Original subtitles

Two black holes circle each other

in a dance of death.

They spiral inwards,

their immense gravities pulling them ever closer.

When they finally collide,

it's one of the most powerful events

since the big bang.

This explosive mystery

sends ripples across the world of science.

But can it also answer

one of the most pressing questions in cosmology?

How do supermassive black holes grow so large?

...Captions by Vitac... www.Vitac.Com

Captions paid for by Discovery communications

In the known universe,

there are roughly 2,000 billion galaxies.

Each one has a different shape and size.

But they may all have one feature in common...

a supermassive black hole buried at their center.

As its name says, it is supermassive.

And here, we're talking about objects

that are millions or billions of times

the mass of the Sun.

Supermassive black holes

are so big that we need a special scale

for measuring them.

A solar mass is the mass of the Sun.

So when we study the universe,

we have to use the tools that we have in hand.

And what's the most massive thing that we have around us?

It's the Sun. And so we refer to things

in multiples of the mass of the Sun

because it just makes it easier to wrap our heads around.

However, if you have something that's 17 billion times

the mass of the Sun, that's pretty difficult

to wrap your head around anyway.

But we know that those kinds of black holes

live in the centers of galaxies.

The supermassive black hole

the milky way, is called Sagittarius "a" -star.

It weighs in at 4 million solar masses.

But compared to the other supermassive black holes

out there, it's puny.

This is probably one of the only contexts

where you would think that our supermassive black hole

isn't very supermassive.

The supermassive black hole

in our neighboring galaxy, Andromeda,

is 25 times larger than Sagittarius "A" -star,

coming in at 100 million solar masses.

But compared to the largest monsters

out in the universe, it's a runt.

O.J. 287's primary supermassive black hole

weighs in at 18 billion solar masses.

And the black hole in the core of galaxy NGC 4889

in the coma cluster weighs 21 billion solar stars.

That's over 5,000 times larger

than Sagittarius "a" -star.

These are incredible things

that are more massive than some galaxies.

Now astronomers may have ma

a giant, new supermassive black hole

that's a mind-blowing 30 billion times

the mass of the Sun.

It's a huge puzzle.

And we have simply no idea how it got so big.

It's a huge mystery how black

we started finding black holes with millions

and billions of times the Sun's mass.

No one expected that.

And we have no idea how they got to be so big.

It's not entirely clear at this point

how supermassive black holes can get to be the masses

that they are today.

Regular-sized black holes form

when large stars over 20 times the mass of our sun

crash and burn.

When a large star runs out of fuel,

the core stops generating enough outward force

to counteract the power of gravity crushing inwards.

As the star collapses,

the outer part explodes in a supernova.

The inner core shrinks

from a sphere millions of miles wide

to one just 10 miles across.

It's like shrinking the earth down

to the size of a golf ball.

This rapid collapse creates a black hole.

So we now have seen black holes

that are solar-mass black holes

and black holes that are million

or billion-solar-mass black holes.

And the question is, how do you get from one to the other?

Do the giants somehow grow

from a solar-mass black hole?

One of the big puzzles today is,

how do you make one of these supermassive black holes?

One idea is, you get there by starting

with a solar-mass black hole,

having it grow through a stage

of being an intermediate-mass black hole

and then eventually getting to be

a supermassive black hole.

Theoretically, intermediate-mass black holes

should be between 100 and 100,000 solar masses.

But we've never seen one.

Part of the mystery of supermassive black holes

is that black holes seem to occur in two flavors.

You have ones that are only a couple times

the mass of the Sun.

And you have ones that are millions

or billions of times the mass of the Sun.

So we have small and extra large.

If we think of the stellar-mass black hole

as sort of the baby black holes,

and the supermassive black holes as the grown-up black holes,

we're missing the teenage black holes.

Where are these black holes that have masses

that are between stellar mass and supermassive?

They're sort of like a holy grail

for black hole hunters.

Where are these things? Where can we find them?

And how do you make them?

Then astronomers caught a break.

They picked up a burst of energy

coming from the NGC 1399 galaxy.

It was the death throes

of a star being eaten by a black hole.

When they measured its size, they discovered it was

an elusive intermediate-mass black hole.

The missing link had been found.

But when scientists did the math

to see if such an intermediate-mass black hole

could grow into a supermassive black hole,

they hit a snag.

There hasn't been enough time

since the birth of the universe

for an intermediate-mass black hole

to eat enough stars

to grow into a supermassive black hole.

