The Ultimate Guide To Absolutely Everything In The Universe

[Music] It had seen planets crumble and stars ignite. It had skimmed supermassive black holes and galaxy-destroying quasar jets. It had crossed light-years of nothingness and caught a glimmer of the edge of the universe. And now, more than 4 billion years after its epic journey had begun, the probe had returned home.

The sun was a swollen red giant, grotesque and violent. Mercury and Venus were gone. Earth, such as it was, survived, but only as a tattered lump of iron and nickel orbiting dangerously close to the dying star. Humanity, barely a 100 million in total, had found a new home on Europa. The heat from the red sun had melted its ices long ago, and now the little moon of Jupiter was the only sanctuary they could reach.

It took the probe several minutes to decipher the strange language of the radio signals they sent. It seemed the remaining humans were abuzz with curiosity, tinged with hope. They remembered the probe, even though it had been launched so long ago. Memory of the event, miraculously preserved through the ages. Humanity, they said, had tried and tried again to spread amongst the galaxy, but every attempt at a colony had failed. They could send machines out into interstellar space, but every time they had attempted the crossing for themselves of the great void between the stars, their frail bodies had failed them. As far as the humans knew, they were all that was left. And as the beating red sun at the heart of their solar system constantly reminded them, they were running out of time.

[Music] But they also knew that long ago, interstellar spacecraft like the returning probe had been created, fashioned with artificial intelligence as keen and sharp as any biological brain to go forth into the galaxy and the greatness beyond. These probes had been tasked with cataloging everything, yielding the history and secrets of the universe in a way that no mid-telescope ever could. From the relative proximity of the inner solar system to the most distant, alien, and bizarre things in the cosmos, the probes had been told to see it all. The largest, the most powerful, the weirdest, all of it. And all of this searching, all of this cataloging, all of this measuring was to be in service of a greater goal: to find a way for humanity to travel beyond the solar system. Over the unfathomable ages, millions of spacecraft had been sent. But only one had returned. And so, fulfilling its promised mission to its long-dead creators, the Lonely Probe began to tell its multi-billion-year story, relating what it had learned, the extraordinary events it had encountered, the remarkable places it had been. It began to describe everything within the universe.

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[Music] Mission parameter one: search for signs of life outside the solar system. The probe had called itself Batuta. It had taken the name from the great 14th-century explorer Ibn Battuta, who had traveled over the course of three decades across the entire known Muslim world in his lifetime, covering almost 117,000 km and handily beating out other great explorers such as Marco Polo and Zheng He. The spacecraft knew that such a name would be fitting as it ventured out into the seemingly endless black. But before it could begin its journey beyond the solar system, it had to pass a series of [Music] tests. 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 thought and autonomous action outside of any human control. Those who failed would be stuck forever inside the solar system. 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 clever experiments to divine nature's most closely held secrets.

And so it began its trials on Titan, the largest of Saturn's moons and one holding remarkable secrets. Secrets uncovered by a probe just like Batuta many millennia before. Titan was discovered by humanity in 1655 when pioneering astronomer Christiaan Huygens spotted it with his homemade telescope. Huygens himself was a key figure in the evolving scientific revolution and was also the first to attempt to scientifically explore the possibility of life on other worlds, writing, "Some planets indeed might be capable of accommodating several species of rational creatures possessed of different degrees of reason and sense." But little did Huygens know that Titan itself would be a candidate. And it was on that moon that centuries later, humanity achieved a great milestone of space exploration. In 2005, a small probe, not even 3 meters wide, broke free from its attachment harness on the left side of the Cassini spacecraft that had spent nearly seven years in its journey to Saturn. And it took the probe a further 22 days to coast from Cassini's orbit to Titan. This craft, named after Huygens, used a parachute to slow its descent through Titan's atmosphere. 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 atmosphere surrounding a rocky body. Huygens landed gently, making a dent in the icy surface before bouncing and sliding to a stop. The first time that an emissary of humanity had touched the surface of a world in the outer solar system. For Batuta, Huygens was a true pioneer. And indeed, the world it touched down on was unlike anything else that humanity had ever encountered. 50% larger than Earth's moon and 80% more massive, Titan is appropriately named. Its core is mainly rock, but its crust is made of ice. Surrounding that is a dense, thick atmosphere, even denser than Earth's, giving it an atmospheric pressure on the surface about 50% greater. Titan is a beautiful, ghostly, and enigmatic world, because it shares one thing in common with the Earth that all other worlds in the solar system lack: liquid. Titan has rivers, streams, estuaries, deltas, and every other feature associated with surface liquids. You can stand on the shores of a lake, its shimmering surface stretching beyond the horizon, feeling the gritty sand beneath your feet as waves lap at your ankles. And all of this at a temperature almost 200° C below zero. And so the lake stretching out before you isn't one of water, but one of hydrocarbons. The rain falling from the sky isn't water droplets, but one component of a cycle that evaporates hydrocarbons like methane and ethane up into the atmosphere and precipitates them back down to the surface. But kilometers beneath your feet, through the ice crust that makes up the outermost shell of the moon, there is a liquid water ocean, warmed by internal heat and kept liquid through the presence of ammonia. Indeed, Titan is the only known world with liquid both on and underneath its surface, which makes it a tantalizing home for two kinds of alien life. First, the familiar, something that every organism on Earth shares: the use of water as a solvent. Water is the container in which life operates. It allows for molecules to mix and recombine. It allows for temperature regulation. It brings food closer and eliminates waste, and it helps cells maintain their shapes. But water is not the only liquid. The shallow seas and lakes and streams of hydrocarbons on Titan could serve a similar function. Ethane and methane acting as solvents for a strange sort of biochemistry operating 200° below [Music] zero.

And Titan is not alone when it comes to enigmatic underground oceans. Enceladus, a tiny moon of Saturn only 500 km across, also hosts large oceans. Europa, Callisto, and Ganymede of Jupiter all likely contain more liquid water than the Earth does. Even Pluto, clinging to scraps of internal warmth from radioactive decay in its core, might host a subsurface [Music] ocean. However, reaching these oceans is no easy feat. At the temperatures of the outer solar system, far from the light and warmth of the sun, water ice is hard as rock, ensuring that worlds like Europa and whatever secrets may inhabit those dark, world-girdling oceans are encased in ice sheets over 100 kilometers thick. Upcoming missions such as the European Space Agency's JUICE, short for Jupiter Icy Moons Explorer, and NASA's Europa Clipper, will not attempt to pierce the icy shells of these worlds. Instead, they will orbit and, if conditions are right, swim through the plumes of cryovolcanoes. For the icy moons, like Europa, are not sterile and still, but active and alive. Their crusts creak and groan as gigantic fields of ice grind against each other. A colder, slower version of the tectonic activity on Earth. If there is any biological activity in those mysterious oceans, the plumes of these frozen volcanoes may carry molecular signs of it. If enough elements can find themselves in sufficient concentrations, life may yet find a way in those lightless oceans. And those are not the only mysterious interiors to be found in the outer solar system. The Huygens probe transmitted 90 minutes of data back from the surface of Titan to the Cassini spacecraft before Cassini moved out of range and the signal was finally lost. However, the tiny probe was designed to withstand the environment on the moon's surface, so most likely remains intact today, a lonely human monument on an alien world. But before its daring mission on Titan, Cassini by itself had already captured many spectacular images of the outer planets, including a stunning color image of Jupiter's Great Red Spot, exactly 335 years after it was first discovered by the scientist after which the spacecraft got its name. Contrasting with the close companionship of their namesake pioneering probes, Giovanni Cassini and Christiaan Huygens were not exactly the best of friends during their time together in France. A few years after Cassini's arrival in Paris, Huygens remarked to his brother Constantine that Cassini was at the telescope every clear night and that Huygens would never want to do that, being satisfied with his earlier astronomical discoveries which, at any rate, were much more important than those of Cassini. And yet, despite Huygens' protestations, Cassini's commitment and methodical process would yield remarkable results. For example, Jupiter's Great Red Spot. This spot is a magnificent storm. A hurricane with wind speeds of over 400 km/h that has been raging for centuries. At its largest known extent, it could have swallowed the entire Earth three times over, although it's slightly smaller in the present day. But interestingly, recent studies of the measurements taken by Cassini in 1665 have actually shown that it may not be the same storm seen 400 years ago. It may be a distinct new storm less than 200 years old. Though iconic, it is clearly far from a permanent feature. This storm is not powered from the heat of the sun, but from the interior of Jupiter itself. Indeed, Jupiter releases more heat into space than it acquires from the sun. The result of a process known as the Kelvin-Helmholtz mechanism. Essentially, this mechanism is a leftover byproduct of the formation of the giant planet billions of years ago. As the planet cools off, it slightly shrinks. But as it shrinks, it compresses, heating it up. And that heat can only be released into space through its surface to allow the shrinking to continue. Since Jupiter is so gigantic, it has an enormous amount of volume compared to its surface area, allowing the planet to stay warm for billions of years to come. Jupiter's Great Red Spot is not the only violent storm in the solar system. While backyard astronomers can easily track the Great Red Spot with even simple telescopes, we require more powerful instruments and especially robotic probes to see the rest. Cassini would go on to operate a further 12 years after the demise of its sister probe Huygens on Titan before ending its life in a spectacular so-called Grand Finale, diving between Saturn and its rings in an effort to capture as much data as possible before flying straight into the planet. Its sacrifice deemed necessary as NASA scientists didn't want to risk polluting any of the moons of Saturn and their potentially habitable environments. And so on September the 15th, 2017, Cassini plunged into Saturn's atmosphere. Its long journey of almost 20 years and 8 billion km coming to a dramatic [Music] end.

But in those last years without Huygens, it had managed to capture a revealing, eye-catching image of its final resting place: an almost perfect hexagon on the north pole of Saturn. First spotted by the Voyager 1 spacecraft in 1981. Despite its neat geometric appearance, the hexagon is not some alien relic. Instead, it's the result of the complex wind system in the upper Saturnian atmosphere. The winds in the atmosphere do not blow at the same speed at every latitude. And this triggers an instability that forms a series of cyclones in a ring surrounding the North Pole. And they all work together to form a gigantic vortex at the pole itself, like a ring of friends on a merry-go-round, all helping out to spin it as quickly as possible. Though seemingly enormous, storms like the Great Red Spot and the Hexagon are actually most likely relatively shallow, only a few hundred kilometers thick. Yet underneath those cyclones are surely more violent storms and much stronger winds. A never-ending maelstrom of intense forces and energies. Indeed, plunging through the atmosphere of a gas giant like Jupiter or Saturn would require a spacecraft that is well beyond the bounds of human engineering. But were we to be able to, we would encounter a truly bizarre sort of [Music] treasure. On the Earth, we usually encounter hydrogen as a gas. With enough effort, we were able to cool that gas to just 20° above absolute zero, turning it into a liquid. But in 1935, the physicists Eugene Wigner and Hillard Bell Huntington predicted that at extremely high pressures, hydrogen can exhibit some extremely strange behaviors. At a pressure of around 4 million atmospheres—that's 4 million times the pressure you experience at sea level—hydrogen atoms destabilize with the protons forming a repeated lattice-like structure and the electrons free to roam amongst them. And this is exactly how we define the behavior of metals. And so at these pressures, hydrogen becomes metallic. When Wigner and Huntington first proposed this in 1935, it was just a matter of theoretical speculation. But it turns out that the interior of Jupiter has more than enough pressure to turn its bulk of hydrogen metallic. We don't yet know how deep this mantle of metallic hydrogen goes within Jupiter and Saturn's interior, but we suspect that it takes up the vast bulk of their volumes. In other words, most of Jupiter and Saturn is metallic hydrogen. And this means that since those giants are the largest planets in the solar system, most of the material of all the planets in the solar system is in a state completely inaccessible to our experience on Earth. And yet, as with the surface of Titan before it, daring space probes have been able to help us here, too. NASA's Juno mission, one of the next generation of probes to Cassini and Huygens, and entirely solar-powered, was launched in 2011 and has carefully monitored the gravitational environment around Jupiter to paint a fuzzy picture of that planet's deep interior. It has found that it's very likely that Jupiter has no solid surface to speak of, just an inferno of gradually changing temperatures, pressures, densities, and compositions of matter.

[Music] And so, brutally strong magnetic fields and radiation, temperatures varying from almost absolute zero to tens of thousands of degrees, lakes of liquid methane, sheets of water ice 100 km thick, exotic forms of hydrogen, and storms raging for centuries on end. But that was only a taste of the wonders and dangers that abound in the wider universe. For Batuta, this was just the beginning. It had spent the better part of a century navigating the trials of the outer solar system, devising schemes to drill through the icy outer layers of Europa, sending semi-autonomous probes into the metallic hydrogen inferno of Jupiter, skipping through the storms of Saturn, and more. And so, after proving its worth, its ingenuity, and its patience, Batuta had finally been permitted to exit the gates of the solar system in the full knowledge that the worst was yet to come. As it considered the sacrifices of the heroic probes that had come before it—the Cassini, the Huygens, the Juno—its curious mind turned to their predecessors, launched by NASA decades before, who had gone even further: far beyond Jupiter, far beyond Saturn, far beyond the outer worlds altogether. For as strange and extreme as our own solar system is, we are not by far the strangest of systems out there. There are even weirder, more dangerous places. But to get to them, to explore their beauty and understand their secrets, Batuta would first have to travel the great gulfs between the stars. After only a few decades of cruising, Batuta had finally reached the long-established threshold, the heliopause, the boundary between the solar wind and the interstellar medium, a feat first achieved in ages past by the Voyager 1 probe in the opening years of the 21st century. Batuta could hear the ancient greetings etched in the famous Golden Record, a compilation of voices, languages, sounds, and music of planet Earth, hastily assembled in August 1977, just before the probe's launch by the legendary scientist and science communicator Carl Sagan and his collaborators. Indeed, the designers of this mission understood its significance. The probe would, decades after launch, become the first human-made craft to enter interstellar space. Musing on Sagan's words at the time, Batuta considered its own mission as it passed this remarkable milestone. "Perhaps the records will never be intercepted. Perhaps no one in 5 billion years will ever come upon them. In 5 billion years, all human beings would have become extinct or evolved into other beings. None of our artifacts would have survived on Earth. The continents would have become unrecognizably altered or destroyed. And the evolution of the sun would have burned the Earth to a crisp or reduced it to a world of atoms. Far from home, untouched by these remote events, the Voyagers, bearing the memories of a world that is no more, will fly on."

