Is The Universe Already Ending?

How much of the universe is already dead? At first glance, it's a difficult question to answer. Except for the occasional supernova blast or colliding black hole, at the very largest scales, the cosmos appears static and unchanging, a frozen snapshot in a long and unknowable history. But Swedish physicist Eric Holberg decided he wanted to know more. Today his name is largely unknown even to astrophysicists. But in 1940 in a darkened gymnasium in Stockholm he crafted perhaps the most ingenious experiment since the Renaissance days of Galileo. In the process becoming the first scientist to watch millions of years of cosmic history unfold. The idea was simple. In the gymnasium, he and his students set to work placing 74 stations, each one equipped with a bright light bulb and a photo receptor that could measure the light being received by that station. The stations represented portions of a galaxy, an individual bit of mass that could influence its surroundings and be influenced by it in return. The light stood in for the gravitational force, each reducing as the distance squared. He divided the stations into two camps, drawing the rough figures of two spiral galaxies just before a merger. Then slowly, methodically, he set the lamps in motion. He measured the amount and direction of light at each station and used that to estimate where the station should be placed as if that portion had advanced forward a million years in time. After moving all the stations, he repeated the measurement and movement again and again. Holberg had created the world's first simulation of the cosmos built with an analog computer. In watching the evolution of galaxies unfold before his very eyes, Holberg saw exquisite beauty, the stretching the galaxies undergo and the graceful arching of tidal tales that arise during a merger. But he also saw something else in the form of light bulbs and photo receptors arranged carefully on a gymnasium floor. He witnessed how galaxies die. Holberg witnessed a universe that changes. And he saw that the cosmos we have learned to know and love, a universe filled with vibrant galaxies arranged in a grand cosmic web, will not last forever. There was a time billions of years ago when the universe was plunged into darkness before the first stars awoke. And by the harsh laws of cosmic expansion, there will be a time in the distant future when the cosmos returns to that darkness. In his shadowfilled gymnasium, Holberg had witnessed that impermanence. But what he didn't see, what he couldn't see without more advanced tools was that the wider universe was already well on its way to oblivion, well past its peak. For today, following in H Homeberg's pioneering footsteps, we have combined advanced computer simulations with comprehensive observations to learn that much of the universe is already dead. We are already in the age of twilight. What we face in the long future is nothing but decline. In a sense, then this is a story of betrayal because in a changing universe, the very forces that give rise to star formation eventually turn their backs on their creations and kill them from the inside out. In the 1930s, Gro Reeber was rejected from a post at Bell Labs, where he'd hoped to work on scanning the sky for radio signals. And so instead, he built an entire radio telescope in his garden, the most advanced in the world at that point, from among other things, spare parts from a Ford Model T truck. Pretty impressive. And so what would your passion project be? Whether you want to create documentaries, a podcast, a company that sells services, or a brand that sells products, Squarespace can help. Squarespace is super simple to use and easily allows you to build a totally unique online presence for your project. But one great, less well-known feature of Squarespace are the in-built email campaigns. Here you have all the tools you need to engage with your world. You

can schedule emails to arrive at the perfect moment for your recipients and set up email automations to save you time. And all of this seamlessly integrates back into booking, sales, or promotional pages that have beautifully well-designed templates. So head to squarespace. com/history of the universe to save 10% off your first purchase of a website or domain using the code history of the universe and start your passion project today. 4 and a half billion years ago, just after its birth, our Earth was a literal hell. There is an earthshaking rumble permanently echoing across the young globe as the very crust itself shifts and cracks. Molten rock spurts and oozes over the constantly shifting landscape. Temperatures reaching a thousand° on the surface. The atmosphere almost non-existent, barely different in its makeup to the inside of a solar nebula. And in the sky above this molten chaos, is the moon, but not the moon we know. 15 times as large as we see it today and glowing a dull red, this young satellite would have filled the horizon like a dreadful portent. And so what happened between then and now to give us the moon of today, placid and distant? Well, it is very slowly moving away. We know this from what is called lunar laser ranging, a technique where scientists shine laser pulses at the moon and measure the length of time they take