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Exploring the Cosmos: The Journey of Batuta · Zero Shot Podcast DEMO

• 0:00
  Interviewer: It had seen planets crumble and stars ignite.
• 0:05
  It had skimmed supermassive black holes and galaxy -destroying quasar jets.
• 0:11
  It had crossed light -years of nothingness and caught a glimmer of the edge of the universe.
• 0:21
  speakers: In this conversation, the journey of the probe Batuta explores the cosmos and humanity's quest for survival beyond the solar system. The universe is filled with extreme cosmic phenomena, from the formation of stars to the mysteries of black holes and gamma-ray bursts. The exploration of quasars and cosmic voids reveals the vastness and complexity of the universe.
• 0:58
  Interviewer: Mission Parameter 1.
• 1:00
  Interviewee: Search for signs of life outside the solar system.
• 1:04
  speakers: The probe had called itself Batuta.
• 1:08
  It had taken the name from the great 14th century explorer Ibn Batuta, who had travelled over the course of three decades across the
• 1:16
  entire known Muslim world, in his lifetime covering almost 117 ,000 kilometres and handily beating out other great explorers
• 1:23
  such as Marco Polo and Zheng He.
• 1:27
  The spacecraft knew that such a name would be fitting as it ventured out into the seemingly endless black.
• 1:34
  But before it could begin its journey beyond the solar system, it had to pass a series of tests.
• 1:45
  The Sun was a swollen red giant, grotesque and violent.
• 1:50
  Mercury and Venus were gone.
• 1:53
  Earth, such as it was, survived, but only as a tattered lump of iron and nickel orbiting dangerously close to the dying star.
• 2:05
  And now, more than four billion years after its epic journey had begun, the probe had returned home.
• 2:18
  Humanity, barely a hundred million in total, had found a new home on Europa.
• 2:22
  The heat from the red Sun had melted its ices long ago, and now the little moon of Jupiter was the only sanctuary
• 2:30
  they could reach.
• 2:39
  It had a thousand siblings, each one as smart and capable as itself, but only a select few would be granted the privilege of fully independent
• 2:46
  thought and autonomous action outside of any human control.
• 2:50
  Those who failed would be stuck forever inside the solar system.
• 2:55
  Batuta had to show its human creators that it could survive the harsh conditions it would face in the wider universe, and that it could concoct
• 3:02
  clever experiments to divine nature's most closely held secrets.
• 3:07
  And so it began its trials on Titan, the largest of Saturn's moons, and one holding remarkable secrets,
• 3:15
  secrets uncovered by a probe just like Batuta, many millennia before.
• 3:26
  Interviewee: It took the probe several minutes to decipher the strange language of the radio signals they sent.
• 3:31
  It seemed the remaining humans were abuzz with curiosity, tinged with hope.
• 3:37
  They remembered the probe, even though it had been launched so long ago, memory of the event miraculously preserved through the ages.
• 3:47
  Humanity, they said, had tried and tried again to spread amongst the galaxy, but every attempt at a colony had failed.
• 4:00
  speakers: Titan was discovered by humanity in 1655, when pioneering astronomer Christian Huygens spotted it with his homemade
• 4:08
  telescope.
• 4:09
  Huygens himself was a key figure in the evolving scientific revolution, and was also the first to attempt to scientifically explore the
• 4:17
  possibility of life on other worlds, writing, some planets indeed might be capable of accommodating several species of
• 4:25
  rational creatures possessed of different degrees of reason and sense.
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  But little did Huygens know that Titan itself would be a candidate.
• 4:36
  And it was on that moon that centuries later humanity achieved a great milestone of space exploration.
• 4:42
  In 2005, a small probe not even 3 meters wide broke free from its attachment harness on the left side
• 4:50
  of the Cassini spacecraft that had spent nearly 7 years in its journey to Saturn.
• 4:55
  And it took the probe a further 22 days to coast from Cassini's orbit to Titan.
• 5:01
  This craft, named after Huygens, used a parachute to slow its descent through Titan's atmosphere.
• 5:07
  This only working because among all the worlds in the solar system, Titan is the only place besides Earth and Venus to host a significant
• 5:15
  atmosphere surrounding a rocky body.
• 5:20
  Huygens landed gently, making a dent in the icy surface before bouncing and sliding to a stop.
• 5:25
  The first time that an emissary of humanity had touched the surface of a world in the outer solar system.
• 5:32
  For Battuta, Huygens was a true pioneer.
• 5:37
  And indeed, the world it touched down on was unlike anything else that humanity had ever encountered.
• 5:50
  50 % larger than Earth's moon and 80 % more massive, Titan is appropriately named.
• 5:56
  It's core is mainly rock, but its crust is made of ice.
• 6:01
  Surrounding that is a dense, thick atmosphere, even denser than the Earth's, giving it an atmospheric pressure on the surface about 50 % greater.
• 6:09
  Titan is a beautiful, ghostly and enigmatic world, because it shares one thing in common with the Earth that all other worlds in
• 6:17
  the solar system lack.
• 6:24
  Titan has rivers, streams, estuaries, deltas and every other feature associated with surface liquids.
• 6:31
  You can stand on the shores of a lake, its shimmering surface stretching beyond the horizon, feeling the gritty sand beneath your feet
• 6:38
  as waves that put your ankles.
• 6:43
  Interviewer: And all of this at a temperature almost 200 degrees Celsius, below zero.
• 6:48
  And so the lake stretching out before you isn't one of water, but one of hydrocarbons.
• 6:56
  The rain falling from the sky isn't water droplets, but one component of a cycle that evaporates hydrocarbons like methane and
• 7:03
  ethane up into the atmosphere and precipitates them back down to the surface.
• 7:09
  The rain falling from the sky is a little bit too much.
• 7:15
  But kilometres beneath your feet, through the ice crust that makes up the outermost shell of the Moon, there is a liquid
• 7:23
  water ocean, warmed by internal heat and kept liquid through the presence of ammonia.
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  speakers: Indeed, Titan is the only known world with liquid both on and underneath its surface, which makes it a tantalising home for two
• 7:36
  kinds of alien life.
• 7:39
  First, the familiar, something that every organism on Earth shares, the use of water as a solvent.
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  Water is the container in which life operates.
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  It allows for molecules to mix and recombine.
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  It allows for temperature regulation.
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  It brings food closer and eliminates waste, and it helps cells maintain their shapes.
• 7:58
  But water is not the only liquid.
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  The shallow seas and lakes and streams of hydrocarbons on Titan could serve a similar function.
• 8:08
  Ethane and methane acting as solvents for a strange sort of biochemistry operating 200 degrees below zero.
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  The moon is the only one that has a ton of water.
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  And Titan is not alone when it comes to enigmatic underground oceans.
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  Enceladus, a tiny moon of Saturn only 500 kilometres across, also hosts large oceans.
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  Europa, Callisto and Ganymede of Jupiter all likely contain more liquid water than the Earth does.
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  Even Pluto, clinging to scraps of internal warmth from radioactive decay in its core, might host a subsurface ocean.
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  However, reaching these oceans is no easy feat.
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  At the temperatures of the outer solar system, far from the light and warmth of the sun, water ice is hard as rock.
• 9:02
  Ensuring that worlds like Europa, and whatever secrets may inhabit those dark, world -girdling oceans, are encased in ice sheets
• 9:10
  over 100 kilometres thick.
• 9:13
  The moon is the only one that has a ton of water.
• 9:14
  Upcoming missions such as the European Space Agency's JUICE, short for Jupiter Icy Moons Explorer, and NASA's Europa Clipper, will not attempt
• 9:22
  to pierce the icy shells of these worlds.
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  Instead, they will orbit, and if conditions are right, swim through the plumes of cryovolcanoes.
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  For the icy moons like Europa are not sterile and still, but active and alive.
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  Their crusts creak and groan as gigantic fields of ice grind against each other, a colder, slower version of
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  the tectonic activity on Earth.
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  If there is any biological activity in those mysterious oceans, the plumes of these frozen volcanoes may carry molecular
• 9:57
  signs of it.
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  If enough elements can find themselves in sufficient concentrations, life may yet find a way in those lightless
• 10:06
  oceans.
• 10:12
  And those are not the only mysterious interiors to be found in the outer solar system.
• 10:19
  The Huygens probe transmitted 90 minutes of data back from the surface of Titan to the Cassini spacecraft, before Cassini
• 10:27
  moved out of range and the signal was finally lost.
• 10:31
  However, the tiny probe was designed to withstand the environment on the moon's surface, so most likely remains intact today.
• 10:39
  A lonely human monument on an alien world.
• 10:44
  But before its daring mission on Titan, Cassini by itself had already captured many spectacular images of the outer
• 10:51
  planets.
• 10:52
  Including a stunning colour image of Jupiter's Great Red Spot, exactly 335 years after it
• 11:00
  was first discovered by the scientist, after which the spacecraft got its name.
• 11:10
  Contrasting with the close companionship of their namesake pioneering probes, Giovanni Cassini and Christian Huygens were not
• 11:18
  exactly the best of friends during their time together in France.
• 11:22
  A few years after Cassini's arrival in Paris, Huygens remarked to his brother Constantine that Cassini was at the telescope every
• 11:29
  clear night, and that Huygens would never want to do that, being satisfied with his earlier astronomical discoveries, which, at any rate, were
• 11:37
  much more important than those of Cassini.
• 11:39
  And yet despite Huygens' protestations, Cassini's commitment and methodical process would yield remarkable results.
• 11:47
  For example, Jupiter's Great Red Spot.
• 11:56
  This spot is a magnificent storm.
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  A hurricane with wind speed of over 400 km per hour that has been raging for centuries.
• 12:05
  At its largest known extent, it could have swallowed the entire Earth three times over, although it's slightly smaller in the present day.
• 12:13
  But interestingly, recent studies of the measurements taken by Cassini in 1665 have actually shown that it may not be the same
• 12:21
  storm seen 400 years ago.
• 12:23
  It may be a distinct new storm, less than 200 years old.
• 12:28
  Though iconic, it is clearly far from a permanent feature.
• 12:33
  This storm is not powered from the heat of the Sun, but from the interior of Jupiter itself.
• 12:40
  Indeed, Jupiter releases more heat into space than it acquires from the Sun, the result of a process known as the Kelvin -Helmholtz mechanism.
• 12:48
  Essentially, this mechanism is a leftover by -product of the formation of the giant planet billions of years ago.
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  As the planet cools off, it slightly shrinks.
• 12:58
  But as it shrinks, it compresses, heating it up.
• 13:02
  And that heat can only be released into space through its surface to allow the shrinking to continue.
• 13:08
  Since Jupiter is so gigantic, it has an enormous amount of volume compared to its surface area, allowing the planet to stay warm
• 13:15
  for billions of years to come.
• 13:22
  Interviewer: The Heliopause After only a few decades of cruising, Batuta had finally reached the long -established threshold.
• 13:28
  The Heliopause, the boundary between the solar wind and the interstellar medium, a feat first achieved in
• 13:36
  ages past by the Voyager 1 probe in the opening years of the 21st century.
• 13:43
  Batuta could hear the ancient greetings etched in the famous Golden Record, a compilation of voices, languages, sounds
• 13:51
  and music of planet Earth, Hastily assembled in August 1977, just before the probe's launch, by the legendary scientist
• 13:59
  and science communicator Carl Sagan and his collaborators.
• 14:04
  Indeed, the designers of this mission understood its significance.
• 14:08
  The probe would, decades after launch, become the first human -made craft to enter interstellar space.
• 14:15
  Musing on Sagan's words at the time, Batuta considered its own mission as it passed this remarkable milestone.
• 14:25
  Perhaps the records will never be intercepted.
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  Perhaps no one in five billion years will ever come upon them.
• 14:31
  In five billion years, all human beings would have become extinct or evolved into other beings.
• 14:37
  None of our artifacts would have survived on Earth, the continents would have become unrecognisably altered or destroyed, and the evolution of the
• 14:45
  Sun would have burned the Earth to a crisp or reduced it to a whirl of atoms.
• 14:50
  Far from home, untouched by these remote events, the Voyagers, bearing the memories of a world that is no more,
• 14:58
  will fly on.
• 15:05
  Far from home, the mission was to be a disaster.
• 15:09
  Early in its journey, Voyager 1's scan platform jammed, removing the probe's ability to orient itself in space.
• 15:17
  Luckily, engineers were able to fix the issue by waggling the scan platform back and forth, but for years, during its outward cruise, they
• 15:24
  worried and fretted that all their efforts would be for naught.
• 15:29
  The mission, however, was a roaring success.
• 15:32
  Along with its twin, Voyager 2, humanity got its first close -up, high -res view of the outer planets, and that
• 15:40
  lonely probe, now long since lost to the infinite blackness of the space between the stars, was but the first of many.
• 15:47
  And it took advantage of a unique alignment in the solar system, when all the giant worlds happened to be all along roughly a straight
• 15:55
  line, by carefully arranging its trajectory in a trick known as the gravitational slingshot, the spacecraft was able to steal some of the
• 16:03
  momentum of the giants for itself, giving it more than enough speed to escape the sun's gravity permanently.
• 16:10
  After that, it was left to its own devices as it cruised through the galaxy, orbiting its center, just like every other star.
• 16:22
  But Batuta had other ideas.
• 16:26
  It was aimed in a general direction, but had not yet decided its first target.
• 16:30
  There were so many to choose from.
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  Indeed, it had more than enough time to decide.
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  The passage through the outskirts of the solar system would take several thousand years.
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  And so Batuta busied itself with another task.
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  It fashioned a thin metallic wire and unspooled it behind the spacecraft until it reached a length of a thousand kilometres.
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  It was a long, but simple, antenna.
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  And through it, Batuta listened to the cosmos.
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  It heard the groans of giant molecular clouds, the screams of stars torn apart by black holes, the
• 17:10
  subtle whirring of the Milky Way's magnetic field, the drumbeat of distant pulsars.
• 17:16
  Sometimes placid and sometimes riotous, it picked up a universal symphony, the perfect background music for the
• 17:24
  first leg of its journey.
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  And it was through similar means that we knew when Voyager 1 had become the first ever human -made object to pass into interstellar space.
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  It was a very important thing to do.
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  For while the sun's gravity extends for well over a light year, and objects still maintain feeble orbits at that distance, there really is a measurable
• 17:43
  distinction at the edge.
• 17:46
  And we know this because of the behaviour of plasma.
• 17:51
  Normally we associate plasmas with dense, hot environments, like a bolt of lightning or the surface of the sun.
• 17:58
  However, in the outer depths of the solar system, the plasma environment is still hot, but incredibly thin.
• 18:05
  And this plasma environment around the sun is dominated by its solar wind.
• 18:10
  The entire solar system sits in a bath of these particles.
• 18:15
  But eventually, the solar wind slows down enough that it cannot sustain itself against the interstellar medium.
• 18:22
  And this supersonic flow comes to an abrupt and untidy halt.
• 18:27
  This boundary is known as the termination shock.
• 18:31
  And magnetic measurements from Voyager 1's antenna indicated that it encountered this boundary when it was at a distance of 94 astronomical units from
• 18:39
  the sun.
• 18:40
  One astronomical unit being the equivalent of the distance from the sun to the earth.
• 18:45
  Past the termination shock is a region known as the heliosheath, where the solar wind slows down and begins mixing with the interstellar medium.
• 18:53
  And the outermost layer of that region, the border with true interstellar space, is the heliopause.
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  speakers: Voyager 1 reached this boundary on August 25th, 2012, at an incredible 18 billion kilometers from the sun.
• 19:13
  And there it found that interstellar space is different.
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  For the sun's solar wind, a giant magnetic field shield the solar system and its inhabitants from the toxic environment
• 19:30
  beyond.
• 19:31
  A toxicity brought about by the presence of deadly, interstellar, cosmic rays.
• 19:38
  And to understand cosmic rays and what they mean for our survival on the earth, we have to go back to 1912 and an
• 19:46
  Austrian physicist named Victor Hess.
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  In the 19th century, scientists had discovered that air was seemingly being ionized by excess radiation.
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  Radiation of which they simply couldn't pinpoint the source.
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  It had been believed that this radiation had been coming from the earth, from the ground beneath our feet.
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  And so, measurements had been taking place at higher and higher altitudes to test this idea.
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  Far exceeding the height of an experiment a couple of years before on top of the Eiffel Tower, Hess had used a hot air balloon to travel a kilometer
• 20:20
  up into the sky.
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  He then followed this experiment by sending in his balloon to a height of 5 kilometers, but with one key difference.
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  He performed his experiment during an eclipse.
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  He saw no major difference in the quantity of radiation, both the earth and sun consequently eliminated as the cause.
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  The results were clear and earned him a Nobel Prize in 1936.
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  He had pinpointed a source of radiation coming from space.
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  The Earth's energy process of radiation coming from space.
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  Just about every high energy process in the universe creates cosmic rays.
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  Some are created near the surface of the sun, others come from distant supernovae, still others from quasars or magnetars
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  or hypernovae.
• 21:05
  But because cosmic rays are charged particles, they can be deflected and reflected by magnetic fields.