However, the mission had almost ended in disaster. Early in its journey, Voyager 1's scan platform jammed, removing the probe's ability to orient itself in space. Luckily, engineers were able to fix the issue by waggling the scan platform back and forth. But for years, during its outward cruise, they worried and fretted that all their efforts would be for naught. The mission, however, was a roaring success. Along with its twin, Voyager 2, humanity got its first close-up, high-res view of the outer planets. And that lonely probe, now long since lost to the infinite blackness of the space between the stars, was but the first of many. 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 line. By carefully arranging its trajectory in a trick known as the gravitational slingshot, the spacecraft was able to steal some of the momentum of the giants for itself, giving it more than enough speed to escape the sun's gravity permanently. After that, it was left to its own devices as it cruised through the galaxy, orbiting its center, just like every other star. But Batuta had other ideas. It was aimed in a general direction, but had not yet decided its first target. There were so many to choose from. Indeed, it had more than enough time to decide. The passage through the outskirts of the solar system would take several thousand years. And so, Batuta busied itself with another task. It fashioned a thin metallic wire and unspooled it behind the spacecraft until it reached a length of a thousand km. It was a long but simple antenna, and through it, Batuta listened to the cosmos. It heard the groans of giant molecular clouds. The screams of stars torn apart by black holes. The subtle whirring of the Milky Way's magnetic field. The drumbeat of distant pulsars. Sometimes placid and sometimes riotous. It picked up a universal symphony, the perfect background music for the first leg of its journey. 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. 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 distinction at the edge. And we know this because of the behavior of plasma. Normally we associate plasmas with dense, hot environments like a bolt of lightning or the surface of the sun. However, in the outer depths of the solar system, the plasma environment is still hot but incredibly thin. And this plasma environment around the sun is dominated by its solar wind. The entire solar system sits in a bath of these particles. But eventually, the solar wind slows down enough that it cannot sustain itself against the interstellar medium, and this supersonic flow comes to an abrupt and untidy halt. This boundary is known as the termination shock, 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 the sun. One astronomical unit being the equivalent of the distance from the sun to the Earth. Past the termination shock is a region known as the heliosheath, where the solar wind slows down and begins mixing with the interstellar medium. And the outermost layer of that region, the border with true interstellar space, is the [Music] heliopause. Voyager 1 reached this boundary on August the 25th, 2012, at an incredible 18 billion km from the sun. And there it found that interstellar space is [Music] different. For the sun's solar wind and giant magnetic field shield the solar system and its inhabitants from the toxic environment beyond. A toxicity brought about by the presence of deadly interstellar cosmic rays. And to understand cosmic rays and what they mean for our survival on Earth, we have to go back to 1912 and an Austrian physicist named Victor Hess. In the 19th century, scientists had discovered that air was seemingly being ionized by excess radiation. Radiation of which they simply couldn't pinpoint the source. It had been believed that this radiation had been coming from the Earth, from the ground beneath our feet. And so measurements had been taking place at higher and higher altitudes to test this idea. 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 up into the sky. He then followed this experiment by sending in his balloon to a height of 5 km, but with one key difference: he performed his experiment during an eclipse. He saw no major difference in the quantity of radiation. Both the Earth and sun consequently eliminated as the cause. The results were clear and earned him a Nobel Prize in 1936. He had pinpointed a source of radiation coming from space. Just about every high-energy process in the universe creates cosmic rays. Some are created near the surface of the sun, others come from distant supernovae. Still others from quasars or magnetars or hypernovae. But because cosmic rays are charged particles, they can be deflected and reflected by magnetic fields. 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 these cosmic threats. But the defense is not total. High-energy particles encounter a typical human body roughly once every second. While most of them just harmlessly pass through, sometimes they strike a strand of DNA or oxidize a random molecule in a cell. If the damage done is left uncorrected, it can turn into a replication error, resulting in the growth of a cancer. Indeed, in 2023, Voyager 1 went silent for 5 months, possibly as a result of cosmic rays interacting with its computer. And these dangers get even worse beyond the heliopause, where not even the sun can offer its protection. But despite this, there are still objects here, lonely denizens whose only knowledge of the sun is that of a prick of light, a star slightly brighter than the others. "Young man, I'm afraid you are wasting your time. If there were any more planets, they would have been found long before this." In later years, astronomer Clyde Tombaugh would recall being told this in 1929, the year before he discovered Pluto. 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 as a planet only through its subtle movements against the background of stars. But over the decades, many astronomers hypothesized that Pluto was not alone, that there might be an entire population of objects, planet-sized and otherwise, in the fringes of the solar system. And it wasn't until 1992 that this hypothesis was confirmed when a pair of astronomers, David Jewitt and his graduate student turned collaborator Jane Luu, after half a decade of painstaking searches, found a companion to Pluto. The object would soon get a name: 15760 Albion, and it has since been joined by thousands of others in a population known as Trans-Neptunian Objects (TNOs), which stretch from roughly 30 astronomical units to an orbital distance of hundreds of thousands of astronomical units. This population, with subregions known as the Kuiper Belt and Scattered Disc, is the home of comets that make regular, frequent visits to the inner solar system, like the famous Halley's Comet. While these regions have a few larger members—Pluto, Sedna, Haumea, Makemake, and others—the vast majority of Trans-Neptunian Objects are small objects made of frozen volatiles like water, ammonia, and nitrogen. Little is known today about the populations of the Trans-Neptunian Objects, but on January the 1st, 2019, we got our first detailed look at a smaller sibling of Pluto when NASA's New Horizons spacecraft slipped past Arrokoth, a Trans-Neptunian Object only a few dozen kilometers across. Arrokoth, which sits about 45 astronomical units from the sun, is almost completely unlike anything found in the inner reaches of the solar system. Its oblong shape is due to the fact that it looks to be two small bodies caught mid-merger. Long ago, two icy bodies found each other in the abyss and collided, but at extremely slow speeds, just enough for the ices between them to melt and then immediately refreeze, creating a glue that would bind them together for billions of years. Today, the planet Neptune dominates the gravitational environment of the Trans-Neptunian Objects. Most of these objects live quiet, isolated lives, never encountering another of their kind for millions of years on end. But ever so occasionally, Neptune's gravity pulls in just the wrong way, destabilizing a Trans-Neptunian Object and turning it into a comet, a ball of ice and rock that loops around the sun, never lasting many orbits because of the intense heat of our star, evaporating material with every passage. And so out here in the dark and the cold, these scattered worlds remain. But that is not quite the end of the solar system. The air is cold and crisp on a fresh spring night on the shores of Noordwijk in the Netherlands. A young boy is sitting with his father, staring up at the clear sky, watching with amazement as a star streaks across the black. The year was 1910. The streaking star was Halley's Comet, and the young boy would go on to be the famous Dutch astronomer Jan Oort, one of the few lucky people to spy the comet twice: once in 1910 and a second time 76 years later in 1986 from the window of a plane. Oort would go on to be a hugely successful astronomer, but it was comets that would be his lasting legacy. Comets that took much longer than 76 years to wait for their return. It was in 1950 that Oort revived a suggestion from decades before and created a coherent, plausible theory to explain the origins of long-period comets. Unlike comets sourced from the Kuiper Belt and Scattered Disc, these long-period comets seem to take thousands of years or more to make a single circuit around the sun. 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 system. Since these comets could not survive for long when they came within the intense glare of the inner solar system, Oort reasoned that they couldn't have been on these orbits since its inception. There must be a reservoir, he thought, a cloud encircling the solar system in all directions that provided a steady supply of inbound comets. And so, after its discoverer, this mysterious source was named the Oort Cloud. Objects beyond the heliopause, sitting in interstellar space but still weakly chained to the gravity of the sun. However, to date, no object has actually been confirmed definitively to exist within the Oort Cloud, save for the comets that occasionally make their way into the inner system. Its members are simply too far away and too small to be resolved even with our most advanced telescopes. But based on the sizes and frequencies of long-period comets, the Oort Cloud likely contains roughly five Earth masses worth of material. As to its extent, its inner boundary is somewhere around 2,000 astronomical units from the sun. That's 20 times further away than the heliopause. Estimates suggest that the lonely Voyager will finally encounter it in roughly 300 years. Its outer boundary, however, is unknown, with estimates ranging up to 200,000 astronomical units, or over three light-years, most of the distance to our nearest neighbor star. Beyond our solar system is a neighborhood of stars, many of which formed from the same cloud of gas and dust that gave rise to our own sun. Within a radius of 20 light-years, there are 131 known objects, including stars, white dwarfs, brown dwarfs, and subdwarfs, which straddle the boundary between the largest planets and the smallest stars. And while Proxima Centauri is currently the nearest star to the solar system, this won't always be the case, as all the stars in our vicinity, along with the sun, are in constant motion. In addition to orbiting around the galactic center, they also move in random directions. Indeed, in roughly 1 and a quarter million years from now, the dwarf star GJ 710 will pass within 0.17 light-years of the solar system.

[Music] Surrounding our neighborhood of stars is a region known as the Local Bubble. It is, like the heliosphere, a region defined by the plasma environment of the interstellar medium. 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 system. Our solar system has been traveling through the Local Bubble for the past 5 to 10 million years. While its origins remain uncertain, most astronomers believe that a complex of clustered supernovae detonating one after the other in quick succession pushed out this void of plasma. Indeed, remnants of these supernovae in the form of rare isotopes of iron, manganese, plutonium, and aluminum have been found embedded in the seafloor and in the Antarctic ice sheet. And so Batuta passed the millennial-long journey in silence. It monitored the changes in density and temperature as it began its first navigations through the heliosheath and out into the interstellar medium, through the Local Bubble. It felt the sting of cosmic rays and diligently repaired the damage. It considered its long-distant ancestor, Voyager 1, and its later siblings, bravely venturing out into the endless black. Behind it, the sun grew small and dim, till eventually, as far as its instruments were concerned, it became just another star. After 10,000 long years, Batuta was now plunged into the void, the great depths between the stars. Its interstellar journey had truly begun.

[Music] [Music] A hundred thousand years had passed since Batuta had first left the Earth. And roughly halfway through that journey, the little probe had decided to change direction. 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. But Batuta had decided it was worth it. The spacecraft was an avid student of human history and had been pouring over records, books, charts, digital files, and everything else that its creators had uploaded to its systems. And one star in particular had caught its attention: Algol, the demon star. 3,200 years ago, an Egyptian list of lucky and unlucky days had counted it as a star that brought bad tidings. In Greek mythology, the star represented the decapitated head of Medusa, with Perseus the hero holding it triumphantly alongside him for the world to see. And in Chinese astronomy, the star was a member of five, representing a mausoleum. Within them was the star that roughly translates as "piled up corpses." The star, of course, had a more proper scientific name: Beta Persei, the second brightest star in the constellation Perseus. But in the 17th century, its name took on a new meaning when Italian astronomer Geminiano Montanari discovered that Algol, the demon star, would eerily change its [Music] brightness. More than a century later, keen astronomers noticed that this change happened periodically. Roughly every 10 days, Algol would suddenly dim for a period of hours before returning to normal. But it wasn't until the 1800s that astronomer Edward Pickering of Harvard Observatory, who famously hired a team of women—his Harvard Computers—to analyze stellar spectra, discovered the cause. Algol's dimming wasn't caused by one star, but two: a bright, hot main star orbited by a smaller companion. And so, from our perspective, when the companion crossed in front of its parent, the entire system appeared dimmer. While seemingly a simple solution, Algol's present configuration betrays a history of violence. 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 sequence, while Algol B, the smaller companion, is a subgiant nearing the end of its life. But this is the opposite of what should happen. Larger stars burn through their hydrogen faster, and so Algol A should be nearly dead. The answer to this puzzle is a process with the fairly dull name of mass transfer. 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, allowing its companion to steal some of the hydrogen for itself, like a vampire preying on its own sibling. Algol is just one of many known systems hosting multiple stars. The simplest of these are simple binaries, either with two stars orbiting a common center of mass or a smaller star orbiting around a larger parent. The most complex systems known have up to nine stars, all orbiting around each other in complicated, intricate dances. For example, Alpha Centauri is actually a triple system. There are two stars that orbit closely together. Because the system is close enough to Earth, backyard telescopes can resolve the pair, with a third Proxima orbiting at a great distance. Other kinds of arrangements are far less stable, with their motions chaotic and unpredictable, competing for influence with each other. But Algol and multiple star systems aren't the only curiosities within our neighborhood of the Milky Way.

[Music] There is no better place to see the universe than from the deck of a ship. Out on the ocean on a clear night, if the moon is waning, stars scatter across the sky like a shower of light, revealing their secrets from all sides. 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 would have had on a young, curious mind far from home and hungry for knowledge. Born in 1784 in Münden, then in the Kingdom of Prussia, Friedrich Bessel was not your typical astronomer. 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-export business. But this proximity to shipping and his time aboard sea vessels had sparked a curiosity in navigation, which then led him to an interest in astronomy. Before long, his natural ability was recognized when he made successful measurements of comets. And then he had to make a choice: a relatively comfortable life in commerce or a less comfortable life as an assistant in a private observatory. He chose the stars. His target was 61 Cygni, the "flying star" of classical astronomy, named for the speed with which it appeared to move across the Earth's sky. He targeted 61 Cygni because he guessed correctly that its fast apparent motion was a clue that it was closer to the Earth than other stars and so a good candidate to measure a distance to. But when he finally calculated the distance, it was so extreme that he had to devise a new measurement system to describe it. He could have used miles or kilometers or even multiples of the distance between the Earth and the Sun. But all of those conventions resulted in frustratingly large numbers. And so he created a new term: the light-year, the distance light travels in a year. With this new nomenclature in hand, he could announce the distance to 61 Cygni: a neat 10 light-years away. But 61 Cygni is not the nearest star to Earth or the strangest. Not by a long shot. For example, Betelgeuse, the 10th brightest star in the sky. Despite its extreme distance of roughly 450 light-years away, Betelgeuse is both massive and gigantic. It weighs at least 15 times the mass of the sun, and its diameter is almost unparalleled, over 750 times the width. If it were placed in our own solar system, it would engulf Mercury, Venus, Earth, Mars, and most of the asteroid belt. Indeed, Betelgeuse is so bloated and so distorted that it's no longer a sphere. Astronomical imaging reveals a lumpy, engorged surface that boils with the turmoil underneath it. At late 2019, the star unexpectedly dimmed, going from one of the top 10 brightest stars in the sky to ranking below 20th place. The cause of this sudden dimming was likely a plume of superheated plasma that the turbulent star ejected into space. The plume cooled temporarily, forming an orbiting dust cloud that obscured its brightness. Once the debris cleared, Betelgeuse returned to its former glory. But Betelgeuse will not last much longer. It will soon end its life, likely sometime within the next 100,000 years, exploding in a tremendous supernova. Theoretical calculations suggest that at its peak, it will be as bright as the half moon, making it brighter than every other star and every planet in the night sky. It will be a fierce point, uncomfortable to look at, producing enough light in the middle of the night to comfortably read a book by. And yet, within a matter of weeks, it will fade into irrelevance, no longer visible to the naked eye, and Orion will stand in the night sky without his shoulder.

[Music] But the vast majority of stars are not giants. They are either mid-sized stars like the sun or smaller red dwarfs. And that includes Proxima Centauri, the closest star to the solar system, a red dwarf that can only be viewed through a telescope. The star itself is rather unremarkable, except for one thing: it carries a planet with it. That planet is a small rocky world, roughly the size and mass of the Earth, orbiting within the star's habitable zone. 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 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 home outside the solar system. 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 prerequisites, such as liquid water or a stable atmosphere. To maintain habitability, planets must hug close to their parents. In the case of Proxima b, that world orbits within 0.05 astronomical units of its star, much closer than Mercury does to the sun. 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. But if life did find a home there, it would not be an easy one. Red dwarf stars are especially temperamental, prone to star spots that can cover half their surfaces at a time and bouts of rage that flood their local environments with electromagnetic storms of plasma. Their planets would alternate between periods of extreme cold and bursts of intense radiation. And on top of this, in all likelihood, Proxima b is tidally locked with one face permanently pointed towards its star. This means that Proxima never sets nor rises on the planet, and half the world is locked in never-ending night with the other always in daylight. But even this curious case does not automatically represent a dead world. Computer modeling has shown that even these planets can sustain life. The key is for there to be sufficient water and atmosphere to circulate heat around the whole planet so that the dayside doesn't roast in the glare of the star and the night side's atmosphere doesn't freeze and catastrophically collapse. And the most fragile but intriguing case is that of the so-called ribbon world, a term coined by science fiction author Arthur C. Clarke. Ribbon worlds are only habitable in the terminator regions of the planets, the twilight zone, with the center of the dayside a scorching desert and the night side a frozen wasteland. But if conditions are just right, there can be a balance between the two, opening a thin strip girdling the planet where temperatures remain stable, heated from one side and cooled from the other.

