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00:00Black holes tearing apart enormous stars,
00:04pulsars spinning at incredible speeds and emitting powerful beams of energy,
00:08colorful nebulae with fireworks of newborn stars,
00:11galaxies of every possible color and size,
00:14all of these are found within our universe.
00:17But it's not infinite.
00:19It has a boundary, a literal wall.
00:22And beyond that, there's an absolute nothingness.
00:25Right now, we're going to make a journey to that wall.
00:28But first things first, our universe is like a humongous nesting doll.
00:33If you open it up, there's a smaller one inside.
00:35It's a galaxy.
00:37Inside that is an even smaller doll.
00:39That's our solar system.
00:41And the smallest doll of all is the Earth.
00:44Each of these dolls has boundaries that we're going to cross.
00:47For that, we'll need a spaceship and a big one.
00:51It also has to be able to move 100 times faster than the speed of light.
00:55You get on board and start the engines.
00:5762 miles above sea level is our first boundary.
01:01That's 10 times higher than passenger planes fly.
01:04This point is called the Kármán line.
01:07It separates the atmosphere of the Earth from outer space.
01:10Now we fly further to the edge of our solar system.
01:14We turn on the hyperdrives and fly past Mars, Jupiter, Saturn, Uranus, and Neptune.
01:19We've traveled a distance of 100 astronomical units.
01:23One AU is the distance from Earth to the Sun.
01:26And here's the boundary of our solar system, the heliosphere.
01:30Here, the speed of the solar wind decreases rapidly.
01:34First, it drops from 620,000 miles per hour to the speed of sound.
01:38Then, there's a layer called the heliopause.
01:41This is where the wind almost vanishes.
01:44And then, our ship experiences a bow wave.
01:47This is where we feel the force of the interstellar wind, which collides with the boundary of our solar system.
01:53When you pass this boundary, you find yourself in the dark of interstellar space.
01:58And here, you can find two human-made objects that made this trip for the first time in history.
02:04They're Voyager 1 and Voyager 2.
02:07Voyager 1 crossed that boundary in 2004.
02:10Voyager 2 did it in 2007.
02:13These space probes discovered that the heliosphere is not a perfect ball around the Sun.
02:17Its southern boundary is 10 AUs closer to the star than the northern one.
02:22So, we're moving in interstellar space and will soon approach a stone wall around our solar system.
02:29200,000 AUs further, and there it is.
02:32This wall of rock is the Oort Cloud.
02:35In fact, it's a pile of asteroids that surround our world.
02:39Scientists speculate that the Oort Cloud could be the source of comets and meteorites that fall to Earth.
02:44But they're so sparse that we easily fly between them.
02:47Now, we're in complete darkness.
02:49The Milky Way is about 106,000 light-years wide.
02:53In a conventional rocket, it would take billions of years to fly across that distance.
02:58But you throttle to the max.
03:00You masterfully fly past the stars and planets as if on a racetrack.
03:04And within minutes, you're at the edge of our galaxy.
03:07There's no more interstellar wind.
03:09All you see are bright dots somewhere in the distance.
03:13These dots are huge galaxies.
03:15We need to look at a map to make a route to the edge of our entire universe.
03:20You're here, near the Milky Way galaxy.
03:23It's part of a cluster of galaxies called the Linnea Kea supercluster.
03:27But even this huge thing is like a little street in a big city.
03:31Zooming out, we find Hydrus and Taurus supercluster.
03:35Thousands of galaxies on the map look like little dots.
03:39Maximum zoom out!
03:40This is our entire observable universe.
03:43We thought it was infinite.
03:45But we may have proof that it has a boundary.
03:48It's here, 10 billion light-years away from our home.
03:52Even if you travel at the speed of light,
03:54a trip there would take twice as long as our whole planet has existed.
03:58During that time, the sun will either fade away or explode like a supernova,
04:03destroying our entire solar system.
