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00:00Two black holes circle each other in a dance of death.
00:09They spiral inwards, their immense gravities pulling them ever closer.
00:17When they finally collide, it's one of the most powerful events since the Big Bang.
00:25This explosive mystery sends ripples across the world of science.
00:31But can it also answer one of the most pressing questions in cosmology?
00:37How do supermassive black holes grow so large?
00:58In the known universe, there are roughly 2,000 billion galaxies.
01:04Each one has a different shape and size.
01:08But they may all have one feature in common, a supermassive black hole buried at their
01:15center.
01:17As its name says, it is supermassive.
01:20And here we're talking about objects that are millions or billions of times the mass
01:24of the sun.
01:29Supermassive black holes are so big that we need a special scale for measuring them.
01:37A solar mass is the mass of the sun.
01:39So when we study the universe, we have to use the tools that we have in hand.
01:45And what's the most massive thing that we have around us?
01:47It's the sun.
01:48And so we refer to things in multiples of the mass of the sun because it just makes
01:52it easier to wrap our heads around.
01:54However, if you have something that's 17 billion times the mass of the sun, that's pretty
01:59difficult to wrap your head around anyway.
02:02But we know that those kinds of black holes live in the centers of galaxies.
02:08The supermassive black hole at the center of our galaxy, the Milky Way, is called Sagittarius
02:14A star.
02:15It weighs in at 4 million solar masses.
02:19But compared to the other supermassive black holes out there, it's puny.
02:25This is probably one of the only contexts in where you would think that our supermassive
02:29black hole isn't very supermassive.
02:34The supermassive black hole in our neighboring galaxy, Andromeda, is 25 times larger than
02:40Sagittarius A star, coming in at 100 million solar masses.
02:47But compared to the largest monsters out in the universe, it's a runt.
02:51OJ 287's primary supermassive black hole weighs in at 18 billion solar masses.
02:59And the black hole in the core of galaxy NGC 4889 in the Coma Cluster weighs 21 billion
03:07solar stars.
03:09That's over 5,000 times larger than Sagittarius A star.
03:15These are incredible things that are more massive than some galaxies.
03:22Now, astronomers may have made a remarkable discovery, a giant new supermassive black
03:28hole that's a mind-blowing 30 billion times the mass of the sun.
03:34It's a huge puzzle, and we have simply no idea how it got so big.
03:40It's a huge mystery how black holes have become so enormous.
03:44We started lining black holes with millions and billions of times the sun's mass.
03:48No one expected that, and we have no idea how they got to be so big.
03:54It's not entirely clear at this point how supermassive black holes can get to be the
03:59masses that they are today.
04:08Regular-sized black holes form when large stars over 20 times the mass of our sun crash
04:14and burn.
04:17When a large star runs out of fuel, the core stops generating enough outward force to counteract
04:23the power of gravity crushing inwards.
04:26As the star collapses, the outer part explodes in a supernova.
04:37The inner core shrinks from a sphere millions of miles wide to one just 10 miles across.
04:47It's like shrinking the Earth down to the size of a golf ball.
04:53This rapid collapse creates a black hole.
04:57So we now have seen black holes that are solar-mass black holes and black holes that are million
05:03or billion solar-mass black holes.
05:05And the question is, how do you get from one to the other?
05:09Do the giants somehow grow from a solar-mass black hole?
05:16One of the big puzzles today is, how do you make one of these supermassive black holes?
05:22One idea is, you get there by starting with a solar-mass black hole, having it grow through
05:27a stage of being an intermediate mass black hole, and eventually getting to be a supermassive
05:31black hole.
05:33Theoretically, intermediate-mass black holes should be between 100 and 100,000 solar masses,
05:41but we'd never seen one.
05:43Part of the mystery of supermassive black holes is that black holes seem to occur in
05:47two flavors.
05:49You have ones that are only a couple times the mass of the sun, and you have ones that
05:53are millions or billions of times the mass of the sun.
05:56So we have small and extra-large.
05:59If we think of the stellar-mass black hole as sort of the baby black holes, and the supermassive
06:04black holes as the grown-up black holes, we're missing the teenage black holes.