It doesn't seem like there's enough time

for black holes to get as big as we see them.

But supermassives are everywhere we look.

How did they get there?

And how did they grow so huge?

In our universe, we've detected small black holes.

And we've seen monsters,

supermassive black holes

billions of times the mass of our sun.

But we'd found almost none in between.

So how do you get from a small black hole to a giant one?

One of the most important outstanding questions

in cosmology is,

how did supermassive black holes

get as big as they are?

And when did that happen?

Black holes are normally S

an all-you-can-eat buffet.

One of the best ideas for how black holes grow

is that black holes do

what we expect black holes to do,

and that is eat stuff.

For a black hole,

it's almost as if the universe is its restaurant.

And on its menu, you'll find stars, planets,

and clouds of gas and dust.

So is binge-eating the answer

to growing a supermassive black hole?

Theoretically, black holes should keep on growing forever

as they consume more and more food.

But recent discoveries suggest

that the universe puts them on a diet,

controlling how much they eat.

Black holes are hungry. They like to eat.

But sometimes, they eat too much,

and they burp it up.

February 2015.

Astronomers report something unusual

in the galaxy NGC 2276.

It looked like something had taken a bite

out of one of its spiral arms.

Sitting alone in the void

was an intermediate-mass black hole,

about 50,000 times the mass of the Sun.

One theory was that the black hole

had eaten everything around it,

creating the dead zone.

But the detection of a burst of energy

from the black hole suggests

it may have tried to eat too much

and, in the process, destroyed its food source,

burping so hard, its food was blasted away.

Turns out that black holes are actually very messy

a lot of matter gets thrown off as it tries to absorb it.

So things move in, gets hot.

But then a lot of it gets thrown all the way back out.

Black holes are not vacuums in space.

They do not just eat everything around them.

And so they are messy. Some things get in.

And they take that on. And it grows their mass.

And some things are just flung out as they're eating.

The enormous gravity of black holes sucks gas, dust,

and even stars towards them.

Everybody's been to an all-you-can-eat buffet.

But let's be honest. There really is a limit

to how much you can eat.

Black holes are gluttons. They're greedy.

They don't really know when they've eaten too much.

They just keep on cramming in more and more food.

It doesn't just fall in.

It has to go down the drain, more or less.

And so it forms this disk around the hole.

And as it does that, there's a lot of turbulence

and magnetic fields

and a witch's brew of forces going on there

that get it really hot.

As the gas and dust swirls around,

it heats up,

pushing temperatures to millions of degrees Fahrenheit.

This swirl, called the accretion disk,

also generates powerful magnetic fields.

These fields are dragged by the SPiN of the black hole

and become focused above the poles.

As energy builds up,

the magnetic fields become so compressed

they blast out super-energized particles.

These beams can actually be incredibly violent.

Matter is flung out

at a large fraction of the speed of light.

It's a tremendous wind

that blows very hard away from the black hole.

The jet hits the gas clouds

surrounding the black hole,

blowing the buffet away.

If they eat too much,

they can basically blow everything

that's in their vicinity away.

They lose their food supply. And then they're gonna starve.

They can kind of shoot themselves in the foot.

With no food available,

the black hole stops growing.

Astronomers think that's what happened

to the intermediate-mass black hole

they discovered in the dead zone.

These burps may regulate star formation

and stop the black hole from getting obese.

But over time,

the black hole will start eating again

as gas falls back towards it.

But can an intermediate-mass black hole eat enough

to become a supermassive black hole

weighing billions of solar masses?

Could that black hole become so obese by eating?

That's a really interesting question.

You'd have to eat a heck of a lot

to get that fat.

When you think about it, if you imagine an average galaxy

has 100 billion stars,

the black hole would have to eat one

in every five stars in the galaxy.

The universe is old. But is it really old enough

that black holes have had time to consume billions of stars?

That seems kind of unlikely.

It doesn't seem to add up.

We need some other way

to make these supermassive black holes.

And the question is, what is that?

Maybe we've been making this all too complicated.

Maybe to get a big black hole is to start big in the first place.

So how can black holes start big?

To answer that question, scientists had to journey back

to the very start of the universe,

to a mysterious time called the dark ages.

As we look out into the universe,

we're seeing farther and farther back in time.

We have now looked back

over 12 billion years

to the time when the cosmos was still an infant.

And what we found was a huge surprise.