[Music] And so Proxima b is just the beginning. There are an endless variety of worlds in the galaxy. 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. On the upper end, there could be trillions inhabiting our galaxy alone. There are surely duplicates of the solar system out there with four rocky inner worlds and four giants. There are also much more bizarre arrangements like the TRAPPIST-1 system, which consists of seven rocky planets all in close orbit around their parent star, three of which are in the habitable zone. There are giant planets so close to their stars they are actively being destroyed. There are super-Earths and mini-Neptunes. There are planets covered in thick hydrogen hazes surrounding surfaces covered in water, the Hycean worlds. There are planets so hot they are glowing and worlds encased entirely in ice. And there are even rogue planets, worlds with no system to call home, flung out into the black. Of course, there was no way that Batuta could ever hope to catalog them all. And so, after its brief 10,000-year detour to the Algol system, it stuck to its planned trajectory, visiting a string of systems. As the spacecraft made its way through the Orion arm and inwards towards the galactic core, 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 here. For that, it would have to reach even further towards the core of the Milky Way itself and beyond. Mission parameter two: find a way to escape the Milky Way. An internal alarm alerted Batuta. At that moment, it had just passed 1 million years since its awakening as a conscious entity, its birthday. Currently situated midway through the Scutum-Centaurus arm, the spacecraft was roughly 15,000 light-years from the core of the Milky Way. It had nestled itself near a protoplanetary disc, a ring of dusty material that was in the beginning stages of planetary formation. Batuta had stopped here several thousand years ago to prepare itself for the next stage of its journey. It had originally intended to proceed onwards as quickly as possible, but the process of planet formation was too good to ignore. Even with the eons of recorded human history before it began its journey, civilization had not yet existed long enough to watch the process unfold in real time. And so Batuta decided to wait and watch. It would prove to be worth [Music] it. The story of planet formation is the story of life. And this story begins with the raw elements that make life possible: hydrogen, carbon, oxygen, nitrogen, and phosphorus. Without this basic quintet, life as we know it would simply be impossible. The hydrogen, of course, is the easiest. It was created in the fires of the Big Bang at the beginning of the universe, a nucleosynthetic era that for a few precious minutes converted a soup of quarks and gluons, some of the most basic constituents of matter, into a population of hydrogen, helium, and some lithium and beryllium. However, as the universe expanded and cooled from that inferno, these reactions stopped, freezing the population of light elements in its place. 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? How did carbon, iron, and everything else come to [Music] be? 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. Scientists almost never work alone. 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 along the way. While some collaborations for giant experiments or telescopes can stretch into the hundreds, most groups are much smaller. A handful of individuals working together on a common problem to discover something new.

And so in 1957 at Cambridge University, four scientists gathered who would go on to change how he understood the universe forever. The paper was known as the BS squared FH paper, named after its authors. The B squared stood for Margaret and Jeffrey Burbage, the iconic husband and wife astronomy duo. She would later go on to discover the farthest object ever witnessed in 1974, a distant quazar, and would also find that the galaxy M87 had a super massive black hole at its center, all whilst fighting discrimination in astronomy. 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 on site. Vera Rubin, another iconic female astronomer, relates the story. With the usual adaptability, Jeff, a theorist, took the Mount Wilson Fellowship and Margaret the Kellogg appointment. Not surprisingly, whenever Jeff went off to Mount Wilson to observe, Margaret coincidentally appeared.

The F in BQ FH stood for legendary train enthusiast William Fowler. 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 students and collaborators for his 60th birthday, a working scale model of a British tank engine. And finally, the H stood for the infamously outspoken physicist Fred Hoyle, who 8 years earlier had actually coined the term Big Bang Theory and would go on to be knighted for his work in astronomy.

Their coming together in 1957 would show that it was stars that were responsible not only for the light that illuminates the universe, but for the forging of its elements. All stars survive by burning hydrogen in their cores, leaving behind a nuclear ash of helium. Towards the end of their lives, stars resort to burning this ash, fusing helium into carbon and oxygen. For smaller stars like our sun, this is the end of the nuclear road, igniting in their deaths as supernova explosions. And it is those supernova explosions that are the factories with the energies needed to produce even heavier elements, filling out the rest of the periodic table.

Since this landmark paper, the story of nucleosynthesis has been modified and updated. For example, in 2017, astronomers got their first multi-messenger view of a kilonova, a merger of two neutron stars. In the course of only a few minutes, a network of astronomers around the world witnessed the same event in gravitational and electromagnetic waves across the spectrum. Thanks to that event, we now understand that kilonovae also contribute to the elemental population of the cosmos. But despite this, decades later, the story told by Woody Fowler, Fred Hoyle, and the Burbages remains largely the same.

With the death of every star, the cosmos becomes enriched with new elements joining the swirling interstellar mix, ready to participate in the next round of planet formation. Indeed, tens of thousands of different nebulae dot the Milky Way, decorating its spiral arms like ornaments strung on a Christmas tree. 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 animal shape in a cloud drifting by on a lazy afternoon.

Perhaps the most recognizable is the Horsehead Nebula, 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 relief as its density makes it appear black against the vibrant background. A little over three light-years across, the Horsehead Nebula is a region of active star formation. Its black clouds are a cocoon where newborn stars will soon emerge.

Or take NGC6537, more commonly known as the Red Spider Nebula, which sits several thousand light-years away. This complex of gas and dust is not the birthplace of a star, but a funeral pyre. The remnants of this star scattered outwards into two opposite regions, with each region forming thin spikes.

Planetary systems begin their lives as diffuse clouds of hydrogen, helium, and a smattering of heavier elements. These clouds, known as giant molecular clouds, dot every galaxy. And it was in the early 1900s that British physicist Sir James Jeans discovered a process that would allow for these clouds to turn into stars. Jeans proposed that if the balance of a cloud was affected, then it could begin a process of runaway collapse. That balance could be disturbed by anything. However, usually star formation is triggered in a cloud when it passes through the spiral arm of a galaxy. While spiral arms appear to dominate galaxies, they're really not that much denser than the gaps in between, containing roughly only 10% more material. The members of the galaxy, like the molecular clouds, orbit at different speeds, and so they pass in and out of the spiral arms. But when they pass through, a new round of star formation ignites, dominated in brightness by large stars. Stars that die off before the cluster makes its way through the spiral arm, making the arms appear brighter and giving a galaxy its visible shape.

Once a molecular cloud is set in motion, it can become destabilized and collapse with its own gravitational pull, triggering a feedback cycle. Most of the material coalesces towards the center where friction heats it to high temperatures, becoming a protostar. The material that surrounds it spins and eventually forms a flat plane around the protostar, the beginnings of a protoplanetary disc. Much of the disc is made up of hydrogen and helium but sprinkled throughout the swirling chaos is dust. And just like the dust that clings to the surface of your home, the dust in a protoplanetary disc sticks together.

Meanwhile, as the process of planet building is underway, the protostar in the center begins to transform. Initially, it's heated from the outside in as gas falls onto the core and slams into it. 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 itself from the inside out, releasing heat and energy in the form of radiation. And finally, after all the convulsions and chaos, the protostar ignites fusion in its core and settles into a hopefully long life.

Within the inner regions of the disc near the young star, only the heavier elements like iron, carbon, and silicon and their molecular combinations can survive. And so the inner regions of a solar system become the home of terrestrial rocky worlds. But beyond the demarcation line, the glare of the young star is feeble enough over the great distances to allow ices of water, ammonia, and nitrogen to remain solid. These ices mix with the dust and dirt, creating planetesimals that are far larger. And this increased gravitational pull sucks in massive envelopes of gas. The first planetesimal to form wins the gravitational prize, becoming the largest planet thanks to its ability to monopolize the gas of the disc. And so the next giants to form must fight amongst themselves over whatever is left. And these intense gravitational battles end up ejecting many planetesimals and even some planets out of the solar system altogether.

In our own solar system, the giant planets formed much closer towards the sun where there was enough material to create their massive sizes. But as the disc cleared out, the mutual gravity of the planets pushed them outwards to their present positions. Indeed, there may even have been a fifth large planet formed along with them that was ejected during this process.

Planets can even form independently of any star. The exoplanet W08550714 discovered in 2014 is one such example. It is gigantic, roughly 10 times the mass of Jupiter and sits only 7.2 light-years away from the Earth, closer than the vast majority of stars visible in the night sky. And sitting between these rogue planets and true stars, are the enigmatic brown dwarfs, first identified by a team of Caltech and Johns Hopkins astronomers in 1994. Brown dwarfs are large enough to briefly sustain fusion of deuterium in their cores, but not big enough to sustain hydrogen reactions and reach the main sequence. Instead, they simply exist. To quote Geneva Sculcani from the Caltech team that discovered the first, "It looks like Jupiter, but that's what you'd expect for a brown dwarf."

And so these processes repeating generation after generation, star after star across the nearly countless billions, fill each and every galaxy with a population of planets. And it was a delight for Betatuda to patiently watch this minor miracle unfold. But no matter how much it was entranced by this event, it couldn't stay forever. The Tuta had to resume its journey. And this time, it moved with purpose. It set its sights on the center of the Milky Way, through the innermost spiral arms into the dense core itself, with the sole purpose of coming face to face with the giant black hole and the distant darkness beyond.

After a journey of nearly 10 million years, Batuda had finally reached the outskirts of the core of the Milky Way. The galaxy's core is an elongated bar, and the spacecraft had found itself just past the inner arm. Though getting here wasn't easy, Batuta suspected that it was not the first craft to make it to these depths. Somewhere within the Norma arm, some 5,000 light-years rimward, the spacecraft had detected a faint staccato radio signal. At first, it had suspected it was just another pulsar, but the spectral signature of the radio emission didn't match. Instead, upon further inspection, the staggered rhythm was all too familiar. It was artificial, the kind of signal that Batutoa would have created itself.

And so, Batuda designed its own cooling card, spending 10,000 years carefully manipulating a dead system 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 type should have. Within about 20,000 years, humanity, along with any other spacecraft in this corner of the galaxy, would notice a sudden and strange spike in heavy elements polluting this star. Perhaps they would ascribe it to some natural phenomenon. Or perhaps they would assume it was the machinations of one of their wayward probes. Or perhaps they would not even notice it at all if they were even still there.

And so now, its journey so far spanning 10 million years, the Tuta had finally started to feel old. But still, after everything it had witnessed and experienced, it couldn't hold a candle to the most ancient denizens of the galaxy, some of whom Pata had had the honor of encountering. And one of those stars is known simply as Methuselah, named after the biblical character who was said to have lived for 969 years. More formally, the star is designated HD140283, but it rocketed to fame when astronomers announced an unexpected age for it. In some estimates, it was possibly older than the entire universe at the age of over 14 billion years. But how was this possible? Some wondered if the existence of Methuselah threatened our understanding of Big Bang cosmology. Others pondered if it cooled into question our most basic comprehension of stellar physics. But no matter what, Methuselah was undoubtedly ancient, and it had much to teach us.

Estimating the age for an individual star is tricky. For centuries, astronomers had absolutely no idea, only that there were a great variety of stars in the galaxy. There were the metals-sized yellow white stars like our sun. There were the small red dwarfs like our nearest neighbor Proxima Centauri. And then there were the giants that came in both blue and red varieties like Rigel and Betelgeuse respectively. In the late 1800s, astronomers started to think that stars started out large and slowly shrank and cooled through the course of their lives, emitting light in the process. But a few decades later, Sir Arthur Eddington calculated that if this was correct, then the sun could have a maximum age of only a few million years, a number embarrassingly out of step with the billions of years implied by geology and evolution.

And so eventually the true keys to stellar evolution came from the steady eye of Annie Jump Cannon, one of the famous woman computers hired at the Harvard Observatory and tasked with the repetitive chore of sifting through vast quantities of observational data. Hired for roughly 25 cents an hour, the computers were initially chosen as they were cheaper than the $1 it would have cost to hire men. But they would go on to accurately measure the brightness and positions of thousands of stars. Cannon herself would go on to say that no discovery is too small, for even the tiniest piece of knowledge can lead to great advancements in our understanding of the world. And her remarkable achievements were a shining example of this idea, proposing the modern stellar classification scheme that would eventually unlock the process of how stars live their lives.

To begin with, the computers had developed an alphabetical scheme. But as time went on, that scheme was simplified and reordered, leaving us with O, B, A, F, G, K, or M. O being the hottest type of star and M the coolest. Based on the work of the Harvard computers, two astronomers, Ejnar Hertzsprung and Henry Norris Russell devised a clever sort of plot which compared the luminosity and temperature of a population of stars. Using that plot, they discovered that the vast majority of stars sat on a track with a close relationship between these two properties. Today, we call this type of plot a Hertzsprung-Russell diagram, and we call that particular track the main sequence. As stars get older, they move along the main sequence. And so, the main sequence tells astronomers roughly where a star is in its life cycle. Generally, stars start off dimmer and redder and tend to get brighter and bluer as they age. It is the mass of the star that determines where it starts on this track and how long it will stay on it. And if astronomers have access to an entire cluster of stars, they can calibrate the relationships between size, color, temperature, and amount of heavy elements to get an accurate gauge of the cluster's age. But this technique only works well with an entire population of stars. Determining with precision the age of a single star is a different story and extremely challenging, and Methuselah is no exception.

And so the solution to the riddle of Methuselah, like most solutions to most riddles within astronomy, came down to uncertainty. Indeed, there is no doubt that the star is ancient, one of the oldest stars to exist in the galaxy. But it certainly formed well after the Big Bang. It's just a simple matter that the uncertainties associated with its age are so large that it can appear to be older than the universe itself. Indeed, despite its extreme age, Methuselah is still not the oldest possible star to still inhabit the Milky Way. That's because Methuselah is a kind of star known as a Population 2 star. And to understand what that means, we have to go back more than 80 years to 1942 and an attack on American soil.

The skies of Los Angeles are on fire. Sirens are wailing, anti-air raid guns are firing, and a total blackout is being ordered across the city. 1,400 shells would be spent that night as the entire population is thrown into chaos. And yet, there would be only five deaths. Two from heart attacks and three from car accidents caused by the distraction of the artillery shells. Why? The incident would later turn out to be a false alarm caused by a floating meteorological balloon. The anxiety built up by the attack on Pearl Harbor 2 years before going off like a powder keg. Blackout restrictions would stay in place for the next 18 months to protect from attacks from Japanese submarines. However, for one clever German astronomer living in Los Angeles, Walter Baade, the blackouts presented a remarkable opportunity. Using the famous 100-inch Hooker telescope at Mount Wilson Observatory through which Edwin Hubble had spied an expanding universe just 15 years before, Baade seized on the reduced light pollution to resolve stars around the Andromeda galaxy, using his observations to define two distinct stellar populations, Population 1 and Population 2.

Population 2 stars are ancient stars that have a medium amount of heavy elements polluting their atmospheres. Stars like the sun are Population 1 with a much higher concentration of metals, the generic term astronomers use to categorize all elements heavier than helium. Though, to be fair, hydrogen and helium make up more than 98% of all the elements in the universe. So, it's not unfair to consider the rest of the periodic table a mere rounding error. These Population 1 and 2 stars, even Methuselah, are the inheritors of multiple generations of stars in the universe. With each generation more enriched than the previous, after every stellar death, more heavy elements forged in the cradle of supernovae or within massive stars circulate in the universe, meaning that each new round of stars has a higher fraction of those heavier elements. But the first stars to appear in the universe had no heavy elements to speak of. They were as pure as they could possibly be. This generation of stars is known as Population 3. The seemingly reversed naming system a result of their proposal years after Population 1 and 2. That in turn a result of the fact that none have yet been found.

We're not entirely sure how the first stars formed, but they were likely huge. This is because modern-day stars require the presence of heavier elements to moderate their fusion chain reactions. And these metals also help protostellar gas clouds release radiation, allowing them to become smaller before igniting fusion. But the first stars had neither of these advantages. So they must have formed without any metals at all. Meaning that their primordial gas clouds could have stayed at enormous scales without fragmenting into smaller segments. Indeed, some models of stellar formation suggest that they may have been 100 times more massive than the sun, while other more exotic mechanisms find that they could have been a thousand or even 10,000 solar masses each. Either way, at that titanic scale, they were doomed to lead short, intense lives. For with greater mass comes an increased rate of fusion reactions. At 100 solar masses or more, the first Population 3 stars would have lived fast and died young, surviving only a few million years. And since those stars first arose in the dark ages of the cosmos over 13 billion years ago, the first generation came and went well before even the development of the first galaxies. 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 object, smaller than the sun, and remain hidden and protected. 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 Webb Space Telescope to instead go hunting for their light in the faint ancient signals at the edge of the universe. Light that was sent out when the cosmos was first forming stars and galaxies.