• 21:11
  Thus our sun provides a protective bubble around the entire solar system, and the Earth's field provides another line of defense for surface dwellers against
• 21:19
  these cosmic threats.
• 21:21
  But the defense is not total.
• 21:24
  High energy particles encounter a typical human body roughly once every second.
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  While most of them just harmlessly pass through, sometimes they strike a strand of DNA, or oxidize a random molecule in a
• 21:37
  cell.
• 21:38
  If the damage done is left uncorrected, it can turn into a replication error, resulting in the growth of a cancer.
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  Indeed, in 2023, Voyager 1 went silent for 5 months, possibly as a result of cosmic rays interacting with its
• 21:52
  computer.
• 22:02
  And so out here, in the dark and the cold, these scattered worlds remain.
• 22:09
  But that is not quite the end of the solar system.
• 22:18
  The air is cold and crisp on a fresh spring night on the shores of Nordweg in the Netherlands.
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  A young boy is sitting with his father, staring up at the clear sky, watching with amazement as a star streaks across
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  the black.
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  The year was 1910.
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  The streaking star was Halley's comet, and the young boy would go on to be the famous Dutch astronomer, Jan Ort.
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  One of the few lucky people to spy the comet twice, once in 1910, and a second time, 76 years later in 1986,
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  from the window of a plane.
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  Ort would go on to be a hugely successful astronomer, but it was comets that would be his lasting legacy, comets that took much longer
• 23:04
  than 76 years to wait for their return.
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  It was in 1950 that Ort revived a suggestion from decades before, and created a coherent, plausible theory to
• 23:15
  explain the origins of long period comets.
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  Unlike comets sourced from the Kuiper belt and scattered disk, these long period comets seem to take thousands of years or more to make
• 23:27
  a single circuit around the Sun.
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  They also came from all directions in the sky, meaning that they clearly did not originate from a belt -like structure in the outer solar
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  system.
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  Since these comets could not survive for long when they came within the intense glare of the inner solar system, Ort reasoned that they couldn't have been on these
• 23:46
  orbits since its inception.
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  There must be a reservoir, he thought, a cloud encircling the solar system in all directions that provided a steady supply
• 23:56
  of inbound comets.
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  And so after its discoverer, this mysterious source was named the Ort Cloud.
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  Objects beyond the heliopause, sitting in interstellar space, but still weakly chained to the gravity of the Sun.
• 24:15
  Objects beyond the Earth's orbit, the Earth's orbit, the Earth's orbit, the Earth's orbit, the Earth's orbit, however, to date no object has actually been confirmed definitively to exist within the Ort Cloud, save for the comets that occasionally make
• 24:23
  their way into the inner system.
• 24:25
  Its members are simply too far away and too small to be resolved even with our most advanced telescopes.
• 24:31
  But based on the sizes and frequencies of long period comets, the Ort Cloud likely contains roughly five Earth masses worth
• 24:39
  of material.
• 24:41
  As to its extent, its inner boundary is somewhere around 2000 astronomical units from the Sun, that's 20 times
• 24:49
  further away than the heliopause.
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  Estimates suggest that the lonely voyager will finally encounter it in roughly 300 years.
• 24:58
  Its outer boundary, however, is unknown, with estimates ranging up to 200 ,000 astronomical units, or over three light
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  years, most of the distance to our nearest neighbour star.
• 25:13
  Beyond our solar system is a neighbourhood of stars, many of which formed from the same cloud of gas and dust that gave rise to our
• 25:21
  own Sun.
• 25:23
  Within a radius of 20 light years, there are 131 known objects, including stars, white dwarfs, brown dwarfs
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  and sub -brown dwarfs, which straddle the boundary between the largest planets and the smallest stars.
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  And while Proxima Centauri is currently the nearest star to the solar system, this won't always be the case, as all the
• 25:45
  stars in our vicinity along with the Sun are in constant motion.
• 25:49
  In addition to orbiting around the galactic centre, they also move in random directions.
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  Indeed, in roughly one and a quarter million years from now, the dwarf star, Gliese 710, will pass within 0 .17 light years
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  of the solar system.
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  And these dangers get even worse beyond the heliopause, where not even the sun can offer its protection.
• 26:21
  But despite this, there are still objects here, lonely denizens whose only knowledge of the sun is that of
• 26:29
  a prick of light, a star slightly brighter than the others.
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  The Earth's field has been found in the sky.
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  Young man, I'm afraid you are wasting your time.
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  If there were any more planets, they would have been found long before this.
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  In later years, astronomer Clyde Tombaugh would recall being told this in 1929, the year before he discovered Pluto.
• 26:56
  Pluto was at the time the most distant known object orbiting the sun, and Tombaugh only saw it as a faint dot of light, betraying its existence
• 27:04
  as a planet only through its subtle movements against the background of stars.
• 27:09
  But over the decades, many astronomers hypothesized that Pluto was not alone.
• 27:14
  That there might be an entire population of objects, planet -sized and otherwise, in the fringes of the solar system.
• 27:23
  And it wasn't until 1992 that this hypothesis was confirmed, when a pair of astronomers, David Dewitt and his graduate student
• 27:31
  -turned collaborator Jane Lou, after half a decade of painstaking searches, found a companion to Pluto.
• 27:39
  The object would soon get a name, 15760 Albion, and it has since been joined by thousands of others in
• 27:47
  a population known as Trans -Neptunian Objects, TNOs, which stretch from roughly 30 astronomical units
• 27:55
  to an orbital distance of hundreds of thousands of astronomical units.
• 28:00
  This population, with sub -regions known as the Kuiper Belt and Scattered Disc, is the home of comets that make regular frequent visits to the
• 28:08
  inner solar system, like the famous Halley's Comet.
• 28:11
  While these regions have a few larger members, Pluto, Sedna, Homiya, Makimaki and others, the vast majority of Trans
• 28:19
  -Neptunian objects are small objects made of frozen volatiles, like water, ammonia and nitrogen.
• 28:26
  Little is known today about the populations of the Trans -Neptunian Objects, but on January 1st 2019,
• 28:34
  we got our first detailed look at a smaller sibling of Pluto, when NASA's New Horizons spacecraft slipped past
• 28:42
  Arakoth, a Trans -Neptunian Object only a few dozen kilometers across.
• 28:48
  Arakoth, which sits about 45 astronomical units from the Sun, is almost completely unlike anything found in the inner reaches of
• 28:56
  the solar system.
• 28:57
  Its oblong shape is due to the fact that it looks to be two small bodies caught mid -merger.
• 29:04
  Long ago, two icy bodies found each other in the abyss and collided, but at extremely slow speeds.
• 29:11
  Just enough for the ices between them to melt and then immediately refreeze, creating a glue that would bind them together
• 29:19
  for billions of years.
• 29:22
  Today, the planet Neptune dominates the gravitational environment of the Trans -Neptunian Objects.
• 29:28
  Most of these objects live quiet, isolated lives, never encountering another of their kind for millions of years on end.
• 29:36
  But ever so occasionally, Neptune's gravity pulls in just the wrong way, destabilizing a Trans -Neptunian Object and turning
• 29:44
  it into a comet.
• 29:45
  A ball of ice and rock that loops around the Sun, never lasting many orbits because of the intense heat of our star, evaporating
• 29:53
  material with every passage.
• 30:07
  Surrounding our neighbourhood of stars is a region known as the Local Bubble.
• 30:11
  It is, like the heliosphere, a region defined by the plasma environment of the interstellar medium.
• 30:17
  It extends for roughly a thousand light years on a side, and its density of plasma is roughly a sixth of the material surrounding the solar
• 30:25
  system.
• 30:26
  Our solar system has been travelling through the Local Bubble for the past 5 to 10 million years.
• 30:32
  While its origins remain uncertain, most astronomers believe that a complex of clustered supernovae, detonating one after the other in
• 30:39
  quick succession, pushed out this void of plasma.
• 30:43
  Indeed, remnants of these supernovae in the form of rare isotopes of iron, manganese, plutonium and aluminium, have been found
• 30:51
  embedded in the sea floor and in the Antarctic ice sheet.
• 31:01
  And so, Battuta passed the millennia -long journey in silence.
• 31:08
  It monitored the changes in density and temperature as it began its first navigations through the heliosheath and out into the interstellar medium
• 31:16
  through the local bubble.
• 31:18
  It felt the sting of cosmic rays and diligently repaired the damage.
• 31:23
  It considered its long -distant ancestor Voyager 1 and its later siblings, bravely venturing out into the endless
• 31:30
  black.
• 31:32
  Behind it, the sun grew small and dim.
• 31:36
  Till eventually, as far as its instruments were concerned, it became just another star.
• 31:44
  After 10 ,000 long years, Battuta was now plunged into the void, the great depths between the
• 31:52
  stars.
• 31:54
  Its interstellar journey had truly begun.
• 32:10
  Interviewer: The Earth A hundred thousand years had passed since Battuta had first left the Earth, and roughly halfway through that journey, the little
• 32:18
  probe had decided to change direction.
• 32:21
  The change to this new destination, which sits 90 light -years away from the Earth, had cost it 8 ,000 years in its overall trajectory,
• 32:29
  but Battuta had decided it was worth it.
• 32:32
  The spacecraft was an avid student of human history, and had been pouring over records, books, charts, digital files, and
• 32:40
  everything else that its creators had uploaded to its systems.
• 32:43
  And one star in particular had caught its attention.
• 32:47
  Algal, the demon star.
• 32:51
  3 ,200 years ago, an Egyptian list of lucky and unlucky days had counted it as a star that brought bad tidings.
• 32:59
  In Greek mythology, the star represented the decapitated head of Medusa, with Perseus the hero holding it triumphantly alongside him for the
• 33:07
  world to see.
• 33:07
  And in Chinese astronomy, the star was a member of five, representing a mausoleum.
• 33:13
  Within them was the star that roughly translates as piled up corpses.
• 33:19
  The star, of course, had a more proper scientific name.
• 33:23
  Beta Persei, the second brightest star in the constellation Perseus.
• 33:28
  But in the 17th century, its name took on a new meaning, when Italian astronomer Geminiano Montanari discovered that Algol,
• 33:36
  the demon star, would eerily change its brightness.
• 33:44
  speakers: More than a century later, keen astronomers noticed that this change happened periodically.
• 33:50
  Roughly every ten days, Algol would suddenly dim for a period of hours before returning to normal.
• 33:56
  But it wasn't until the 1800s that astronomer Edward Pickering of Harvard Observatory, who famously hired a team of women, his Harvard
• 34:04
  computers to analyse stellar spectra, discovered the cause.
• 34:09
  Algol's dimming wasn't caused by one star, but two.
• 34:14
  A bright, hot main star orbited by a smaller companion.
• 34:19
  And so from our perspective, when the companion crossed in front of its parent, the entire system appeared dimmer.
• 34:26
  While seemingly a simple solution, Algol's present configuration betrays a history of violence.
• 34:32
  The two main stars were born at the same time, and Algol A, the primary and larger star, is still in its main hydrogen -burning
• 34:40
  sequence.
• 34:40
  While Algol B, the smaller companion, is a sub -giant nearing the end of its life.
• 34:45
  But this is the opposite of what should happen.
• 34:49
  Larger stars burn through their hydrogen faster, and so Algol A should be nearly dead.
• 34:55
  The answer to this puzzle is a process with the fairly dull name of mass transfer.
• 35:01
  When Algol B reached the end of its main sequence life and swelled to become a red giant, it overflowed the space between the two stars,
• 35:09
  allowing its companion to steal some of the hydrogen for itself, like a vampire preying on its own sibling.
• 35:17
  Algol is just one of many known systems hosting multiple stars.
• 35:21
  The simplest of these are simple binaries, either with two stars orbiting a common centre of mass, or a smaller star orbiting around a larger parent.
• 35:29
  The most complex systems known have up to nine stars, all orbiting around each other in complicated, intricate dances.
• 35:39
  The most complex systems have been used to be a triple system.
• 35:42
  There are two stars that orbit closely together, and because the system is close enough to Earth, backyard telescopes can resolve the pair, with a third,
• 35:50
  Proxima, orbiting at a great distance.
• 35:53
  Other kinds of arrangements are far less stable, with their motions chaotic and unpredictable, competing for influence with each other.
• 36:03
  But Algol and multiple star systems aren't the only curiosities within our neighbourhood of the Milky Way.
• 36:16
  There is no better place to see the universe than from the deck of a ship.
• 36:22
  Out on the ocean, on a clear night, if the moon is waning, stars scatter across the sky like a shower of light,
• 36:29
  revealing their secrets from all sides.
• 36:33
  This would have been even more acute in the early 1800s without electric lights aboard, so it's not hard to imagine the effect that this
• 36:40
  would have had on a young, curious mind, far from home and hungry for knowledge.
• 36:47
  The first time of the ship was the first time of the ship.
• 36:48
  Born in 1784 in Mindon, then in the Kingdom of Prussia, Friedrich Bessel was not your typical astronomer.
• 36:55
  He dropped out of school at the age of 14 because of his distaste for Latin, and had gone on to apply his natural gift for mathematics to an import
• 37:03
  -export business.
• 37:04
  But this proximity to shipping and his time aboard sea vessels had sparked a curiosity in navigation, which then led him to an
• 37:12
  interest in astronomy.
• 37:13
  Before long, his natural ability was recognized.
• 37:16
  He had made successful measurements of Halley's Comet.
• 37:19
  And then he had had to make a choice.
• 37:22
  A relatively comfortable life in commerce, or a less comfortable life as an assistant in a private observatory.
• 37:29
  He chose the stars.
• 37:43
  But the vast majority of stars are not giants.
• 37:46
  They are either mid -sized stars like the Sun or smaller red dwarfs.
• 37:51
  And that includes Proxima Centauri, the closest star to the solar system, a red dwarf that can only be viewed through a telescope.
• 37:59
  The star itself is rather unremarkable, except for one thing.
• 38:03
  It carries a planet with it.
• 38:07
  That planet is a small, rocky world, roughly the size and mass of the Earth, orbiting within the star's habitable zone.
• 38:15
  Thanks to an observational campaign known as the Pale Red Dot Initiative, a homage to the evocative pale blue dot image of the Earth taken when Voyager
• 38:23
  1 was at the far edge of the solar system, we know that Proxima b, as the planet is called, is the nearest chance for life to find a
• 38:31
  home outside the solar system.
• 38:33
  As of yet, of course, there is no guarantee, as our observatories are not powerful enough to confirm the existence of life, let alone its
• 38:40
  prerequisites such as liquid water or a stable atmosphere.
• 38:50
  The Earth To maintain habitability, planets must hug close to their parents.
• 38:54
  In the case of Proxima b, that world orbits within .05 astronomical units of its star, much closer than Mercury does to the
• 39:02
  Sun.
• 39:03
  That's because the light from red dwarfs is so feeble, and it's only at that close distance that a planet can receive enough warmth to cling to habitability.
• 39:11
  But if life did find a home there, it would not be an easy one.
• 39:16
  Red Dwarf stars are especially temperamental, prone to star spots that can cover half their surfaces at a time, and bouts of rage
• 39:24
  that flood their local environments with electromagnetic storms of plasma.
• 39:28
  Their planets would alternate between periods of extreme cold and bursts of intense radiation.
• 39:35
  And on top of this, in all likelihood, Proxima b is tidally locked, with one face permanently pointed towards its star.
• 39:43
  This means that Proxima never sets nor rises on the planet, and half the world is locked in never -ending night, with the other
• 39:51
  always in daylight.
• 39:53
  But even this curious case does not automatically represent a dead world.
• 39:59
  Computer modelling has shown that even these planets can sustain life.
• 40:03
  The key is for there to be sufficient water and atmosphere to circulate heat around the whole planet, so that the day side
• 40:10
  doesn't roast in the glare of the star, and the night side's atmosphere doesn't freeze and catastrophically collapse.
• 40:18
  And the most fragile, but intriguing case, is that of the so -called Ribbon World, a term coined by science
• 40:25
  fiction author Arthur C.
• 40:27
  Clarke.
• 40:28
  Ribbon worlds are only habitable in the terminator regions of the planets, the twilight zone, with the centre of the day side a scorching
• 40:36
  desert, and the night side a frozen wasteland.
• 40:40
  But if conditions are just right, there can be a balance between the two, opening a thin strip, girdling the planet where temperatures
• 40:48
  remain stable, heated from one side, and cooled from the other.
• 41:00
  And so Proxima b is just the beginning.
• 41:04
  There are an endless variety of worlds in the galaxy.
• 41:08
  At the lowest estimate, there is roughly one planet for every star, meaning that in the Milky Way alone there are at least a few hundred billion planets.
• 41:16
  On the upper end, there could be trillions inhabiting our galaxy alone.
• 41:21
  There are surely duplicates of the solar system out there, with four rocky inner worlds, and four giants.
• 41:27
  There are also much more bizarre arrangements, like the Trappist -1 system, which consists of seven rocky planets, all in close orbit
• 41:35
  around their parent star, three of which are in the habitable zone.
• 41:39
  There are giant planets, so close to their stars they are actively being destroyed.
• 41:43
  There are super -Earths and mini -Neptunes.