We had made the assumption

that as you look farther out into the universe,

the black holes would be smaller.

They haven't had much time to grow.

But now we've found a 12-billion-solar-mass

black hole that's actually less

than a billion years into the universe.

How did this thing form so early?

How did it grow so fast?

This is like walking into a delivery room

and finding a 100-pound baby.

I mean, how does that even happen?

It doesn't make any sense.

Physics tells us no black hole could swallow enough stuff

to get that big that quickly.

There really wasn't enough time between the big bang

and when we're studying these things

for them to grow to such large sizes

just by eating matter around them.

So if there's not enough time

maybe they're born supermassive.

To understand how, we have to travel back even farther,

to not long after the birth of the universe.

The early universe was definitely

a much more compact

and richer place for material.

It was smaller, and it was denser.

Things were much closer. It was hotter.

It was just a much more intense place to be.

Clouds of hydrogen and helium gas clumped together.

As the clouds grew, so did their gravity,

sucking in more and more gas.

Eventually, the ball of gas

became so dense, it collapsed,

triggering nuclear fusion.

A star was born.

These massive first stars

are called population III stars.

Because there was so much food around,

these stars were huge,

many times bigger than any stars that exist today.

We think a lot of these population III stars

probably were incredibly massive,

incredibly short-lived, and just blew up right away.

They would've left massive black holes behind.

With so much food available,

these young, ravenous black holes, called quasars,

started binge-eating

and became incredibly bright.

Billions of years later, we can still see their gluttony.

The most luminous, bright objects in the universe

are things called quasars.

And it may seem kind of ironic.

But what these really are are supermassive black holes.

There's so much stuff trying to cram itself down

the black hole that everything gets very hot, very energetic.

And you can see them clear across the universe.

But when we measured the size of the young quasars,

we discovered they were already billions of solar masses.

There's not enough time,

a billion years after the universe was created,

for them to get to a billion solar masses in...

it's just too short a time.

So the question becomes,

that are this big in that small amount of time?

We need some other way

of growing these supermassive black holes.

There needs to be some other mechanism

that allows them to get that massive so early.

But what is that?

A clue can be found in the very early universe.

The early universe is still so much of a mystery to us.

We know that conditions were very different.

It was denser. There was a lot more material.

This period is called the dark ages.

During the dark age, we know

that there was basically nothing happening.

Matter existed.

We think that there was hydrogen and helium gas

but really not much else.

There were a few stars around,

but nothing large enough to form giant black holes.

But there were huge clouds of gas.

And because the universe was much smaller and denser,

the clouds were much thicker.

The idea is that from these basic ingredients,

gravity and gas,

the cosmos built massive black holes.

Somehow, the universe has created a shortcut

to the black hole.

We've typically thought of it as,

cloud of gas collapses into a star,

star evolves, star dies, leaves behind a black hole.

Perhaps the universe has found a way

to skip the star phase

and go directly to the black hole.

Clouds of gas may have built massive black holes

in a process called direct collapse.

As they collapsed, they never even formed a star.

They just collapsed straight into a giant black hole.

Through this direct collapse theory,

you can form really big black holes.

Imagine what it's like seeing one of these giant clouds

of gas collapsing down into a black hole.

You might think you start with, okay,

cloud of gas slowly collapsing,

and, boop, it's a black hole.

That wouldn't be the case. It would be more like

giant cloud of gas starts collapsing,

then... aah! Black hole.

It's believed that direct collapse

could have created black holes

up to a million times the mass of the Sun,

much bigger than from the collapse

of a single star.

These early black holes

are sort of like the galaxies that never were.

They were gonna make galaxies.

But instead, they collapsed into very massive black holes.

For direct collapse to form a black hole,

the conditions need to be precise.

The clouds must be very symmetrical,

forming a smooth ball.

If you have a ball of gas that isn't quite a ball,

that's not quite homogeneous,

as it collapses, it'll fragment.

And it'll fragment into objects that won't form black holes.

So you want it to be hot enough

that it stays one big, giant thing.

But it does need to cool a little bit, right,

so that you get it to collapse in on itself.

You have to get uniform collapse over time

of a very large amount of hydrogen gas, presumably,

which is the original matter in the universe,

collapsing spherically symmetrically,

without fragmenting,

over a period of less than 500 million years.

Direct collapse may have created black holes

a million times the mass of the Sun.

But it can't completely explain

the 12 billion solar-mass supermassive black holes

we see in the early universe.