to arrive. The accuracy of these measurements improved by optical receptors placed on its surface during the Apollo missions. Scientists have calculated that our moon is receding at a speed of 1 in per year and would only finish its journey in roughly 15 billion years were that not well after the time at which the sun will have consumed the earth. Our night sky is far from static. And on cosmic scales, our retreating moon is just a tiny example. Our entire universe is full of change at scales great and small. Indeed, the astronomers of the Renaissance were astonished to discover that the heavens were just as tumultuous as our lives here on Earth. Once they realized that the Earth itself moved, the idea of the permanent, fixed, and unchanging cosmos crumbled, comets were discovered not to be just strange atmospheric phenomena. They were temporary visitors from the outer solar system. The bright guest stars that flickered and flared in the night sky were really the deaths of massive suns. The nebula were not mere clouds. They were either the birthplaces or the cemeteries of stars. The universe was revealed to be constantly shifting. And because of that change, there are things that have not yet had enough time to be born and things whose time has passed. Things that we will never see again. To start with, the extremely early universe was very different to today. Due to the immense energies present, it may have fashioned exotic particles like magnetic monopoles or even stranger topological defects like cosmic strings. They are all gone now. There was a time when the universe itself was capable of creating new elements. Primordial hydrogen and helium condensed out of the thick nuclear soup within minutes after the big bang. Generation after generation of stars lived and died, forming new elements and enriching the cosmos, eventually giving rise to planets, oceans, and the complex molecules that would make life possible. But those that form in a star are heavier elements. Except in a few rare cases, all the hydrogen the cosmos will ever have formed in roughly a dozen tumultuous minutes just after the universe began. There was a time when the cosmos was completely unrecognizable. Roughly 380,000 years after the Big Bang, the universe was so different that it was a completely distinct state of matter. The gas of the universe had a temperature of over 10,000 Kelvin that was hot enough to convert it into a plasma. When the universe changed from that state to something more similar to the cosmos of today, it released a flood of radiation, a one-time only event that created the cosmic microwave background that we can just barely still see today. That will never happen again. There was a time before stars, a time before galaxies, a time before planets. There was a time

when the first stars poured their light out, a super massive type of sun that seems to have totally vanished in the modern universe. There was a time when the universe was flooded with new stars and chaotic colliding galaxies. Now that time is done. This story of cosmological change began with Edwin Hubble's discovery that the galaxies around us were moving and in fact most of them moved in only one direction away. The universe was expanding. And this was key for an astic universe. Everything would be the same. No matter where we look, no matter how deep our observations, we would always see roughly the same things. the same kinds of galaxies, the same kind of activity, the same kinds of cosmic structures. And that is not what we found. But as well as this, Hubble was also able to increase the size of the universe by some distance. Before Hubble, many had believed that the Milky Way was all there was. But his measurements of distant galaxies ballooned the universe from 300,000 lightyear to a breadth of 280 million lightyear. Over the first half of the 20th century, estimates continued to grow as astronomers used more powerful telescopes and more effective methods of judging distance, reaching over 4 billion lightyears across by 1955. But then in the late 1950s, the first radio surveys were done of the sky and all previous estimates were blown out of the water. Physicists found strange sources of radio emission, pinpoints of almost impossible intensity, dubbing them quasi stellar objects or QSOs. The name would eventually be shortened to quazars, and they would turn out to be stupendously, almost impossibly bright, compact sources of electromagnetic energy that could outshine a million galaxies at once. And that was not all. For the key discovery was that there were no nearby quazars. They were all billions of light years away. And so once again, the universe ballooned in size. The most distant quazar found in the 1960s being 12 billion lightyears away. The furthest we have seen in modern times, closer to 13 billion, existing barely 600 million years after the Big Bang. Quazars were primarily creatures of the distant universe, which meant they were only active in the young universe, existing at a time billions of years before the modern epoch. In a static universe, we would expect to have some quazars as nearby neighbors, but there were none. We could only conclude that the universe was different in the past with just the right conditions to give rise to quaazars. a