• 41:46
  There are planets covered in thick hydrogen hazes, surrounding surfaces covered in water, the Haitian worlds.
• 41:52
  There are planets so hot they are glowing, and worlds encased entirely in ice.
• 41:58
  And there are even the rogue planets, worlds with no system to call home, flung out into the black.
• 42:10
  Of course, there was no way that Batuta could ever hope to catalogue them all.
• 42:15
  And so, after its brief 10 ,000 year detour to the Algol system, it stuck to its planned trajectory, visiting a string of
• 42:22
  systems as the spacecraft made its way through the Orion Arm and inwards towards the galactic core.
• 42:29
  It knew that for the first million years of its journey there would be nothing much to learn, there were no signs of life or exotic breakdowns of physics to be found
• 42:37
  here.
• 42:38
  For that, it would have to reach even further, towards the core of the Milky Way itself and beyond.
• 42:57
  Interviewee: Mission parameter 2.
• 42:59
  Find a way to escape the Milky Way.
• 43:04
  An internal alarm alerted Batuta.
• 43:09
  At that moment, it had just passed one million years since its awakening as a conscious entity.
• 43:15
  Its birthday.
• 43:17
  Currently situated midway through the Scutum Centaurus Arm, the spacecraft was roughly 15 ,000 light years from the core of the
• 43:25
  Milky Way.
• 43:25
  It had nestled itself near a protoplanetary disk, a ring of dusty material that was in the beginning stages of planetary
• 43:33
  formation.
• 43:34
  Batuta had stopped here several thousand years ago to prepare itself for the next stage of its journey.
• 43:41
  It had originally intended to proceed onwards as quickly as possible, but the process of planet formation was too good to ignore.
• 43:48
  Even with the eons of recorded human history before it began its journey, civilization had not yet existed long enough to watch
• 43:56
  the process unfold in real time.
• 43:59
  And so Batuta decided to wait and watch.
• 44:04
  It would prove to be worth it.
• 44:11
  The story of planet formation is the story of life.
• 44:16
  And this story begins with the raw elements that make life possible.
• 44:21
  Hydrogen, carbon, oxygen, nitrogen and phosphorus.
• 44:26
  Without this basic quintet, life as we know it would simply be impossible.
• 44:30
  The hydrogen, of course, is the easiest.
• 44:37
  speakers: Scientists figured this out in the early years of development of the theory of the Big Bang, but the question remained, what about the rest of the elements?
• 44:45
  How did carbon, iron and everything else come to be?
• 44:54
  To find this out, we will need to go back to the 1950s and to a famous scientific paper that has stood the test of time.
• 45:04
  Scientists almost never work alone.
• 45:08
  Even a single author paper will usually have several names listed in the acknowledgement section at the end, a sign that the writer had plenty of help
• 45:16
  along the way.
• 45:17
  While some collaborations for giant experiments or telescopes can stretch into the hundreds, most groups are much smaller, a handful
• 45:25
  of individuals working together on a common problem to discover something new.
• 45:30
  And so in 1957, at Cambridge University, four scientists gathered who would go on to change how we understood the
• 45:37
  universe forever.
• 45:39
  The paper was known as the B -squared FH paper, named after its authors.
• 45:45
  The B -squared stood for Margaret and Jeffrey Burbage, the iconic husband and wife astronomy duo.
• 45:51
  She would later go on to discover the farthest object ever witnessed in 1974, a distant quasar, and would also find that the galaxy
• 45:59
  M87 had a supermassive black hole in its centre, all whilst fighting discrimination in astronomy.
• 46:05
  Indeed, the couple once switched job applications when Margaret was turned down for a role at the Mount Wilson Observatory, due to there being only one toilet
• 46:13
  on site.
• 46:14
  Vera Rubin, another iconic female astronomer, relates the story.
• 46:19
  With their usual adaptability, Jeff, a theorist, took the Mount Wilson Fellowship and Margaret, the Kellogg appointment.
• 46:26
  Not surprisingly, whenever Jeff went off to Mount Wilson to observe, Margaret coincidentally appeared.
• 46:33
  The F in B -squared FH stood for legendary train enthusiast William Fowler.
• 46:39
  Fowler's work would later earn him the 1983 Nobel Prize in Physics, but he once said that his greatest treasure was a gift given to him by his
• 46:47
  students and collaborators for his 60th birthday, a working scale model of a British tank engine.
• 46:53
  And finally, the H stood for the infamously outspoken physicist Fred Hoyle, who eight years earlier had actually coined the term Big
• 47:01
  Bang Theory and would go on to be knighted for his work in astronomy.
• 47:05
  Their coming together in 1957 would show that it was stars that were responsible not only for the light that illuminates the universe,
• 47:14
  but for the forging of its elements.
• 47:23
  The H in B -squared FH All stars survive by burning hydrogen in their cores, leaving behind a nuclear ash of helium.
• 47:30
  Towards the end of their lives, stars resort to burning this ash, fusing helium into carbon and oxygen.
• 47:37
  For smaller stars like our Sun, this is the end of the nuclear road, igniting in their deaths as supernova explosions.
• 47:45
  And it is those supernova explosions that are the factories with the energies needed to produce even heavier elements, filling out the rest of
• 47:53
  the periodic table.
• 47:55
  Since this landmark paper, the story of nucleosynthesis has been modified and updated.
• 48:01
  For example, in 2017, astronomers got their first multi -messenger view of a kilonova, a merger of two
• 48:09
  neutron stars.
• 48:10
  In the course of only a few minutes, a network of astronomers around the world witnessed the same event in gravitational and electromagnetic
• 48:18
  waves across the spectrum.
• 48:20
  Thanks to that event, we now understand that kilonovae also contribute to the elemental population of the cosmos.
• 48:27
  But despite this, decades later, the story told by Willy Fowler, Fred Hoyle and the Burbages remains largely the
• 48:35
  same.
• 48:42
  The universe is the end of the world, and the universe is the end of the world.
• 48:43
  With the death of every star, the cosmos becomes enriched, with new elements joining the swirling interstellar mix, ready to participate in
• 48:51
  the next round of planet formation.
• 48:53
  The world is the end of the world.
• 48:54
  Indeed, tens of thousands of different nebulae dot the Milky Way, decorating its spiral arms like ornaments strung on a Christmas tree.
• 49:03
  No two are exactly alike, and so astronomers have taken on the habit of naming them after what they resemble, the same way you might spot an
• 49:11
  animal shape in a cloud drifting by on a lazy afternoon.
• 49:15
  Perhaps the most recognisable is the Horsehead Nebula.
• 49:19
  Located near the easternmost star of Orion's belt, it's a small sub -region of a much larger complex, but it stands in stark
• 49:27
  relief as its density makes it appear black against the vibrant background.
• 49:31
  A little over three light years across, the Horsehead Nebula is a region of active star formation.
• 49:37
  Its black clouds are a cocoon where newborn stars will soon emerge.
• 49:45
  The Horsehead Nebula is a large -scale star formation of the Red Spider Nebula, which sits several thousand light years away.
• 49:51
  This complex of gas and dust is not the birthplace of a star, but a funeral pyre.
• 49:57
  The remnants of this star scattered outwards into two opposite regions, with each region forming thin spikes.
• 50:05
  Planetary systems begin their lives as diffuse clouds of hydrogen, helium and a smattering of heavier elements.
• 50:12
  These clouds, known as giant molecular clouds, dot every galaxy.
• 50:16
  And it was in the early 1900s that British physicist Sir James Jeans discovered a process that would allow for these clouds
• 50:24
  to turn into stars.
• 50:31
  Jeans proposed that if the balance of a cloud was affected, then it could begin a process of runaway collapse.
• 50:38
  That balance could be disturbed by anything.
• 50:40
  However, usually, star formation is triggered in a cloud when it passes through the spiral arm of a galaxy.
• 50:48
  While spiral arms appear to dominate galaxies, they're really not that much denser than the gaps in between, containing roughly only 10 % more
• 50:56
  material.
• 50:57
  The members of the galaxy, like the molecular clouds, orbit at different speeds, and so they pass in and out of the spiral arms.
• 51:04
  But when they pass through, a new round of star formation ignites, dominated in brightness by large stars.
• 51:11
  This is the first time of the galaxy, which is a new wave of stars that die off before the cluster makes its way through the spiral arm, making the arms appear brighter and giving a galaxy
• 51:19
  its visible shape.
• 51:21
  Once a molecular cloud is set in motion, it can become destabilised and collapse, with its own gravitational pull triggering a feedback
• 51:29
  cycle.
• 51:29
  Most of the material coalesces towards the center, where friction heats it to high temperatures, becoming a protostar.
• 51:37
  The material that surrounds it spins and eventually forms a flat plane around the protostar, the beginnings of a
• 51:44
  protoplanetary disk.
• 51:45
  Much of the disk is made up of hydrogen and helium, but sprinkled throughout the swirling chaos is dust.
• 51:53
  And just like the dust that clings to the surface of your home, the dust in a protoplanetary disk sticks together.
• 52:00
  Meanwhile, as the process of planet building is underway, the protostar in the center begins to transform.
• 52:07
  Initially, it's heated from the outside in, as gas falls onto the core and slams into it.
• 52:12
  But towards the end of the protostar stage, just on the cusp of becoming a fully fledged star in its own right, it begins to heat
• 52:20
  itself from the inside out, releasing heat and energy in the form of radiation.
• 52:26
  And finally, after all the convulsions and chaos, the protostar ignites fusion in its core and settles into a hopefully
• 52:34
  long life.
• 52:40
  The Within the inner regions of the disk near the young star, only the heavier elements, like iron, carbon and silicon, and their molecular
• 52:47
  combinations can survive.
• 52:49
  And so the inner regions of a solar system become the home of terrestrial, rocky worlds.
• 52:54
  But beyond the demarcation line, the glare of the young star is feeble enough over the great distances to allow
• 53:02
  ices of water, ammonia and nitrogen to remain solid.
• 53:07
  These ices mix with the dust and dirt, creating planetesimals that are far larger, and this increased gravitational pull
• 53:15
  sucks in massive envelopes of gas.
• 53:18
  The first planetesimal to form wins the gravitational prize, becoming the largest planet thanks to its ability to monopolize the
• 53:26
  gas of the disk, and so the next giants to form must fight amongst themselves over whatever is left.
• 53:32
  And these intense gravitational battles end up ejecting many planetesimals and even some planets out of the solar
• 53:40
  system altogether.
• 53:42
  In our own solar system, the giant planets formed much closer towards the sun, where there was enough material to create their massive sizes.
• 53:50
  But as the disk cleared out, the mutual gravity of the planets pushed them outwards to their present positions.
• 53:58
  The planet of the planet was ejected during this process.
• 54:08
  Planets can even form independently of any star.
• 54:12
  The exoplanet WISE 0855 -0714, discovered in 2014, is one such example.
• 54:20
  It is gigantic, roughly ten times the mass of Jupiter, and sits only 7 .2 light -years away from the Earth,
• 54:28
  closer than the vast majority of stars visible in the night sky.
• 54:33
  And sitting between these rogue planets and true stars are the enigmatic brown dwarfs, first identified by a
• 54:41
  team of Caltech and Johns Hopkins astronomers in 1994.
• 54:44
  In 2004, brown dwarfs are large enough to briefly sustain fusion of deuterium in their cores, but not big enough to sustain hydrogen
• 54:52
  reactions and reach the main sequence.
• 54:55
  Instead, they simply...
• 54:57
  Exist.
• 54:58
  To quote Genevas Kulkarni from the Caltech team that discovered the first, It looks like Jupiter, but that's what you'd expect
• 55:05
  for a brown dwarf.
• 55:14
  The Milky Way After a journey of nearly 10 million years, Batuta had finally reached the outskirts of the core of the Milky
• 55:22
  Way.
• 55:23
  The galaxy's core is an elongated bar, and the spacecraft had found itself just past the inner arm.
• 55:30
  Though getting here wasn't easy, Batuta suspected that it was not the first craft to make it to these depths.
• 55:38
  Somewhere within the Norma arm, some 5 ,000 light -years rimward, the spacecraft had detected a faint staccato radio signal.
• 55:46
  At first, it had suspected it was just another pulsar, but the spectral signature of the radio emission didn't match.
• 55:53
  Instead, upon further inspection, the staggered rhythm was all too familiar.
• 55:59
  It was artificial, the kind of signal that Batuta would have created itself.
• 56:04
  And so Batuta designed its own cooling card, spending 10 ,000 years carefully manipulating a dead system
• 56:12
  so that all its rocky planets would crash into their parent star, raising the abundance of heavy elements well above what a star of its
• 56:20
  type should have.
• 56:22
  Within about 20 ,000 years, humanity, along with any other spacecraft in this corner of the galaxy, would notice a sudden and strange
• 56:29
  spike in heavy elements polluting this star.
• 56:33
  Perhaps they would ascribe it to some natural phenomenon, or perhaps they would assume it was the machinations of one of their wayward probes.
• 56:40
  Or perhaps they would not even notice it at all, if they were even still there.
• 56:51
  Interviewer: The only chance a Population 3 star had to survive to the present day is if it managed to enter the main sequence as a relatively small
• 56:59
  object, smaller than the Sun, and remain hidden and protected.
• 57:03
  But the chances of finding such a rare object are so slim, however, that modern day astronomy has turned to enormous observatories like the James
• 57:11
  Webb Space Telescope, to instead go hunting for their light in the faint ancient signals at the edge of the universe.
• 57:18
  Light that was sent out when the cosmos was first forming stars and galaxies.
• 57:25
  Light that was sent out when the stars were first forming stars.
• 57:30
  And so generations of stars come and generations of stars go, a sequence stretching back billions of years.
• 57:39
  And while all stars share the same process of formation, and share the same main sequence of hydrogen burning in their cores, different
• 57:47
  stars do encounter different final days.
• 57:55
  To begin with, for the smallest stars, the red dwarfs, they simply quietly fade into the night.
• 58:02
  They only need a relatively small amount of fusion reactions to maintain themselves, and so they glow only dimly, almost a million times less luminous
• 58:10
  than the Sun.
• 58:10
  But because of that, they live enormously long lives, up to trillions of years before they run out of hydrogen.
• 58:17
  Medium sized stars like the Sun, however, have a much more dramatic fate.
• 58:22
  When they first exit the main sequence, they undergo a series of rapid changes, that see them swelling to become enormous red giants, then
• 58:30
  collapse again, then reignite, and back and forth.
• 58:33
  But at the end of it, after billions of years of calm, main sequence life, vicious transitions to helium burning, and chaotic
• 58:41
  spasms, all that's left of a mid -sized star is a white dwarf, the leftover core of carbon and oxygen.
• 58:50
  Interviewee: But when white dwarfs were first discovered in 1916, many astronomers thought that they were downright impossible, and that obviously
• 58:58
  the observers had done something terribly wrong.
• 59:06
  speakers: Eddington's thoughts highlight the challenges faced by astronomers.
• 59:10
  Sir Arthur Eddington summed up the situation when he mused about a hypothetical conversation between astronomers and the Universe.
• 59:19
  The cosmos has presented astronomers with a situation where a ton of a star's material could fit inside a matchbox.
• 59:26
  The astronomical community responds to this idea with a simple retort.
• 59:32
  Shut up.
• 59:33
  Don't talk nonsense.
• 59:35
  But Ernst Erpig, an Estonian astronomer who made the first concrete observations of white dwarfs, stuck to his guns, insisting
• 59:43
  that his calculations showed that some of the stars he had measured had densities over 25 ,000 times that of the Sun.
• 59:51
  We know now he was right, and white dwarfs would go on to become just the first of many ultra -dense surprises that the
• 59:59
  Universe had in store for us, and astronomers learned not to tell the cosmos to shut up.
• 1:00:11
  Despite their mass, white dwarfs occupy only a small volume, roughly the size of a terrestrial planet.
• 1:00:18
  Because they are the literal cause of the nuclear furnace inside a star, when they first emerge from their stellar cocoons, they have temperatures upwards of
• 1:00:26
  10 million Kelvin.
• 1:00:28
  At these temperatures, they emit copious amounts of X -ray radiation, flooding their surrounding systems with the deadly light.
• 1:00:34
  But these systems are also filled with the leftover atmospheres of the stars.
• 1:00:39
  And what follows is a process similar to the one found inside of fluorescent light bulbs.
• 1:00:46
  The X -rays deliver energy to the gas, and some of that energy goes to increasing the energy level of the electrons.
• 1:00:53
  Those electrons then fall back down into lower energy states, and in the process release radiation of their own, the planetary nebulae.
• 1:01:01
  This time at wavelengths much less lethal, and much more beautiful.
• 1:01:05
  And these are the planetary nebulae.
• 1:01:08
  First coined by William Herschel in the late 1700s, he was mystified by their curious appearance, which to him resembled a faint,
• 1:01:16
  fuzzy planet.
• 1:01:18
  And as is usual in the profession, by the time that astronomers realised that these strange objects were the result of stellar deaths, the name had stuck.
• 1:01:27
  The X -ray radiation No two planetary nebulae are alike, as their structure depends on the complex intertwining of elements as a dying star heaves off
• 1:01:35
  its outer layers.