04:05And if you can live that long and then return home,
04:07you will see that our galaxy is there no more.
04:11It's long since collided with the Andromeda galaxy
04:14and merged into one big cosmic body.
04:17Luckily, your ship is able to warp space-time
04:20so that this journey will literally take a few seconds.
04:23Boom!
04:25Congratulations!
04:26You've arrived at your destination,
04:28the Eridanus Supervoid.
04:30Some scientists believe this location
04:32is the evidence of collisions of our universe
04:35with something big enough to leave such a large scar.
04:38The Eridanus Supervoid is an empty and cold space
04:42one billion light-years wide.
04:44If you think of this void as a cup,
04:46it would fit at least 10,000 galaxies.
04:49And it appeared after an accident of gigantic proportions.
04:53The object that crashed into our universe was...
04:56another universe!
04:57Yes, other universes may actually exist.
05:01Imagine that our entire universe is a huge bubble
05:04that contains all the clusters of galaxies
05:06in the observable universe.
05:08There could be an infinite number of such bubbles.
05:10They could have been born during the Big Bang.
05:13These universes may be different from ours.
05:15They may have other galaxies and nebulae.
05:18But these bubbles could also be parallel universes.
05:21This means that if you chose cereal
05:24over oatmeal in the morning,
05:25in another universe,
05:26your twin would choose the oatmeal!
05:29Every choice you ever made in life
05:30had completely different consequences
05:32in a parallel universe.
05:34And because the number of choices are infinite,
05:36there's a whole infinity of parallel universes.
05:39So, like a regular bubble,
05:41our universe has a wall
05:43that is near the Eridanus Supervoid.
05:45Long ago, another bubble flew past ours.
05:48As they approached each other,
05:50their gravitational fields began to interact.
05:52Our boundary wall began to deform
05:55and pull toward the other universe.
05:57The same thing happened on the other side.
05:59Then the walls of our universes came into contact.
06:02But as these bubbles moved,
06:04their connection began to break.
06:05And the other universe just ripped a huge chunk of ours.
06:09A cold void was formed at the point of collision.
06:11And that was the Eridanus Supervoid.
06:14The problem is that the universe looks the same to the observer,
06:18regardless of point of view.
06:19For example, imagine a basketball hanging in the air.
06:23Now, if we put an ant on the ball
06:25and tell it to find the edge of the ball,
06:27it will start running around it,
06:29making an infinite number of circles.
06:30But the landscape around the ant will not change.
06:33All it will see is a rounded horizon.
06:36That's because the ball remains the same
06:38from any point of view.
06:39The same thing happens to us
06:41when we try to find the edge of our universe,
06:43all because we imagine the world in three-dimensional space,
06:47and our view is limited.
06:48For example, you see an ordinary square in two-dimensional space.
06:53But if you add depth and change the point of view,
06:56voila, it's a cube.
06:57If we could see the universe in four-dimensional space,
07:01a square might be something completely different.
07:03But maybe we can leave our home bubble.
07:06The key to traveling to another universe
07:08might be inside a black hole.
07:10A black hole is one of the most mysterious objects in the universe.
07:14They're so heavy, they warp not only space, but time as well.
07:18It's like putting a heavy boulder on a net.
07:21The net will sag, and the closer you get to the boulder,
07:24the stronger the curvature is.
07:26Once you're in the gravitational field of a black hole,
07:28you can't leave it.
07:30We still don't know what might actually be in the heart of a black hole.
07:34Some scientists speculate that white holes also exist.
07:37Theoretically, they should be born along with black holes.
07:40Except for the color, they're the exact opposite of black holes.
07:44Nothing can come close to a white hole.
07:46At the moment, there's no data on such objects.
07:50But general relativity theory suggests they do exist.
07:53There's also a theory that a black hole
07:56may be a passage to another universe.
07:58When you get into a black hole,
08:00you can come out the other side,
08:01through the event horizon of the white hole.