set of conditions that no longer exists. And this is a change in the universe driven by its expansion. But that is not all. For as well as this, we've discovered that it's not just the quazars that are different, but the galaxies themselves. Observations of the distant cosmos with the James Webb Space Telescope reveal a youthful universe inhabited by smaller, dimmer galaxies than the grand spirals and massive ellipticals that we see today. The earliest yet found Z14 is from roughly 282 million years after the Big Bang and is full of new stars being born. Although it isn't actually that small, still roughly the size of the small melanic cloud, giving it the potential over the intervening 13 1/2 billion years to our time to grow into a galactic leviathan. Indeed, deep cosmic surveys find that the cosmic web, the grand arrangement of galaxies on the very largest of scales, took time to evolve from a diffuse collection of gas into the intricate lattis work of the modern era. And so a changing cosmos tells us that the universe has a finite age. But the neverending expansion of the cosmos also means something else. That the universe will die. And in actual fact, it is well on its way. We put the relevant laws of physics into a computer, set up some initial conditions at an early cosmic epoch, add in all the ingredients we know of, and

let it all churn. Eric Holberg was the first to recreate our cosmos in a laboratory, but he would be far from the last. For today, 80 years later, all over the world, gymnasiums have been replaced by supercomputers and light bulbs by N bodies. One of the largest such simulations created by Vulkarus Springer and Lars Hearnquist and dubbed Illustrous TNG or Illustrous, the next generation, requires the use of nearly a 100,000 computer cores working in tandem to recreate just a tiny portion of the visible universe. Their code can follow the evolution of gas as it converts into stars and trace the formation of structure across the cosmos. And one of the primary goals of illustrous TNG is to understand the detailed mechanisms behind the rise and slow decline of galaxies. We know from observations with the James Webb Space Telescope that galaxies first appeared within a few hundred million years after the Big Bang. At first, they were small, less than 1% of the modern-day mass of the Milky Way and relatively dim. But those first galaxies grew quickly, feasting on their surrounding material. Simulations like illustrious TNG show us what only observations cannot as they reveal the hidden flows of gas onto the galaxies and how they intermix and mingle to form stars. And it is clear that things move slowly, not with a crash nor a bang, but a whisper, secret movements in the dark, the conspiracy of gravity, and slim slivers of matter. The universe was young then, only 100 million years old, a fraction of its modern size, and all was darkness. The fires of nucleiosynthesis that gave rise to the first elements, all of it was a dim and distant memory. But over time, the dim, diffuse clouds found each other. And with their stronger gravity, they pulled on their surroundings, drawing them closer. And then in some unknown region of the vast and cooling cosmos, the first spark of revolution was lit. The primordial elements had through the inevitable crushing of their own gravitational weight ignited into fusion. The first release of energy in a 100 million years. The first new light to shine upon the universe. Change had come. The cosmic dawn had broken. But those first stars were not yet done. Clumps of gas and newborn stars gathered together, huddling their warmth against the cold and dark universe beyond. More gas piled onto these safe havens, these refuges, these cities. Beginning as simple spheres, then flattened discs. Then, as pertubations did their work, ripples of gravitational influence working their way through their volumes, spiral arms of dazzling starlight emerged. The first galaxies were born, and they would rise to dominate the visible universe. One of the best places to see the night sky is in the deep desert. There, far away from any source of light pollution or interference, you can enjoy a crisp, clear view of the heavens, and the Atakama Desert of Northern Chile is one such special place. Spanning hundreds of kilometers along the western spine of the Andes Mountains, the rugged elevated desert makes for worldclass observing. There, once you allow your vision to adjust to the darkness, your naked eye can discern up to 3,000 individual stars. And so, as you can imagine, this makes the ideal place for a telescope array. The Atakama Large Millm Subm Array, or ALMA, is situated 5,000 m above this bone dry desert. It is an intererometer, an assemblage of 66 individual dishes, all working in tandem to pierce deep into the sky. And in 2015, Japanese astronomer Hideki Umahhata used it to find a set of proto galaxies. Using ALMA, he was able to map out the SSA22 protocluster of galaxies. A monster being born nestled in a cradle of filamentary matter nearly 12 billion

