. What lies beyond them? How do they die? Most of all, what is the biggest
black hole in the universe? Well, it's likely that TON-618 is up there with the biggest,
if not the biggest in the universe. And while we don't know much about it, its profile is truly
magnificent. This radio-loud and hyper-luminous quasar was once thought to be just another star,
but we now know that this monster's photonic relay is emanating and calling us from much further
across space, over 10 billion light years into the void. But is this beacon of the cosmos really what
we think it is? With so many properties that push the limits of our understanding of black holes,
TON-618 may have another explanation. But until we find that, we must assume that this
record-breaking beast is the largest and most massive black hole in the entire known universe.
TON-618 is the catalogue title of a quasar located at the centre of a galaxy approximately
10.37 billion lightyears from the Earth, in the constellation of Canes Venatici near the galactic
north pole. Despite this unfathomable distance, it shines with an absolutely visual magnitude
of -30.7. This number equates to the brightness of over 140 trillion Suns, making it one of the
brightest objects in the known universe. We will cover more about why this is in a moment, but in
a quasar, the mass of the black hole usually indirectly corresponds to the amount of light
it produces, and as such, numerous calculations have estimated the mass of this engine black
hole to be over 66 billion times the mass of our home star. This mass is both hard to visualise
and hard to fully appreciate, but for some sort of perspective, the Milky Way's central black hole is
approximately 4.3 million times more massive than the Sun. Sagittarius A* is largely responsible for
anchoring our entire galaxy together, which could be up to 200 thousand lightyears in diameter, yet
its mass doesn't equal a thousandth of the total mass of TON-618. Thanks to our study of black
holes, we can use what we already know about its mass to estimate its actual size, and the results
will truly blow your mind. If our calculations are correct, then the radius of its event horizon,
the boundary beyond which nothing can escape, including light, which gives it the distinctive
black colour, will be approximately 194 billion kilometres. This radius dwarfs even our entire
Solar System, and then some. The Solar System's diameter is a little under 300 billion kilometres,
whereas TON-618's event horizon diameter is close to 400 billion kilometres. But then,
how is this black hole so massive if the emissions we see are from billions of years ago,
when the universe was significantly younger? The universe is about 13.8 billion years old,
and this likely formed between 1 and 2 billion years after the Big Bang, meaning it could have
been as young as a billion years or less when the light reaching us now began its intergalactic
journey. This is very young for a black hole, and yet the signals are indicating that a black
hole this young is larger than anything we've been able to find in the universe billions of years on.
After all this time, there's no telling how big it has become, but there are a few ideas on how
it might have come to be so large at such a young age. The main theory is that this black hole was
a direct collapse black hole. Instead of being formed by a dying star's supernova, this black
hole was formed by a supermassive gas cloud collapsing under its own gravity. This process
can also take all the stars within the gas cloud with it, rapidly consuming thousands more distinct
sources of mass. This process would generate an inconceivably large supermassive black hole,
and is the expected way in which most engine black holes at the centre of galaxies formed.