06:08Where are these black holes that have masses that are between stellar-mass and supermassive?
06:13They're sort of like a holy grail for black hole hunters.
06:19Where are these things?
06:20Where can we find them?
06:21And how do you make them?
06:23Then, astronomers caught a break.
06:27They picked up a burst of energy coming from the NGC 1399 galaxy.
06:34It was the death throes of a star being eaten by a black hole.
06:40When they measured its size, they discovered it was an elusive intermediate-mass black
06:45hole.
06:46The missing link had been found.
06:50But when scientists did the math to see if such an intermediate-mass black hole could
06:55grow into a supermassive black hole, they hit a snag.
07:00There hasn't been enough time since the birth of the universe for an intermediate-mass black
07:05hole to eat enough stars to grow into a supermassive black hole.
07:12It doesn't seem like there's enough time for black holes to get as big as we see them.
07:17But supermassives are everywhere we look.
07:21How did they get there?
07:22And how did they grow so huge?
07:35In our universe, we have detected small black holes, and we've seen monsters.
07:42Supermassive black holes, billions of times the mass of our sun.
07:46But we'd found almost none in between.
07:50So how do you get from a small black hole to a giant one?
07:55One of the most important outstanding questions in cosmology is how did supermassive black
08:00holes get as big as they are, and when did that happen?
08:05Black holes are normally surrounded by gas and stars, an all-you-can-eat buffet.
08:11One of the best ideas for how black holes grow is that black holes do what we expect
08:16black holes to do, and that is eat stuff.
08:19For a black hole, it's almost as if the universe is its restaurant, and on its menu you'll
08:23find stars, planets, and clouds of gas and dust.
08:29So is binge eating the answer to growing a supermassive black hole?
08:34Theoretically, black holes should keep on growing forever as they consume more and more
08:39food.
08:41But recent discoveries suggest that the universe puts them on a diet, controlling how much
08:46they eat.
08:48Black holes are hungry, they like to eat, but sometimes they eat too much and they burp
08:51it up.
09:01February 2015.
09:04Astronomers report something unusual in the galaxy NGC 2276.
09:11It looked like something had taken a bite out of one of its spiral arms.
09:18Sitting alone in the void was an intermediate-mass black hole, about 50,000 times the mass of
09:24the sun.
09:27One theory was that the black hole had eaten everything around it, creating the dead zone.
09:32But the detection of a burst of energy from the black hole suggests it may have tried
09:37to eat too much and, in the process, destroyed its food source.
09:42Burping so hard, its food was blasted away.
09:52Turns out that black holes are actually very messy eaters.
09:54A lot of matter gets thrown off as it tries to absorb it.
09:58So things move in, gets hot, but then a lot of it gets thrown all the way back out.
10:04Black holes are not vacuums in space.
10:06They do not just eat everything around them.
10:08And so they are messy.
10:10Some things get in and they take that on and it grows their mass.
10:12And some things are just flung out as they're eating.
10:17The enormous gravity of black holes sucks gas, dust, and even stars towards them.
10:24Everybody's been to an all-you-can-eat buffet, but let's be honest, there really is a limit
10:27to how much you can eat.
10:30Black holes are gluttons.
10:32They're greedy.
10:33They don't really know when they've eaten too much.
10:35They just keep on cramming in more and more food.
10:40It doesn't just fall in.
10:41It has to go down the drain, more or less.
10:44And so it forms this disk around the hole.
10:47And as it does that, there's a lot of turbulence and magnetic fields and a witch's brew of
10:52forces going on there that get it really hot.
10:56As the gas and dust swirls around, it heats up, pushing temperatures to millions of degrees
11:02Fahrenheit.
11:07This swirl, called the accretion disk, also generates powerful magnetic fields.
11:14These fields are dragged by the spin of the black hole and become focused above the poles.
11:22As energy builds up, the magnetic fields become so compressed they blast out super-energized
11:27particles.
11:30These beams can actually be incredibly violent.
11:34Matter is flung out at a large fraction of the speed of light.
11:37It's a tremendous wind that blows very hard away from the black hole.