lightyears away. As galactic wounds go, SSA22 is gigantic, reaching over 200 million lightyears on a side. Individual gas bubbles carved out by the infant and temperamental galaxies are 400,000 lightyears across each. Indeed, it was one of the first such protoclusters of galaxies to emerge on the cosmic scene, and Elma brought us right into the delivery room. And yet within a mere 3 billion years, these galaxies would start to collapse. But for a time, they were truly glorious. As galaxies first began to grow, the gas pervading the universe fractured and fragmented, collapsing itself into a series of filaments and sheets. At the nodes of these filaments, their points of intersection, sat the newly constructed galactic cities. Each growing galaxy was attached to the wider universe through numerous umbilical cords, drawing from the resources of their surroundings to grow themselves and in the process produced stars at an astounding rate. Indeed, at their peak, roughly 3 to 3 and 1/2 billion years after the Big Bang, galaxies produce stars at a rate over 10 times greater than they do today. Indeed, if any intelligent creatures lived on the surface of any planets in that young universe, they wouldn't need a telescope like Elma to witness stellar miracles. They could simply look up. Our own sky is dominated by the light of only the largest stars. A stars like our sun are too dim and feeble to be seen over interstellar distances with the naked eye. But if you were to take this same view back to the era of peak star formation, you would instead see over 30,000 stars, perhaps more. This era is known as cosmic known, the peak of the universe. And it was glorious, but it wouldn't last long. Our modern universe is different. Star formation takes place in only a few selected sites, special places where the right conditions are met, like the nebula NGC 346, a star forming cluster in the large melanic cloud. Otherwise, the vast majority of galaxies are quiet, sleepy places. Even our own Milky Way galaxy is only able to create about seven stars a year. Just seven across its entire expanse of over a 100,000 lightyear. Indeed, in your entire lifetime, there'll only be around 500 new stars awakening within our galaxy. And astronomers consider the Milky Way to be relatively active for our time. And so, what happened in the 10 or so billion years that separates us from the cosmic known? Why is the universe so much less fruitful today than it was then? Hastily built, the galaxies of the early universe were small and inelegant. They'd not yet formed grand spiral arms or settled into stately equilibrium. Within them, titanic forces competed for dominance, like rival street gangs, waging wars for control of their neighborhoods. Some material drifted down from all directions, but most of it came rushing along the filaments that connected each galaxy to the broader cosmos. As the new gas rushed in, gravitational pertubations triggered collapse of nearby gas clouds. Those clouds, once destabilized, would fragment into hundreds or even thousands of star forming clumps at once. And those first rounds of star formation were the subject of the J ADES or James Webb Space Telescope Advanced Deep Extragalactic Survey. The team co-led by Marsha Reek of the University of Arizona in Tucson searched for what amounted to tiny smudges of light. And yet those smudges represented millions or billions of newborn stars lighting up all at once. When those stars died, they exploded in massive supernova blasts, which in turn sent new shock waves racing through their neighborhoods. And round and around the feedback cycle went as new material led to new star formation. Each galaxy was a picture in turbulence. And not just within themselves. For many early galaxies didn't just gather new material quietly from their surroundings. Sometimes they stole it from each other. The candle survey was designed to watch

this cosmic drama unfold. The largest project the Hubble Space Telescope ever took part in, the survey used about 60 days worth of observing time between 2010 and 2013 and provided deep imaging of over 250,000 galaxies. The young universe was smaller with the average distance between galaxies far shorter than it is today. And so as these galaxies grew, they battled over resources. When galaxies interact with each other, it's not always slow waltzes and gentle dances. Sometimes it's complete consumption. Our own Milky Way hides many such remnants of its violent history. It's not obvious to the casual observer with one stellar population looking largely like any other. But thanks to the enormous cataloges of billions of stars we've been able to gather over the years, careful astronomers have been able to tease apart the populations, deducing which stars were born natively in the Milky Way and which are later arrivals. These investigations have revealed dozens of merger remnants. Some more recent ones are still partially intact with small cores of high density remaining. What we call the globular clusters. Some are stretched and elongated like the Sagittarius stream. The mighty gravity of the Milky Way distorting them beyond recognition. And others are completely assimilated. The leftover populations of stars scattered far and wide within the galaxy. Like the entertainingly named Gaia Sausage. Each of these encounters, merges, and collisions led to fresh rounds of star formation. When a larger galaxy would consume a smaller one, it would lead to cannibalism. But when a galaxy encountered another one of an