Maybe gigantic supermassive black holes were created

by strange, unseen forces.

Maybe they were created

by the mysterious dark universe.

Astronomers looking deep into the early universe

have discovered gigantic supermassive black holes.

This is a pretty deep mystery.

There are these supermassive black holes

that exist in the very early universe.

And by all accounts, they should not exist.

According to the normal laws of physics,

it shouldn't have been possible

for them to grow so big so quickly.

For astrophysicists, understanding how black holes

have grown to be so large is one of our biggest mysteries.

We need some other way

of growing these supermassive black holes.

There needs to be some other mechanism

that allows them to get that massive so early.

But what is that?

Everything we can see in the night sky

makes up just 4.8%

of all the matter in the cosmos.

The rest is the dark universe,

including dark matter.

We can't see it, feel it, or detect it directly.

But we know dark matter is there.

Its gravity is tugging on everything around it.

And we're beginning to understand

it plays a fundamental role

in the formation of the universe.

Most of the stuff that collects together

gravitationally is dark matter.

So perhaps black holes form

somehow with the inclusion of dark matter.

One way of looking at it is there's six times

as much dark matter as normal matter.

So there's six times as much food out there

for the black holes to eat

if they're able to tap into this dark stuff.

Maybe these supermassive black holes are growing

by eating dark matter.

There are some tantalizing clues.

The largest supermassive black holes

don't live in the galaxies with the most regular matter.

They live in the galaxies with the most dark matter.

The one thing we know about dark matter right now

is that it has gravity.

And a black hole runs on gravity.

It attracts anything with mass.

So there's no reason to assume

that black holes would only eat regular matter.

And now we know that there's far more dark matter out there.

Maybe dark matter helps the black holes eat.

Maybe in some ways, dark matter is a feeder

for these supermassive black holes.

Perhaps what really grows a supermassive black hole

is all of the regular matter being directed into the center

by the dark matter around it.

Maybe the dark matter's powerful gravity

sucks in regular matter

and funnels it into the black hole.

In a sense, the dark matter is greasing the wheels.

It's sort of tilting the table up

so that that food can slide right in.

But now scientists think the dark matter

may create gigantic black holes directly

by igniting dark stars.

Some believe that dark matter

sparked early universe super stars.

When they die, they leave behind supermassive black holes.

Dark stars sound like

they come from the fertile imagination

of some Sci-Fi writer.

But Dr. Katie Freese believes they may explain

how early supermassive black holes grew so fast.

Dark stars are amazing.

So, when we first had this idea,

we got excited really quickly,

because this is a new type of star

that has never been seen before.

Dark stars may have been some of the first stars

to form in the universe.

They sparked into life

when the universe was just 200 million years old.

But how could dark stars form really massive black holes?

A newborn black hole can't weigh more than its parent star.

So in order to give birth to a really massive black hole,

the parent star has to be supermassive, as well.

These early objects are really strange.

They're very cool.

And they're really, really big.

The size of these things is 10 times the distance

between the Sun and the earth.

But how is that possible?

Regular stars have an upper size limit.

A star is a battle between gravity pushing inwards

and nuclear fusion pushing out.

When the star grows too big,

its gravity becomes overwhelming.

The delicate balance

between gravity and fusion is broken.

Gravity wins out, and the star collapses.

But dark stars may have a work-around

that lets them become supermassive.

So, they are made of ordinary matter.

They're made of hydrogen and helium.

But they're powered by dark matter.

We don't know what dark matter is made from.

But we do have theories on how it might power a star.

One of the best ideas we have for dark matter

is that it's made of weakly interacting massive particles,

or wimps for short.

So, these wimps are their own antimatter.

And that means, whenever they encounter each other,

they annihilate and turn into something else.

That means a lot of heat is released, a lot of energy.

And it's that energy that could power stars.

The energy from the wimps' annihilations

keeps the star from collapsing like a normal star.

So it's possible that, in some stars,

their internal reactions

are actually being powered by dark matter.

If that's the case, then you could imagine situations

where, when that burns out,

you produce very massive black holes.

So it could be that dark matter,

the physics of dark matter,

plays really important roles in creating black holes

and their prevalence in the universe.

The energy from the dark matter

allows the dark stars to grow huge.

When they first form, they're small.

They're about the mass of the Sun.

But because they're so cool,

they keep accumulating matter

and growing, growing, growing.