11:43The jet hits the gas clouds surrounding the black hole, blowing the buffet away.
11:49They eat too much.
11:51They can basically blow everything that's in their vicinity away.
11:55They lose their food supply, and then they're going to starve.
12:00They can kind of shoot themselves in the foot.
12:03With no food available, the black hole stops growing.
12:07Astronomers think that's what happened to the intermediate-mass black hole they discovered
12:11in the dead zone.
12:18These burps may regulate star formation and stop the black hole from getting obese.
12:25But over time, the black hole will start eating again as gas falls back towards it.
12:31But can an intermediate-mass black hole eat enough to become a supermassive black hole
12:37weighing billions of solar masses?
12:40Could that black hole become so obese by eating?
12:44That's a really interesting question.
12:46You'd have to eat a heck of a lot to get that fat.
12:51When you think about it, if you imagine an average galaxy has a hundred billion stars,
12:56the black hole would have to eat one in every five stars in the galaxy.
13:01The universe is old, but is it really old enough that black holes have had time to consume
13:05billions of stars?
13:07That seems kind of unlikely.
13:10That doesn't seem to add up.
13:11We need some other way to make these supermassive black holes, and the question is, what is that?
13:18Maybe we've been making this all too complicated.
13:21Maybe to get a big black hole is to start big in the first place.
13:25So how can black holes start big?
13:29To answer that question, scientists had to journey back to the very start of the universe,
13:35to a mysterious time called the Dark Ages.
13:48As we look out into the universe, we're seeing farther and farther back in time.
13:54We have now looked back over 12 billion years, to the time when the cosmos was still an infant.
14:02And what we found was a huge surprise.
14:07We had made the assumption that the universe was still alive.
14:13We had made the assumption that as you look farther out into the universe,
14:18the black holes would be smaller.
14:20They haven't had much time to grow.
14:22But now we've found a 12 billion solar mass black hole.
14:26That's actually less than a billion years into the universe.
14:29How did this thing form so early?
14:31How did it grow so fast?
14:33This is like walking into a delivery room and finding a 100 pound baby.
14:37I mean, how does that even happen?
14:39It doesn't make any sense.
14:41Physics tells us no black hole could swallow enough stuff to get that big that quickly.
14:48There really wasn't enough time between the Big Bang and when we're studying these things
14:52for them to grow to such large sizes just by eating matter around them.
14:58So if there's not enough time for them to grow so large, maybe they're born supermassive.
15:06To understand how, we have to travel back even farther,
15:11to not long after the birth of the universe.
15:16The early universe was definitely a much more compact and richer place for material.
15:23It was smaller and it was denser.
15:25Things were much closer. It was hotter.
15:27It was just a much more intense place to be.
15:32Clouds of hydrogen and helium gas clumped together.
15:37As the clouds grew, so did their gravity, sucking in more and more gas.
15:44Eventually, the ball of gas became so dense, it collapsed, triggering nuclear fusion.
15:52A star was born.
15:55These massive first stars are called Population III stars.
16:01Because there was so much food around, these stars were huge,
16:06many times bigger than any stars that exist today.
16:11We think a lot of these Population III stars probably were incredibly massive,
16:15incredibly short-lived, and just blew up right away.
16:19They would have left massive black holes behind.
16:26Population III stars
16:33With so much food available, these young, ravenous black holes, called quasars,
16:39started binge-eating and became incredibly bright.
16:44Billions of years later, we can still see their gluttony.
16:49The most luminous, bright objects in the universe are things called quasars.
16:54And it may seem kind of ironic, but what these really are, are supermassive black holes.
16:59There's so much stuff trying to cram itself down the black hole
17:02that everything gets very hot, very energetic, and you can see them clear across the universe.
17:07But when we measured the size of the young quasars,
17:10we discovered they were already billions of solar masses.
17:14There's not enough time, a billion years after the universe was created,
17:19for them to get to a billion solar masses in just too short a time.
17:24So the question becomes, how do you make black holes that are this big in that small amount of time?
17:30We need some other way of growing these supermassive black holes.
17:34There needs to be some other mechanism that allows them to get that massive so early.