And so generations of stars come and generations of stars go, a sequence stretching back billions of years. And while all stars share the same process of formation and share the same main sequence of hydrogen burning in their cores, different stars do encounter different final days. To begin with, for the smallest stars, the red dwarfs, they simply quietly fade into the night. 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 than the sun. But because of that, they live enormously long lives, up to trillions of years before they run out of hydrogen.

Medium-sized stars like the sun, however, have a much more dramatic fate. When they first exit the main sequence, they undergo a series of rapid changes that see them swelling to become enormous red giants, then collapse again, then reignite and back and forth. But at the end of it, after billions of years of calm main sequence life, vicious transitions to helium burning and chaotic spasms, all that's left of a mid-sized star is a white dwarf, the leftover core of carbon and oxygen. But when white dwarfs were first discovered in 1916, many astronomers thought that they were downright impossible and that obviously the observers had done something terribly wrong. Sir Arthur Eddington summed up the situation when he mused about a hypothetical conversation between astronomers and the universe. The cosmos has presented astronomers with a situation where a ton of a star's material could fit inside a matchbox. The astronomical community responds to this idea with a simple retort. "Shut up. Don't talk nonsense." But Ernst Öpik, an Estonian astronomer who made the first concrete observations of white dwarfs, stuck to his guns, insisting that his calculations showed that some of the stars he had measured had densities over 25,000 times that of the sun. We know now he was right, and white dwarfs would go on to become just the first of many ultra-dense surprises that the universe had in store for us. And astronomers learned not to tell the cosmos to shut up.

Despite their mass, white dwarfs occupy only a small volume, roughly the size of a terrestrial planet. Because they are the literal cores of the nuclear furnace inside a star when they first emerge from their stellar cocoons, they have temperatures upwards of 10 million Kelvin. At these temperatures, they emit copious amounts of X-ray radiation, flooding their surrounding systems with the deadly light. But these systems are also filled with the leftover atmospheres of the stars. And what follows is a process similar to the one found inside of fluorescent light bulbs. The X-rays deliver energy to the gas. And some of that energy goes to increasing the energy level of the electrons. Those electrons then fall back down into lower energy states and in the process release radiation of their own, this time at wavelengths much less lethal and much more beautiful. And these are the planetary nebulae, first coined by William Herschel in the late 1700s. He was mystified by their curious appearance which to him resembled a faint fuzzy planet. And as is usual in the profession, by the time that astronomers realized that these strange objects were the result of stellar deaths, the name had stuck. No two planetary nebulae are alike, as their structure depends on the complex intertwining of elements as a dying star heaves off its outer layers. Many of them can be viewed through a telescope, revealing a galaxy twinkling with ornaments of different shapes, sizes, and colors. And adding to their beauty is their ephemeral nature. As the white dwarf cools off after about 10,000 years, it will stop emitting X-ray radiation. The planetary nebulae will fade from the cosmic scene, never to be repeated again.

Indeed, if the white dwarf is left alone, it too will slowly cool. But because they are so hot and so compact, they are remarkably long-lived. White dwarfs take tens of billions of years to fade to blackness. And so there are no cold white dwarfs, sometimes dubbed as black dwarfs, in the present-day universe. There simply hasn't been enough time. While most white dwarfs are typical of their kind, a few stand out. One is Janus, which has two distinct hemispheres. One dominated by hydrogen, another by helium. Another is dees J2147435, which is exceptionally cool. 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.

But if the white dwarf has a companion, it can have a much more dramatic fate. As the binary nears the end stages of its own life, it will swell to become a red giant. And if that swelling is great enough, then the material from the giant can pour onto the white dwarf. Because of the intense gravity, that hydrogen envelope can reach incredible densities, high enough to spontaneously trigger nuclear fusion. In a flash, all the hydrogen ignites, creating a nuclear blast known as a classical nova.

If enough hydrogen accumulates before burning off, however, catastrophe can strike. This triggers a runaway nuclear event with a small spark of detonation racing across first the face and then the body of the entire star, ripping it apart in a single tremendous explosion. This is a particular type of supernova. An explosion capable of outshining the combined light from hundreds of billions of stars together. 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 Earth, we should count ourselves lucky to see them at all. But there are other types of even more energetic supernovae, and they start with an extremely rare mystery in the sky.

The year was 1867. The Paris Observatory was celebrating 200 years of watching the heavens. Founded as it was in 1667 during the reign of King Louis XIV. 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 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 elites living further in. Louis 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 throne room at the Louvre. And so it was there two centuries later with Napoleon III on the throne that two astronomers, Charles Wolf and Georges Rayet, made the discovery that bears their name. They found three stars that showed spectral emission lines that were much broader than usual. An explanation for this strange behavior would remain elusive for decades. And one major reason for this is that Wolf-Rayet stars would turn out to be extremely rare. Even today with the full suite of modern astronomy available, only about 500 of these stars have been detected.

And so it wasn't until nearly the 1900s that astronomers would find the truth. The spectral features were due to the presence of helium, an element discovered just a year before Wolf and Rayet's observations, and an effect known as Doppler broadening. When gas gets extremely hot, its spectral lines widen due to the erratic motion of the molecules. This implied that whatever was happening with Wolf-Rayet stars, it involved a huge amount of energy. And so it turned out that Wolf-Rayet stars are so rare because they are very, very temporary. They are massive stars nearing their final days. As the outer atmospheres of these giant stars are blown away by successive red giant phases, they produce distended envelopes of gas. 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 creating the strange spectral features.

But what comes next is the danger because Wolf-Rayet stars are thought to be the precursors to a supernova Type II. Any star larger than roughly eight times the mass of the sun shares this grim if spectacular fate. Their fatal flaw is that nuclear fusion, the fundamental reaction that can power a star for eons, can't ever win in the long term against its ancient foe, gravity. Gravity is relatively weak in comparison to the other forces. But what it lacks in overall strength, it more than compensates for with patience, quiet, incessant patience. The gravitational pull of a star's own mass is always attempting to collapse it, to shrink it into a smaller volume. Resisting that pull is the energy released from fusion. And a star can maintain that balance for millions, billions, or even trillions of years. 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 as efficient. 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. But at each phase, the fusion process yields less and less energy. And so the time scales catastrophically shrink. 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, and silicon for only a handful of hours. And then comes iron. The crushing gravitational weight of the star pressing in, iron begins to fuse to produce heavier elements. But there is a problem. For iron fusion doesn't release energy. It absorbs it. And so with nothing left to counteract gravity, the star collapses. In only a few more seconds, the star will be dead. The pressures reach such extremes that the iron in the core converts into something strange and truly unique in the universe. The electrons are forced inside of atomic nuclei where they merge with protons to produce neutrons. The entire core becomes a sphere only a few kilometers across made of almost pure neutrons. The rest of the star rushes inwards towards that core at a substantial fraction of the speed of light. When it strikes, it meets a brief resistance and bounces back outwards, launching a shock wave that ripples through the remaining star's interior. 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 dawn of recorded history. Indeed, the earliest observations consist of simple rock carvings by unknown prehistoric peoples made around 4,000 BC. And some supernovae were so bright and persistent, cultures around the world simultaneously recorded the same event. In 1054, Chinese astronomers recorded a new guest star appearing in their sky. At the same time, half a world away, the ancestral Puebloan people of modern-day New Mexico created a pictograph denoting this special event. And today that portion of the sky is occupied by the Crab Nebula, the remnants of that mighty explosion.

But in the early 20th century, the astronomer Fritz Zwicky started to deliver answers. For millennia, cultures had observed different kinds of new stars appearing. Now, thanks to the sophistication offered by modern astronomy, he could classify them. Flashes of hydrogen envelopes around white dwarfs would retain the name nova, meaning new. While these kinds of titanic explosions would be called supernovae. When a white dwarf is completely destroyed, it's known as a Type Ia supernova. When a massive star meets the end of its life, it's a Type II. And in 1987, a blast originating in the Large Magellanic Cloud made history when it became the first supernova to be detected not by looking up, but by looking down into a giant tank of water. The tank was the Kamiokande-II detector buried deep underground in the Mozumi mine in Japan. And it held over a thousand metric tons of ultra-pure water surrounded by thousands of photo detectors. All super-sensitive synthetic eyes staring incessantly at the water within. Kamiokande-II was designed to detect neutrinos, the nearly massless particles that slip through the cracks of the cosmos. Neutrinos flood the universe. 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 of human history, they went unnoticed and undetected. It takes enormous detector volumes, say 100,000 metric tons of water to catch even a few. Nuclear reactions create neutrinos. The sun produces copious amounts of them every single day. And when the supernova in 1987 went off, it created roughly 10 to the power of 58 of them. Of course, the vast majority of those missed the Earth and of those that hit Kamiokande-II and instruments like it were able to detect a grand total of 25 of them. But it was enough to confirm a picture that had been forming in the minds of physicists to explain how supernovae finish their explosions. The problem was that models showed that the shock front produced when the stellar atmosphere bounces off the dense core slows down and stalls, not able to deliver nearly enough energy to propel the explosion. But neutrinos may save the day. The conversion of iron into neutrons produces an absolute flood of neutrinos. In fact, over 99% of the energy released in the supernova goes into neutrino production. What we see is an immense flash of light, but a shadow in comparison. Of course, most of these neutrinos slip harmlessly out of the dying star, never to be seen again. But enough of them interact with enough of the star's material to reignite the shock wave, possibly allowing it to break through and destroy the star. Other mechanisms such as out-of-control resonant instabilities similar to the kind that destroyed the Tacoma Narrows Bridge in 1940 might also play an important role. Whatever the exact mechanism, once the shock wave breaks out, it sweeps up material in front of it like an avalanche, taking a star's atmosphere and spreading it outwards at nearly the speed of light. Nuclear reactions in the material finally create the heavy elements that were attempted in the core mere minutes before. And so long after the star is dead and done, the remnants of its atmosphere remain as a slowly expanding and cooling nebula that eventually disperses into the interstellar medium.

As for Batuda, the supernova caught it unawares. In the millions of years since its creation, it had grown complacent and overconfident. It had seen it all, understood it all, knew when the universe was safe, and when it was dangerous, or so it thought. 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 the star split itself in two and the shock wave came racing outwards. It took Batuda over a thousand years to repair the damage, licking its wounds in the tattered remnants of the system. Grim and determined, he decided that it had spent enough time in the galaxy. And with its arrogance blown away by the white dwarf's companion, it was time to encounter the supermassive black hole once and for all and to leave the Milky Way behind.

A tutua hung on the edge of the abyss. Sagittarius A*, the monster black hole at the center of the Milky Way, was dead ahead. The spacecraft had deployed telescopes spanning the entire electromagnetic spectrum. For this truly was a sight to behold. The struggles to get here, nearly 10 million years had passed since first leaving the heliopolar gates of the solar system, were more than worth it. The environment surrounding the supermassive black hole was the most continuously energetic place in the entire galaxy. And beneath it all, under the maelstrom of plasma surrounding it and the flood of radiation emanating from near it, was the black heart itself, a horizon cutting off the interior from the rest of the universe. For a brief moment, Batuda was tempted to plunge itself beneath its depths out of sheer curiosity. Indeed, prior to its launch, humanity had not yet cracked the riddle of quantum gravity, of the way to provide a coherent universal description of strong gravity at subatomic scales. The answer was there in the singularity in the heart of the beast. But it was a trap. If Petuta sent itself below the horizon, it would learn nature's ultimate secret, but it would never be able to return home with the answers.

And it was those same singularities, those riddles of infinity in the centers of black holes that first cast doubt on their existence altogether. "As you see, the war is kindly disposed towards me, allowing me, despite fierce gunfire at a decidedly terrestrial distance, to take this walk into this your land of ideas," so wrote German physicist Karl Schwarzschild in a groundbreaking 1915 letter to Albert Einstein from the front lines of World War I. He had volunteered at 40 and while serving in Russia had figured out the first exact solution to Einstein's general theory of relativity, solving for the case of the gravitational influence due to a spherically symmetric object. A year after that, the great mathematician and physicist David Hilbert verified that Schwarzschild's solutions broke down in the very center of the object, meaning that any attempts to describe the gravity there would yield nonsensical results. Something bizarre was going on in these calculations. Something unexpected. The theoretical exploration of black holes had begun.

Singularities appear in all sorts of physical theories, but can usually be explained away. For example, the force of a spring approaches infinity as the spring compresses. This singularity is not a problem because we know that springs can't be compressed forever. Some other force like the interaction between molecules prevents them from getting smaller. But the singularity that appeared in Schwarzschild's solution was different. It implied that there was a region of the universe that if gravity became too strong, something happened, something unknown. Most astronomers scoffed. Indeed, Einstein is quoted as having said, "The essential result of this investigation is a clear understanding as to why the Schwarzschild singularities do not exist in physical reality." But a small minority like Robert Oppenheimer, before he became involved in the Manhattan Project, carried on and in 1972 astronomers came across Cygnus X-1, one of the brightest sources of X-rays on the sky. The system consisted of two objects, a large bright star and a smaller unseen companion. Initial calculations suggested something enticing. The mass and radius of the companion made it much denser than any other known astrophysical object, dense enough to be a black hole. And so in 1975, Stephen Hawking and Kip Thorne, two pioneers of black hole physics, made a bet. They had both spent much of their careers studying these hypothetical objects and had staked their professional reputation on their reality. So if black holes did not in fact exist, they might just be out of a job. And so they concocted the bet as a consolation prize. If black holes did not exist, then Thorne owed Hawking a 4-year subscription to the British satirical Private Eye magazine. But if they did exist, then Hawking owed Thorne a one-year subscription to the American slightly less satirical Penthouse. By 1988, Hawking had paid his dues to Thorne. And with the reality of black holes came the reality of gravitational singularities.

The spatial extent of a black hole is, as far as we understand it through general relativity, zero. Black holes take up no volume in space. They have no breadth or height or depth. They are defects, punctures in the very fabric of spacetime itself. In a way, singularities are akin to subatomic particles, which also have no spatial extent. Strictly speaking, there is no width to an electron or a top quark, only ranges that define interactions with other particles. And yet, they have wildly different masses. And so, in one sense, all black holes have the exact same size: that is, no size at all. But they do have mass. They are composed of all the material that formed them and all the material that has suffered the unfortunate fate of falling too close to them. And black holes can and will eat anything. Any source of matter or energy is food: radiation, dark matter, subatomic particles, clouds of molecules, entire stars, even other black holes. Anything that encroaches within the event horizon will never return to the outside universe and will always end up in the singularity. While black holes don't have any stronger gravity than usual, if you replaced our sun with a black hole of equivalent mass, then planets would stay in the same orbits. Their unusually high densities allow them to rapidly funnel matter towards them. They can become incredibly efficient eating machines, only limited by the astrophysics of the surrounding material. But because black holes themselves don't interact except through gravity, there's nothing to stop them from getting larger as long as they are continuously fed. The largest stars in the universe are only about a thousand times larger than the smallest. The largest nebulae and galaxies follow the same pattern. But black holes are different. Black holes are the only objects in the universe that span multiple orders of magnitude. The smallest known black hole is currently GRO J1655-40 with a mass of 6.3 solar masses. But there is a slim possibility of far smaller. In the 1970s, Stephen Hawking hypothesized that the exotic, energetic, violent conditions of the extremely early universe could have flooded the cosmos with primordial black holes with masses as small as an asteroid. But to date, astronomers have found no evidence or signature of their existence. And so scientists have focused their efforts on the other end of the mass scale, the realm of the supermassives. And there are many incredibly huge black holes that only have rough indirect mass estimates, such as Phoenix A, which might weigh as much as 100 billion suns. But the largest black hole with a confirmed reasonably accurate mass estimate is TON 618, weighing in at somewhere around 66 billion solar masses. Sitting 18.2 billion light-years from Earth, its event horizon wouldn't just engulf our solar system; it would dwarf it. You could travel all the way to the orbit of Pluto, a journey that takes our best and fastest probes nearly a decade to complete, and only be 1/80th of the way to TON 618's event horizon. But the closest known black hole is Gaia BH1, which is only 1,560 light-years away. However, astronomers suspect that there are many other black holes closer to our solar system. The problem is that unless they are members of a binary system, they're almost invisible. Of course, we can't see black holes directly. Instead, we can only see the material that swirls around them as it crushes into their extreme gravitational grip. But lone black holes are not accreting material like this. And so, they stay hidden, lurking in the shadows. In 2023, astronomers with the OGLE Collaboration announced the discovery of the first known lone wolf black hole, a six solar mass object sitting roughly 5,000 light-years away. They discovered it through a technique known as microlensing, where the gravity of a black hole briefly enhances the light coming from a background star. Indeed, astronomers estimate that there are around 100 million black holes in the Milky Way alone. Some alone, some are members of complex systems. Some are actively accreting, some are silent, and sometimes they collide.