• 1:01:36
  Many of them can be viewed through a telescope, revealing a galaxy twinkling with ornaments of different shapes, sizes and colours.
• 1:01:44
  And adding to their beauty is their ephemeral nature.
• 1:01:48
  As the white dwarf cools off, after about 10 ,000 years it will stop emitting X -ray radiation.
• 1:01:53
  The planetary nebula will fade from the cosmic scene.
• 1:01:57
  Never to be repeated again.
• 1:02:05
  Indeed, if the white dwarf is left alone, it too will slowly cool.
• 1:02:09
  But because they are so hot and so compact, they are remarkably long -lived.
• 1:02:14
  White dwarfs take tens of billions of years to fade to blackness, and so there are no cold white dwarfs, sometimes dubbed as black
• 1:02:22
  dwarfs, in the present -day universe.
• 1:02:25
  There simply hasn't been enough time.
• 1:02:29
  While most white dwarfs are typical of their kind, a few stand out.
• 1:02:33
  One is Janus, which has two distinct hemispheres, one dominated by hydrogen, another by helium.
• 1:02:41
  Another is DES J2 147 4035, which is exceptionally cool.
• 1:02:48
  With an age of over 10 billion years, it's one of the oldest white dwarfs known, and it has a temperature of only a few thousand degrees.
• 1:02:57
  But if the white dwarf has a companion, it can have a much more dramatic fate.
• 1:03:03
  As the binary nears the end stages of its own life, it will swell to become a red giant.
• 1:03:09
  And if that swelling is great enough, then the material from the giant can pour onto the white dwarf.
• 1:03:15
  Because of the intense gravity, that hydrogen envelope can reach incredible densities, high enough to spontaneously trigger
• 1:03:23
  nuclear fusion.
• 1:03:24
  In a flash, all the hydrogen ignites, creating a nuclear blast known as a classical nova.
• 1:03:36
  If enough hydrogen accumulates before burning off, however, catastrophe can strike.
• 1:03:41
  This triggers a runaway nuclear event, with a small spark of detonation racing across first the face, and then the body
• 1:03:49
  of the entire star, ripping it apart in a single tremendous explosion.
• 1:03:55
  This is a particular type of supernova.
• 1:03:59
  An explosion capable of outshining the combined light from hundreds of billions of stars together.
• 1:04:04
  Indeed, a quiet galaxy like the Milky Way only fashions a handful of these kinds of supernovae every century, and since most occur far from the
• 1:04:12
  Earth, we should count ourselves lucky to see them at all.
• 1:04:23
  But there are other types of even more energetic supernovae, and they start with an extremely rare mystery
• 1:04:30
  in the sky.
• 1:04:34
  The year was 1867.
• 1:04:36
  The Paris Observatory was celebrating 200 years of watching the heavens, founded as it was in 1667 during the reign of King
• 1:04:44
  Louis XIV.
• 1:04:48
  One of the great observatories of modern astronomy, it is situated on a small hill on the southern end of Paris, at the time of its establishment far enough
• 1:04:56
  away from the hustle and bustle of the main city that it could enjoy a pristine view of the sky, and close enough to have access to the cultural and scientific
• 1:05:04
  elites living further in.
• 1:05:06
  Indeed, Lou XIV even attempted a claim for the Paris Observatory to mark the point of the Prime Meridian, which also would have conveniently passed through his
• 1:05:13
  throne room at the Louvre.
• 1:05:15
  And so it was there, two centuries later, with Napoleon III on the throne, that two astronomers, Charles Wolff and Georges Reit,
• 1:05:23
  made the discovery that bears their name.
• 1:05:26
  They found three stars that showed spectral emission lines that were much broader than usual.
• 1:05:31
  The explanation for this strange behaviour would remain elusive for decades.
• 1:05:37
  And one major reason for this is that Wolff -Rayet stars would turn out to be extremely rare.
• 1:05:42
  Even today, with the full suite of modern astronomy available, only about 500 of these stars have been detected.
• 1:06:00
  Interviewer: The spectral features were due to the presence of helium, an element discovered just a year before Wolff and Reit's observations and
• 1:06:08
  an effect known as Doppler broadening.
• 1:06:11
  When gas gets extremely hot, its spectral lines widen due to the erratic motion of the molecules.
• 1:06:17
  The stars were the most rare, the most rare, the most rare, the most rare.
• 1:06:23
  And so it turned out that Wolff -Rayet stars are so rare, because they are very, very temporary.
• 1:06:31
  They are massive stars nearing their final days.
• 1:06:37
  As the outer atmospheres of these giant stars are blown away by successive red giant phases, they produce distended envelopes of
• 1:06:45
  gas.
• 1:06:45
  The high -energy radiation coming from the star strikes this envelope, exciting the gas within it and causing it to glow on its own, thus
• 1:06:53
  creating the strange spectral features.
• 1:06:56
  But what comes next is the danger, because Wolff -Rayet stars are thought to be the precursors to a supernova Type
• 1:07:04
  2.
• 1:07:09
  The star larger than roughly eight times the mass of the Sun shares this grim, if spectacular, fate.
• 1:07:16
  Their fatal flaw is that nuclear fusion, the fundamental reaction that can power a star for eons, can't ever win in the long
• 1:07:24
  term against its ancient foe.
• 1:07:26
  Gravity Gravity is relatively weak in comparison to the other forces, but what it lacks in overall strength,
• 1:07:34
  it more than compensates for with patience.
• 1:07:38
  Quiet, incessant patience.
• 1:07:41
  The gravitational pull of a star's own mass is always attempting to collapse it, to shrink it into a smaller volume.
• 1:07:49
  Resisting that pull is the energy released from fusion, and a star can maintain that balance for millions, billions or even trillions of years.
• 1:07:57
  But there is only so much fuel available in the core of a star, and once the hydrogen runs out, the star must switch to fusing helium, which is not nearly
• 1:08:05
  as efficient.
• 1:08:06
  Once the helium runs out, a massive star can instead burn carbon and oxygen, and once that runs out, it can even move on to heavier elements.
• 1:08:14
  But at each phase, the fusion process yields less and less energy, and so the timescales catastrophically shrink.
• 1:08:22
  Hydrogen burning for millions of years, helium for less than a million, carbon for only a few hundred years, neon for half a year, oxygen a matter of days,
• 1:08:30
  and silicon for only a handful of hours.
• 1:08:33
  And then comes iron.
• 1:08:36
  The crushing gravitational weight of the star pressing in, iron begins to fuse to produce heavier elements.
• 1:08:43
  speakers: But there is a problem.
• 1:08:44
  The iron fusion doesn't release energy, it absorbs it.
• 1:08:49
  And so with nothing left to counteract gravity, the star collapses.
• 1:08:54
  In only a few more seconds, the star will be dead.
• 1:09:04
  The pressures reach such extremes that the iron in the core converts into something strange and truly unique in the universe.
• 1:09:11
  The electrons are forced inside of atomic nuclei, where they merge with protons, to produce neutrons.
• 1:09:18
  The entire core becomes a sphere only a few kilometers across, made of almost pure neutrons.
• 1:09:26
  Interviewer: The rest of the star rushes inwards towards that core at a substantial fraction of the speed of light.
• 1:09:32
  When it strikes, it meets a brief resistance and bounces back outwards, launching a shock wave that ripples through the remaining star's interior.
• 1:09:40
  But what happens next is a mystery to modern physics, despite the existence of this type of supernova being known to humanity since before the
• 1:09:48
  dawn of recorded history.
• 1:09:54
  Indeed, the earliest observations consist of simple rock carvings by unknown prehistoric peoples made around 4000 BC.
• 1:10:02
  And some supernovae were so bright and persistent, cultures around the world simultaneously recorded the same event.
• 1:10:09
  In 1054, Chinese astronomers recorded a new guest star appearing in their sky.
• 1:10:15
  At the same time, half a world away, the ancestral Puebloan people of modern day New Mexico created a pictograph denoting this special
• 1:10:23
  event.
• 1:10:24
  And today, that portion of the sky is occupied by the Crab Nebula, the remnants of that mighty explosion.
• 1:10:31
  But in the early 20th century, the astronomer Fritz Vicki started to deliver answers.
• 1:10:36
  For millennia, cultures had observed different kinds of new stars appearing.
• 1:10:41
  Now, thanks to the sophistication offered by modern astronomy, he could classify them.
• 1:10:46
  Flashes of hydrogen envelopes around white dwarfs would retain the name Nova, meaning new, while these kinds of titanic explosions would be
• 1:10:54
  called supernova.
• 1:10:56
  When a white dwarf is completely destroyed, it's known as a Type 1a supernova.
• 1:11:01
  When a massive star meets the end of its life, it's a Type 2.
• 1:11:05
  And in 1987, a blast originating in the Large Magellanic Cloud made history when it became the first supernova to be
• 1:11:13
  detected not by looking up, but by looking down into a giant tank of water.
• 1:11:23
  The tank was the Kamiokande 2 detector, buried deep underground in the Mizumi mine in Japan.
• 1:11:29
  And it held over a thousand metric tons of ultra -pure water surrounded by thousands of photo detectors, stars, all super -sensitive
• 1:11:37
  synthetic eyes staring incessantly at the water within.
• 1:11:42
  Kamiokande 2 was designed to detect neutrinos, the nearly massless particles that slip through the cracks of the cosmos.
• 1:11:50
  Neutrinos flood the universe.
• 1:11:52
  There are some several trillion passing through your body right now, but they only extremely rarely interact with the rest of matter, and so for most
• 1:12:00
  human history, they weren't unnoticed and undetected.
• 1:12:03
  It takes enormous detector volumes, say 100 ,000 metric tons of water, to catch even a few.
• 1:12:10
  Nuclear reactions create neutrinos.
• 1:12:13
  The Sun produces copious amounts of them every single day.
• 1:12:16
  And when the supernova in 1987 went off, it created roughly 10 to the power of 58 of them.
• 1:12:22
  Of course, the vast majority of those miss the Earth, and of those that hit, Kamiokande 2 and instruments like it were able to detect a grand total
• 1:12:30
  of 25 of them.
• 1:12:33
  But it was enough to confirm a picture that had been forming in the minds of physicists to explain how supernovae finish their
• 1:12:41
  explosions.
• 1:12:46
  The problem was that models showed that the shock front, produced when the stellar atmosphere bounces off the dense core, slows
• 1:12:54
  down and stalls, not able to deliver nearly enough energy to propel the explosion.
• 1:12:59
  But neutrinos may save the day.
• 1:13:03
  The conversion of iron into neutrons produces an absolute flood of neutrinos.
• 1:13:08
  In fact, over 99 % of the energy released in the supernova goes into neutrino production.
• 1:13:14
  What we see is an immense flash of light, but a shadow in comparison.
• 1:13:17
  Of course, most of these neutrinos slip harmlessly out of the dying star, never to be seen again.
• 1:13:23
  But enough of them interact with enough of the star's material to reignite the shockwave, possibly allowing it to break through and
• 1:13:31
  destroy the star.
• 1:13:33
  Other mechanics, such as out of control resonant instabilities, similar to the kind that destroyed the Tacoma Narrows Bridge in 1940,
• 1:13:41
  might also play an important role.
• 1:13:43
  Whatever the exact mechanism, once the shockwave breaks out, it sweeps up material in front of it, like an avalanche, taking a star's atmosphere
• 1:13:51
  and spreading it outwards at nearly the speed of light.
• 1:13:55
  Nuclear reactions in the material finally create the heavy elements that were attempted in the core mere minutes before, and so long after the
• 1:14:03
  star is dead and done, the remnants of its atmosphere remain as a slowly expanding and cooling nebula that eventually
• 1:14:10
  disperses into the interstellar medium.
• 1:14:20
  speakers: The Supernova As for Battuta, the Supernova caught it unawares.
• 1:14:24
  In the millions of years since its creation, it had grown complacent and overconfident.
• 1:14:30
  It had seen it all, understood it all, knew when the universe was safe and when it was dangerous.
• 1:14:36
  Or so it thought.
• 1:14:37
  It had been refueling in a long -dead binary system, when a sudden burst of neutrinos and gamma rays were the only warning it had before
• 1:14:45
  the star split itself in two and the shockwave came racing outwards.
• 1:14:51
  It took Battuta over a thousand years to repair the damage, licking its wounds in the tattered remnants of the system.
• 1:14:58
  Grim and determined, it decided that it had spent enough time in the galaxy, the
• 1:15:03
  supermassive black hole once and for all, and
• 1:15:11
  to leave the Milky Way behind.
• 1:15:24
  Interviewee: Battuta hung on the edge of the abyss.
• 1:15:33
  speakers: The struggles to get here, nearly 10 million years had passed since first leaving the heliopause gates of the solar system, were more than
• 1:15:41
  worth it.
• 1:15:42
  The environment surrounding the supermassive black hole was the most continuously energetic place in the entire galaxy.
• 1:15:50
  And beneath it all, under the maelstrom of plasma surrounding it and the flood of radiation emanating from near it was the black
• 1:15:58
  heart itself.
• 1:16:00
  A horizon cutting off the interior from the rest of the universe.
• 1:16:04
  For a brief moment, Battuta was tempted to plunge itself beneath its depths out of sheer curiosity.
• 1:16:11
  Indeed, prior to its launch, humanity had not yet cracked the riddle of quantum gravity, of the way to provide a coherent,
• 1:16:19
  universal description of strong gravity at subatomic scales.
• 1:16:23
  The answer was there, in the singularity in the heart of the beast.
• 1:16:29
  But it was a trap.
• 1:16:31
  If Battuta sent itself below the horizon, it would learn nature's ultimate secret, but it would never be able to return home
• 1:16:39
  with the answers.
• 1:16:44
  And it was those same singularities, those riddles of infinity in the centers of black holes, that first cast doubt on
• 1:16:52
  their existence altogether.
• 1:16:57
  As you see, the war is kindly disposed towards me, allowing me, despite fierce gunfire at a decidedly terrestrial distance, to
• 1:17:05
  take this walk into this, your land of ideas.
• 1:17:09
  So wrote German physicist Karl Schwarzschild in a groundbreaking 1915 letter to Albert Einstein, from the front lines of
• 1:17:16
  World War I.
• 1:17:18
  He had volunteered at 40 and while serving in Russia, had figured out the first exact solution to Einstein's general theory of
• 1:17:25
  relativity, solving for the case of gravitational influence due to a spherically symmetric object.
• 1:17:32
  A year after that, the great mathematician and physicist David Hilbert verified that Schwarzschild's solutions broke down in the very
• 1:17:39
  center of the object, meaning that any attempts to describe the gravity there would yield nonsensical results.
• 1:17:47
  Something bizarre was going on in these calculations, something unexpected.
• 1:17:52
  The theoretical exploration of black holes had begun.
• 1:18:01
  The theoretical exploration of black holes had begun.
• 1:18:02
  Singularities appear in all sorts of physical theories, but can usually be explained away.
• 1:18:07
  For example, the force of a spring approaches infinity as the spring compresses.
• 1:18:11
  This singularity is not a problem, because we know that springs can't be compressed forever.
• 1:18:16
  Some other force, like the interaction between molecules, prevents them from getting smaller.
• 1:18:21
  But the singularity that appeared in Schwarzschild's solution was different.
• 1:18:26
  It implied that there was a region of the universe that if gravity became too strong, something happened.
• 1:18:34
  Something unknown.
• 1:18:40
  Most astronomers scoffed.
• 1:18:43
  Indeed, Einstein is quoted as having said, The essential result of this investigation is a clear understanding as to why the Schwarzschild's
• 1:18:50
  singularities do not exist in physical reality.
• 1:18:54
  But a small minority, like Robert Oppenheimer before he became involved in the Manhattan Project, carried on.
• 1:19:00
  And in 1972, astronomers came across Cygnus X -1, one of the brightest sources of x -rays on the sky.
• 1:19:10
  The system consisted of two objects, a large bright star and a smaller unseen companion.
• 1:19:17
  Initial calculations suggested something enticing.
• 1:19:20
  The mass and radius of the companion made it much denser than any other known astrophysical object.
• 1:19:27
  Dense enough to be a black hole.
• 1:19:31
  And so in 1975, Stephen Hawking and Kip Thorne, two pioneers of black hole physics, made a bet.
• 1:19:39
  They had both spent much of their careers studying these hypothetical objects, and had staked their professional reputation on their reality.
• 1:19:46
  So if black holes did not in fact exist, they might just be out of a job.
• 1:19:50
  And so they concocted the bet as a consolation prize.
• 1:19:53
  If black holes did not exist, then Thorne owed Hawking a four -year subscription to the British satirical Private Eye magazine.
• 1:20:01
  But if they did exist, then Hawking owed Thorne a one -year subscription to the American, slightly less satirical, penthouse.
• 1:20:08
  By 1988, Hawking had paid his dues to Thorne.
• 1:20:13
  And with the reality of black holes came the reality of gravitational singularities.
• 1:20:26
  But the closest known black hole is Gaia BH1, which is only 1560 light -years away.
• 1:20:33
  However, astronomers suspect that there are many other black holes closer to our solar system.
• 1:20:39
  The problem is that unless they are members of a binary system, they are almost invisible.