And some of them will get to be a million times

as massive as the Sun and a billion times as bright.

But these giants don't live for long.

Eventually, the dark matter particles

wipe each other out completely.

And there is no more fuel

to keep the massive amount of ordinary matter

from collapsing.

And then that's it.

There's nothing to sustain this big, puffy object.

If it's big enough, you collapse directly to a black hole.

A monster supermassive black hole.

It's really fun to think about the possibility

that the physics of dark matter

is actually helping to power stars.

If so, it would bring, you know,

a whole new window into our understanding

of stars and their evolution.

At the moment, dark stars are just theoretical.

But when the powerful James Webb telescope

comes online in 2018,

we may get our first glimpse.

We're gonna do an observing run and look f

and so we're very excited.

If you would find an entirely new type of star,

that would be huge.

While Katie Freese looks for dark stars,

another team is investigating another radical idea

that offers new insight

into how supermassive black holes grow so huge.

They detect the faint echoes

of a violent event from across the universe,

the remnants of an extraordinary collision,

a supremely energetic event

that reveals black holes are cannibals.

Our universe is filled with enormous

supermassive black holes that defy explanation.

Supermassive black holes are one of the things

in the universe that, when you run the physics,

when you run the math of how did they evolve,

they really shouldn't be there.

It's still a profound mystery.

The universe hasn't been around long enough

for regular black holes

to eat enough matter to get supermassive.

So how did they get so big?

The most logical answer

is that large black holes are born large,

around 1 to 2 billion solar masses.

But that's still over 10 times smaller

than the largest supermassive black holes out there.

Given the time scales, it doesn't seem to add up.

We need some other way

to make these supermassive black holes.

And the question is, what is that?

A clue came from a large, isolated galaxy

200 million light-years away

in a quiet part of the universe.

Nestling alone was a supermassive black hole

with a mass of 17 billion suns.

Normally, such monsters

are found in dense regions of space

with lots of galaxies and lots of stars.

This black holes doesn't match its surroundings at all.

It's kind of like driving to the middle of a desert

and coming across the empire state building.

Now, the empire state building belongs in the middle of a city.

And a black hole this big

belongs in a rich cluster of galaxies.

This is the first time astronomers have found

such a giant object

lurking in such a relatively empty area of the universe.

So you got to ask the question,

if there's nothing else around, how exactly do you grow

a 17-billion-solar-mass black hole?

One possible answer is the stuff of nightmares.

Maybe the story of this black hole

is actually a little more scary than we thought.

Maybe it's all alone

because it ate all of its neighbors.

Maybe it was eating more than galaxies.

Maybe it was eating its own kind.

The thing about black holes is they're omnivores.

They'll eat anything.

Anything that gets close them, they'll gobble up.

One way black holes can grow so large

is by eating other black holes.

So in a sense, they may be cannibals.

Cannibal black holes were just theoretical.

We'd never actually seen them eat each other.

Then scientists detected the faint echoes

of actual ripples in space-time.

When engineers turned on

the laser interferometer gravitational-wave observatory,

or LIGO for short,

they immediately picked up

the faint signal of gravitational waves.

Gravitational waves are created

by huge explosions in space.

To make them, you need an almost unimaginably energetic event,

something really, really big...

...something like merging black holes.

A black hole merger is the most violent,

the most energetic thing

that happens in the universe, period.

Picture the scene, 1.3 billion years ago.

Two black holes circle each other in a dance of death.

The larger black hole pulls the smaller one inwards

until they're locked together in a spiral.

Very, very slowly, that orbit is decaying.

They're getting closer and closer and closer.

And then they will merge into one giant black hole,

truly one of the most dramatic events in the universe.

Finally, they collide

in one of the largest bangs since the big bang.

I would have loved to have been able

to safely view the collision

of these two black holes up close.

Imagine these two black holes

as they spiral in toward each other,

going faster and faster and faster and faster.

And then, suddenly, where there appears to be nothing

or just distortions in space in front of you,

suddenly, there is this enormous burst of energy.

And everything just rains around you.

By measuring the frequency

we can calculate the size of the objects causing them.

When those two black holes,

weighing 29 solar masses

and 36 solar masses, collided,

they created a black hole around twice the size.

In some ways, it's very elegant and simple.

You take two black holes. You spiral them in together.

And you end up with one big black hole.

The event showed that black holes can double their mass

through cannibalism... Almost.