In the future, we may re-estimate the black hole's measurements and come up with new figures,
but for now, these are the facts. But what about its history? How was this black hole discovered,
and when did we realise that it was the most massive we had ever found? TON-618 was first
noted in a survey of faint blue and white dwarf stars around the Milky Way's galactic plane,
in 1957. Back then, it was mistaken for another blue star, and was listed as the 618th member
of the Tonatzintla Catalogue of globular star cluster, thus coining its name. However, moving in
the 1960s, astronomers started noticing radio-loud sources of light which appeared similar to stars,
but the source of this radio energy was unknown, and thus the term quasi-stellar, meaning starlike,
was contracted to "quasar," and astronomers continued to investigate them. It was Martin
Schmidt who then noted that the Doppler shift lines in the quasar spectrums were not consistent
with the expected redshift of stars, and so these sources of light must be emanating from
much further away. Gradually, higher-resolution images and X-ray photographs indicated that these
radio-loud sources were emitting were from the centres of distant galaxies,
and the light reaching us from these quasars is billions of years old. But how does a black hole,
scientifically the darkest thing in the universe, even shine in the first place,
let alone shine brighter than stars a millionth of a percent of the distance away? Well,
the process begins with the immense gravity of central galactic black holes. When gas and
matter orbit the black hole, they orbit the area at immense speeds, as do the light and
energy released when they consume things. When a black hole destroys something, it does through the
trivially named process of "spaghettification." As you get closer to the singularity, or centre,
of the black hole, the gravity becomes so exponential in its increase that even
a difference of a few million kilometres can lead to vastly different levels of pull on the object,
essentially shredding up whatever it is. The gases from this shredded matter form a disk around the
event horizon of the black hole and can orbit it, forming an accretion disk. When this disk orbits,
it swells at unimaginably fast speeds, and the friction generated releases unbelievable
amounts of heat. The accretion disk can also cause black holes to emit plumes of plasma,
known as relativistic jets. These jets travel away from the black hole at nearly the speed of light,
and this and the accretion disk shine so brightly together that the light we can
observe drowns out the surrounding galaxy. When it does this, we call these illuminated black holes
"active galactic nuclei." A sufficiently luminous active galactic nucleus is classified as a quasar.
With our growing understanding of quasars, the hunt was on for more, and before long,
a radio survey in Bologna in 1970 detected strong radio emissions from this so-called star,
and thus it was reclassified as a quasar. Marie-Helene Ulrich then collected the spectral
data from the object. She detected the spectral emission lines, the lines in the spectrum that
occur from the emission of light from an object. From an analysis of the redshift of the spectra,
Ulrich determined that TON-618 must be very far away, and hence it ranked among some of the most
luminous quasars ever detected. Furthermore, the emission lines detected on the spectrum were very
wide, including in the broad line region of the quasar, a spectrum of light from an orbital area
beyond the accretion disk. These are indicative that to the light-emanating gases around the
black hole must be traveling incredibly quickly, calculated be a terrifying 7 thousand kilometres
every single second. This told scientists that the black hole creating the quasar must be exerting an
intensely strong gravitational influence. The size of the broad line region can be reverse engineered
from the brightness of the quasar, and given the size of the region and its orbital speed,
this black hole had to be one of the largest ever discovered. The mass was estimated at
its official figure of 66 billion times the mass of our Sun, and using black hole equations, the
event horizon radius was determined from this to be over 1300 astronomical units, thus giving it a
new classification - an ultra-massive black hole. But in spite of all these mind-blowing statistics,
it's important to remember that we cannot see TON-618's surrounding galaxy, and therefore
we know very little about it. While the math suggests these massive figures for the black hole,
some have argued that we may be mistaken. There are questions over its true nature, and if its
sheer size so early on in its life wasn't enough to cast doubt, the nature of quasars does too.