17:38But what is that?
17:41A clue can be found in the very early universe.
17:46The early universe is still so much of a mystery to us.
17:49We know the conditions were very different. It was denser. There was a lot more material.
17:54This period is called the Dark Ages.
17:58During the Dark Age, we know that there was basically nothing happening.
18:02Matter existed. We think that there was hydrogen and helium gas, but really not much else.
18:09There were a few stars around, but nothing large enough to form giant black holes.
18:14But there were huge clouds of gas, and because the universe was much smaller and denser,
18:20the clouds were much thicker.
18:24The idea is that from these basic ingredients, gravity and gas,
18:29the cosmos built massive black holes.
18:33Somehow the universe has created a shortcut to the black hole.
18:36We've typically thought of it as cloud of gas collapses into a star, star evolves, star dies, leaves behind a black hole.
18:43Perhaps the universe has found a way to skip the star phase and go directly to the black hole.
18:52Clouds of gas may have built massive black holes in a process called direct collapse.
19:01As they collapsed, they never even formed a star. They just collapsed straight into a giant black hole.
19:07Through this direct collapse theory, you can form really big black holes.
19:11Imagine what it's like seeing one of these giant clouds of gas collapsing down into a black hole.
19:16You might think you start with, okay, cloud of gas slowly collapsing, and then, boop, it's a black hole.
19:21That wouldn't be the case. It would be more like giant cloud of gas starts collapsing, and then, ah, black hole.
19:28It's believed that direct collapse could have created black holes up to a million times the mass of the sun,
19:35much bigger than from the collapse of a single star.
19:39These early black holes are sort of like the galaxies that never were.
19:42They were going to make galaxies, but instead they collapsed into very massive black holes.
19:49For direct collapse to form a black hole, the conditions need to be precise.
19:55The clouds must be very symmetrical, forming a smooth ball.
20:00If you have a ball of gas that isn't quite a ball, that's not quite homogeneous,
20:05as it collapses, it'll fragment, and it'll fragment into objects that won't form black holes.
20:09So you want it to be hot enough that it stays one big giant thing,
20:15but it does need to cool a little bit, right, so that you get it to collapse in on itself.
20:21You have to get uniform collapse over time of a very large amount of hydrogen gas, presumably,
20:29which is the original matter in the universe, collapsing spherically, symmetrically,
20:35without fragmenting, over a period of less than 500 million years.
20:41Direct collapse may have created black holes a million times the mass of the sun,
20:46but it can't completely explain the 12 billion solar mass supermassive black holes we see in the early universe.
20:54Maybe gigantic supermassive black holes were created by strange unseen forces.
21:03Maybe they were created by the mysterious dark universe.
21:18Astronomers looking deep into the early universe have discovered gigantic black holes.
21:26Astronomers looking deep into the early universe have discovered gigantic supermassive black holes.
21:34This is a pretty deep mystery.
21:37There are these supermassive black holes that exist in the very early universe,
21:41and by all accounts, they should not exist.
21:44According to the normal laws of physics, it shouldn't have been possible for them to grow so big so quickly.
21:51For astrophysicists, understanding how black holes have grown to be so large is one of our biggest mysteries.
21:57We need some other way of growing these supermassive black holes.
22:01There needs to be some other mechanism that allows them to get that massive so early.
22:05But what is that?
22:07Everything we can see in the night sky makes up just 4.8% of all the matter in the cosmos.
22:15The rest is the dark universe, including dark matter.
22:20We can't see it, feel it, or detect it directly, but we know dark matter is there.
22:27Its gravity is tugging on everything around it.
22:31We're beginning to understand it plays a fundamental role in the formation of the universe.
22:37Most of the stuff that collects together gravitationally is dark matter.
22:41So, perhaps black holes form somehow with the inclusion of dark matter.
22:47One way of looking at it is there's six times as much dark matter as normal matter.
22:50So, 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.
22:57Maybe these supermassive black holes are growing by eating dark matter.
23:02There are some tantalizing clues.
23:05The largest supermassive black holes don't live in the galaxies with the most regular matter.
23:11They live in the galaxies with the most dark matter.