08:03So, you bypass the boundary of the universes
08:06and find yourself in a completely different world.
08:09But, we may have proof that a white hole exists.
08:13In 2006, scientists discovered an unusual burst of energy
08:17somewhere 1.6 billion light-years away from Earth.
08:20This burst was unique.
08:21It didn't look like a supernova explosion
08:24or even the merger of two black holes.
08:26Some astronomers believe it was the birth of a white hole.
08:30But because it was unstable,
08:31it was destroyed almost immediately.
08:33This process was reminiscent of the birth of our entire universe,
08:37the Big Bang.
08:38So, scientists called it the Little Bang.
08:45This is our home planet Earth and its satellite, the Moon.
08:49Zooming out, and here's our solar system.
08:52A bit more, the Milky Way galaxy.
08:54And we're a small dot among an infinite number of stars.
08:58Now, even farther out, a cluster of galaxies.
09:01Dots and swirls in the endless space.
09:04Further, there's Laniakia, supercluster.
09:07That little dot here is our galaxy.
09:09Moving on, Hydra-Centaurus supercluster.
09:13Huge clusters comprising thousands of galaxies are no more than a speck from here.
09:18Next, Pavo-Indu supercluster.
09:20This is an area 200 million light-years wide.
09:24We can zoom out until we see the entire observable universe.
09:28Each little dot in here actually contains thousands of galaxies and quadrillion stars.
09:33Scientists speculate that our universe may look like a bubble.
09:37And that bubble might collide with another universe.
09:40Yes, other universes could exist.
09:43Actually, even a whole infinite number of those.
09:46All of them could have appeared after the Big Bang.
09:49The collision between them isn't impossible either.
09:52At least, it might have happened before.
09:54And the proof is here, in the direction of the constellation Eurydynus.
09:58This place is called Eurydynus supervoid.
10:00It's about 1 billion light-years wide.
10:03By comparison, the width of our entire galaxy is only about 100,000 light-years.
10:09There's absolutely nothing in this place.
10:12And it may be a trace from an old collision between our universe and another.
10:16Scientists think they were passing by each other.
10:19When the distance between them was minimal,
10:21the gravitational forces of the bubbles began to pull toward each other.
10:25Just like two drops of water trying to connect when they're close.
10:28But the speed of the universes was too high for them to continue interacting.
10:33So, the other universe just tore out a piece of our bubble.
10:36There might have been about 10,000 galaxies in that void.
10:40And all of them were either destroyed or taken over by the other universe.
10:44Let's travel to the edge of our universe to see how this collision might have taken place.
10:49We're 10 billion light-years away from our home.
10:51Here, in another galaxy, we see amazing nebulae of different colors and shapes.
10:57And if you look in the other direction, there's a huge wall moving at us.
11:02All these bright sparks on it are enormous galaxies about to collide with us.
11:07But in fact, it's a humongous mirror that only reflects our universe.
11:11Here, space-time is distorted and begins to be pulled into another universe at a tremendous speed.
11:17The usual law of physics may simply stop working at this point.
11:22Gravity may disappear.
11:23And with it, all the stars would explode and people on the surface of planets would hang in weightlessness.
11:29But if the universes didn't go at a tangent but crashed directly into each other, things would be much scarier.
11:37The enormous amount of collision energy would probably cause an incredible explosion.
11:42Its force would simply destroy everything in our bubbles.
11:45Still, the two bubbles might begin to merge, too.
11:48At first, all galaxies at the edge of the universes would be torn apart.
11:52But then, the merger would begin.
11:55The galaxies would start moving chaotically.
11:57They would fly past each other, break apart, collide, and explode.
12:02The collision of two galaxies is an accident of enormous proportions.
12:06And it might happen to our home quite soon, in space terms.
12:10The Andromeda galaxy is heading our way.
12:12It's a spiral galaxy about twice the size of ours.