The final black hole was less than the sum of its parts.

There were 3 solar masses missing.

That may not sound like a lot.

So let's put it in context.

Our sun is burning

about 100 billion hydrogen bombs every second.

And over its 10-billion-year lifetime,

it will convert less than maybe 1% of the mass

of the Sun to energy.

In 2/10 of a second,

3 times the mass of the Sun in matter

got converted to energy in that collision.

It was 36 septillion yottawatts.

What does that mean? A lot of freaking energy.

That's more energy in that 2/10 of a second

than is emitted by all the stars

in the visible universe in the same time.

In its first run, LIGO detected two collisions.

This suggests that cannibal black holes

are relatively common

and that each feast builds a larger black hole.

But so far, the largest black hole

these mergers have produced

is 62 solar masses,

not close to the largest supermassives we've found.

It's hard to imagine, in 13.8 billion years,

that there'd be enough collisions of 30-solar-mass

black holes to build up to form a billion-solar-mass black hole.

That's 100 million collisions.

So maybe small black holes eating each other

isn't the solution.

Maybe supermassive black holes

are eating each other.

If so, could the supermassive black hole

at the heart of our own galaxy be on the menu?

We've found supermassive black holes

so large, they defy explanation.

They're too big to have grown

by simply eating the matter around them.

They can't form the same way that regular black holes do.

There must be something else that happens that lets them grow

to such enormous mass.

Too large to have grown from dark stars

and too big to have grown from regular black holes

simply eating each other.

Merging black holes almost certainly play a role

in our understanding of supermassive black holes.

We think that supermassive black holes themselves also merge

and have merged regularly over the course of the universe.

Now, whether this merging activity itself

is enough to make them that big,

the jury is still out on that.

Now a newly discovered type of galaxy

may provide an answer.

It's called w2246-0526.

And we can't see it.

But we can detect the heat it gives off.

This galaxy is an example

of a rare class of objects called hot dogs.

One of the funnier terms f

is a hot dog galaxy.

And no, this is not some delicious sausage snack.

In fact, it means "hot, dust-obscured galaxy."

It's called obscured because it's shrouded

in so much dust and gas, the only light that escapes

is infrared in the form of heat.

All this heat must be coming from somewhere.

So in the core, there is a cauldron,

a seething supermassive black hole,

the likes of which we can't even imagine.

Of all the supermassive black holes we know of,

the ones that are obscured in these hot dog galaxies

may be the ones that are the most ravenous,

consuming many millions of times

the mass of the Sun.

Scientists theorize that hot dogs

could be the offspring

of cannibal giant black holes.

When the monstrous black holes merge,

they drag gas and dust with them.

This brings more food to the table,

allowing the new black hole to gorge itself.

When you have these two galaxies merging,

they have all-new food.

It's a brand-new dinner plate,

a brand-new buffet of food to eat.

The combination of cannibalism and fresh food

allows the black holes to grow super large.

Perhaps this is how the supermassive black hole

at the center of our galaxy

grew when it was young.

But what's the future

of our supermassive Sagittarius "a" -star?

As far as supermassive black holes go,

Sagittarius "a" -star

is actually still kind of in the minor leagues.

It's small. But it's not done yet.

It's still eating. It's still growing.

And in around 4 billion years,

it's going to become 25 times larger,

because it's going to be eaten by its neighbor.

The giant Andromeda galaxy is heading our way.

And it's going to engulf our milky way.

When galaxies merge,

their central supermassive black holes merge.

Andromeda's huge supermassive black hole

will drag Sagittarius "a" -star into orbit...

...gradually drawing it closer and closer

until it devours it.

The new supermassive black hole will weigh

around 100 million solar masses.

But the disruption to the new galaxy

will provide the new supermassive black hole

with plenty to eat

and the opportunity to grow a whole lot bigger.

At present, there are many theories

of how supermassive black holes get so big.

Most likely, it's a combination of them all.

But however it happens, we can be pretty sure

it's one of the most spectacular things in the universe.

The jury's still out on exactly

how supermassive black holes become so massive.

Making all the black holes we see

probably requires a pretty diverse cookbook.

So any physicist who's looking for a really simple,

single answer for how they get made,

they're probably gonna be disappointed.

It's probably a pretty complex thing that's going on.

It could be through eating.

It could be through eating and merging.

And usually, the answer is somewhere in the middle.

So they will merge with other black holes.

And they'll also have a few snacks between mergers.