23:15One thing we know about dark matter right now is that it has gravity.
23:19And a black hole runs on gravity.
23:21It attracts anything with mass.
23:23So, there's no reason to assume that black holes would only eat regular matter.
23:27And now we know that there's far more dark matter out there.
23:31Maybe dark matter helps the black holes eat.
23:36Maybe in some ways dark matter is a feeder for these supermassive black holes.
23:41Perhaps what really grows a supermassive black hole is all of the regular matter being directed into the center by the dark matter around it.
23:50Maybe the dark matter's powerful gravity sucks in regular matter and funnels it into the black hole.
23:58In a sense, the dark matter is greasing the wheels.
24:01It's sort of tilting the table up so that that food can slide right in.
24:06But now, scientists think the dark matter may create gigantic black holes directly by igniting dark stars.
24:17Some believe that dark matter sparked early universe superstars.
24:21When they die, they leave behind supermassive black holes.
24:27Dark stars sound like they come from the fertile imagination of some sci-fi writer.
24:32But Dr. Katie Fries believes they may explain how early supermassive black holes grew so fast.
24:40Dark stars are amazing.
24:41So, 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.
24:50Dark stars may have been some of the first stars to form in the universe.
24:55They sparked into life when the universe was just beginning to move.
24:59They sparked into life when the universe was just 200 million years old.
25:05But how could dark stars form really massive black holes?
25:10A newborn black hole can't weigh more than its parent star.
25:15So, in order to give birth to a really massive black hole, the parent star has to be supermassive as well.
25:24These early objects are really strange.
25:26They're very cool and they're really, really big.
25:31The size of these things is ten times the distance between the sun and the earth.
25:39But how is that possible?
25:42Regular stars have an upper size limit.
25:46A star is a battle between gravity pushing inwards and nuclear fusion pushing out.
25:56When the star grows too big, its gravity becomes overwhelming.
26:02The delicate balance between gravity and fusion is broken.
26:07Gravity wins out and the star collapses.
26:12But dark stars may have a workaround that lets them become supermassive.
26:19So, they are made of ordinary matter.
26:21They're made of hydrogen and helium, but they're powered by dark matter.
26:27We don't know what dark matter is made from, but we do have theories on how it might power a star.
26:35One of the best ideas we have for dark matter is that it's made of weakly interacting massive particles, or WIMPs for short.
26:44So, these WIMPs are their own antimatter, and that means whenever they encounter each other, they annihilate and turn into something else.
26:53That means a lot of heat is released, a lot of energy, and it's that energy that could power stars.
27:02The energy from the WIMPs' annihilations keeps the star from collapsing like a normal star.
27:08So, it's possible that in some stars, their internal reactions are actually being powered by dark matter.
27:16If that's the case, then you can imagine situations where when that burns out, you produce very massive black holes.
27:23So, 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.
27:38The energy from the dark matter allows the dark stars to grow huge.
27:46When they first form, they're small. They're about the mass of the sun.
27:51But because they're so cool, they keep accumulating matter and growing, growing, growing,
27:56and some of them will get to be a million times as massive as the sun and a billion times as bright.
28:02But these giants don't live for long.
28:04Eventually, the dark matter particles wipe each other out completely,
28:09and there is no more fuel to keep the massive amount of ordinary matter from collapsing.
28:15And then that's it. There's nothing to sustain this big puffy object.
28:19If it's big enough, you collapse directly to a black hole.
28:24A monster, supermassive black hole.
28:29It's really fun to think about the possibility of a black hole.
28:32It's really fun to think about the possibility that the physics of dark matter is actually helping to power stars.
28:38If so, it would bring a whole new window into our understanding of stars and their evolution.
28:47At the moment, dark stars are just theoretical.
28:51But when the powerful James Webb telescope comes online in 2018, we may get our first glimpse.
28:59We're going to do an observing run and look for these things.
29:03And so we're very excited. If you would find an entirely new type of star, that would be huge.
29:09While Katie Fries looks for dark stars, another team is investigating another radical idea
29:16that offers new insight into how supermassive black holes grow so huge.