12:16And there are about a trillion stars there, which is twice more than in our Milky Way.
12:21At the very center of this bright galaxy lurks a dark beast, a black hole.
12:27Its weight is two and eight zeros of the sun's mass.
12:31Red giants hundreds of times larger than our sun.
12:35Pulsars emitting enormous amount of energy like spotlights.
12:38Rogue stars and many large and small black holes.
12:42This soup of dangerous objects is moving toward us at 68 miles per second.
12:48A trip to New York to Los Angeles at that speed would only take half a minute.
12:52The disk of Andromeda can already be seen with the unaided eye on moonless nights.
12:57As time goes on, it'll get even bigger.
13:00As Andromeda gets closer to us, its gravitational force will begin to stretch the arms of our spiral galaxy.
13:07It'll begin to unwind.
13:09The stars and planets will lose their orbits.
13:12One possible scenario is that an unknown asteroid or even a dwarf planet from the Andromeda galaxy will crash into
13:19the Earth at an incredible speed.
13:21Our planet will explode just like a balloon from this impact.
13:25Oh, goody.
13:25Another option involves stellar collisions.
13:28Our sun would face another star.
13:30The bigger star will slowly begin to consume the smaller one.
13:34First, it will steal the light upper layers from it, and then it will eat it just like spaghetti.
13:40Or even like rigatoni.
13:42When a large star reaches its critical weight, it will burst.
13:46This explosion will destroy everything around it, including our solar system.
13:51Perhaps the shockwave will even reach other neighboring stars.
13:54Yet another scenario is that our solar system will be thrown into dark space.
13:59Imagine a tennis ball tied to a rope.
14:02You take the rope and spin the ball over your head like a sling.
14:05Then you let go of the rope and send the ball flying.
14:08This is what will happen to the sun and all the planets around it.
14:12We'll find ourselves in dark and cold space.
14:15But life on Earth will not be affected.
14:18We'll still have our bright star to keep us warm.
14:20The only disadvantage is that all the stars in our night sky will disappear.
14:25And the most likely possibility is that the merger of two giant galaxies will have no effect on us at
14:32all.
14:32The thing is, the distance between stars and planets in space is enormous.
14:37So they can all just mix together and form one giant cloud.
14:42It would be like shoveling fine sand through a big sieve.
14:45The objects won't interact with each other.
14:47But the most interesting thing will happen to the black holes in the centers of our galaxies.
14:52Right now, there's a dense cluster of stardust and stars around them.
14:57As Andromeda and the Milky Way come closer together, they will begin to dance with each other.
15:02Gee, will it be the twist or the foxtrot?
15:05And when the black holes get close together, they'll begin to swallow all matter around one another.
15:11Billions of tons of colored stardust, asteroids, and star particles will fly toward the very center of either black hole.
15:19It might seem like this process happens very slowly, but it's an illusion.
15:23Super-heavy objects like black holes warp the space-time grid.
15:27So, time is much slower near black holes.
15:30And all objects that seemingly stay on the event horizon for weeks, or even months, are actually long gone.
15:37When the black holes finally come together, they merge into one super-giant black hole.
15:42But its mass is slightly less than the combined mass of the two dark monsters.
15:46Some of their weight is transformed into collision energy.
15:50This energy is released so strongly that its waves can be felt in other galaxies.
15:54Now, a huge black hole gathers all this dense and hot core of the two galaxies around itself.
16:01At some point, the black hole feels full and throws out powerful jets of energy into space.
16:07This is called an active galactic nucleus.
16:09It's one of the brightest phenomenon in the universe and the most powerful source of electromagnetic radiation ever known.
16:17These jets may be more than 5,000 light-years long.
16:20By comparison, the distance from Earth to the nearest star, Proxima Centauri, is only 4.2 light-years.
16:27And the explosion that accompanies the jets has the power of 100 supernova explosions.
16:33Wow, blows my mind.