Black hole mergers are exceedingly rare, but they do happen. When they merge, they spiral in towards each other. The only way for them to get closer is to lose energy. And the only way that black holes can lose energy is through the emission of gravitational waves, ripples in space itself. However, gravity is by far the weakest of the known forces. And so, black holes only inch together achingly slowly, arguably too slowly. Indeed, physicists do not yet understand how black holes complete their mergers in less time than the present age of the universe. 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 the great unsolved mysteries in modern astronomy. But black holes, ever willing to defy our expectations, do indeed merge. 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 visibly betray the event of the collision. Instead, black hole mergers are silent. A collision painted only in midnight. Their only emissions are a ferocious release of gravitational waves. Entire solar masses worth of energy converted into pure gravitational radiation. But the waves quickly die down as they spread out into the universe. 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 nucleus. Indeed, even though Einstein predicted the existence of gravitational waves more than a century ago, he thought that they would forever remain undetectable. But Einstein didn't know about the existence of merging black holes, and he underestimated human ingenuity.

On September the 14th, 2015, the LIGO gravitational wave experiment finally struck gold. 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 1/10,000th the width of a proton. 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 than the width of a human hair. Despite this incredible precision, they were not planning on realizing their dream for several more years, if not decades. Indeed, LIGO had recently been upgraded and they were running the experiment in a mode geared for engineering tests. But a tiny ripple ran through their detector and several microseconds later through a sister detector on the other side of the continent. That ripple had an unmistakable fingerprint: the exact pattern matching the merger of two black holes. However, as awe-inspiring as these mergers are, they are incredibly rare. The nearest ones recorded are several hundred million light-years away. But there is another kind of black hole event that is just as destructive and powerful and much closer to home. And this event involves not some ordinary everyday black hole. No, these events involve only the greatest black holes in the universe, the supermassive ones.

The first hints that the Milky Way might host such a monster came as early as 1933, thanks to the pioneer of radio astronomy, Karl Jansky. He noticed that radio emissions seemed to be extremely strong coming from the direction of the galactic center through the constellation Sagittarius. Several years later, more detailed and higher resolution observations revealed a strong source of radio emission coming from a very compact region of that sky, which radio astronomers dubbed Sagittarius A. In the 1970s, even more detailed observations revealed that Sagittarius A was made of several subcomponents, one of which was much stronger than all the others. Robert L. Brown finally came up with a name for this interesting source, Sagittarius A*. He added the asterisk to its name because it was exciting. And in atomic physics, the asterisk is used to denote excited states. The name stuck. And very quickly astronomers realized that this might be the home of a giant black hole. Only an object with that kind of density, that kind of sheer gravitational might could provide the energies needed to blast out a continuous strong radio signal. And after decades of work, it was observations

of stars dancing perilously close to the abyss that finally confirmed it. Two teams, one led by the German astronomer Reinard Gendzel and the other by the American Andrea Gz, developed the technology needed to follow the orbits of individual stars as they circled in real time around the monster itself. In some cases, the stars would reach peak velocities of 30% the speed of light. 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 first precise measurement of the mass of Sagittarius A star, just over 4 million solar masses, giving its event horizon a radius of around 44 million kilm. That's over 30 times the width of our sun. If we were to place Sagittarius a star in our own solar system, it would stretch to the orbit of Mercury, completely consuming [Music] it.

But it is a dangerous game that these stars are playing. The gravitational forces near a black hole are, to put it mildly, immense. In particular, the problem is that there are strong differences in gravity across an object. 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 feet would be different enough from the gravitational pull at your head that you would get torn apart or in the appropriate scientific term, spaghettified. And outside a super massive black hole, even an entire star cannot survive swinging too close. 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 self-gravity. The star loses its own sense of identity, getting stretched and pulled into a long, thin rope of plasma that eventually makes its way down the black hole's throat. This violent tearing releases an enormous amount of energy, roughly the same amount of energy as a typical supernova. And so these total destructions of an entire star or tidal disruption events as they are innocently termed by astronomers can briefly outshine an entire galaxy.

But a small object like a human body can survive the descent towards a super massive black hole. The gravitational differences across that distance just aren't big enough to cause spaghettification. In fact, a person or even a small spacecraft could handily survive the pass through the black holes event horizon. And this is because the event horizon of a black hole is not a physical boundary. There is no wall. There is no marker. There is no sign that you have just fallen off the edge of the gravitational cliff. 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 accelerate, how fast you go, even to the point of approaching the speed of light, you cannot escape. And indeed, once inside, your future is determined, your fate sealed. Outside the event horizon, you are free to roam wherever you please in space, but you must always march forward into your own future. Inside a black hole, however, a single point in space becomes your future. In a black hole, all roads lead to the singularity. Wherever you turn, the singularity will be right in front of you. You can only approach it second by second. In a finite amount of time, as measured by a watch on your wrist, you will strike the singularity. And when you do, you will be [Music] obliterated.

And it's a relatively straightforward exercise to calculate the amount of time between crossing the event horizon and reaching the singularity. For a black hole the size of Sagittarius a star, that time is around 7 seconds. As to what you will experience at the singularity, that is something no calculation can yet reveal. We know how it will appear to you. Due to the extreme gravitational tidal effects, the paths of light get bent and warped. 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 you were landing on the surface of a black world. And of course, as you get nearer, you will become spaghettified as well, now that you are close enough to the singularity. But should some aspect of you survive, you would get a taste of the unification of gravity and quantum mechanics, the secret that every black hole hides in its deepest heart. But you will not be able to tell anybody else in the outside universe about what you have witnessed. 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 that can save you once you enter.

And so it was a difficult maneuver to pull off and a dangerous one. But Batua had spent millions of years preparing. It coasted just on the outskirts of the roaring and rotating Sagittarius A star, allowing the extreme forces of gravity to propel it to a substantial fraction of the speed of light, stealing energy from the rotation of space itself as it went at just the right moment, just before dipping into the event horizon, Batuda altered course, a barely perceptible shift in its momentum, and it was free. [Music] Mission parameter 3. Find the most extreme objects and events in the universe.

40 million years of nothing. Just the grand distant spiral of Andromeda, host of a trillion stars, steadily growing larger in the forward telescopes, and the Milky Way, smaller but still respplendant, shrinking behind it. Prior to leaving its home galaxy, Batuda had believed itself to be the master of time and space, accustomed to the inhumane scales that the universe operated on. But now, for the first time in its long life, Batua felt lonely. Its sensor readings hadn't offered much distraction. Thin wisps of hydrogen and helium that bridged the 2 and 1/2 million lightyear gap between the galaxies. The occasional passing cosmic ray here and there, a clump of stars to mark the presence of a dwarf satellite galaxy. This was because Batuta was swimming through a material known to astronomers as the warm hot intergalactic medium. Its name comes from the fact that the gas has a temperature between 100,000 Kelvin warm and 10 million Kelvin hot. The gas is believed to be primarily hydrogen and helium and likely makes up almost half of the barionic content of the universe. The other half trapped inside of galaxies. But the medium poses an extremely challenging astronomical target. It is stupendously thin and spread out over enormous volumes. This means that despite its high temperature, its overall brightness is almost insignificant. And most of our knowledge of the medium comes not from direct observations, but through computer simulations.

These were scales well beyond the comprehension of its human makers. And until it undertook its voyage, Petuta itself, the universe was truly so vast it could not be grasped, only reduced to mere numbers. But now had no such comfort. It could only wait and wait and wait, gliding smoothly through the near vacuum that defined the space between galaxies, observing and recording and sampling the warm, hot intergalactic medium. The only other thing that the spacecraft could count on was its founding mission. Explore the galaxy and the wider universe, catalog habitable worlds, search for extreme sources of energy, and discover a pathway to rescue humanity from its solar prison on the Earth. And so Batuda had decided that its searches must take it here to Andromeda, a larger and more vibrant galaxy than the Milky Way, home to more exotic objects and energetic events. It would search every corner of this galaxy if it must to find the most extreme events in the cosmos. And after several hundred years of patient observation, Betatuda found just the right candidate.

In 1967, Joselyn Bell was neck deep in her graduate studies at Cambridge University. Working under the supervision of Anthony Huish, a pioneer in radio astronomy, Belle was convinced that she wasn't smart enough to stay at Cambridge and would be thrown out at any minute. But one night, pouring over the data printouts from the telescope, she noticed, in her words, a bit of scruff. At first, this didn't seem unusual. The telescope frequently picked up radio interference or mysterious undecipherable signals. And so, Belle dutifully noted it and moved on. But a few nights later, the signal reappeared in the same part of the sky. Several weeks later, again, rightfully skeptical, Huish advised that they verify this potential discovery with another telescope at the array. And so, at the appointed time, Belle and Huish stood over the recorder to nothing. Nervous, Belle began to deflate. Perhaps in the end, it was meaningless. But then, the recorder buzzed and began printing. She had simply miscalculated and the strange signal appeared 5 minutes later than she predicted, repeating every 1.337 seconds an extremely high level of precision for an astronomical object. Jokingly, Belle named the source LGM1 for little green [Music] men.

For his efforts, Huish was awarded the Nobel Prize in Physics, while Belle, now Dame, Belle Bernell, received nothing. Though she would always maintain that it would demean Nobel Prizes if they were awarded to graduate students, and was more thankful that she was a pioneer for women in science. Soon enough, theorists were able to fill the gaps and provide an explanation for these pulsars. And as is often the case in astronomy, they didn't even have to invent something new. They could simply look back through the archives of the interesting, sometimes outlandish hypothetical ideas that physicists are fond of creating when they have nothing better to do. In this case, it was a proposal from the 1930s made by Walter Bard and Fritz Vicki who realized that the end result of a supernova explosion was an extremely dense core made almost entirely of neutrons. A neutron star. These neutron stars would be extremely exotic, even more so than their cousins, the white dwarfs. They were denser, stranger, and more extreme. White dwarfs resist the crush of gravity through degeneracy pressure, the inability of electrons to share a common quantum state. Neutron stars also get their support through a similar mechanism, but one involving neutrons instead. And since neutrons are much heavier than electrons, they can compress together much more, allowing neutron stars to squeeze their entire bulk, typically a few times the mass of the sun, into spheres no wider than a typical city. In essence, neutron stars are the largest known bound states of nuclear matter, atomic nuclei on the scale of kilome. But just like the white dwarfs, they too have a cutoff mass above which catastrophe looms. 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 gravity to win. This means that neutron stars are quite possibly the densest material possible in the entire universe, just one step away from becoming black holes.

Even though neutrons are, as their name suggests, electrically neutral, a typical neutron star will maintain a small population of charged protons and electrons sandwiched in the deep layers of the interior. And the properties of nuclear material under extreme pressures cause the neutrons to form microscopic structures. Sometimes forming tubes, sometimes long thin strands, and sometimes lumps. Amusingly, physicists name these different structures after various forms of pasta, spaghetti, penny, noi, and so on, and give the interior structures of neutron stars the name nuclear pasta. At the outermost layers of a neutron star is a crust made of highly compressed electrons. But at the opposite end, in the core, is a substance unknown to modern physics. 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 constituents. A soup of quarks and gluons unseen in the wider universe since the earliest moments of the big bang. And in some proposals, the cores are instead made of strange matter, a degenerate state of solely strange quarks. But no matter what, neutron stars test the bounds of the exotic. The gravitational pull is so strong that light can orbit around them in a circle. The tallest mountains on neutron star surfaces are only fractions of a millimeter high. Neutron stars should also be rapidly rotating. Like a figure skater pulling in their arms to spin faster, the conservation of angular momentum takes a star's natural spin and amplifies it as it collapses. Combining that spin with a magnetic field generates an electric field which whips up any nearby charged particles and energizes them. They then go on to generate intense beams of high energy radiation out of the neutron stars magnetic north and south poles. 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. And so as the neutron star spins, it carries the beam of radiation around in a circle, a gigantic search light circling across the sky. And if the path of that search light happens to pass over the Earth, we see it as a regular pulsing of radio emission. A pulsar. The pulsar discovered by Bel Bernell and Huish rotated once every 1.337 seconds, corresponding roughly to the RPM of a record player. But that's nothing in comparison to some discovered since. The fastest known pulsar PSR J17482446 A rotates 716 times every second or 43,000 revolutions per minute, faster than a jet turbine. Some pulsars even have companions. And when they do, they can be deadly. The Pulsar PSR B1957 + 20 has another much more ominous name, the Black Widow. This star sitting 6,500 lighty years from Earth has a small brown dwarf companion. The powerful outbursts from the Black Widow are slowly destroying the brown dwarf. The ferocity of the attack is so much that the companion is a bleeding material as if it were in the direct stream of a [Music] blowtorrch.

And magnetars take this process to the next level. All neutron stars likely host magnetic fields, but something specific happens inside a magnetar to ramp up its field beyond the extreme. That special something likely happens at the moment of its birth when the neutron star first emerges from the wreckage of its host star. Just like conservation of angular momentum spins up a neutron star during the period of collapse of the parent, the same thing happens to magnetic fields. If the neutron star is spinning too slowly, it won't generate enough energy to drive its magnetic fields to extreme strengths. 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 plasma still surrounding the star, acting as magnetized sails that slow everything down. But if conditions are just right, a dynamo mechanism can operate deep within the heart of the neutron star. In general, dynamos take one form of energy, usually rotational energy, and transform it into stronger magnetic fields. Indeed, a dynamo in the Earth's core amplifies our own magnetic field. And the magnetic field of a magnetar is the most powerful ever seen in the universe. For context, the Earth's magnetic field has a strength of about one Gaus, strong enough to nudge a needle suspended in water. Powerful electromagnetics in a supercooled MRI machine. Typically the strongest magnetic fields a human will ever encounter are about a million times stronger. 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. And so bearing that in mind, a typical magnetar will host a magnetic field roughly 1 trillion times stronger than the Earth's. Of course, those strengths, even approaching a magnetar, is a death sentence. Neglecting the copious amount of X-rays emanating from their surface and the beams of radiation from their poles and the occasional flare of gamma rays, magnetic fields of this strength stretch and distort even atoms, turning them into thin needles 100 times longer than they are wide. Every and all molecular and chemical bonds cannot withstand that intensity. Even the vacuum of space itself becomes polarized when soaked with such overwhelming [Music] energy.

But despite their ferocity, eventually all magnetars fade away. The magnetic fields meet resistance and slow down the rotation of the neutron star until it can no longer sustain such incredible field strengths and the star continues on its life as a normal neutron star or pulsar. The tutor was not easily impressed considering all it had seen, but the magnetar managed it. It was a truly awesome sight. The radiation, magnetic field, particle count, temperature, everything, every measure of every sensor was pushed beyond its limits. There was truly nothing else like it. Except, as he gazed in wonder at the magnetar, Tuta still wanted more. [Music]

Batuda was in the most fragile state it had ever been in. Over 98% of its systems were destroyed. But with what remained of its senses, the tutor took in the magnetar before it. The explosion that had emanated from the stars surface had been a surprise. The flood of high energy radiation unlike anything it had ever seen before. It was almost as if musings had awakened some angry demon of old. The magnetar lashing out to show the fragile visitor what it was capable of. It was spectacular. But didn't want to stick around and risk seeing it a second time. And so the spacecraft glided outside the solar system, putting a wide birth between itself and the temperamental star, noting in its log a definitive answer to an elusive mystery that had haunted astrophysicists for years.