equal size, it led to a period of star formation unlike anything either galaxy had seen before. In the most intense cases, the gravitational interactions between merging galaxies was so strong that their newly merged form could create stars at a rate over a thousand times greater than the Milky Way of today. But perhaps the most important ingredient of all, the central player that regulated the rate of star formation, the key component that determined how efficiently early galaxies could convert gas into stars was dust. Humble simple dust. Today, dust is everywhere in the universe. These are small, usually microscopic grains of various chemical compounds. Indeed, it's so abundant that you can even see it. The diffused dust in the solar system is responsible for the phenomenon known as zodiacal light. If you look to the western sky shortly after sunset on a clear dark night, you might be able to see a faint glow stretching from the horizon and narrowing to a point higher in the sky. That is the sunlight scattering of the dust contained in the space between the planets. Indeed, there's so much dust that every year the Earth accumulates several tons of new mass simply by swimming through it. And dust plays a crucial role in star formation. To make a star, you can't simply bring together a reservoir of gas. Indeed, it will just sit there existing as a nebula for millions of years. That's because that gas is hot and its temperature prevents it from collapsing into the small sizes needed to become a star. But dust can cool off a nebula by efficiently emitting radiation. By cooling through dust, a gas cloud can continually contract until its central densities reach the critical thresholds needed to ignite nuclear fusion and become a star. But the extremely young universe was completely devoid of dust as only hydrogen, helium, and trace amounts of lithium and burillium were made in the primordial big bang. Indeed, this is one of the reasons why we don't quite understand how early stars formed. But once those stars lived and died, they enriched their surrounding environments with heavier elements. And those elements would quickly mix together to create the first dust. Observations with ALMA reveal a dusty universe with small galaxies obscured in relatively thick hazes. All this frenetic activity led to more stars being produced in the first 3 billion years after the big bang than in the 10 billion years since. Cosmic noon was the peak, the point when star formation reached its zenith. And then it stopped. Cosmic noon ticked over to cosmic

afternoon. And today in the modern universe, we are in cosmic evening. Galaxies continued to form. They grew larger and more massive. The modern Milky Way is 10 times bigger than it was at cosmic noon. And the building of galaxies continues even to the present day with dim clumps of gas still emerging into small galaxies. But the pace is much more sedate, much more measured. Except for the occasional rare exception, the galaxies of our time are much calmer, quieter places, and some are completely dead. 80 years ago, if Eric Holberg had been able to tune his crude simulation to a specific scenario with a particular pair of galaxies with just the right masses merging together in just the right way, he might have seen something familiar. This is because his studies laid the groundwork for a new understanding of a certain type of galaxy with farreaching consequences for how the universe evolves and the ultimate fate of everything within it. If Eric Holberg had just the right setup in his gymnasium laboratory, he could have recreated M87. When astronomer Charles Messier published his famous catalog of deep sky objects in 1781, he had no idea that number 87 was different from any of the others. And in fact, he wasn't particularly bothered whether it was or not. Forsier had only one thing on his mind. He really liked comets. To quote French astronomer Francois Argo, "At a time when he was in expectation of discovering a comet, his wife took ill and died. While attending upon her, being withdrawn from his observatory, Montana demoj anticipated him by discovering the comet. Messier was in despair. A friend visiting him began to offer some consolation. " Messier, thinking only of his comet, exclaimed, "I had discovered 12. Alas, that I should be robbed of the 13th by Montana. " Then remembering that it was necessary to mourn for his wife whose remains were still in the house, he exclaimed, "H this poor woman. " And again wept for his comet. Messier was an avid comet hunter and not wanting to fool himself or his fellow astronomers with false observations, he had created his catalog of objects to help clarify fuzzy things in the sky. Fuzzy things that were definitely not comets. And M87 was definitely not a comet. M87 was and still is an elliptical galaxy. And as well as this, M87 was not just any elliptical galaxy, but a true Leviathan containing several trillion stars. And even with backyard telescopes, you can tell that it looks different. There is no flattened disc and no majestic swirl of spiral arms. This is because M87 is different to the Milky Way. Different even to most of the galaxies surrounding the Milky Way. M87 is dead. A graveyard of galactic proportions. Billions of years ago, it was ablaze