16:35The blast wave from this event could even reach the edges of a new galaxy.
16:39And this outburst would be visible from millions of light-years away.
16:43Now, there are dense clouds of multicolored dust at the center of merged galaxies.
16:48The weight of these clouds is so great that they begin to shrink and take on a round shape.
16:54Gradually, they become so heavy that they compress the core and nuclear reactions start inside them.
16:59The temperature begins to rise and soon, boom, there's a supernova.
17:04It's a veritable firework show at the center of the galaxies.
17:08Stars erupt from the fog and form new hot worlds.
17:11At this point, the arms of the two galaxies that were previously pulled out slowly return to their former shape.
17:18The super-heavy center of our galaxy has such a gravitational force that it affects stars and nebula hundreds of
17:26thousands of light-years away.
17:27The galaxy's arms twist again, and we see the new and finished galaxy, the Milcomeda.
17:34Or Milkdromeda.
17:37Hey, how about the Andromilky-Meda way?
17:40Blah, blah, blah.
17:41Well, that's hard to say.
17:46Empty space is not really empty.
17:48At least, that's what quantum field theory says.
17:51It's actually filled with tiny vibrations that can turn into virtual particles if they have enough energy.
17:57These virtual particles can produce packets of light with low energy called photons.
18:04Now, there's something every black hole has.
18:07An event horizon.
18:09An event horizon.
18:09It's a point of no return.
18:10That means once something crosses that point, it can never get away.
18:15Not even light.
18:16And there's an insanely strong gravitational force around the event horizon.
18:21Black holes survive by gobbling up gas and stars around them.
18:25In most cases, a black hole has a swirling disk of material that surrounds it, called an accretion disk.
18:31It glows brightly, as all those things that come too close to an event horizon get heated up and torn
18:37apart before the black hole swallows them all.
18:40As material comes closer, it starts to travel and move faster and faster, going all around the black hole.
18:47This makes the accretion disk glow and at the same time, outlines the shadow of the black hole, which is
18:54basically the very event horizon we're talking about.
18:57Black holes might even want to hide, but they do so awfully badly.
19:02According to Einstein's theory of general relativity, gravity bends and warps space and time.
19:08It means that the closer you come to this extremely powerful gravitational pull around the black hole, the more twisted
19:16space and time around it become.
19:18That's what Stephen Hawking was talking about nearly 50 years ago, and it doesn't stop there.
19:23He also suggested that if these particles find a way to escape a black hole, they steal some of its
19:29energy.
19:29And because of these thieves, the black hole loses its energy as time goes by until it, at one point,
19:35completely disappears.
19:38He suggested that black holes release energy in the form of thermal energy or heat, which is called Hawking radiation.
19:45And this radiation doesn't carry any information.
19:48It means that when a black hole evaporates, it destroys all information it had about the star that created the
19:54black hole.
19:55That way, we can't know what really happened.
20:00And it's kind of confusing because the laws of quantum mechanics say the information can't be destroyed.
20:06This conflict is something we call the Hawking information paradox.
20:11According to Hawking, all this information isn't really lost, but is stored in a cloud of all those zero energy
20:17particles that surround the black hole.
20:19He called that soft hair.
20:21Now, there's this new study as a possible solution to this paradox.
20:27Maybe Hawking radiation is non-thermal.
20:30Instead of just releasing plain heat, it's possible the black hole sends out a message in the form of radiation.
20:38This message contains important information about the black hole's past, the stars that formed it, and other details we thought
20:45were lost forever.
20:46It's like a secret code that tells us all about the history of the black hole.
20:51But let's go back to Hawking's theory, where a black hole can eventually disappear.
20:56It says that not only black holes produce Hawking radiation, any object with enough mass can.
21:03Researchers actually studied a process called the Schwinger Effect 2.
21:07It's when an electromagnetic field creates strong distortions, and in that way, forms matter.
21:12They applied this idea to Hawking's theory of black hole radiation.