29:21They detect the faint echoes of a violent event from across the universe.
29:27The remnants of an extraordinary collision.
29:31A supremely energetic event that reveals black holes are cannibals.
29:52Our universe is filled with enormous supermassive black holes that defy explanation.
29:59Supermassive black holes are one of the things in the universe that when you run the physics,
30:04when you run the math of how they evolve, they really shouldn't be there.
30:08It's still a profound mystery.
30:11The universe hasn't been around long enough for regular black holes to eat enough matter to get supermassive.
30:18So how did they get so big?
30:19The most logical answer is that large black holes are born large, around 1 to 2 billion solar masses.
30:27But that's still over 10 times smaller than the largest supermassive black holes out there.
30:33Given the time scales, it doesn't seem to add up.
30:37We need some other way to make these supermassive black holes. And the question is, what is that?
30:42A clue came from a large, isolated galaxy 200 million light years away in a quiet part of the universe.
30:52Nestling alone was a supermassive black hole with a mass of 17 billion suns.
31:01Normally, such monsters are found in dense regions of space with lots of galaxies and lots of stars.
31:09This black hole doesn't match its surroundings at all.
31:12It's kind of like driving through the middle of a desert and coming across the Empire State Building.
31:16Now the Empire State Building belongs in the middle of a city,
31:19and a black hole this big belongs in a rich cluster of galaxies.
31:23This is the first time astronomers have found such a giant object lurking in such a relatively empty area of the universe.
31:32So you've got to ask the question, if there's nothing else around,
31:34how exactly do you grow a 17 billion solar-mass black hole?
31:40One possible answer is the stuff of nightmares.
31:44Maybe the story of this black hole is actually a little more scary than we thought.
31:48Maybe it's all alone because it ate all of its neighbors.
31:54Maybe it was eating more than galaxies.
31:57Maybe it was a supermassive black hole.
32:00Maybe it was eating more than galaxies.
32:03Maybe it was eating its own kind.
32:08The thing about black holes is they're omnivores.
32:11They'll eat anything. Anything that gets close to them, they'll gobble up.
32:15One way black holes can grow so large is by eating other black holes.
32:18So in a sense, they may be cannibals.
32:21Cannibal black holes were just theoretical.
32:24We'd never actually seen them eat each other.
32:28Then, scientists detected the faint echoes of actual ripples in space-time.
32:36When engineers turned on the Laser Interferometer Gravitational Wave Observatory, or LIGO for short,
32:44they immediately picked up the faint signal of gravitational waves.
32:49Gravitational waves are created by huge explosions in space.
32:58To make them, you need an almost unimaginably energetic event.
33:03Something really, really big.
33:08Something like merging black holes.
33:14A black hole merger is the most violent, the most energetic thing that happens in the universe, period.
33:23Picture the scene 1.3 billion years ago.
33:27Two black holes circle each other in a dance of death.
33:31The larger black hole pulls the smaller one inwards until they're locked together in a spiral.
33:38Very, very slowly, that orbit is decaying. They're getting closer and closer and closer.
33:43And then they will merge into one giant black hole.
33:47Truly one of the most dramatic events in the universe.
33:50Finally, they collide in one of the largest bangs since the Big Bang.
33:58I would have loved to have been able to safely view the collision of these two black holes up close.
34:05Imagine these two black holes as they spiral in toward each other, going faster and faster and faster and faster.
34:10And then suddenly, where there appears to be nothing, or just distortions in space in front of you,
34:16suddenly there's this enormous burst of energy and everything just rings around you.
34:21By measuring the frequency of the gravitational waves, we can calculate the size of the objects causing them.
34:29When those two black holes, weighing 29 solar masses and 36 solar masses, collided,
34:37they created a black hole around twice the size.
34:43In some ways, it's very elegant and simple.
34:45You take two black holes, you spiral them in together, and you end up with one big black hole.
34:52The event showed that black holes can double their mass through cannibalism.
34:58Almost.
35:00The final black hole was less than the sum of its parts.
35:05There were three solar masses missing.
35:08That may not sound like a lot, so let's put it in context.