with activity with new stars made every year by the hundreds. But not anymore. And among astronomers, elliptical galaxies like M87 have a nickname, the red and dead galaxies. There are still stars, but not like there once were. They mostly small dwarfs, dim and red, only a fraction of the luminosity of the sun. And among those still clinging to some feeble form of life, elited the bones of their ancestors, the black holes, places of infinite gravity and no escape. The neutron stars and white dwarves, leftover cores, once capable of fusion energies now laying in slumber as they slowly, slowly cool off. And finally, the oblivious planets and asteroids and other debris of once magnificent solar systems. There are hundreds of billions of red and dead elliptical galaxies just like it scattered around the cosmos like the rotting corpses left from a great battle. Indeed, somewhere between 20 to 30% of all galaxies are ellipticals. And yet, because they can grow to such massive sizes, roughly 40% of all stars in the universe call an elliptical home.

And soon enough, their numbers will grow. It's the fate of every galaxy in the universe. Even our own Milky Way, as calm and steady and sedate as it's been for billions of years, will someday become one of these red and dead galactic graveyards. Just as astronomers do not fully understand what led to the great rise in star formation rates in the early universe, they do not yet understand exactly how galaxies die. But observations of dead galaxies reveal that it's not usually one single culprit, but a series of events that strangle a galaxy of newborn stars. And these culprits are not any new process or event that a galaxy undergoes, but the very same ones that gave rise to rapid star formation in the first place. In other words, it seems that galaxies are a victim of their own success. The technical term that astronomers use to describe the death of a galaxy is quenching. Some series of processes that extinguish the fire of stars, preventing new ones from forming and relegating the galaxy to the rest of the Red and Dead collection. There are any number of mechanisms that can participate in the quenching process, and any of them might be more or less important in the evolution of any particular galaxy, but in the end, all galaxies end up sharing the same fate. Sooner or later, some mechanism, some process goes out of control within the galaxy and snuffs out star formation. Sometimes it's just a period of too intense star formation that does the trick. During these periods, thousands of giant bright stars can appear at once. Those stars pump out enormous amounts of particles and radiation, creating stellar winds to accompany their high energy starlight. These winds, stronger than any hurricane, but so thin that you can feel them rushing by you, can combine together and sweep through the galaxies, heating up the rest of the interstellar gas. And when the gas is hot, it can't cool off to collapse, condense, and form stars. Sometimes the stellar winds can be so strong that they drive material out of the galaxies altogether, stopping accretion in its tracks. Loosened from the gravitational grip of the galaxy like dandelion seeds in the wind, that material can no longer participate in the game of star formation. And this process of galactic expulsion goes double when supernovi go off. Since clusters of massive stars tend to be born together, they have roughly the same age and life cycle, meaning that they also tend to die as a group. When hundreds or thousands of supernovi explode nearly simultaneously, they launch massive plumes of material that blow out holes in interstellar gas like the local bubble that the solar system is currently sailing through. And in 2016, a team of curious astronomers led by Arti Camps Ferina went out in search of these explosive events. After weeks of searching, they found a beautiful site. A series of three bubbles surrounding M33, the triangulum galaxy, nested together like Russian dolls. Even the very mechanisms that lead to accretion can quench a galaxy, preventing it from forming further stars. As galaxies drift through the intergalactic medium, they enjoy a steady supply of infalling material that gently rains down, mixing with its own interstellar medium and refilling its reservoirs of star forming fuel. The galaxies aren't moving around randomly in the universe. They are drawn towards each other and drawn towards larger clusters. In the modern universe, the clusters are home to a thousand galaxies or more. Bound together with their mutual gravitational attraction. Threading and winding its way through those galaxies is a hot plasma. That plasma is thin enough to register as a vacuum in Earth laboratories, but can still play an important role. When a new galaxy enters into a cluster, it encounters the hot gas. The movement of the galaxy through the hot gas heats up the material within the galaxy itself, which shuts off the formation of new stars. As well as this, the simple act of friction between the gas in the cluster and galaxy rips away material. Astronomers can even see this process affecting galaxies right now. When they do, they observe long tentacles of gas streaming behind the galaxies. And of course, the astronomers call them jellyfish galaxies.