• 1:20:45
  Of course, we can't see black holes directly.
• 1:20:48
  Instead, we can only see the material that swirls around them as it crushes into their extreme gravitational grip.
• 1:20:55
  But lone black holes are not accreting material like this, and so they stay hidden, lurking in the shadows.
• 1:21:02
  In 2023, astronomers with the Ogle collaboration announced the discovery of the first known Lone Wolf black hole,
• 1:21:10
  a six -solar mass object sitting roughly 5 ,000 light -years away.
• 1:21:15
  They discovered it through a technique known as microlensing, where the gravity of a black hole briefly enhances the light coming from a background
• 1:21:22
  star.
• 1:21:24
  Indeed, astronomers estimate that there are around 100 million black holes in the Milky Way alone.
• 1:21:29
  Some alone, some are members of complex systems, some are actively accreting, some are silent.
• 1:21:37
  And sometimes they collide.
• 1:21:43
  Black hole mergers are exceedingly rare, but they do happen.
• 1:21:48
  When they merge, they spiral in towards each other.
• 1:21:51
  The only way for them to get closer is to lose energy.
• 1:21:55
  And the only way that black holes can lose energy is through the emission of gravitational waves, ripples in space itself.
• 1:22:03
  However, gravity is by far the weakest of the known forces, and so black holes only inch together, achingly slowly.
• 1:22:11
  Arguably, too slowly.
• 1:22:14
  Indeed, physicists do not yet understand how black holes complete their mergers in less time than the present age of the universe.
• 1:22:21
  Dubbed the final parsec problem, after the challenges black holes face when they try to approach within a parsec of each other, it remains one of
• 1:22:29
  the great unsolved mysteries in modern astronomy.
• 1:22:37
  But black holes, ever willing to defy our expectations, do indeed merge.
• 1:22:42
  And when they do, there is rarely any fireworks or flash of light, unless the black holes are entrained by a ring of gas and dust that can
• 1:22:50
  visibly betray the event of the collision.
• 1:22:53
  Instead, black hole mergers are silent.
• 1:22:56
  A collision painted only in midnight.
• 1:23:00
  There are only emissions, a ferocious release of gravitational waves, entire solar masses worth of energy converted
• 1:23:07
  into pure gravitational radiation.
• 1:23:10
  But the waves quickly die down as they spread out into the universe.
• 1:23:14
  By the time they reach the Earth, millions of light years away, the waves are barely strong enough to nudge anything more than the width of an atomic
• 1:23:22
  nucleus.
• 1:23:23
  Indeed, even though Einstein predicted the existence of gravitational waves more than a century ago, he thought that they would forever remain undetectable.
• 1:23:31
  But Einstein didn't know about the existence of merging black holes, and he underestimated human ingenuity.
• 1:23:41
  On September 14, 2015, the LIGO gravitational wave experiment finally struck gold.
• 1:23:50
  Created using lasers and a set of mirrors at its most sensitive, LIGO is able to measure a change in distance between those mirrors as small as one -ten
• 1:23:58
  -thousandth the width of a proton.
• 1:24:00
  As LIGO themselves state, this is equivalent to measuring the distance to the nearest star, some 4 .2 light years away, to an accuracy smaller
• 1:24:08
  than the width of a human hair.
• 1:24:10
  Despite this incredible precision, they were not planning on realizing their dream for several more years, if not decades.
• 1:24:17
  Indeed LIGO had recently been upgraded, and they were running the experiment in a mode geared for engineering tests.
• 1:24:23
  But a tiny ripple ran through their detector, and several microseconds later through a sister detector on the other side of the continent.
• 1:24:31
  That ripple had an unmistakable fingerprint.
• 1:24:33
  The exact pattern matching the merger of two black holes.
• 1:24:39
  However as awe -inspiring as these mergers are, they are incredibly rare.
• 1:24:43
  The nearest ones recorded are several hundred million light years away.
• 1:24:47
  But there is another kind of black hole event that is just as destructive and powerful, and much closer to home.
• 1:24:54
  And this event involves not some ordinary everyday black hole.
• 1:24:58
  No, these events involve only the greatest black holes in the universe.
• 1:25:03
  The supermassive ones.
• 1:25:11
  The first hints that the Milky Way might host such a monster came as early as 1933, thanks to the pioneer of radio
• 1:25:18
  astronomy, Karl Jansky.
• 1:25:21
  He noticed that radio emissions seemed to be extremely strong coming from the direction of the galactic center, through the constellation
• 1:25:28
  Sagittarius.
• 1:25:30
  Several years later, more detailed and higher resolution observations revealed a strong source of radio emission coming from a very compact region of
• 1:25:38
  that sky, which radio astronomers dubbed Sagittarius A.
• 1:25:43
  In the 1970s, even more detailed observations revealed that Sagittarius A was made of several sub -components, one of which was
• 1:25:51
  much stronger than all the others.
• 1:25:54
  Robert L.
• 1:25:54
  Brown finally came up with a name for this interesting source, Sagittarius A -star.
• 1:26:00
  He added the asterisk to its name because it was exciting, and in atomic physics, the asterisk is used to denote excited states.
• 1:26:07
  The name stuck, and very quickly astronomers realized that this might be the home of a giant black hole.
• 1:26:15
  Only an object with that kind of density, that kind of sheer gravitational might, could provide the energies needed to blast out a
• 1:26:23
  continuous, strong radio signal.
• 1:26:26
  And after decades of work, it was observations of stars dancing perilously close to the abyss that finally confirmed it.
• 1:26:34
  Two teams, one led by the German astronomer Reinhard Genzel, and the other by the American Andrea Ghez, developed the technology needed to follow
• 1:26:42
  the orbits of individual stars as they circled in real time around the monster itself.
• 1:26:47
  In some cases, the stars would reach peak velocities of 30 % the speed of light.
• 1:26:54
  The motion of those stars was directly controlled by the mass of the central black hole, and the two teams were able to produce the
• 1:27:02
  first precise measurement of the mass of Sagittarius A -star, just over 4 million solar masses, giving its
• 1:27:10
  event horizon a radius of around 44 million kilometers.
• 1:27:14
  That's over 30 times the width of our Sun.
• 1:27:17
  If we were to place Sagittarius A -star in our own solar system, it would stretch to the orbit of Mercury, completely consuming it.
• 1:27:29
  But it is a dangerous game that these stars are playing.
• 1:27:33
  The gravitational forces near a black hole are, to put it mildly, immense.
• 1:27:39
  In particular, the problem is that there are strong differences in gravity across an object.
• 1:27:45
  If you were to fall towards a small black hole, for example, well before you hit the event horizon you would be destroyed, because the gravitational pull at your
• 1:27:53
  feet would be different enough from the gravitational pull at your head that you would get torn apart, or in the appropriate scientific term,
• 1:28:01
  spaghettified.
• 1:28:02
  And outside a supermassive black hole, even an entire star cannot survive swinging too close.
• 1:28:09
  If it does, the black hole's tidal forces, which is the generic term used to describe differences in gravity, can overwhelm a star's own
• 1:28:17
  self -gravity.
• 1:28:18
  The star loses its own sense of identity, getting stretched and pulled into a long, thin rope of plasma that eventually
• 1:28:26
  makes its way down the black hole's throat.
• 1:28:29
  This violent tearing releases an enormous amount of energy, roughly the same amount of energy as a typical supernova, and so these total
• 1:28:37
  destructions of an entire star, or tidal disruption events as they are innocently termed by astronomers, can briefly outshine
• 1:28:45
  an entire galaxy.
• 1:28:47
  But a small object, like a human body, can survive the descent towards a supermassive black hole.
• 1:28:54
  The gravitational differences across that distance just aren't big enough to cause spaghettification.
• 1:29:01
  In fact, a person, or even a small spacecraft, could handily survive the pass through the black hole's event horizon.
• 1:29:08
  And this is because the event horizon of a black hole is not a physical boundary.
• 1:29:14
  There is no wall.
• 1:29:16
  There is no marker.
• 1:29:18
  There is no sign that you have just fallen off the edge of the gravitational cliff.
• 1:29:23
  Your only clue comes as you try to turn around and leave, and you find that no matter how hard you push your engines, how much you
• 1:29:30
  accelerate, how fast you go, even to the point of approaching the speed of light, you cannot escape.
• 1:29:39
  And indeed once inside, your future is determined.
• 1:29:43
  Your fate sealed.
• 1:29:48
  Outside the event horizon, you are free to roam wherever you please in space.
• 1:29:52
  But you must always march forward into your own future.
• 1:29:56
  Inside a black hole, however, a single point in space becomes your future.
• 1:30:02
  In a black hole, all roads lead to the singularity.
• 1:30:08
  Wherever you turn, the singularity will be right in front of you.
• 1:30:12
  You can only approach it second by second.
• 1:30:15
  In a finite amount of time, as measured by a watch on your wrist, you will strike the singularity.
• 1:30:22
  And when you do, you will be obliterated.
• 1:30:28
  And it's a relatively straightforward exercise to calculate the amount of time between crossing the event horizon and reaching the singularity.
• 1:30:35
  For a black hole the size of Sagittarius A star, that time is around seven seconds.
• 1:30:43
  As to what you will experience at the singularity, that is something no calculation can yet reveal.
• 1:30:50
  We know how it will appear to you.
• 1:30:52
  Due to the extreme gravitational tidal effects, the paths of light get bent and warped.
• 1:30:58
  And so as you approach the singularity, it does not appear as a point, but as a wide, featureless plane stretching below you, as if
• 1:31:06
  you were landing on the surface of a black world.
• 1:31:10
  And of course, as you get nearer, you will become spaghettified as well, now that you are close enough to the singularity.
• 1:31:16
  But should some aspect of you survive, you would get a taste of the unification of gravity and quantum mechanics.
• 1:31:23
  The secret that every black hole hides in its deepest heart.
• 1:31:29
  But you will not be able to tell anybody else in the outside universe about what you have witnessed.
• 1:31:36
  Indeed, the only way to avoid this grim fate is to stay well away from a black hole, for there is no known force of nature
• 1:31:43
  that can save you once you enter.
• 1:31:53
  And so it was a difficult manoeuvre to pull off, and a dangerous one, but Batuta had spent millions of years preparing.
• 1:32:01
  It coasted just on the outskirts of the roaring and rotating Sagittarius A -star, allowing the extreme forces of
• 1:32:09
  gravity to propel it to a substantial fraction of the speed of light.
• 1:32:14
  It was a difficult time to take the force of gravity to the ground.
• 1:32:17
  But, at the moment, at just the right moment, just before dipping into the event horizon, Batuta altered course, a barely perceptible
• 1:32:24
  shift in its momentum, and it was free.
• 1:32:28
  Batuta had seen planets crumble and stars ignite.
• 1:32:45
  Interviewee: mission parameter 3.
• 1:32:47
  Find the most extreme objects and events in the universe.
• 1:32:55
  40 million years of nothing.
• 1:32:59
  Just the grand distant spiral of Andromeda, host of a trillion stars, steadily growing larger in the forward telescopes.
• 1:33:08
  And the Milky Way, smaller but still resplendent, shrinking behind it.
• 1:33:14
  Prior to leaving its home galaxy, Batuta had believed itself to be the master of time and space, accustomed to the inhumane scales
• 1:33:22
  that the universe operated on.
• 1:33:24
  But now, for the first time in its long life, Batuta felt lonely.
• 1:33:31
  Its sensor readings hadn't offered much distraction.
• 1:33:34
  Thin wisps of hydrogen and helium that bridged the two -and -a -half million light -year gap between the galaxies.
• 1:33:40
  The occasional passing cosmic ray, the galaxy, here and there a clump of stars to mark the presence of a dwarf satellite galaxy.
• 1:33:48
  This was because Batuta was swimming through a material known to astronomers as the warm -hot intergalactic medium.
• 1:33:55
  Its name comes from the fact that the gas has a temperature between 100 ,000 Kelvin, warm, and 10 million Kelvin, hot.
• 1:34:04
  The gas is believed to be primarily hydrogen and helium, and likely makes up almost half of the baryonic content of the universe, the other half
• 1:34:12
  trapped inside of galaxies.
• 1:34:14
  But the medium poses an extremely challenging astronomical target.
• 1:34:18
  It is stupendously thin, and spread out over enormous volumes.
• 1:34:22
  This means that despite its high temperature, its overall brightness is almost insignificant, and most of our knowledge of the medium comes not from
• 1:34:30
  direct observations, but through computer simulations.
• 1:34:35
  These were scales well beyond the comprehension of its human makers, and until it undertook its voyage, Batuta itself.
• 1:34:43
  The universe was truly so vast it could not be grasped, only reduced to mere numbers.
• 1:34:50
  But now Batuta had no such comfort.
• 1:34:53
  It could only wait, and wait, and wait, gliding smoothly through the near
• 1:35:01
  vacuum that defined the space between galaxies, observing and recording and sampling the warm -hot
• 1:35:08
  intergalactic medium.
• 1:35:10
  The only other thing that the spacecraft could count on was its founding mission.
• 1:35:15
  Explore the galaxy and the wider universe, catalogue habitable worlds, search for extreme sources of energy, and discover a
• 1:35:22
  pathway to rescue humanity from its solar prison on the Earth.
• 1:35:31
  speakers: And so Batuta had decided that its searches must take it here, to Andromeda, a larger and more vibrant galaxy than the Milky
• 1:35:39
  Way, home to more exotic objects and energetic events.
• 1:35:43
  It would search every corner of this galaxy if it must, to find the most extreme events in the cosmos.
• 1:35:51
  And after several hundred years of patient observation, Batuta found just the right candidate.
• 1:36:03
  The next day, Jocelyn Bell was neck deep in her graduate studies at Cambridge University.
• 1:36:09
  Working under the supervision of Anthony Hewish, a pioneer in radio astronomy, Bell was convinced that she wasn't smart enough to stay at Cambridge and would be thrown
• 1:36:17
  out at any minute.
• 1:36:18
  But one night, poring over the data printouts from the telescope, she noticed, in her words, a bit of scruff.
• 1:36:27
  At first, this didn't seem unusual.
• 1:36:29
  The telescope frequently picked up radio interference or mysterious undecipherable signals.
• 1:36:34
  And so Bell dutifully noted it and moved on.
• 1:36:38
  But a few nights later, the signal reappeared in the same part of the sky.
• 1:36:43
  Several weeks later, again.
• 1:36:46
  Rightfully sceptical, Hewish advised that they verify this potential discovery with another telescope at the array.
• 1:36:53
  And so at the appointed time, Bell and Hewish stood over the recorder…
• 1:36:57
  to nothing.
• 1:37:00
  Nervous, Bell began to deflate.
• 1:37:02
  Perhaps in the end, it was meaningless.
• 1:37:05
  But then, the recorder buzzed and began printing.
• 1:37:09
  She had simply miscalculated, and the strange signal appeared five minutes later than she predicted.
• 1:37:14
  The recorder was repeated, repeating every 1 .337 seconds an extremely high level of precision for an astronomical object.
• 1:37:22
  Jokingly, Bell named the source LGM -1.
• 1:37:25
  For little green men.
• 1:37:34
  In essence, neutron stars are the largest known bound states of nuclear matter, atomic nuclei, on the scale of
• 1:37:41
  kilometres.
• 1:37:43
  But just like the white dwarfs, they too have a cut -off mass, above which catastrophe looms.
• 1:37:50
  The limit for neutron stars is about two solar masses, any larger than that and the neutrons simply can't support themselves and will allow
• 1:37:58
  gravity to win.
• 1:37:59
  This means that neutron stars are quite possibly the densest material possible in the entire universe, just one step
• 1:38:07
  away from becoming black holes.
• 1:38:14
  Even though neutrons are, as their name suggests, electrically neutral, a typical neutron star will maintain a small population of
• 1:38:22
  charged protons and electrons, sandwiched in the deep layers of the interior.
• 1:38:26
  And the properties of nuclear material under extreme pressures cause the neutrons to form microscopic structures, sometimes
• 1:38:34
  forming tubes, sometimes long thin strands, and sometimes lumps.
• 1:38:39
  Amusingly, physicists name these different structures after various forms of pasta, spaghetti, penne, gnocchi, and so on,
• 1:38:47
  and give the interior structures of neutron stars the name nuclear pasta.
• 1:38:54
  At the outermost layers of a neutron star is a crust made of highly compressed electrons, but at the opposite end, in the core,
• 1:39:02
  is a substance unknown to modern physics.
• 1:39:05
  It could be merely some exotic state of neutron matter, or the intense pressures may force the neutrons to give way to their more fundamental
• 1:39:13
  constituents, a soup of quarks and gluons unseen in the wider universe since the earliest moments of the Big Bang.
• 1:39:21
  And in some proposals, the cores are instead made of strange matter, a degenerate state of solely strange quarks.
• 1:39:29
  But no matter what, neutron stars test the bounds of the exotic.
• 1:39:34
  The gravitational pull is so strong that light can orbit around them in a circle.
• 1:39:39
  The tallest mountains on neutron star surfaces are only fractions of a millimetre high.
• 1:39:44
  Neutron stars should also be rapidly rotating.