Quasars are a phenomenon more typical of the early universe, hence their massive distances from the
Earth. They are more commonly found between nine and ten billion lightyears away, because the
universe was teeming with them at that age. While we do see some nearer to us, the number is nowhere
near as high. This is because in the billions of years since these quasars occurred, the engine
black holes of the universe have had time to consume much more matter, increasing their event
horizons. Once a supermassive black hole exceeds a certain size, they are large enough to completely
swallow matter up rather than shredding it. Less shredded material orbits within the accretion
disk, and the light generated no longer outshines its host galaxy. So, there's a contradiction,
is there not? If smaller black holes have more favourable conditions for quasars to occur,
why is one occurring around the largest black hole in the universe? Unfortunately, we currently don't
even have a way of knowing. Perhaps the galaxy surrounding TON-618 was immature at the time,
and there was enough matter orbiting it to be shredded up despite the size. Or perhaps,
its gravitation influence is just too great so that the swirling gases are orbiting too fast not
to shine. But this isn't the only argument against TON-618 as a quasar, either. Some point to the
theoretical size limit of black holes, predicted by some to be 50 billion solar masses. Any larger,
and they suggest black holes would start to lose their accretion disks, and would not be
able to maintain their size and constant intake of matter. Anything larger wouldn't shine at all,
and of course, the fact that this black hole was so massive so early on cast doubt. Even
if it is explainable by direct collapse black holes, why don't we observe other black holes
of this size from that area of the universe? Why does TON-618 stand out as such an anomaly? Well,
although it does display all the expected properties of a quasar, the true explanation
behind it could not yet be known. Most quasars happen to look like stars in the Milky Way,
so who's to say that quasars don't also look like some sort of other undiscovered phenomenon
in the early universe? Who's even to say that we understand quasars and black holes? Perhaps it's
not our estimations that are inconsistent, but our knowledge of the phenomena themselves. After all,
we propose the theoretical size limit for structures in the universe based on a uniform
distribution of matter at the Big Bang to be about 2 billion lightyears, and yet we have observed
quasar groups, superclusters, and gamma-ray burst rings which vastly exceed this size. Perhaps the
problem lies in the limit of our imagination, and the true size and mass benchmarks of these
phenomena are much, much higher than we expected. It's impossible to cast a clear judgement as we're
missing some quite major pieces of observational evidence. If something similar was discovered,
we may be able to compare two abnormal quasars and gain a better understanding. But quasars are the
brightest things in the universe - it's unlikely that there's another one of this magnitude just
hiding out there that we have yet to discover. So for the time being, we just don't know.
TON-618 may be the largest black hole in the known universe, but what are the other
giants it is vying for the privilege with? Well, the previous record-holder was the obscurely
named S5 0014+81. This was another quasar which was reverse-engineered, and the measurements
suggest a black hole of around 40 billion solar masses. However, some believe this figure may be
exaggerated. When a relativistic jet fires in the direction of the Earth, its quasar appears even
brighter, so much so that it has own type of name, known as a blazar. This black hole's jet
happens to be pointing in the direction of Earth, and so some suggest its unusual brightness is not
actually indicative of more mass. But whatever the case, it's still an enormous black hole.
Another enormous black hole lies about 1 billion lightyears from us, and it is the monster at the
centre of the largest known galaxy. IC 1101 is a supergiant elliptical galaxy over 6 million
lightyears in diameter, and it is estimated to contain over 100 trillion stars. We cannot see
inside the galaxy due to its luminosity, and it is too far away to be directly measured,
but IC 1101's black hole has often been considered the biggest by default, because galaxy size is
typically representative of black hole size. But due to the variables, the estimation comes in
quite broad, between 40 billion and 100 billion solar masses. This is likely another one we'll
never be able to properly assess and measure, but once again we know what lies inside is probably
larger than our entire Solar System. And then finally, there's HD1821+643. This black hole is
also a quasar that lies much closer to us, and was considered the most massive in 2014. Unlike the
others, its mass has been precisely measured to be around 30 billion solar masses. This gives it an
event horizon radius of 1150 astronomical units, approximately 14 times the distance of Pluto from
the Sun. Black holes are scary enough on a stellar scale, but they can grow to sizes so great it
becomes hard to imagine how something cannot lie beyond the boundary of its singularity. We can
speculate on the size and mass of TON-618 all day long, but don't forget that this was its size 10
and a half billion years ago. Largest black hole or not, its size now must be truly incredible. Or
is it? Perhaps it has reached the size limit for black holes, and what happens to a black hole when
it cannot consume and begins to break down is anybody's best guess. With all these questions,
it's depressing to think that they will probably always remain just that: questions. But who knows
what discoveries are waiting for us in the future? The observable universe is growing,
and eventually its radius is expected to expand beyond 60 billion lightyears. With that extra
distance, who knows what else could be waiting for us in the shadows of the early universe. And
if you thought that was big, the area beyond the observable universe could be millions of times
bigger. While TON-618 may be the largest black hole in the known universe, just take a minute
to consider what else might be lurking out there, trillions of lightyears into the void.