Once again, the twin effects of heating of gas and removing it from the galaxy couldn't quench it, turning it from blue and bright to red and dead. But arguably the greatest galaxy killer of all isn't the thunderous winds from supernovi explosions or the friction and heat generated from mergers. Instead, it is perhaps the black heart that sits in the center of almost every galaxy. When we look at red and dead galaxies in radio wavelengths, we can see a long thin jet of intense radiation piercing nearly 5,000 light years through the body of the galaxy. And when the astronomers of the event horizon telescope trained their globe spanning instrument on the center of M87, they captured the shadow of an enormous super massive black hole. A super massive black hole surrounded by a disc of intense radiation. A jet of energetic particles capable of reaching thousands of light years and an empty ghost town of a gigantic galaxy. It looked like the scene of a crime with the knife still thrust into the victim and the sinister villain hiding in the shadows. Super massive black holes, those with more than a million suns worth of mass, appear very early in cosmic history. By some observations, just a few hundred million years after the Big Bang. And this means that the story of galaxy evolution is twinned with the story of black hole evolution. Like a dark secret that they try to hide their whole lives, a dark secret that ultimately destroys them. For the most part, the rest of the galaxy do not really notice the black hole at the center. While each black hole is individually massive, they still weigh less than 1% of the entire mass of a galaxy. Except that is when black holes awaken. And these are the quazars. The same radiointense objects that astronomers first noticed in the 1950s and cemented our understanding of a changing evolving universe. Indeed, soon after quazars were first discovered, Fred Hy, the same astronomer that railed against the idea of an expanding universe until his death, suggested that super massive stars might power them with their intense gravity. But then Richard Fineman pointed out that such gigantic stars couldn't support themselves. Eventually, astronomers realized that it must be a black hole at the center. But as with the big bang theory, despite his idea not gaining wider acceptance, Hy's choice of name stuck and so they became known as super massive black holes. When a clump of matter comes too close to the central super massive, either in the form of a wandering gas cloud or an errant star, the extreme gravity of the black hole rips it to shreds, forming its material into a thin disc. The disc of accreting material reaches temperatures of trillions of Kelvin. And at that temperature, it emits a flood of high energy radiation, ultraviolet, and X-rays that rush through the rest of the galaxy. Indeed, the quazars powered by super massive black holes don't just heat a patch of neighborhood interstellar gas. They can literally inflate an entire galaxy, puffing it up like a hot air balloon. The hot gas can't collect, can't condense, can't fragment, can't form stars. It can only drift aimlessly and uselessly in a superheated ionized state. Of course, as with all things galactic, in moderation, an active quazar can be a good thing. It can slow down star formation, preventing a galaxy from using up all its gas at once. In this way, the central black hole, despite its relative smallness compared to the galaxy at large, plays an outsized role in galactic evolution, serving as a critical regulator to keep a galaxy running at its most efficient level. But when a quazar gets too bright, it can destroy a galaxy from the inside out. This typically occurs after a major merger event. When a galaxy meets another of equal size, their twin black holes quickly find each other and merge. And that unified black hole makes its way to the center of the new galaxy. Once planted there, it engorges on the feast of material crashing around, taking advantage of this short-lived surplus. Once this quazar activates, it drives so much gas to a high temperature that it doesn't just slow down star formation

but stops it completely. So much material leaves the galactic environment that only a feeble trickle remains. And if that weren't enough, the quazar has one more trick up its sleeve. The jet. The ferocious energies of the accretion disc whip up enormously strong magnetic fields. But these magnetic fields are so strong that some of the accreting gas follows their influence instead, following twisting corkcrew paths around the black hole and eventually emerging in the form of powerful jets racing out of the poles of the system. The particles in the jet are accelerated to nearly the speed of light and a cocoon of magnetic fields keep the jet straight and narrow for up to tens of thousands of light years. Any loose material in the