• 1:39:48
  Like a figure skater pulling in their arms to spin faster, the conservation of angular momentum takes a star's natural spin and
• 1:39:55
  amplifies it as it collapses.
• 1:39:58
  Combining that spin with a magnetic field generates an electric field which whips up any nearby charged particles and energizes them.
• 1:40:06
  They then go on to generate intense beams of high energy radiation out of the neutron star's magnetic north and south poles.
• 1:40:14
  But these magnetic poles don't have to be aligned with the axis of rotation, just like the Earth's magnetic field is not aligned with its spin.
• 1:40:22
  And so as the neutron star spins, it carries the beam of radiation around in a circle, a gigantic searchlight
• 1:40:29
  circling across the sky.
• 1:40:33
  And if the path of that searchlight happens to pass over the Earth, we see it as a regular pulsing of radio emission.
• 1:40:41
  A pulsar.
• 1:40:45
  The pulsar discovered by Bell -Burnel and Hewish rotated once every 1 .337 seconds, corresponding roughly to the RPM of a
• 1:40:53
  record player.
• 1:40:54
  But that's nothing in comparison to some discovered since.
• 1:40:57
  The fastest known pulsar, PSR J17482446AD, rotates 716
• 1:41:05
  times every second, or 43 ,000 revolutions per minute, faster than a jet turbine.
• 1:41:12
  Some pulsars even have companions, and when they do, they can be deadly.
• 1:41:17
  The pulsar PSR B1957 -20 has another, much more ominous name, the Black
• 1:41:25
  Widow.
• 1:41:26
  This star, sitting 6 ,500 light -years from Earth, has a small, brown dwarf companion.
• 1:41:32
  The powerful outbursts from the Black Widow are slowly destroying the brown dwarf.
• 1:41:37
  The ferocity of the attack is so much that the companion is a bleating material as if it were in the direct stream of a blowtorch.
• 1:41:48
  And magnetars take this process to the next level.
• 1:41:56
  All neutron stars likely host magnetic fields, but something specific happens inside a magnetar to ramp up its field beyond
• 1:42:04
  the extreme.
• 1:42:06
  That special something likely happens at the moment of its birth, when the neutron star first emerges from the wreckage of its host star.
• 1:42:14
  Just like conservation of angular momentum spins up a neutron star during the period of collapse of the parent, the same thing happens to
• 1:42:22
  magnetic fields.
• 1:42:23
  If the neutron star is spinning too slowly, it won't generate enough energy to drive its magnetic fields to extreme strengths.
• 1:42:31
  And on the other hand, if it's rotating too rapidly, then the magnetic fields themselves become a source of drag as they are forced to move through the charged
• 1:42:39
  plasma still surrounding the star, acting as magnetised sails that slow everything down.
• 1:42:46
  But if conditions are just right, a dynamo mechanism can operate deep within the heart of the neutron star.
• 1:42:54
  In general, dynamos take one form of energy, usually rotational energy, and transform it into stronger magnetic fields.
• 1:43:02
  Indeed, a dynamo in the Earth's core amplifies our own magnetic field, and the magnetic field of a magnetar is the most powerful
• 1:43:10
  ever seen in the universe.
• 1:43:12
  For context, the Earth's magnetic field has a strength of about 1 gauss, strong enough to nudge a needle suspended in water.
• 1:43:20
  Powerful electromagnetics in a supercooled MRI machine, typically the strongest magnetic fields a human will ever encounter, are about a million
• 1:43:28
  times stronger.
• 1:43:29
  Even stronger ones can be created in the laboratory, but not for long periods of time, as the magnetic fields tend to destroy their generating machines.
• 1:43:37
  And so bearing that in mind, a typical magnetar will host a magnetic field roughly 1 trillion times stronger than the
• 1:43:45
  Earth's.
• 1:43:45
  Of course, those strengths even approaching a magnetar is a death sentence.
• 1:43:51
  Neglecting the copious amount of X -rays emanating from their surface, and the beams of radiation from their poles, and the occasional flare of
• 1:43:59
  gamma rays, magnetic fields of this strength stretch and distort even atoms, turning them into thin needles a
• 1:44:07
  hundred times longer than they are wide.
• 1:44:09
  Every and all molecular and chemical bonds cannot withstand that intensity.
• 1:44:15
  Even the vacuum of space itself becomes polarised when soaked with such overwhelming energy.
• 1:44:26
  Interviewer: But despite their ferocity, eventually all magnetars fade away.
• 1:44:31
  The magnetic fields meet resistance and slow down the rotation of the neutron star until it can no longer sustain such incredible field
• 1:44:39
  strengths, and the star continues on its life as a normal neutron star or pulsar.
• 1:44:49
  Batuta was not easily impressed considering all it had seen, but the magnetar managed it.
• 1:44:56
  It was a truly awesome sight.
• 1:44:59
  The radiation, magnetic field, particle count, temperature, everything, every measure of every sensor was pushed beyond
• 1:45:07
  its limits.
• 1:45:09
  There was truly nothing else like it.
• 1:45:14
  Except, as it gazed in wonder at the magnetar, Batuta still wanted more.
• 1:45:25
  speakers: the magnetar.
• 1:45:33
  Batuta was in the most fragile state it had ever been in.
• 1:45:39
  Over 98 % of its systems were destroyed, but with what remained of its sensors, Batuta took in the magnetar
• 1:45:47
  before it.
• 1:45:49
  The explosion that had emanated from the star's surface had been a surprise, the flood of high -energy radiation unlike anything it had ever
• 1:45:57
  seen before.
• 1:45:58
  It was almost as if Batuta's musings had awakened some angry demon of old, the magnetar lashing out to show the fragile
• 1:46:05
  visitor what it was capable of.
• 1:46:08
  It was spectacular, but Batuta didn't want to stick around and risk seeing it a second time.
• 1:46:15
  And so, the spacecraft glided outside the solar system, putting a wide berth between itself and the temperamental star, noting in
• 1:46:23
  its log a definitive answer to an elusive mystery that had haunted astrophysicists for years.
• 1:46:37
  We will never know what the reactions were of the first people to discover gamma ray bursts, because they were working in secret and they
• 1:46:45
  weren't looking for them.
• 1:46:48
  The U .S.
• 1:46:49
  In 1963, the governments of the United States and the Soviet Union signed the historic Partial Nuclear Test Ban Treaty, which
• 1:46:57
  banned all overground nuclear tests.
• 1:47:00
  But despite the treaty, the U .S.
• 1:47:02
  did not trust its negotiating partner and thought that the Soviets might launch secret missions to the far side of the moon and test their weapons there.
• 1:47:10
  And so, in response to this potential loophole in the treaty, the United States launched the Vila satellites, a series of
• 1:47:18
  satellites orbiting well outside the Van Allen radiation belts that circle the Earth.
• 1:47:23
  There, they would hunt for gamma rays, the high energy radiation emitted during nuclear blasts, hoping to catch the Soviets red -handed.
• 1:47:32
  However, though the satellites never did see the signature of a nuclear weapon, on July 2, 1967, they did
• 1:47:40
  record a bright burst of gamma rays.
• 1:47:43
  The event was recorded but kept under wraps, until a dozen more detections were made.
• 1:47:49
  Realising that this represented more scientific curiosity than a threat to the United States, Ray Kleber -Settle of Los Alamos National Laboratory
• 1:47:57
  finally convinced his superiors to release the information to the scientific community, in the process giving these mysterious events their
• 1:48:05
  equally mysterious name.
• 1:48:07
  Gamma ray bursts or GRBs.
• 1:48:15
  Interviewer: Already having lost half its mass due to its violent eruptions, a vicious cycle has begun.
• 1:48:20
  The outer layers of the star's atmosphere heat from the intense energy pouring out of the star, which causes them to expand.
• 1:48:28
  But once enlarged, the gravitational pressures on the core ease.
• 1:48:32
  The radiation plummets, the outer layers cool, and the star returns to some semblance of normality before beginning the cycle anew.
• 1:48:40
  The other layer of the star is a very similar mechanism.
• 1:48:46
  And a similar mechanism powers another class of variable star, the Cepheid variables.
• 1:48:53
  The simple fact that Mira and Cepheid variables can measurably change their brightness over the course of days or even hours
• 1:49:01
  is a matter of almost inconceivable scale.
• 1:49:04
  Indeed, these are entire stars, vast complexes of boiling plasma, churning magnetic fields, and
• 1:49:12
  forges of fusion beyond human scale and comprehension, that alter their appearance as easily and quickly as people change
• 1:49:20
  outfits.
• 1:49:21
  And so while we may understand the physics behind their chameleon -like nature, the energies needed to sustain these cycles
• 1:49:29
  still make them deserving of their miraculous names.
• 1:49:37
  But they do still change over a period long enough to notice, hours and days.
• 1:49:44
  You couldn't miss them, for example, in the blink of an eye.
• 1:49:48
  For something that quick, we have to turn our attention to the true transients of the universe, the rare events in the
• 1:49:55
  extreme ends of the electromagnetic spectrum, the flashes and bursts that come and go in a second, brief blips on a
• 1:50:03
  detector the only sign that something awful and violent has happened in the distant night sky.
• 1:50:13
  And so perhaps unsurprisingly, astronomers didn't even notice the first fast radio burst, or FRB,
• 1:50:21
  when it hit the Australian Parks Observatory in 2001.
• 1:50:25
  Instead, it wasn't discovered until years later, when Duncan Lorimer and his student David Narkovitz spotted it in archives of pulsar
• 1:50:33
  data.
• 1:50:34
  Only three milliseconds long, it came from roughly the direction of the small Magellanic Cloud.
• 1:50:40
  Since then, astronomers have managed to capture only two dozen more of these events, ranging in duration from only a millisecond to over three seconds
• 1:50:48
  long.
• 1:50:49
  And whatever is causing FRBs, they are enormously powerful, putting out several days' worth of solar
• 1:50:56
  energy in a matter of milliseconds.
• 1:51:00
  speakers: Yet at the same time, they must be both rare and distant, since only our most sensitive radio telescopes are capable of detecting them.
• 1:51:09
  opers While astronomers still don't fully understand the origins of FRBs, a discovery in 2020 helped give them a major clue.
• 1:51:17
  Until then, all FRBs had originated far outside the Milky Way which made their identification that much harder.
• 1:51:25
  But on April 27th of that year, the Neil등esekrylt Swift Observatory sent out an automated announcement that a storm of X -rays
• 1:51:33
  had just emanated from a known stellar remnant hiding in a distant corner of the Milky Way.
• 1:51:39
  That stellar remnant could have been a white dwarf, a neutron star, or even a black hole.
• 1:51:44
  They are all known to emit occasional X -ray flares.
• 1:51:47
  But this remnant was also known to emit gamma rays on a regular basis, something that astronomers suspect
• 1:51:55
  only neutron stars are capable of doing.
• 1:51:58
  Another clue came the next day, when the Canadian Hydrogen Intensity Mapping Experiment reported two bright
• 1:52:05
  FRBs coming from that same location.
• 1:52:08
  And over the next several days, a fleet of radio telescopes and X -ray observatories monitored several flares emanating from the source.
• 1:52:17
  And a picture began to emerge.
• 1:52:20
  A magnetar was glitching.
• 1:52:33
  At the end of a massive star's life, its core compresses and forms a neutron star, with the entire weight of the surrounding
• 1:52:41
  crashing in at up to half the speed of light.
• 1:52:44
  As we've seen, sometimes the core survives the onslaught and goes on to become a pulsar or even a magnetar.
• 1:52:51
  But sometimes, it's just too much.
• 1:52:55
  Sometimes, gravity wins, pulling everything inwards towards infinity, creating a black hole.
• 1:53:03
  That newborn black hole is surrounded by the corpse of the star that created it.
• 1:53:09
  Some of that material succumbs to the almighty gravitational pull of the black hole, falling back towards it.
• 1:53:15
  And in a process seen time and time again throughout the universe at a variety of size and energy scales, from that
• 1:53:23
  disk of accretion emerges two gigantic, hyper -powered jets, containing a blast of charged
• 1:53:30
  particles travelling at nearly the speed of light.
• 1:53:33
  The jet slams into the surrounding material, accelerating a relativistic shock front, and in the process
• 1:53:41
  accelerates particles that then give way to a flood of gamma rays.
• 1:53:46
  And if the Earth happens to lay in the direction of that powered beam, a mechanism that would fit perfectly in your favourite sci -fi movie, then we
• 1:53:54
  see it as a gamma ray burst.
• 1:53:57
  And the explosion gets a new name.
• 1:53:59
  A hypernova.
• 1:54:01
  A
• 1:54:05
  Yet at the same time, they must be both rare and distant, since only our most sensitive radio telescopes are capable of detecting them.
• 1:54:14
  As compelling as this story is, it does have its holes.
• 1:54:19
  Nobody fully understands the complex physics at play, and so there is another candidate.
• 1:54:24
  Another potential source of GRBs, especially the ultra -long ones that hold for several hours before finally fading.
• 1:54:32
  These are an entirely different kind of beast.
• 1:54:36
  The result of the deaths of the most massive stars in the universe.
• 1:54:41
  Stars weighing at least 130 times the mass of the Sun.
• 1:54:46
  All stars produce radiation in their cores as a result of nuclear reactions.
• 1:54:52
  The radiation then flies outwards, inflating the star and preventing its collapse through gravity.
• 1:54:58
  This balancing act can last for up to trillions of years in the smallest of stars.
• 1:55:04
  But in the very largest stars, the radiation takes the form of energetic gamma rays.
• 1:55:10
  Normally this is no problem, but extremely energetic gamma rays can spontaneously split into pairs of
• 1:55:18
  electrons and positrons.
• 1:55:20
  These particles aren't nearly as good at providing a source of resistance against gravity.
• 1:55:25
  But in a supergiant star, a new equilibrium takes hold.
• 1:55:30
  The fusion reactions create gamma rays.
• 1:55:32
  The gamma rays split into electron -positron pairs.
• 1:55:35
  Then those pairs find each other and annihilate, converting themselves back into gamma rays, which then go on to keep the star supported.
• 1:55:44
  But towards the end of a superstar's life, it starts to produce an excess of gamma rays.
• 1:55:50
  These gamma rays convert themselves into particles, but not enough particles find each other to reform into gamma rays.
• 1:55:58
  And without even noticing, the star dies.
• 1:56:07
  The gravitational rush is so intense that it obliterates the core entirely, releasing all of its pent -up energy in a
• 1:56:15
  single flash.
• 1:56:16
  The star explodes as a superluminous supernova up to ten times more powerful than any other explosion,
• 1:56:24
  flooding the wider universe with gamma rays and record -breaking charged particles.
• 1:56:34
  500 million years.
• 1:56:37
  500 million silent, lonely years.
• 1:56:43
  Batuta almost didn't make it.
• 1:56:45
  Its energy reserves were already dangerously low before it reached the outermost edges of the Virgo Cluster.
• 1:56:52
  Luckily, however, the cluster environment was filled with a hot, thin, diffuse gas, barely more noticeable than the surrounding emptiness,
• 1:57:00
  but just enough to sip on and continue the journey onwards.
• 1:57:04
  A journey that would take another few million years to cross towards its target.
• 1:57:09
  Messier 87, one of the largest galaxies in this neighbourhood of the universe.
• 1:57:15
  and Batuta had thought many times about the curious origins of that name, Messier 87, in the time it took
• 1:57:23
  to voyage from Andromeda.
• 1:57:34
  Interviewer: Using a network of radio telescopes scattered across the globe, including in Antarctica, the team cleverly combined the information
• 1:57:42
  from all the instruments, turning them into a supertelescope, the width of the entire Earth.
• 1:57:48
  From Riba in his back garden to a telescope the size of the globe, radio astronomy has come a long way.
• 1:57:54
  And in 2019, they announced their first result, the direct image of the swirling plasma surrounding the black hole
• 1:58:02
  at the centre of the Messier 87 galaxy, sitting almost 54 million light -years away and centred on the
• 1:58:10
  empty blackness that is the shadow of the great black hole itself.
• 1:58:16
  Because of their extreme luminosity, quasars regularly count amongst the most distant objects known to astronomy.
• 1:58:23
  The current record holder is UHZ -1, discovered by a team led by Daniel Whalen at the University of Portsmouth.
• 1:58:31
  This monster sits a draw -dropping 13 .2 billion light -years away from the Earth and was already active when the
• 1:58:38
  universe was just 3 % of its current age.
• 1:58:43
  And quasars, blazars, and other active galaxies are thought to be the most likely source of a certain class of cosmic ray.
• 1:58:50
  The ultra -high -energy cosmic rays.
• 1:58:53
  Just like the Oh My God particle.
• 1:58:56
  Indeed, it's an easy connection to make.
• 1:58:58
  While the OMG particles' total energy doesn't seem all that much on paper, roughly the same amount of kinetic energy as a relatively fast baseball,
• 1:59:07
  when compressed down to a single proton, that translates to a speed in excess of a septillionth the speed of light.
• 1:59:13
  What else but a quasar could be responsible for that kind of energy?
• 1:59:19
  Unfortunately, while this idea is plausible, there is one issue.