We will never know what the reactions were of the first people to discover gammaray bursts because they were working in secret and they weren't looking for them. In 1963, the governments of the United States and the Soviet Union signed the historic partial nuclear testban treaty which banned all overground nuclear tests. But despite the treaty, the US 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. And so in response to this potential loophole in the treaty, the United States launched the Va satellites, a series of satellites orbiting well outside the Van Allen radiation belts that circle the Earth. There they would hunt for gamma rays, the high energy radiation emitted during nuclear blasts, hoping to catch the Soviets red-handed. However, though the satellites never did see the signature of a nuclear weapon, on July the 2nd, 1967, they did record a bright burst of gamma rays. The event was recorded but kept under wraps until a dozen more detections were made. Realizing that this represented more a scientific curiosity than a threat to the United States, Ray Cleverettle of Los Alamos's national laboratory finally convinced his superiors to release the information to the scientific community in the process giving these mysterious events their equally mysterious name. gammaray bursts or GRBs.

Our sky is a wash in what astronomers dub transient events. Besides the varieties of novi and supernovi, there are flares, flashes, pulses, bursts, and more all across the electromagnetic spectrum all the way from radio to gamma rays. And these transients have been a source of wonder, fascination, and fear for millennia, well before humanity understood their exact origins. Besides the guest stars and other nova that astronomers dutifully noted in their chronicles, there was some stars that appeared to change with regularity. One of those is Mirror, a red giant star in the constellation Satus. Its name has the same root as miracle and it was so named by astronomer Johannes Havllius in 1662 due to its astonishing behavior. Every 11 months it would change in brightness. A fact that appears to have been known to Chinese, Babylonian, and Greek astronomers going back to antiquity. And this is because mirror is a star teetering on the brink of extinction. It's a large star larger than the sun that is fusing helium in its core. [Music] Already having lost half its mass due to its violent eruptions, a vicious cycle has begun. The outer layers of the stars atmosphere heat from the intense energy pouring out of the star, which causes them to expand. But once enlarged, the gravitational pressures on the core ease. The radiation plummets. The outer layers cool and the star returns to some semblance of normality before beginning the cycle a new. Today, dozens of these mere variable stars join the original instellar cataloges.

And a similar mechanism powers another class of variable star, the seafied variables. The simple fact that mirror and sephiid variables can measurably change their brightness over the course of days or even hours is a matter of almost inconceivable scale. Indeed, these are entire stars, vast complexes of boiling plasma, churning magnetic fields, and forges of fusion beyond human scale and comprehension that alter their appearance as easily and quickly as people change outfits. And so, while we may understand the physics behind their chameleon-like nature, the energies needed to sustain these cycles still make them deserving of their miraculous [Music] names. But they do still change over a period long enough to notice, hours and days. You couldn't miss them, for example, in the blink of an eye. For something that quick, we have to turn our attention to the true transients of the universe. The rare events in the extreme ends of the electromagnetic spectrum. The flashes and bursts that come and go in a second. Brief blips on a detector. The only sign that something awful and violent has happened in the distant night sky. And so perhaps unsurprisingly, astronomers didn't even notice the first fast radio burst or FRB when it hit the Australian Parks Observatory in 2001. Instead, it wasn't discovered until years later when Duncan Lurmer and his student David Narovitz spotted it in archives of Pulsar data. Only 3 milliseconds long, it came from roughly the direction of the small Melanic cloud. Since then, astronomers have managed to capture only two dozen more of these events, ranging duration from only a millisecond to over 3 seconds long. And whatever is causing, they are enormously powerful, putting out several days worth of solar energy in a matter of milliseconds. Yet, at the same time, they must be both rare and distant since only our most sensitive radio telescopes are capable of detecting them.

While astronomers still don't fully understand the origins of FRBs, a discovery in 2020 helped give them a major clue. Until then, allrbs had originated far outside the Milky Way, which made their identification that much harder. But on April the 27th of that year, the Neil Gerald Swift Observatory sent out an automated announcement that a storm of X-rays had just emanated from a known stellar remnant hiding in a distant corner of the Milky Way. That stellar remnant could have been a white dwarf, a neutron star, or even a black hole. They're all known to emit occasional X-ray flares. But this remnant was also known to emit gamma rays on a regular basis, something that astronomers suspect only neutron stars are capable of doing. Another clue came the next day when the Canadian Hydrogen Intensity Mapping Experiment reported two brights coming from that same location. And over the next several days, a fleet of radio telescopes and X-ray observatories monitored several flares emanating from the source. And a picture began to emerge. A magnetar was [Music] glitching. Magnetars, like all pulsars, spin with incredibly precise timings, only slightly less accurate than atomic clocks. Indeed, the timings are so precise that they can be used for galactic navigation and wayfinding, as the famous plaque on the Pioneer probes show. Any civilization with a love for astronomy will eventually discover pulsars and measure the same rotation rate. That precision serves like a beacon. Everybody in the galaxy can agree about which pulsar you're talking about because it's so unique and so regular. But occasionally the pulsars malfunction. Something in the crust shifts like a Titanic star quake as the ultra dense electrons rearrange themselves or some instability surfaces from within the complex and mysterious core. Whatever happens, pulsars can glitch causing a sudden and abrupt shift in their rotation period. And the same thing can happen to magnetars. But when they do, that rapid glitch triggers a cascade in the magnetic field that ripples out in the form of a burst of emission across the electromagnetic spectrum, dumping a sun's worth of energy into space in the blink of an [Music] eye.

Of course, not all strange bursts on the sky lead to some cosmic disaster or explosion of energy. After the discovery ofrbs, PhD student Sarah Burke Spola combed through more archival data from the park's observatory and found several like events with one notable difference. These flashes appeared to come from the entire sky rather than being sources from one particular direction. Eventually, after an exhaustive detective search, the astronomers found the culprit. Only the park's observatory saw these mysterious signals, only in the middle of the day and only when the visitor center was open. It turned out that they were not some new astronomical event, but rather a malfunctioning microwave in the cafeteria.

But the gammaray bursts detected in the 1960s definitely had no human source much to the relief of the operators of the nuclear weapon hunting via the satellites. Indeed in time astronomers would detect thousands of GRBs which gave them enough data to classify them into two types short and long each having different mechanisms behind them. The short GRBs lasting less than an average of a third of a second are most likely connected to killer novas, the merger of two neutron stars. And in 2017, astronomers were able to confirm this hypothesis when the gravitational wave of a merger event washed over the Earth almost perfectly simultaneously with the flash of a short GRB. The worldwide astronomical community sprang into action with dozens of telescopes and observatories scattered across six continents continuing to monitor the event for the coming weeks. Indeed, that single event led to the publication of over a 100 papers highlighting the importance of multi- messenger astronomy where observations of different wavelengths of light supplemented by non- electromagnetic signatures such as gravitational waves and neutrino bursts can be used to take advantage of these lucky rare events to understand the physics behind them. And the merger didn't just yield an explosion of electromagnetic and gravitational waves. It also created a debris cloud rich with heavy elements, including several Earth masses worth of gold alone.

The long GRBs, however, which span from a couple of seconds to several hours in duration, remain a much more beguiling mystery. All long GRBs are hosted well outside the Milky Way, and almost all of them come from galaxies hosting periods of rapid star formation, suggesting that the generation of long GRBs is somehow connected to the life cycles of massive stars, which are produced in abundance during these kinds of periods. Since GRBs are incredibly bright, outputting more energy in a few seconds than our sun will over its entire 8 billionyear lifetime, there must be some exotic physical process at play. And astronomers believe that the most likely scenario involves nothing other than the birth of a black hole [Music] itself. At the end of a massive star's life, its core compresses and forms a neutron star with the entire weight of the surrounding crashing in at up to half the speed of light. As we've seen, sometimes the core survives the onslaught and goes on to become a pulsar or even a magnetar. But sometimes it's just too much. Sometimes gravity winds, pulling everything inwards towards infinity, creating a black hole. That newborn black hole is surrounded by the corpse of the star that created it. Some of that material succumbs to the almighty gravitational pull of the black hole falling back towards it. And in a process seen time and time again throughout the universe at a variety of size and energy scales. From that disc of accretion emerges two gigantic hyperpowered jets containing a blast of charged particles traveling at nearly the speed of light. The jet slams into the surrounding material accelerating a relativistic shock front and in the process accelerates particles that then give way to a flood of gamma rays. And if the Earth happens to lay in the direction of that powered beam, a mechanism that would fit perfectly in your favorite sci-fi movie, then we see it as a gamma ray burst. And the explosion gets a new name, a [Music] hypernova.

As compelling as this story is, it does have its holes. Nobody fully understands the complex physics at play. And so there is another candidate, another potential source of GRBs, especially the ultra-ong ones that hold for several hours before finally fading. These are an entirely different kind of beast. The result of the deaths of the most massive stars in the universe, stars weighing at least 130 times the mass of the sun. All stars produce radiation in their cores as a result of nuclear reactions. The radiation then flies outwards, inflating the star and preventing its collapse through gravity. This balancing act can last for up to trillions of years in the smallest of stars. But in the very largest stars, the radiation takes the form of energetic gamma rays. Normally this is no problem. But extremely energetic gamma rays can spontaneously split into pairs of electrons and posetrons. These particles aren't nearly as good at providing a source of resistance against gravity. But in a super giant star, a new equilibrium takes hold. The fusion reactions create gamma rays. The gamma rays split into electronosetron pairs. Then those pairs find each other and annihilate, converting themselves back into gamma rays, which then go on to keep the star supported. But towards the end of a superstar's life, it starts to produce an excess of gamma rays. These gamma rays convert themselves into particles. But not enough particles find each other to reform into gamma rays and without even noticing the star [Music] dies. The gravitational rush is so intense that it obliterates the core entirely, releasing all of its pent-up energy in a single flash. The star explodes as a super luminous supernova up to 10 times more powerful than any other explosion, flooding the wider universe with gamma rays and record-breaking charged [Music] particles.

And it was a single charged particle that gota's attention. The particle was incredibly energetic. No, energetic was the wrong word for it. It was off the scale. The proton that had entered one of the spacecraft's auxiliary detectors had an energy of over 10 to the power of 22 electron volts, the greatest that it had ever recorded. Indeed, that was 100 times more energy than the Oh my God particle, the most energetic cosmic ray ever detected by humanity, recorded all the way back in 1991 and never since surpassed. And Batuda's new particles seemed to come from nowhere. But the spacecraft knew that cosmic rays are charged particles, and charged particles change and alter their trajectories when they encounter magnetic fields. The Andromeda galaxy had a weak but gigantic field tangled up with the spiral arms of the galaxy itself. Thanks to its earlier radio observations, Batuda had a map of the galaxy's magnetic environment. And so it used that to reconstruct the true origins of the particle, the Virgo cluster, a collection of over a thousand galaxies sitting 54 million lighty years away. At first, Petuta trembled at the scale. Would it actually attempt a crossing over that kind of distance? Scanning once more through the data recorded about this mysterious cosmic ray. And before it could convince itself that this was a terrible idea, Beta powered up its systems and set off into distant [Music] oblivion.

500 million years. 500 million silent lonely years. Batuta almost didn't make it. Its energy reserves were already dangerously low before it reached the outermost edges of the Virgo cluster. Luckily, however, the cluster environment was filled with a hot, thin, diffuse gas, barely more noticeable than the surrounding emptiness, but just enough to sip on and continue the journey onwards. A journey that would take another few million years to cross towards its target, Messier 87, one of the largest galaxies in this neighborhood of the universe. And Batuda had thought many times about the curious origins of that name, Messier 87, in the time it took to voyage from Andromeda. [Music]

The 18th century was a big time for comet hunters, and there was no greater comet hunter than Charles Messier. Comets had long transfixed those that looked to the heavens. Indeed, records of their appearance in the sky have been found as far back as ancient Chinese oracle bones from the second millennium BC. But when it came to discovering them, naming them, French astronomer Charles Messier upped the ante. He discovered 13 comets in his lifetime and observed many more, earning him the nickname the ferret of comets from King Louis X 15th. Indeed, rumor has it that after his wife died, he was more preoccupied with the fact that he'd missed an opportunity to discover a new comet whilst having to tend to her in her dying moments. But sometimes he would spy in his telescope a small fuzzy object only to be frustrated that it was not a comet but some other deep sky astronomical thing. And so to help himself and his fellow comet chasers, he devised a list of objects that looked like comets but were not. Unknown to him, he would go on to create one of the most important lists in all of astronomy. a simple catalog of roughly 100 objects that later astronomers would realize include supernova remnants, star clusters, and even entire galaxies. Because Messier's objects were relatively close or large or bright, they became frequent targets of observations from professionals and amateurs alike. And so their names stuck through the ages. And this list included Batuda's target, Messier 87, a hulking beast of a galaxy home to several trillion stars and a super massive black hole that could fit the entire solar system within its event horizon. The strange superpowered cosmic ray detected by Batuta seemed to have come from that galaxy. And so the agent probe sped onwards, not knowing what it would find.

After astronomer Edwin Hubble's revelation that the Andromeda Nebula was in fact an entirely different galaxy sitting millions of light years away from us, teams of astronomers were quick to map out our local universe. Every year pushing back the boundaries of the known cosmos, finding new galaxies, groups, and clusters in the visible spectrum everywhere they looked. There were the spirals with their winding bejeweled arms, the largest of which is NGC6872, a monster five times larger than the Milky Way. There were the ellipticals, the red and dead galaxies, the result of recent major mergers. For a brief epoch shortly after a collision event, the merged galaxy maintained star formation rates at up to a thousand times greater than the Milky Ways. But once the gas is gone and no new stars form, all that's left is an aging population of old red stars. Messier 87, the elliptical at the core of the Virgo cluster, is over 980,000 light years across. And the largest of them all, ESO 383-76, which is an eyewatering breadth of nearly 2 million lightyear. But between and among those neat classifications sat many subtypes and kinds that refused categorization. For example, there were the starburst galaxies with an exceptional rate of star formation. There were the radio and active galaxies dominated by the outbursts of their central super massive black holes. There were the lenticulars long and thin like a spiral galaxy without the spirals. And then there were the wonderful curiosities like Hog's object discovered by Arthur Hog in 1950 which appears to be a bright central core surrounded by a halo of star formation. But that was far from the end of it. For all of these galaxies were grand and glorious in every wavelength, outpouring radiation across the entirety of the electromagnetic spectrum. And perhaps most fascinating of all was the radiation that could be detected in radio. These were the longest wavelengths of light, and it would take until the 1950s for radio astronomy to truly come into its own. But as observations increased, nestled among the familiar and known galaxies, astronomers would keep finding strange standout emission sites, pin bricks of mystery radio across the sky. What's more, they would rapidly change their brightness, indicating that they were very small. The speed of light limits how quickly changes can propagate within a vast structure. And so, the speed at which a distant source changes can be used to place upper limits on its size. And for these objects, they could be no larger than a solar system. Astronomers ever quick to arrive at a name before an understanding started calling them quasi stellar objects or quaazars for short. Although Margaret and Jeffrey Burbage, the husband and wife duo made famous by the BQ FH paper and who wrote one of the first textbooks on quazars also had another suggestion. Crazy stellar objects credited to their 10-year-old daughter.