35:12Our sun is burning about 100 billion hydrogen bombs every second.
35:18And over its 10 billion year lifetime, it will convert less than maybe 1% of the mass of the sun to energy.
35:24In two tenths of a second, three times the mass of the sun in matter got converted to energy in that collision.
35:33It was 36 septillion yottawatts.
35:37What does that mean?
35:39A lot of freaking energy.
35:40That's more energy in that two tenths of a second than is emitted by all the stars in the visible universe in the same time.
35:50In its first run, LIGO detected two collisions.
35:56This suggests that cannibal black holes are relatively common, and that each feast builds a larger black hole.
36:04But so far, the largest black hole these mergers have produced is 62 solar masses, not close to the largest supermassives we've found.
36:16It'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.
36:25That's 100 million collisions.
36:28So maybe small black holes eating each other isn't the solution.
36:33Maybe supermassive black holes are eating each other.
36:39If so, could the supermassive black hole at the heart of our own galaxy be on the menu?
36:58We found supermassive black holes so large they defy explanation.
37:04They're too big to have grown by simply eating the matter around them.
37:10They can't form the same way that regular black holes do.
37:13There must be something else that happens that lets them grow to such enormous mass.
37:17Too large to have grown from dark stars, and too big to have grown from regular black holes simply eating each other.
37:25Merging black holes almost certainly play a role in our understanding of supermassive black holes.
37:30We think that supermassive black holes themselves also merge and have merged regularly over the course of the universe.
37:37Now whether this merging activity itself is enough to make them that big, the jury is still out on that.
37:42Now, a newly discovered type of galaxy may provide an answer.
37:47It's called W2246-0526.
37:53And we can't see it, but we can detect the heat it gives off.
37:58This galaxy is an example of a rare class of objects called hot dogs.
38:05One of the funnier terms for an exotic type of galaxy is a hot dog.
38:09One of the funnier terms for an exotic type of galaxy is a hot dog galaxy.
38:13And no, this is not some delicious sausage snack.
38:15In fact, it means hot dusk obscured galaxy.
38:20It'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.
38:31All this heat must be coming from somewhere.
38:33So in the core, there is a cauldron, a seething supermassive black hole, the likes of which we can't even imagine.
38:42Of 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.
38:54Scientists theorize that hot dogs could be the offspring of cannibal giant black holes.
39:02When the monstrous black holes merge, they drag gas and dust with them.
39:07This brings more food to the table, allowing the new black hole to gorge itself.
39:14When 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.
39:24The combination of cannibalism and fresh food allows the black holes to grow super large.
39:32Perhaps this is how the supermassive black hole at the center of our galaxy grew when it was young.
39:41But what's the future of our supermassive Sagittarius A star?
39:47As far as supermassive black holes go, Sagittarius A star is actually still kind of in the minor leagues, it's small.
39:54But it's not done yet. It's still eating, it's still growing.
39:58And in around 4 billion years, it's going to become 25 times larger.
40:06Because it's going to be eaten by its neighbor.
40:15The giant Andromeda galaxy is heading our way, and it's going to engulf our Milky Way.
40:22When galaxies merge, their central supermassive black holes merge.
40:27Andromeda's huge supermassive black hole will drag Sagittarius A star into orbit.
40:36Gradually drawing it closer and closer until it devours it.
40:45The new supermassive black hole will weigh around 100 million solar masses.
40:50But 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.
41:03At present, there are many theories of how supermassive black holes get so big.
41:10Most likely, it's a combination of them all.
41:14But however it happens, we can be pretty sure it's one of the most spectacular things in the universe.
41:23The jury's still out on exactly how supermassive black holes become so massive.
41:31Making all the black holes we see probably requires a pretty diverse cookbook.
41:36So any physicist who's looking for a really simple single answer for how they get made, they're probably going to be disappointed.
41:44It's probably a pretty complex thing that's going on.
41:51It could be through eating. It could be through merging. It could be through eating and merging.
41:56And usually the answer is somewhere in the middle.
41:59So they will merge with other black holes and they'll also have a few snacks between mergers.
42:13NASA Jet Propulsion Laboratory, California Institute of Technology