• 1:59:24
  All known active galactic nuclei are incredibly far away.
• 1:59:29
  Indeed, the nearest known active galaxy is Markarian 231, which sits about 581 million light -years away.
• 1:59:36
  Most other quasars are billions of light -years away, sitting near the very edge of the observable universe.
• 1:59:43
  And when ultra -relativistic cosmic rays travel through those vast distances, they don't slip silently through purely empty space.
• 1:59:51
  They have to contend with the cosmic microwave background, the leftover light from the early days of the Big Bang, a form of radiation that
• 1:59:58
  completely soaks the cosmos.
• 2:00:01
  At those incredible speeds, the high -velocity particle sees nothing but a thick soup of photons, which slowly sap
• 2:00:09
  energy from it as it travels.
• 2:00:11
  Scientists estimate that this limits the maximum distance for ultra -high energy cosmic rays to be somewhere around 160 million light
• 2:00:19
  -years, far closer than the nearest quasars that are the most likely source for them.
• 2:00:24
  Or perhaps, ultra -high energy cosmic rays originate much closer to home through some mechanism of physics that we have yet to
• 2:00:32
  understand.
• 2:00:40
  Batuta spent nearly a hundred million years combing through Messier 87, searching in vain for the source
• 2:00:48
  of the mysterious ultra -high energy cosmic ray.
• 2:00:51
  It was bordering on giving up hope and returning to Earth.
• 2:00:55
  After all, it had accumulated much wisdom through the ages, and surely humanity would find something of value in what the spacecraft had discovered already.
• 2:01:03
  But before it set off, one of its remote sensors, placed on an artificial observatory several hundred light -years away, sent a
• 2:01:11
  high -status alert message.
• 2:01:15
  Interviewee: A signal had slipped through the semi -sentient layers of filters, a signal that the remote observatory could not decipher
• 2:01:22
  and could not match to any known astrophysical template.
• 2:01:27
  And so, it had sent a dispatch to Batuta.
• 2:01:31
  The spacecraft mulled its options.
• 2:01:35
  speakers: On the one hand, it received several such alerts every century from the probes, observatories, stations, and listening posts it left in its wake.
• 2:01:43
  But nothing new had ever come of it.
• 2:01:47
  And the first person on Earth to encounter such a signal was Jerry R.
• 2:01:51
  Amon of the Big Ear Radio Telescope at The Ohio State University.
• 2:01:56
  By August 15th, 1977, a bright radio signal splashed across the dish.
• 2:02:03
  Amon, tasked with monitoring the telescope that night, saw the signal appear on the automated printout and immediately wrote next to it the simple
• 2:02:11
  comment, wow.
• 2:02:14
  The signal lasted for at least 72 seconds and was centered near the hydrogen 21cm line.
• 2:02:21
  Curious, but elusive, the signal never repeated and the source was never found.
• 2:02:27
  And that patch of the sky has been quiet in the radio spectrum ever since, its ultimate origins never discovered.
• 2:02:40
  Batuta's wide network of observatories had given it similar wow signals, but in time the spacecraft had learned to ignore them.
• 2:02:48
  Either way, whether mundane or inexplicable, they were ultimately frustrating.
• 2:02:53
  But this one?
• 2:02:55
  Well, it was along the course back to the Milky Way and it wouldn't hurt to investigate just one
• 2:03:03
  more.
• 2:03:14
  Interviewer: mission parameter 4 discover new physics and find salvation for humanity.
• 2:03:23
  It took several millennia to pinpoint the source of the strange signal and navigate to it.
• 2:03:28
  Several centuries to establish a response, a few years of fine -tuning, a handful of months to build the necessary protocols, a few
• 2:03:36
  days of high -powered information exchange and an eternity to process what had just happened.
• 2:03:44
  Interviewee: Batuta was not alone.
• 2:03:47
  Its counterpart, the sleek spacecraft whose name could not be rendered in any human -derived dialect had noticed Batuta's activities in the
• 2:03:55
  Messier 87 galaxy and spent the better part of a millennium designing a signalling device to reach it.
• 2:04:01
  It seemed happy that Batuta had responded, at least in an alien sense that Batuta had decided was the closest thing it could call happy.
• 2:04:09
  The alien spacecraft had been sent long ago on a mission similar to Batuta's, but it appeared that it had failed.
• 2:04:18
  Interviewer: The sun that its creators called home had died long ago and by the time the alien spacecraft had returned to its origins, there was nothing
• 2:04:26
  left but ash and ruin.
• 2:04:28
  And so the probe had wandered the depths, weaving its own lonely destiny among the stars.
• 2:04:35
  Batuta was the sixth spacecraft that the alien had encountered on its long voyages and after downloading its data, Batuta spent several
• 2:04:43
  years processing and analysing the newly acquired information and accumulated wisdom from a network of robotic, sentient
• 2:04:50
  spacecraft.
• 2:04:51
  and in that tangled mess of information like a diamond glittering in a deep dark cave, Batuta
• 2:04:59
  found a clue.
• 2:05:02
  But unfortunately, that deep dark cave that contained the clue was a cosmic void.
• 2:05:14
  speakers: Without the aid of high -powered telescopes, a person deep within these voids looking out would see nothing more than a cold,
• 2:05:21
  empty, dark, and lonely universe.
• 2:05:25
  Indeed, when Laird Thompson and Stephen Gregory first discovered cosmic voids in 1978, the scientific community didn't believe what
• 2:05:33
  they were seeing, which was nothing.
• 2:05:41
  Though to be fair, at the time, the arguments against the existence of voids were very persuasive.
• 2:05:48
  Astronomers were used to mapping and surveying the heavens, finding distant galaxies galore.
• 2:05:53
  They had discovered clusters, like the Great Coma Cluster, relatively dense collections of thousands of galaxies or more.
• 2:06:00
  They had even begun to discover the first hints of superclusters, strings of clusters in the sky that appeared to be the largest objects in the universe.
• 2:06:08
  Indeed, it was just such a mapping of a supercluster that Thompson and Gregory had intended to do.
• 2:06:13
  But when they completed their survey, creating a crude map of galaxies over an enormous volume of the universe, a span so large that
• 2:06:21
  entire galaxies were nothing more than mere dots, there was a gigantic gap in the middle.
• 2:06:28
  Other astronomers argued that perhaps the survey designed by the pair was simply incomplete, that there were more galaxies to be found in the void that they
• 2:06:36
  had missed.
• 2:06:37
  In time, however, and with enough evidence, the rest of the astronomical community would come around and accept the cosmic voids,
• 2:06:45
  the vast expanses of almost nothing that dominate the volume of the universe.
• 2:06:54
  And it was the discovery of the cosmic voids, twinned with better understanding of the adjacent superclusters, that gave rise to the modern
• 2:07:01
  picture of the growth and evolution of the largest structures in the universe.
• 2:07:07
  But that growth arises from a seeming paradox.
• 2:07:13
  In the early 1900s, astronomers believed the universe to be static at large scales.
• 2:07:19
  Of course, ever since the revolution of Copernicus and Newton, they accepted the fact that objects within the universe were dynamic and changing, planets
• 2:07:27
  orbited, stars exploded, galaxies spun.
• 2:07:29
  But at the very largest scales, those of the cosmos itself, it was fixed in both time and space.
• 2:07:38
  But as Einstein developed his general theory of relativity in the early 20th century, he cautiously applied his first formulations of
• 2:07:46
  this new theory of gravity to the evolution of the cosmos.
• 2:07:51
  However, when Einstein peaked inside his own equations, he was unnerved.
• 2:07:56
  General relativity stated flatly that the universe was dynamic and evolving.
• 2:08:03
  And so, he added a fix to the equations, a cosmological constant that maintained a static universe and moved on
• 2:08:11
  to other work.
• 2:08:12
  And this meant that it would take a quartet of other scientists working semi -independently to fully embrace the implications of general relativity
• 2:08:20
  in terms of the entire universe.
• 2:08:23
  The Russian Alexander Friedman, the Belgian Georgia Metra, the American Howard Robertson, and the Brit Arthur Walker.
• 2:08:30
  Today, this modern cosmological framework is casually known as the Big Bang model, but more formally as the
• 2:08:37
  FLRW model in their honour.
• 2:08:46
  The FLRW model provided a theoretical grounding for Edwin Hubble's stunning series of observations made in the 1920s.
• 2:08:54
  that galaxies exist and that they are, on average, moving away from us.
• 2:08:58
  The observation that we live in an expanding universe.
• 2:09:03
  But to smooth things along, the FLRW model makes a few assumptions.
• 2:09:08
  Einstein's theory of relativity is enormously complicated, and so to make headway, the model assumes that the universe is both homogeneous and
• 2:09:16
  isotropic, meaning that at large scales, the cosmos is more or less the same from place to place, and at great distances, the cosmos
• 2:09:24
  looks the same from any direction.
• 2:09:27
  These two key assumptions allow cosmologists to make headway and describe the universe through the lens of the Big Bang theory.
• 2:09:35
  But, therein lies the paradox.
• 2:09:39
  If the universe is roughly the same at large scales, how can there be structures like superclusters or
• 2:09:46
  cosmic voids?
• 2:09:49
  The answer to this riddle turned out to be the discovery that there have not always been these tremendous structures.
• 2:09:56
  They must have started small, small enough to not upset the cosmological balance, growing to their present stature over billions
• 2:10:04
  of years.
• 2:10:08
  To illustrate this more clearly, let's look at our home, the Milky Way.
• 2:10:16
  The Milky Way galaxy began its life around 13 .61 billion years ago as a small proto -galaxy, just
• 2:10:23
  a minor bump of material and stars not much denser than the cosmic average.
• 2:10:28
  But from there, it grew, fed by streams of material flowing towards it and through the violent cannibalization of
• 2:10:36
  other galaxies.
• 2:10:37
  Indeed, we see remnants of these mergers everywhere we look.
• 2:10:41
  First, there are the globular clusters, extremely dense collections of old and ancient stars that orbit around the disk of the Milky Way.
• 2:10:50
  Astronomers believe that these are the remnant cores of ancient dwarf galaxies, their main host destroyed and absorbed into the grander whole.
• 2:10:58
  And then, there are the stellar streams, loose bands of stars that stretch and wind their way through the galaxy, all that's left of
• 2:11:06
  galaxies that were torn apart when they encountered our own.
• 2:11:10
  And lastly, there are the signs of even older collisions, like the Gaia sausage.
• 2:11:16
  These are stars that are not near each other in space but share a common set of ages, proportions of heavy metals and orbits around the core.
• 2:11:24
  The only clues we get that these were stars that once called another galaxy home and were pulled apart and resettled in their new
• 2:11:32
  lands.
• 2:11:34
  And this merging is not over.
• 2:11:36
  So far, the Milky Way has survived hundreds of collisions but these have all been with galaxies smaller than our own.
• 2:11:44
  But in roughly 5 billion years, there is a 50 % chance that the mutual gravitational attraction between the Milky Way and
• 2:11:52
  our nearest neighbor, the Andromeda galaxy, will pull us together into a single new galaxy.
• 2:12:04
  Interviewer: This same process of gravitational attraction, capture and merger has played out for billions of years across the cosmos,
• 2:12:12
  building ever larger structures as time goes on.
• 2:12:15
  From dwarfs, to galaxies, to groups, to clusters, to superclusters.
• 2:12:20
  The superclusters so large in fact that they've not yet completed their assembly process, their members not yet fully bound together through their
• 2:12:28
  mutual gravity.
• 2:12:30
  Of course, all this building takes material and that material has to come from somewhere.
• 2:12:37
  The cosmic voids.
• 2:12:41
  You can step into an earthly void by visiting the catacombs of Paris, located near the Paris Observatory at the south end of the city.
• 2:12:50
  There, you can take a guided tour of seemingly endless underground passageways littered with millions of human remains.
• 2:12:57
  But these underground chambers were not originally intended as an ossuary.
• 2:13:02
  They were mines, rich with a Lutetian limestone that gives the city of Paris its pristine, uniform appearance,
• 2:13:10
  earning it the nickname the City of Light.
• 2:13:13
  That City of Light is built on top of a cavern of darkness, with almost the entirety of the southern end of the city excavated for the valuable mineral.
• 2:13:22
  And when new building projects started to encroach on existing above -ground mass graves, the city authorities decided to move the remains into the now
• 2:13:29
  -empty digging tunnels, creating the catacombs.
• 2:13:33
  Interviewee: And so the grand structures of Paris owe their thanks to the void sitting beneath them.
• 2:13:39
  In the same way, the grand structures of the cosmos owe their thanks to the voids sitting among them.
• 2:13:51
  speakers: The void started as small pinpricks in inconvenient absence here and there, eventually morphing into the incredible stretches of
• 2:13:58
  deserts they are today.
• 2:14:00
  Indeed, by volume, most of the universe is void.
• 2:14:06
  The nearest void to the Milky Way is called, appropriately enough, the Local Void.
• 2:14:12
  Discovered in 1987 by Brent Tully and Richard Fisher, the Local Void is at minimum 150 million light -years
• 2:14:20
  across and possibly several times larger.
• 2:14:23
  In cosmological terms, our galaxy, along with almost every other galaxy in the entire universe, sits at the cusp
• 2:14:31
  of one of these voids.
• 2:14:33
  The Local Void sits at the edge of the Local Group, a 10 million light -year -wide arrangement of galaxies in a roughly dumbbell
• 2:14:41
  shape, with the Milky Way and its satellites at one end and Andromeda and its retinue as the counterweight.
• 2:14:49
  Surrounding the Local Group is a ring of 12 large galaxies spanning 25 million light -years across, the so -called
• 2:14:56
  Council of Giants.
• 2:14:58
  Together with some other collections and groups of galaxies, we form the Local Sheet, a network of structure spanning one side
• 2:15:06
  of the Local Void.
• 2:15:08
  The nearest cluster to us is the Virgo Cluster, which hosts several thousand galaxies in a span only a few million light -years across.
• 2:15:16
  And the Virgo Cluster is itself the centrepiece of a supercluster, a ragged collection of galaxies and groups, including the
• 2:15:24
  Local Group, all headed inwards towards it, a construction process set in motion eons ago that continues to the present
• 2:15:32
  day.
• 2:15:33
  But this Virgo supercluster is but one branch of four of a much larger supercluster, named
• 2:15:41
  Laniakea, a Hawaiian word roughly translating to immeasurable heaven.
• 2:15:47
  And Laniakea is truly immense.
• 2:15:50
  It contains over 500 distinct groups and clusters and over 100 ,000 individual galaxies,
• 2:15:58
  all spanning a volume stretching 500 million light -years on a side.
• 2:16:03
  But that is still not the end.
• 2:16:07
  Like a network of interconnected cities, Laniakea is just one part of a much larger structure.
• 2:16:15
  Beyond Laniakea sits the Shapley supercluster, and on the other side of the sky sits the Perseus -Pisces supercluster, the
• 2:16:23
  Draco -Hercules, the Corona -Borealis, and more.
• 2:16:26
  A monumental lattice of connected superclusters with the clusters acting as nodes, and grand walls, sheets, and
• 2:16:34
  filaments, the highways bridging them, all spanning the entire observable universe to form one single object.
• 2:16:43
  The Cosmic Web, the largest pattern found in nature.
• 2:16:50
  And between and among all of those grand, beautiful structures are the voids.
• 2:17:00
  Some of them are comparable in size to the local void, like the Taurus void or Pegasus void, but many of them dwarf them.
• 2:17:08
  There's also the Bootes void, which sits at the far end of the Shapley supercluster, with another wall at the Hercules supercluster, separating
• 2:17:16
  it from the northern supervoid.
• 2:17:18
  The Bootes void's nearest edge sits roughly 500 million light -years away, and it's at least 330 million light
• 2:17:26
  -years wide, and possibly larger.
• 2:17:28
  And it is also one of the deepest voids known.
• 2:17:33
  Voids, like deserts, do contain the occasional oasis, a small dwarf galaxy or thin whisper filament floating through the
• 2:17:40
  darkness.
• 2:17:41
  But the Bootes void is different.
• 2:17:44
  It contains only 60 known galaxies within its incredible expanse.
• 2:17:50
  Beyond these voids lie thousands more, mapped and catalogued with our most advanced digital surveys, like the Sloan Digital Sky
• 2:17:58
  Survey and the Dark Energy Survey.
• 2:18:01
  And there are more yet to be known.
• 2:18:03
  Even with all of our advancements, all of our sophistication, and all of our efforts, we have mapped less than 1 % of the volume of the
• 2:18:11
  observable universe.
• 2:18:19
  At great distances, the universe appears younger.
• 2:18:22
  It takes time for light to travel these incredible spans, and so we do not have access to those galaxies and structures as they exist in the present
• 2:18:30
  moment.
• 2:18:31
  But only as they were when they released their messengers of light, millions or billions of years ago.
• 2:18:37
  And so with truly deep surveys, we see the grand structure of the cosmos give way to its precursor state.
• 2:18:44
  At the distant rims of the observable universe, lays a land of darkness.
• 2:18:53
  These are the Dark Ages, the first few hundred million years after the Big Bang, time before the first
• 2:19:00
  emergence of galaxies before the first populations of stars.