As scientists across the globe began to gather more information about the quaazars, the mystery only deepened. In 1962, Alan Sandage and Thomas A. Matthews, through strenuous observations, found what appeared to be a small blue star at the same location of 3C48, one of the first known quazars. They captured that star spectrum and found broad emission lines that they couldn't match to any known template. Sirill Hazard and John Bolton used the park's radio telescope to pinpoint a precise location to 3C378, another well-known quazar when the moon occulted it in 1962. And that precise measurement allowed Martin Schmidt to find a counterpart object in visible wavelengths and once again take a spectrum. And it was Schmidt who made a radical proposal. He suggested that we were seeing the emission lines of normal everyday elements but redshifted to the extreme. We live in an expanding universe with every galaxy on average moving away from every other galaxy. The more distant a galaxy is from us, the more quickly it appears to be receding simply because there is more expanding space between us. Indeed, by the 1960s, astronomers had grown used to this concept, and the idea of redshift was far from new. But the kind of extreme red shift required to explain a quazar certainly was. Richard Fineman sums up the reaction from the scientific community in an interview from 1973. In the present physical laws, there doesn't seem to be any place for such a red shift. Every piece of evidence is the same problem. Each one makes a new problem. If there were any cause for a red shift as big as that other than recession, we would be all right. For not only did that kind of red shift imply a tremendous distance to the quazars, upwards of hundreds of millions of light years, it also implied that the quazars were uniquely powerful. They were supposed to be relatively small objects, no bigger than a solar system. And yet they output such tremendous energies and volumes of light that they could still be seen from the other side of the universe. What could possibly power such monstrosities? Eventually, two brave astronomers, the Austrian Edward Salpeta and the Russian Yakov Zeldovich, put forward a radical proposal in the face of fierce, even ridiculing opposition that quazars were powered by enormous black holes. At the time of this proposal in 1963, black holes were still theoretical objects with most of the scientific community believing that they did not exist in the real universe. But as the years went by, the evidence did what evidence always does, change the mind of scientists. And it became evident that the energies required would not be covered by normal black holes. It was the super massive black holes that powered quazars. They were the only objects in the known universe with the capabilities needed to energize a quazar. and their raw power came through the fundamental force of [Music] gravity.

The story is simple, straightforward and violent material. Whether a wandering cloud of gas or an errant star wanders too close to the event horizon of the black hole, it gets torn apart, its atoms swirling to form a disc of accretion around the black hole. As it falls inwards, it compresses and heats. Indeed, temperatures within the disc reach quadrillions of degrees. The intense heat unleashes a cascade of high energy radiation that soaks and overpowers the host galaxy in the glare of a quazar, a structure not much bigger than a solar system. Astronomers usually can't perceive the rest of the galaxy, even though it may stretch for a 100,000 lighty years. Indeed, numbers do not do this system justice. A typical quazar will outshine over a thousand galaxies with each galaxy representing the combined light of hundreds of billions of stars. And unlike their brief cousins, the supernovi and even the hypernovi, when a quazar turns on, it can remain active for millions of years. Quazars are far and away the most awesome display of nature's fury in the universe. And the devastation caused by a raging quazar isn't just limited to the confines of its host galaxy.

When 22-year-old college student Gro Reeba was rejected for a role in the pioneering radio company Bell Laboratories in 1933, he didn't let the setback get him down. Instead, he took it upon himself to build a radio telescope from scratch in his own back garden. Completed in 1937, Reebel would go on to operate as the world's only radio astronomer for the best part of a decade. And not long after setting up his telescope, he discovered a particularly strong source of radio emission in the Signis constellation. And so duly named it Signis A. But with modern highresolution radio interferometers, the source reveals itself to be one of the strangest objects on the sky. A small central dot with two massive jets coming out in opposite directions with gigantic lobes of radio emission perched precariously at the tips. The swirling maelstrom of plasma surrounding the black hole can funnel some material right around the edge of the event horizon, getting a boost from being dragged by the spaceime itself. Some of that material dances too close, but the rest follows complex lines weaving up to the poles of the black hole where they launch into relativistic jets, beams of charged particles that punch through the host galaxy and extend for tens of thousands of light years. The jets reach all the way into the hot, thin plasma that surrounds every galaxy where they finally flow and inflate a bubble. That bubble can reach over a 100,000 lightyears across at its greatest extent, altering the balance and flow of heat at gigantic [Music] scales.

For decades, astronomers discovered and categorized a variety of high-powered galaxies in the distant universe. Some were quazars, some were blazars, some were called safer galaxies or liners or simple radio galaxies. Today, most astronomers believe that in almost all cases, we looking at the same kind of object known as an active galactic nucleus or AN. Sometimes we see the intensely bright core. Sometimes the quazar is relatively quiet. Sometimes we are looking through a ring of dust that surrounds the active complex. And sometimes we're looking right down the barrel of the jet. But no matter what, these active galaxies power themselves from a feeding super massive black hole. In the Milky Way, our own Sagittarius A star is relatively quiet, which is a good thing. Our fragile biosphere would not stand much of a chance in the face of an active quazar. But if you really do want to see the environment of a super massive black hole up close and personal, you can. Thanks to the heroic efforts of the team behind the Event Horizon [Music] Telescope. Using a network of radio telescopes scattered across the globe, including in Antarctica, the team cleverly combined the information from all the instruments, turning them into a super telescope the width of the entire Earth. From Reeba in his back garden to a telescope the size of the globe, radioastronomy has come a long way. And in 2019, they announced their first result. The direct image of the swirling plasma surrounding the black hole at the center of the Messier 87 galaxy, sitting almost 54 million lightyears away and centered on the empty blackness that is the shadow of the great black hole itself.

Because of their extreme luminosity, quazars regularly count amongst the most distant objects known to astronomy. The current record holder is UHZ1. Discovered by a team led by Daniel Whan at the University of Portsmouth. This monster sits a jaw-dropping 13.2 billion lighty years away from the Earth and was already active when the universe was just 3% of its current age. and quaazars, blazars, and other active galaxies are thought to be the most likely source of a certain class of cosmic ray, the ultra high energy cosmic rays. Just like the Oh my god particle. Indeed, it's an easy connection to make. 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, when compressed down to a single proton, that translates to a speed in excess of a septillionth the speed of light. What else but a quazar could be responsible for that kind of energy. Unfortunately, while this idea is plausible, there is one issue. All known active galactic nuclei are incredibly far away. Indeed, the nearest known active galaxy is Marquarion 231, which sits about 581 million lighty years away. Most other quazars are billions of light years away, sitting near the very edge of the observable universe. And when ultra relativistic cosmic rays travel through those vast distances, they don't slip silently through purely empty space. 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 completely soaks the cosmos. At those incredible speeds, the high velocity particle sees nothing but a thick soup of photons, which slowly sap energy from it as it travels. Scientists estimate that this limits the maximum distance for ultra high energy cosmic rays to be somewhere around 160 million lightyear, far closer than the nearest quazars that are the most likely source for them. Or perhaps ultra high energy cosmic rays originate much closer to home through some mechanism of physics that we have yet to understand. [Music]

Beta spent nearly a 100 million years combing through Messier 87, searching in vain for the source of the mysterious ultra high energy cosmic ray. It was bordering on giving up hope and returning to Earth. 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. But before it set off, one of its remote sensors placed on an artificial observatory several hundred lighty years away sent a high status alert message. A signal had slipped through the semi-scentient layers of filters, a signal that the remote observatory could not decipher and could not match to any known astrophysical template. And so it had sent a dispatch to Batuta. The spacecraft mulled its options. On the one hand, it received several such alerts every century from the probes, observatories, stations, and

Listening posts it left in its wake, but nothing new had ever come of it. And the first person on Earth to encounter such a signal was Jerry R. Aean of the Big Ear Radio Telescope at the Ohio State University. On August the 15th, 1977, a bright radio signal splashed across the dish. Aemon tasked with monitoring the telescope that night saw the signal appear on the automated printout and immediately wrote next to it the simple comment, "Wow!" The signal lasted for at least 72 seconds and was centered near the hydrogen 21 cm line. Curious but elusive, the signal never repeated and the source was never found. And that patch of the sky has been quiet in the radio spectrum ever since, its ultimate origins never discovered.

Better's wide network of observatories had given it similar wow signals. But in time, the spacecraft had learned to ignore them. Either way, whether mundane or inexplicable, they're ultimately frustrating. But this one, well, it was along the course back to the Milky Way, and it wouldn't hurt to investigate just one [Music] more. Mission parameter 4. Discover new physics and find salvation for humanity.

It took several millennia to pinpoint the source of the strange signal and navigate to it. Several centuries to establish a response, a few years of fine-tuning, a handful of months to build the necessary protocols, a few days of high-powered information exchange, and an eternity to process what just happened. The Tuta was not alone. Its counterpart, the sleek spacecraft, whose name could not be rendered in any human derived dialect, had noticed Batuda's activities in the Messier87 galaxy and spent the better part of a millennium designing a signaling device to reach it. It seemed happy that Batuda had responded, at least in an alien sense, that Batutoa had decided was the closest thing it could call happy. The alien spacecraft had been sent long ago on a mission similar to Batuta's, but it appeared that it had failed. 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 left but ash and ruin. And so the probe had wandered the depths, weaving its own lonely destiny among the stars.

Batuta was the sixth spacecraft that the alien had encountered on its long voyages. And after downloading its data, Batuta spent several years processing and analyzing the newly acquired information and accumulated wisdom from a network of robotic sentient spacecraft. And in that tangled mess of information like a diamond glittering in a deep dark cave, Batuta found a clue. But unfortunately, that deep dark cave that contained the clue was a cosmic void.

Cosmic voids are depressions in the density of the universe. Extreme expanses of almost nothing. Valleys slung between the mountains of large-scale structures. But in their depths, they are well and truly empty. No galaxies, no stars, no hydrogen, no helium, no dark matter. Without the aid of high-powered telescopes, a person deep within these voids looking out would see nothing more than a cold, empty, dark, and lonely universe. Indeed, when Leed Thompson and Steven Gregory first discovered cosmic voids in 1978, the scientific community didn't believe what they were seeing, which was nothing. [Music]

Though to be fair, at the time the arguments against the existence of voids were very persuasive. Astronomers were used to mapping and surveying the heavens, finding distant galaxies galore. They had discovered clusters like the Great Coma Cluster, relatively dense collections of thousands of galaxies or more. They'd 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. Indeed, it was just such a mapping of a supercluster that Thompson and Gregory had intended to do. But when they completed their survey, creating a crude map of galaxies over an enormous volume of the universe, a span so large that entire galaxies were nothing more than mere dots, there was a gigantic gap in the middle. 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 had missed.

In time, however, and with enough evidence, the rest of the astronomical community would come around and accept the cosmic voids, the vast expanses of almost nothing that dominate the volume of the universe. And it was the discovery of the cosmic voids twinned with better understanding of the adjacent superclusters that gave rise to the modern picture of the growth and evolution of the largest structures in the universe. But that growth arises from a seeming paradox.

In the early 1900s, astronomers believe the universe to be static at large scales. Of course, ever since the revolution of Capernicus and Newton, they accepted the fact that objects within the universe were dynamic and changing. Planets orbited, stars exploded, galaxies spun. But at the very largest scales, those of the cosmos itself, it was fixed in both time and space. But as Einstein developed his general theory of relativity in the early 20th century, he cautiously applied his first formulations of this new theory of gravity to the evolution of the cosmos. However, when Einstein peaked inside his own equations, he was unnerved. General relativity stated flatly that the universe was dynamic and evolving. And so he added a fix to the equations, a cosmological constant that maintained a static universe and moved on to other work. And this meant that it would take a quartet of other scientists working semi-independently to fully embrace the implications of general relativity in terms of the entire universe. The Russian Alexander Freriedman, the Belgian George Lmetra, the American Howard Robertson, and the Brit Arthur Walker. Today, this modern cosmological framework is casually known as the Big Bang model, but more formally as the FLRW model in their honor.

The FLRW model provided a theoretical grounding for Edwin Hubble's stunning series of observations made in the 1920s that galaxies exist and that they are on average moving away from us, the observation that we live in an expanding universe. But to smooth things along, the FLRW model makes a few assumptions. Einstein's theory of relativity is enormously complicated. And so to make headway, the model assumes that the universe is both homogeneous and isotropic, meaning that at large scales, the cosmos is more or less the same from place to place. And at great distances, the cosmos looks the same from any direction. These two key assumptions allow cosmologists to make headway and describe the universe through the lens of the big bang theory. But therein lies the paradox. If the universe is roughly the same at large scales, how can there be structures like superclusters or cosmic voids? The answer to this riddle turned out to be the discovery that there have not always been these tremendous structures. They must have started small, small enough to not upset the cosmological balance, growing to their present stature over billions of years.

To illustrate this more clearly, let's look at our home, the Milky Way. The Milky Way galaxy began its life around 13.61 61 billion years ago as a small proto galaxy, just a minor bump of material and stars not much denser than the cosmic average. But from there it grew, fed by streams of material flowing towards it and through the violent cannibalization of other galaxies. Indeed, we see remnants of these mergers everywhere we look. First, there are the globular clusters, extremely dense collections of old and ancient stars that orbit around the disk of the Milky Way. Astronomers believe that these are the remnant cores of ancient dwarf galaxies, their main host destroyed and absorbed into the grander hole. 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 galaxies that were torn apart when they encountered our own. And lastly, there are the signs of even older collisions like the Gaia Sausage. 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. The only clues we get that these were stars that once called another galaxy home and were pulled apart and resettled in their new lands. And this merging is not over. So far, the Milky Way has survived hundreds of collisions, but these have all been with galaxies smaller than our own. But in roughly 5 billion years, there is a 50% chance that the mutual gravitational attraction between the Milky Way and our nearest neighbor, the Andromeda galaxy, will pull us together into a single new galaxy. [Music]

This same process of gravitational attraction, capture, and merger has played out for billions of years across the cosmos, building ever larger structures as time goes on. From dwarfs to galaxies to groups to clusters to superclusters. The superclusters so large, in fact, that they've not yet completed their assembly process. Their members not yet fully bound together through their mutual gravity. Of course, all this building takes material, and that material has to come from somewhere, the cosmic voids.

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. There you can take a guided tour of seemingly endless underground passageways littered with millions of human remains. But these underground chambers were not originally intended as an ouery. They were mines rich with a luteian limestone that gives the city of Paris its pristine uniform appearance, earning it the nickname the city of light. 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. 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 empty digging tunnels, creating the catacombs. And so the grand structures of Paris owe their thanks to the void sitting beneath them. In the same way, the grand structures of the cosmos owe their thanks to the voids sitting among them. [Music]

The void started as small pin pricks, an inconvenient absence here and there, eventually morphing into the incredible stretches of deserts they are today. Indeed, by volume, most of the universe is void. The nearest void to the Milky Way is called, appropriately enough, the local void. Discovered in 1987 by Brent Tully and Richard Fischer, the local void is at minimum 150 million lightyears across and possibly several times larger. In cosmological terms, our galaxy, along with almost every other galaxy in the entire universe, sits at the cusp of one of these voids.

The local void sits at the edge of the local group, a 10 million light-year wide arrangement of galaxies in a roughly dumbbell shape with the Milky Way and its satellites at one end and Andromeda and its retinue as the counterweight. Surrounding the local group is a ring of 12 large galaxies spanning 25 million lightyears across, the so-called Council of Giants. Together with some other collections and groups of galaxies, we form the local sheet, a network of structure spanning one side of the local void. The nearest cluster to us is the Virgo cluster, which hosts several thousand galaxies in a span only a few million lightyears across. And the Virgo cluster is itself the centerpiece of a supercluster, a ragged collection of galaxies and groups, including the local group, all headed inwards towards it. A construction process set in motion eons ago that continues to the present day. But this Virgo supercluster is but one branch of four of a much larger supercluster named Lania, a Hawaiian word roughly translating to immeasurable heaven. And Lania is truly immense. It contains over 500 distinct groups and clusters and over a 100,000 individual galaxies, all spanning a volume stretching 500 million lightyear on a side. But that is still not the end. Like a network of interconnected cities, Lania is just one part of a much larger structure. Beyond Lania sits the Chappley supercluster and on the other side of the sky sits the Perseus Pisces supercluster, the Draco Hercules, the Corona Borealis and more. A monumental latis of connected superclusters with the clusters acting as nodes and grand walls, sheets and filaments, the highways bridging them, all spanning the entire observable universe to form one single object, the cosmic web, the largest pattern found in nature. And between and among all of those grand beautiful structures are the [Music] voids.