21:15What they found is that the radiation Hawking predicted can actually be created in places with different levels of gravity,
21:22not just around black holes.
21:25Here's the key.
21:26When there are massive objects, like stars or planets, they create a curving effect on space and time.
21:32This curving is there because of their strong gravity.
21:35Even if you're far away from a black hole, there's still some massive object somewhere around that creates the curving
21:41of space,
21:42which can make you feel like you're in that twisted space.
21:46It can create radiation, similar to what happens near black holes.
21:51It means that not only black holes can slowly evaporate,
21:55other massive objects in the universe can also lose their energy in the same way.
22:01If this is true, the energy of everything in the universe will slowly be drained away in the form of
22:07light particles.
22:07That means everything and everyone, including stars, planets, black holes, and us, share the same destiny, and will all eventually
22:19fade away.
22:21This sounds scary at first, but even if this theory is true, it's not going to happen anytime soon.
22:27It would take way longer than the current age of the universe for a supermassive black hole to completely disappear.
22:33So, in the way we measure time, black holes are basically eternal.
22:39And stars could last even longer since many black holes formed after some giant star collapsed upon itself.
22:46Some others belong to a group called primordial black holes.
22:51Hawking, who became famous for talking about black holes, mentioned those ones as well.
22:56And the theory says they probably formed spontaneously in the early universe, not long after the Big Bang happened.
23:04Hawking realized that primordial black holes could have different sizes, from very light to very heavy.
23:11Really small ones would have disappeared by now because of Hawking radiation.
23:16There's a pretty cool idea that these primordial black holes could be dark matter.
23:22It's a mysterious substance that scientists think exists in the universe.
23:27It doesn't give off or reflect light, so we can't see it directly.
23:31Scientists think dark matter might explain why stars and galaxies move in strange ways.
23:36But we still don't have the tools yet to confirm whether these black holes really exist,
23:40or if they're actually made up of dark matter or not.
23:44Hawking also explored the idea that our universe is just one of many universes.
23:49It's a concept called the multiverse.
23:53Some scientists believe it could be that any situation you can imagine in your life
23:57is happening somewhere in some other universe.
24:00Hawking didn't agree with that.
24:02So in his final paper, he proposed a new mathematical framework
24:05that made the multiverse finite instead of infinite.
24:09This means that there would be a limited number of universes rather than an infinite number.
24:15Another thing that's hard to test and prove, but at least he left us with something to think about.
24:22Believe it or not, time travel is not that impossible according to the laws of physics.
24:27Scientists have equations that suggest we could have something called
24:30closed time-like curves that might allow us to go back in time.
24:35Imagine going back to the most embarrassing moments of your life.
24:39Knowing what would happen, you could avoid them.
24:42But here's the tricky part.
24:44Going back in time could cause some big problems.
24:47It could create situations where things would happen in a way that doesn't make sense.
24:52Imagine you're walking down the street and meeting your younger or older self,
24:56or accidentally changing things that have already happened.
25:00Hawking was talking about this part too.
25:02And these are all things that made him concerned about time travel.
25:06He made a guess called the chronology protection conjecture.
25:10He suggested there might be a rule of nature that stops time travel from happening because
25:15it could create all these strange and confusing situations where we would just go back,
25:20fixing our wrong decisions rather than living in the present.
25:23Until we discover traveling through time without causing such disruptions,
25:28I guess all we're left with are life lessons and learning from our mistakes.
25:33In his last years, Hawking talked about the future of humanity,
25:36and we still don't know if he was totally serious.
25:39He mentioned a special particle called the Higgs boson
25:42that could cause a big bubble that could gobble up and eventually even destroy the universe.
25:48He also mentioned things like beings from other planets coming to Earth and conquering us,
25:53or robots becoming smart enough to take over the world.
25:57Some of his ideas turned out to be true,
25:59and time will tell if it will be the same with the others.
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