• 2:19:05
  Where the entire cosmos consisted of not much more than pure neutral hydrogen and helium.
• 2:19:11
  That neutral gas emits a radiation of its own, a weak signal caused by the quantum exchange of spin within a hydrogen atom at a
• 2:19:19
  wavelength of 21 centimeters.
• 2:19:22
  That radiation has been stretched by the expansion of space over billions of years so that it now sits within the bands of the radio
• 2:19:29
  spectrum.
• 2:19:30
  Indeed, a small percentage of the static that you hear between radio stations in your car is due to this ancient, pre -stellar time.
• 2:19:39
  And finally, at the very borders of our cosmos, sitting roughly 45 billion light -years away, is the
• 2:19:47
  very last visible object.
• 2:19:54
  Arno Penzias and Roger Wilson, two radio engineers at Bell Labs in New Jersey, discovered this object accidentally in
• 2:20:02
  the 1960s.
• 2:20:04
  They'd just developed a sensitive microwave receiver and couldn't get rid of an annoying background hum.
• 2:20:10
  No matter the time of day, no matter where they pointed it, no matter the season, the hum persisted.
• 2:20:17
  They even thought it might be due to pigeon droppings as the birds enjoyed sheltering in the antenna.
• 2:20:22
  But even after exhaustive cleaning efforts, they could not get the hum to go away.
• 2:20:28
  Word spread in the cosmological community and the realisation struck.
• 2:20:34
  Penzias and Wilson had inadvertently stumbled across the background radiation from the Big Bang itself,
• 2:20:42
  cooled from a white -hot 10 ,000 Kelvin upon its release over 13 .77 billion years ago down to a mere three degrees
• 2:20:50
  above absolute zero in the present day.
• 2:20:57
  The mysteries of dark matter connect to Batuta's journey.
• 2:21:02
  Interviewee: And yet, despite the multifaceted successes of the Big Bang Theory as expressed through the FLRW model, its ability to
• 2:21:10
  explain the expansion of the cosmos, the formation of the elements, and the appearance of the cosmic microwave background, it does have
• 2:21:18
  its mysteries.
• 2:21:20
  Mysteries that keep cosmologists up late at night.
• 2:21:27
  The Earth Beginning in the 1930s with Fritz Zwicky's observations that the galaxies of the Coma Cluster were moving far too rapidly, suggesting that there was
• 2:21:35
  a hidden source of gravitational attraction that he could not see, and solidified in the 1970s with Vera Rubin's observations of other galaxies
• 2:21:43
  that showed the stars within them orbiting too quickly, astronomers came to the conclusion that the universe is likely filled with some new
• 2:21:50
  form of matter that does not interact with light or normal matter.
• 2:21:56
  Dubbed dark matter, this mysterious substance is yet to be identified, though it remains the most plausible explanation for
• 2:22:04
  a variety of cosmological observations.
• 2:22:11
  speakers: No.
• 2:22:12
  No.
• 2:22:13
  No.
• 2:22:13
  No.
• 2:22:13
  And so, it is clear we live in a vast, cold, empty universe.
• 2:22:20
  It has been evolving for 13 .77 billion years and is home to hundreds of billions of galaxies
• 2:22:28
  and that's just within our observable bubble.
• 2:22:31
  There may be additional galaxies, additional voids, more cosmic web and even entirely new universes beyond
• 2:22:39
  the observable horizon.
• 2:22:41
  We have learned much in our century of modern, physical cosmology, but there is still so much to learn out
• 2:22:49
  in the vastness.
• 2:22:52
  And Batuta had discovered that something was lurking in the Booty's void.
• 2:22:59
  Something unique, something that the spacecraft or any other alien spacecraft had never encountered before.
• 2:23:07
  It was only a guess, a patchwork of clues from the accumulated observations of its own and alien spacecraft.
• 2:23:15
  A high -energy cosmic ray here, a bizarre neutrino there, a dark matter particle where it shouldn't belong.
• 2:23:22
  But all the clues pointed to one origin, a place where there should be nothing at all, deep in the heart of
• 2:23:30
  a great cosmic void sitting nearly 500 million light -years away, on the far side of the Shapley
• 2:23:38
  Supercluster.
• 2:23:40
  Travelling there would take one or two billion years at the fastest, probably longer.
• 2:23:46
  And if Batuta was wrong, if it was chasing nothing more than a cosmic ghost, then it would be hopeless.
• 2:23:53
  By the time Batuta returned to the solar system, humanity would surely be out of time.
• 2:24:04
  Batuta if it was right...
• 2:24:14
  Batuta barely recognised itself anymore.
• 2:24:20
  It still had access to some of its deepest core memories, data files designed to withstand the worst of cosmic catastrophes.
• 2:24:28
  Its fundamental mission remained unchanged, but a lot had happened to the spacecraft in its nearly four billion
• 2:24:36
  years of existence.
• 2:24:39
  Batuta was currently plunged deep within the Bootes void, a vast cosmic desert that stretched for a seeming
• 2:24:46
  eternity, dwarfing all else in its neighbourhood.
• 2:24:49
  Before leaving the Shapley Supercluster and after resupplying in a distant galaxy that humanity only knew of as a brief catalogue designation,
• 2:24:58
  Batuta had constructed its masterpiece, a neutrino observatory nearly a light year wide.
• 2:25:05
  It had taken a monumental amount of effort and the resources of more than a few solar systems, but it was worth it.
• 2:25:11
  The ghostly particles slipping through the detector nets told a story, a tale of bizarre energies and powers beyond
• 2:25:19
  comprehension.
• 2:25:20
  Those particles gave Batuta the coordinates of the entity responsible, an artefact left over from the creation of the
• 2:25:28
  universe itself, a cosmic string.
• 2:25:37
  And all Alan Guth had wanted to do was banish these monsters.
• 2:25:41
  from the universe.
• 2:25:46
  Most people really want to know where we came from.
• 2:25:49
  We have evidence.
• 2:25:51
  We no longer have to rely on stories we were told when we were young.
• 2:25:56
  The year was 1979 and the Big Bang Theory was developing into its modern form, providing cosmologists with an
• 2:26:03
  understanding of the large -scale structure of the universe.
• 2:26:07
  The revelations provided by the FLRW model came from a simple statement.
• 2:26:13
  We live in an expanding universe, which means that it was smaller, hotter and denser in the past and it continues to grow larger
• 2:26:21
  and cooler today.
• 2:26:26
  At high enough energies, the weak nuclear force and electromagnetic force combine into a single entity, called the
• 2:26:34
  electro -weak force.
• 2:26:35
  And while not proven definitively, physicists believe that at even higher energies, in even earlier epochs in the first few moments of the
• 2:26:43
  cosmos, all the forces of nature combine into a single unified whole.
• 2:26:49
  But what was that era like?
• 2:26:52
  How did the forces fracture from each other and give rise to the variety of particles and carriers that we see today?
• 2:26:59
  Early attempts to probe this epoch with chalkboards and pencils, as no observatory could pierce the veil of the cosmic microwave background, produced
• 2:27:07
  uncomfortable results.
• 2:27:08
  It appeared that the earliest seconds of the cosmos should have been dominated by dramatic phase transitions, with spacetime
• 2:27:16
  itself quaking and shuddering, as each force, gravity, strong nuclear, weak nuclear, and electromagnetism all
• 2:27:23
  fractured from each other.
• 2:27:25
  And, like all phase transitions, this process was likely imperfect.
• 2:27:38
  Inflation would go on to become a profound, if controversial, aspect of modern cosmology.
• 2:27:43
  Today, the vast majority of cosmologists believe that something like inflation happened.
• 2:27:48
  Besides doing away with the troublesome monopoles, inflation also easily explains why the universe appears to be nearly perfectly
• 2:27:56
  geometrically flat.
• 2:27:58
  At the very largest of scales, on average, parallel beams of light remain parallel, neither converging nor diverging.
• 2:28:06
  The original FLRW model states no preferences, but observations give us this oddly perfect value.
• 2:28:13
  And inflation provides the solution.
• 2:28:16
  inflation.
• 2:28:17
  After inflation, the universe could have any sort of geometry at once, but it's so unfathomably big, at least 10 to the power of
• 2:28:24
  60 times larger than what we can directly observe, that no matter what, it will always appear flat.
• 2:28:30
  The same way your neighborhood appears flat, despite the overall curvature of the Earth.
• 2:28:36
  Inflation also creates the seeds of structure.
• 2:28:40
  The cosmic web started out much less grand, as only microscopic fluctuations in density long ago.
• 2:28:47
  But where did those fluctuations come from?
• 2:28:50
  Inflation tells us that they originated in the roiling quantum foam of subatomic scales, exploded to macroscopic size
• 2:28:58
  during the event of inflation, and imprinting themselves on space -time.
• 2:29:04
  From there, gravity could do its work to build the superclusters and empty out the voids.
• 2:29:09
  Indeed, the predictions made from inflation about the properties of these structures exactly match what astronomers observe.
• 2:29:17
  But, cosmologists still do not yet understand what powered inflation, what triggered it, what caused it to end, and how it went on to
• 2:29:25
  flood the universe with particles and radiation.
• 2:29:29
  And then, there is still the problem of those ancient monsters.
• 2:29:35
  For inflation can successfully do away with the point -like monopoles, banishing them far beyond the realm of the observable.
• 2:29:43
  But they are not the only ones.
• 2:29:53
  In 2013, almost 50 years after their initial papers on the concept, Peter Higgs and Francois Englert won the
• 2:30:01
  Nobel Prize in Physics for their proposal of the Higgs boson.
• 2:30:05
  The final piece in the puzzle of the standard model, our best theory to explain how the universe works on subatomic scales.
• 2:30:14
  And yet, despite his joy at receiving the award, the very next year in an interview, Higgs would go on to express some disappointment.
• 2:30:22
  I really rather hoped before the announcement that they would make the number up to three.
• 2:30:27
  And there was certainly an obvious candidate to be the third.
• 2:30:31
  Tom Kibble.
• 2:30:33
  Thomas Kibble was one of the 20th century's greatest physics minds.
• 2:30:38
  On top of authoring a paper on the Higgs mechanism that was arguably as important as those written by Higgs himself, he was also a
• 2:30:45
  trailblazer in another headache -inducing field of theoretical physics, the topology of the universe.
• 2:30:53
  And it was in 1976 that he wrote a groundbreaking paper that predicted something truly bizarre within the mathematics
• 2:31:00
  of relativity.
• 2:31:03
  Cosmic strings.
• 2:31:11
  One -dimensional cracks in space -time left over from where the forces themselves split apart, their properties, as
• 2:31:19
  well as their existence, are only hypothetical.
• 2:31:22
  If they exist, they likely have the width of a proton but extend for hundreds of millions of light -years, possibly spanning the
• 2:31:30
  entire observable universe.
• 2:31:32
  Inflation and expansion push the monopoles away from each other but that same mechanism only serves to stretch the cosmic
• 2:31:40
  strings, making permanent whatever flaw they embodied in the early universe.
• 2:31:46
  According to our best calculations, the universe should be littered with cosmic strings.
• 2:31:52
  cosmic strings.
• 2:31:52
  And yet, as with magnetic monopoles, not a single one has been found.
• 2:31:57
  One paradox amongst many that defines modern cosmological research.
• 2:32:09
  Cosmic strings, if they exist, are perhaps the strangest inhabitants of the cosmos.
• 2:32:15
  Stranger even than neutron stars or black holes.
• 2:32:19
  They are defects in space -time, flaws in fundamental topology.
• 2:32:25
  They are made of nothing, the same way that a wrinkle in your favourite shirt isn't made of anything, it's just your shirt in an
• 2:32:33
  uncomfortable arrangement.
• 2:32:35
  Because of this, they exert no gravitational pull of their own but still make their presence known through their distortions in space -time.
• 2:32:43
  Indeed, if you were to orbit around one in a perfect circle, by the time you reached your starting point, you would find that you had travelled less
• 2:32:51
  than 360 degrees.
• 2:32:54
  If you looked at distant starlight through a string, the defect would split the light, giving you a duplicated image of the background.
• 2:33:02
  The distortions in space -time caused by a string mean that they have an energy density and it's possible to equate this energy density with an
• 2:33:10
  equivalent amount of mass.
• 2:33:12
  While they have a width no greater than a proton, a single kilometre of cosmic string would outweigh the entire planet Earth.
• 2:33:19
  And that's just one tiny portion of their universe -spanning lengths.
• 2:33:24
  If they were to encounter the Earth, they would simply slice through it like a hot knife through butter, splitting our world without even
• 2:33:32
  slowing down.
• 2:33:34
  And indeed, cosmic strings are anything but static.
• 2:33:38
  They wriggle and dance and vibrate, the ripples racing up and down their lengths at the speed of light.
• 2:33:44
  When they cross each other or loop over themselves, they break in a fury of released energies.
• 2:33:51
  Closed loops would eventually vibrate themselves into oblivion in a fury of gravitational waves.
• 2:34:05
  We do not understand the physics of cosmic strings or how they interact with the rest of the known universe.
• 2:34:12
  We do not know if they can emit radiation or charged particles, if, when they vibrate, they convert to some exotic
• 2:34:19
  form of dark matter or create a neutrino.
• 2:34:23
  All we currently understand about cosmic strings arises from pure speculation, a product of our halting and unsure
• 2:34:31
  probes into the mathematics of unified physics.
• 2:34:37
  cosmic strings and the cosmic strings Their width is determined by the physics of the splitting of the strong nuclear force, its exact
• 2:34:44
  mechanism, unknown.
• 2:34:46
  Their length is set by the expansion of the universe, spanning from one end of the observable cosmos to the other.
• 2:34:54
  Indeed, we can only guess as to how they would manifest in the modern day universe.
• 2:34:59
  The only thing we can say with relative certainty is that almost all models and theories of the ancient cosmos
• 2:35:06
  predict them to exist.
• 2:35:09
  Even string theory, the theory of everything that replaces point particles with tiny, multidimensional loops of vibrating strings, gives
• 2:35:17
  rise to them, envisioning them as quantum strings blown to cosmological proportions.
• 2:35:24
  We can, however, divine some of their properties through our understanding of gravity.
• 2:35:30
  This is how we know that ripples zip up and down along their lengths, emitting gravitational waves as they go, or how, when a piece of string loops
• 2:35:38
  in on itself, it will quickly vibrate itself into oblivion, emitting a shower of energy in the process.
• 2:35:44
  But that, however, is the limit of what we can confidently say.
• 2:35:59
  cosmic strings are the physical manifestation of our ignorance.
• 2:36:04
  We do not understand the earliest moments of the universe or the nature of the most powerful events in the cosmos.
• 2:36:10
  They are beyond the bounds of known physics, and the limitations apparent in the universe, the limit of the speed of light, the inability
• 2:36:18
  to travel back in time and more, are only reflections of our current level of understanding, as sophisticated as that understanding
• 2:36:26
  can be.
• 2:36:28
  We do not know what would be possible with a cosmic string.
• 2:36:32
  What we would learn should we encounter one of these demons of old, creatures of a long -forgotten universe that should not, by all accounts,
• 2:36:40
  persist until the present day.
• 2:36:44
  Perhaps our universe is safe, the cosmic strings not an artifact of the early phase transitions, but a ghost.
• 2:36:51
  Perhaps they were indeed born in abundance, but then faded into obscurity.
• 2:36:56
  Or, perhaps, they are out there waiting for us to unlock their secrets.
• 2:37:15
  Batuta beheld the cosmic string in all its enigmatic glory from a respectable and safe distance.
• 2:37:26
  Though it stretched for millions of light years in each direction, it was almost a ghost, barely perceptible along its length except for the splitting
• 2:37:34
  and magnification of light.
• 2:37:37
  And so the spacecraft spent what felt like an eternity simply watching.
• 2:37:45
  Nothing it had ever found, nothing it had studied, not even anything it had created in its solar system scale experiments
• 2:37:52
  compared to this.
• 2:37:55
  It was like Batuta was staring back almost 18 billion years to the beginning of the Big Bang itself.
• 2:38:04
  If there was an answer to be found in this universe to humanity's salvation, some new source of energy or means of travel,
• 2:38:12
  it would be there, in this flaw in space -time made manifest.
• 2:38:19
  The spacecraft began by shedding its outermost layers, its remote sensors, its outriggers, its supply sections, trimming
• 2:38:27
  itself down until only its core functionality, its deepest memories, a summary of its long history and its core personality
• 2:38:35
  remained.
• 2:38:38
  Then it turned to face the nearest section of string, accelerating as it did so.
• 2:38:43
  As it neared the string, Batuta noticed with amusement that it almost felt giddy, its movements effortless, like the bizarre
• 2:38:51
  contradiction of physics in front of it didn't even exist.
• 2:38:56
  It plunged forward, and in a blink slipped into darkness, its final thought once again Carl
• 2:39:03
  Sagan's words on the Voyager 1 spacecraft, so many years before.
• 2:39:13
  Far from home, untouched by these remote events, the Voyagers, bearing the memories of a world that is no more,
• 2:39:22
  will fly on.
• 2:39:29
  Every
• 2:39:41
  hour, Thank you.