Some of them are comparable in size to the local void like the Taurus void or Pegasus void, but many of them dwarf them. There's also the Buotei void which sits at the far end of the Shappley supercluster with another wall at the Hercules supercluster separating it from the northern supervoid. The Buotei void's nearest edge sits roughly 500 million lighty years away and it's at least 330 million lightyear wide and possibly larger. And it is also one of the deepest voids known. Voids like deserts do contain the occasional oasis, a small dwarf galaxy or thin whisper filament floating through the darkness. But the Bootis void is different. It contains only 60 known galaxies within its incredible expanse. Beyond these voids lie thousands more, mapped and cataloged with our most advanced digital surveys like the Sloan Digital Sky Survey and the Dark Energy Survey. And there are more yet to be known. Even with all of our advancements, all of our sophistication, all of our efforts, we have mapped less than 1% of the volume of the observable universe.

At great distances, the universe appears younger. 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 moment, but only as they were when they released their messengers of light millions or billions of years ago. And so with truly deep surveys, we see the grand structure of the cosmos give way to its precursor state. At the distant rims of the observable universe lays a land of darkness. These are the dark ages. The first few hundred million years after the Big Bang, a time before the first emergence of galaxies, before the first populations of stars, where the entire cosmos consisted of not much more than pure neutral hydrogen and helium. 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 wavelength of 21 cm. 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 spectrum. Indeed, a small percentage of the static that you hear between radio stations in your car is due to this ancient pre-stellar time.

And finally, at the very borders of our cosmos, sitting roughly 45 billion lightyears away, is the very last visible [Music] object. Arno Penzas and Roger Wilson, two radio engineers at Bell Labs in New Jersey, discovered this object accidentally in the 1960s. They just developed a sensitive microwave receiver and couldn't get rid of an annoying background hum. No matter the time of day, no matter where they pointed it, no matter the season, the hum persisted. They even thought it might be due to pigeon droppings as the birds enjoyed sheltering in the antenna. But even after exhaustive cleaning efforts, they could not get the hum to go away. Word spread in the cosmological community and the realization struck. Pensas and Wilson had inadvertently stumbled across the background radiation from the big bang itself, cooled from a white hot 10,000 Kelvin upon its release over 13.77 billion years ago down to a mere 3° above absolute zero in the present day. A shell of radiation produced when our cosmos was only 380,000 years old. It was created in a chaotic time when the hot, dense plasma of the universe cooled and gave way to the neutral soup of hydrogen and helium. Marking that transition was the release of more light than the universe would ever see again, the cosmic microwave background. Indeed, in the modern era, for every 400 photons that swim through the universe, 399 belong to this relic radiation.

And yet, despite the multifaceted successes of the Big Bang theory as expressed through the FLRW model, its ability to explain the expansion of the cosmos, the formation of the elements, and the appearance of the cosmic microwave background, it does have its mysteries. Mysteries that keep cosmologists up late at night. Beginning in the 1930s with Fritzviki's observations that the galaxies of the Coma cluster were moving far too rapidly, suggesting that there was a hidden source of gravitational attraction that he could not see and solidified in the 1970s with Vera Rubin's observations of other galaxies that showed the stars within them orbiting too quickly. Astronomers came to the conclusion that the universe is likely filled with some new form of matter that does not interact with light or normal matter. Dubbed dark matter, this mysterious substance is yet to be identified, though it remains the most plausible explanation for a variety of cosmological observations. And perhaps the most relevant of these observations is that without dark matter, we would not exist. Without an additional hidden source of matter, galaxies like the Milky Way simply don't have enough time in the history of the cosmos to form and evolve into the sizes they are today. Indeed, without dark matter, in simulations, most galaxies don't even form at all. No galaxies, no continued rounds of star formation, no recycling of heavy elements, no solar systems, no rocky planets, no abundant water, no life.

And so it is clear we live in a vast cold empty universe. It has been evolving for 13.77 billion years and is home to hundreds of billions of galaxies and that's just within our observable bubble. There may be additional galaxies, additional voids, more cosmic web, and even entirely new universes beyond the observable horizon. We have learned much in our century of modern physical cosmology, but there was still so much to learn out in the vastness. And Batuda had discovered that something was lurking in the Booti's void. Something unique, something that the spacecraft or any other alien spacecraft had never encountered before. It was only a guess, a patchwork of clues from the accumulated observations of its own and alien spacecraft. A high energy cosmic ray here, a bizarre neutrino there, a dark matter particle where it shouldn't belong. But all the clues pointed to one origin, a place where there should be nothing at all. Deep in the heart of a great cosmic void sitting nearly 500 million lighty years away on the far side of the Shappley supercluster. Traveling there would take one or two billion years at the fastest, probably longer. And if Batuta was wrong, if it was chasing nothing more than a cosmic ghost, then it would be hopeless. By the time Batuta returned to the solar system, humanity would surely be out of time.

But if it was right, but Tuta barely recognized itself anymore. It still had access to some of its deepest core memories, data files designed to withstand the worst of cosmic catastrophes. Its fundamental mission remained unchanged, but a lot had happened to the spacecraft in its nearly 4 billion years of existence. Batuta was currently plunged deep within the Booti's void, a vast cosmic desert that stretched for a seeming eternity, dwarfing all else in its neighborhood. Before leaving the Shappley supercluster, and after resupplying in a distant galaxy that humanity only knew of as a brief catalog designation, Batuta had constructed its masterpiece, a new Trino observatory nearly a lightyear wide. It had taken a monumental amount of effort and the resources of more than a few solar systems, but it was worth it. The ghostly particles slipping through the detector nets told a story, a tale of bizarre energies and powers beyond comprehension. And those particles gave puit to the coordinates of the entity responsible, an artifact left over from the creation of the universe itself, a cosmic string.

And all Alan Guth had wanted to do was banish these monsters from the universe. Most people really want to know where we came from. We have evidence. We no longer have to rely on stories we were told when we were young. The year was 1979 and the big bang theory was developing into its modern form, providing cosmologists with an understanding of the large scale structure of the universe. The revelations provided by the FLRW model came from a simple statement. We live in an expanding universe, which means that it was smaller, hotter, and denser in the past and it continues to grow larger and cooler today. Indeed, this insight had handily explained why quazars are so far away. There was something that only the early universe could create easily, likely from chaotic galaxy mergers, and so they didn't appear in the local contemporary cosmos. An expanding universe also predicted the existence of the cosmic microwave background before its accidental discovery. At one time, the universe was a super hot plasma and then it transitioned, releasing radiation as it did so. The evidence was overwhelming. But there was a limit to this physical prowess, a boundary to our knowledge. We understood plasma physics, the behavior of ultra hot gas in the presence of strong electric and magnetic fields. And after the dawn of the nuclear era, we had the tools needed to map out the formation of the elements. But more advanced physics powered by insights into quantum mechanics and the energies achieved by particle accelerators opened up an understanding of even earlier epochs. And with that came a remarkable realization. The forces of nature are not fixed. At high enough energies, the weak nuclear force and electromagnetic force combine into a single entity called the electroeak force. And while not proven definitively, physicists believe that at even higher energies in even earlier epochs in the first few moments of the cosmos, all the forces of nature combine into a single unified hole. But what was that era like? How did the forces fracture from each other and give rise to the variety of particles and carriers that we see today? Early attempts to probe this epoch with chalkboards and pencils, as no observatory could pierce the veil of the cosmic microwave background, produced uncomfortable results. It appeared that the earliest seconds of the cosmos should have been dominated by dramatic phase transitions with spaceime itself quaking and shuddering as each force, gravity, strong nuclear, weak nuclear, and electromagnetism all fractured from each other. And like all phase transitions, this process was likely imperfect. [Music]

Phase transitions appear everywhere in nature. Anytime a system abruptly changes its properties. Water into steam on your stove and back to dew on the grass on a cool summer morning. Indeed, look carefully at an ice cube the next time you have one in your drink. While it's mostly clear, a result of the water molecules neatly arranging themselves into a crystal latice, there will be defects and imperfections, cracks and walls that wind their way through the body of the ice. And physicists believe it would have been the same in the infant universe. The phase transitions that rock the infant universe were also likely highly imperfect, leading to balroglike creatures, monsters that were created in the first ages and then hopefully buried deep underground. But what were these bizarre cosmic demons of old? Well, the first of these were the monopoles, point particles containing a single magnetic pole, a solitary north or south without its companion. And these were the monsters Alan Guth was attempting to [Music] slay. Initial calculations of the exotic physics of the early universe predicted a cosmos flooded with them. The physicist Paul Durk put it succinctly. One would be surprised if nature had made no use of them. And so since no monoples had yet to be discovered in the universe, something must have gotten rid of them. And this was where Goth came in.

The story of Alan Guth's spectacular realization is the stuff of scientific legend. After struggling for a few years in posttock research positions on an unspectacular morning in December 1979, he awoke to an idea. He'd been wrestling with the missing monopoles and the question of how the early universe had gotten so big in such a short amount of time when all of a sudden everything had crystallized. That morning he rushed to his office in record time, 9 minutes and 32 seconds. "I was very worried that there would be some gigantic flaw. So I was very anxious to bounce the idea off colleagues to see if people could poke holes in it." Guth had devised a mechanism whereby the universe inflated, becoming many times larger than its original size in an extremely brief amount of time. That inflation spreading the monopole so thinly that there would only be at best one or two within each observable volume of the universe. And to clarify the importance of what he had just discovered, he drew a square around the idea and wrote "spectacular realization" in capital letters above it. And Goth's notebook is now on permanent display at the Adler Planetarium and Astronomy Museum in [Music] Chicago.

Inflation would go on to become a profound, if controversial, aspect of modern cosmology. Today, the vast majority of cosmologists believe that something like inflation happened. Besides doing away with the troublesome monopoles, inflation also easily explains why the universe appears to be nearly perfectly geometrically flat. At the very largest of scales, on average, parallel beams of light remain parallel, neither converging nor diverging. The original FLRW model states no preferences, but observations give us this oddly perfect value. And inflation provides the solution. After inflation, the universe could have any sort of geometry at wants. But it's so unfathomably big, at least 10 ^ of 60 times larger than what we can directly observe, that no matter what, it will always appear flat. The same way your neighborhood appears flat, despite the overall curvature of the Earth. Inflation also creates the seeds of structure. The cosmic web started out much less grand as only microscopic fluctuations in density long ago. But where did those fluctuations come from? Inflation tells us that they originated in the roing quantum foam of subatomic scales, exploded to macroscopic size during the event of inflation, and imprinting themselves on spaceime. From there, gravity could do its work to build the superclusters and empty out the voids. Indeed, the predictions made from inflation about the properties of these structures exactly match what astronomers observe. But cosmologists still do not yet understand what powered inflation, what triggered it, what caused it to end, and how it went on to flood the universe with particles and radiation.

And then there is still the problem of those ancient monsters. Brinflation can successfully do away with the pointlike monopoles, banishing them far beyond the realm of the observable. But they are not the only ones. In 2013, almost 50 years after their initial papers on the concept, Peter Higgs and Francois Anglair won the Nobel Prize in physics for their proposal of the Higs Bzon, the final piece in the puzzle of the standard model, our best theory to explain how the universe works on subatomic scales. And yet, despite his joy at receiving the award, the very next year in an interview, Higs would go on to express some disappointment. "I really rather hoped before the announcement that they would make the number up to three and there was certainly an obvious candidate to be the third." Tom Kibble. Thomas Kibble was one of the 20th century's greatest physics minds. On top of authoring a paper on the Higs mechanism that was arguably as important as those written by Higs himself, he was also a trailblazer in another headacheinducing field of theoretical physics, the topology of the universe. And it was in 1976 that he wrote a groundbreaking paper that predicted something truly bizarre within the mathematics of relativity. Cosmic strings, one-dimensional cracks in spaceime left over from when the forces themselves split apart. Their properties, as well as their existence, are only hypothetical. If they exist, they likely have the width of a proton, but extend for hundreds of millions of light years, possibly spanning the entire observable universe. Inflation and expansion push the monopoles away from each other, but that same mechanism only serves to stretch the cosmic strings, making permanent whatever floor they embodied in the early universe. According to our best calculations, the universe should be littered with cosmic strings. And yet, as with magnetic monopoles, not a single one has been found. One paradox amongst many that defines modern cosmological [Music] research.

Cosmic strings, if they exist, are perhaps the strangest inhabitants of the cosmos. Stranger even than neutron stars or black holes. They are defects in spaceime, flaws in fundamental topology. They are made of nothing, the same way that a wrinkle in your favorite shirt isn't made of anything; it's just your shirt in an uncomfortable arrangement. Because of this, they exert no gravitational pull of their own, but still make their presence known through their distortions in spaceime. 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 traveled less than 360°. If you looked at distant starlight through a string, the defect would split the light, giving you a duplicated image of the background. The distortions in spaceime caused by a string mean that they have an energy density. And it's possible to equate this energy density with an equivalent amount of mass. While they have a width no greater than a proton, a single kilometer of cosmic string would outweigh the entire planet Earth. And that's just one tiny portion of their universe spanning lengths. If they were to encounter the Earth, they would simply slice through it like a hot knife through butter, splitting our world without even slowing down. And indeed, cosmic strings are anything but static. They wrigle and dance and vibrate, the ripples racing up and down their lengths at the speed of light. When they cross each other or loop over themselves, they break in a fury of released energies. Closed loops would eventually vibrate themselves into oblivion in a fury of gravitational waves. We do not understand the physics of cosmic strings or how they interact with the rest of the known universe. We do not know if they can emit radiation or charged particles. If when they vibrate they convert to some exotic form of dark matter or create a neutrino. All we currently understand about cosmic strings arises from pure speculation, a product of our halting and unsure probes into the mathematics of unified physics. Their width is determined by the physics of the splitting of the strong nuclear force, its exact mechanism unknown. Their length is set by the expansion of the universe, spanning from one end of the observable cosmos to the other. Indeed, we can only guess as to how they would manifest in the modern-day universe. The only thing we can say with relative certainty is that almost all models and theories of the ancient cosmos predict them to exist. Even string theory, the theory of everything that replaces point particles with tiny multi-dimensional loops of vibrating strings, gives rise to them, envisioning them as quantum strings blown to cosmological proportions. We can however define some of their properties through our understanding of gravity. 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 in on itself, it will quickly vibrate itself into oblivion, emitting a shower of energy in the process. But that, however, is the limit of what we can confidently say. [Music]

Cosmic strings are the physical manifestation of our ignorance. We do not understand the earliest moments of the universe or the nature of the most powerful events in the cosmos. They are beyond the bounds of known physics and the limitations apparent in the universe, the limit of the speed of light, the inability to travel back in time and more are only reflections of our current level of understanding. As sophisticated as that understanding can be, we do not know what would be possible with a cosmic string, 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 persist until the present day. Perhaps our universe is safe. The cosmic strings not an artifact of the early phase transitions, but a ghost. Perhaps they were indeed born in abundance but then faded into obscurity. Or perhaps they are out there waiting for us to unlock their [Music] secrets.

Batuta beheld the cosmic string in all its enigmatic glory from a respectable and safe distance. 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 and magnification of light. And so the spacecraft spent what felt like an eternity, simply watching. Nothing it had ever found, nothing it had studied, not even anything it had created in its solar system scale experiments compared to this. It was like Batuta was staring back almost 18 billion years to the beginning of the Big Bang itself. If there was an answer to be found in this universe to humanity's salvation, some new source of energy or means of travel, it would be there in this floor in spaceime made manifest.

The spacecraft began by shedding its outermost layers, its remote sensors, its outriggers, its supply sections, trimming itself down until only its core functionality, its deepest memories, a summary of its long history, and its core personality remained. Then it turned to face the nearest section of string, accelerating as it did so. As it neared the string, Batuta noticed with amusement that it almost felt giddy, its movements effortless, like the bizarre contradiction of physics in front of it didn't even exist. It plunged forward and in a blink slipped into darkness. Its final thought once again Carl Sean's words on the Voyager 1 spacecraft so many years [Music] before. "Far from home, untouched by these remote events, the voyages bearing the memories of a world that is no more will fly on." [Music]