path of the jet is simply shoved out of the galaxy wholesale, never to participate in the grand dance of star formation ever again. And so this was the smoking gun that killed M87. Simply too many mergers and a too eager super massive black hole. Though not all galaxies will die through merger, some will be lucky, distant enough from their nearest neighbors that they will escape that fate. But even if they manage to keep producing stars for eons to come, eventually their lights will dim too. The universe is still changing, still expanding, still on average pulling galaxies away from each other. There is only so much material available in the cosmos. Slowly, ever so slowly, all galaxies will simply run out of gas. In this sense, galaxies are temporary creations. They will live for much longer than the current age of the universe, but they still only have a finite lifespan. And so while cosmic noon, the peak of the universe, came and went in the blink of a cosmic eye, a mere 3 billion years after the big bang, cosmic evening and cosmic twilight will last much longer. The automated system awoke the astronomer. She roused herself from deep slumber methodically, but did not rush. Checking the chronometer, she'd been asleep for over 140,000 years. The analysis could wait until after coffee. Her station was a remote one, as were all such stations. It was on the surface of the moon once known as Titan. But long ago, it had cooled and solidified, losing its oceans and its atmosphere in the process. It still orbited what remained of Saturn, which in turn orbited the black dwarf sun, which now had a temperature of just under minus 170° C. The cache of antimatter powered her station and would continue to do so for a long time to come, more than enough to chart the end of the universe. There was no larger purpose to this mission. She knew the recordings she kept, the notes she diligently took, would ultimately be read by nobody and would in time dissolve along with all other microscopic objects. And so the astronomer worked, passing through the meager data collected by her observatory outpost. And there it was, exactly what she had hoped for. After several million years of false positives, this was something. A new star had been born. It was small, a fraction of what the sun once was, but it was still a star. Its formation had been triggered by the random collision of two brown dwarfs, giving the new entity just enough mass to reach the critical fusion densities in its core. After completing an all sky scan, she added the new object to her log book. In total, there were now 78 stars shining in the Milky Way galaxy, the highest number in over a trillion years. Content, the astronomer returned to her stasis chamber to await the next signal. The world of the faristant future of the universe would be utterly alien to us. Accustomed as we are to planets, oceans, living suns and skies full of stars. But in the end, darkness will come for us all. As we know, the universe is expanding. And that expansion has driven a cosmos of change. A change from a hot, dense

plasma into a cool gas. A change from a thin soup of hydrogen into pockets of star formation. A change from the dark ages to the greatness of cosmic known. And now we stand in a universe 13. 8 billion years old in the twilight years of the cosmos. Star formation peaked long ago. And while galaxies have continued to assemble and grow, their total brightness has already begun the long slow decline into oblivion. Advancing this decline like the rapid onset of a terminal disease is dark energy. the mysterious substance that is causing the expansion of the universe to accelerate. We don't know for sure its true nature, its cause, or its ultimate evolution. But if it continues to operate as it's done for over 5 billion years, then the fate of our cosmos is sealed. Accelerated expansion drives everything away from everything else. In doing so, it starves the galaxies of the vital material they need to build new stars. As well as this, in enough time, it will pluck any stars, planets, and remnants away from the galaxies one by one, dissolving them. But the grimness of this fate is offset by its longevity. When we discuss the timeline of the universe, we find that we are still in its infant stages. And what we perceive as the decline from the cosmic noon is really the brief initial flash that set the stage in motion that put all the pieces in place for the cosmos to live a long life. And in those distant time scales when the universe is orders of magnitude older than its present age, new wonders will appear. Strange creatures that we can't possibly fathom in the youthful time of the present. In 100 billion years, the observable universe will disappear from view. Any galaxy not already bound by gravity to our local group will be caught up in the accelerated expansion of dark energy and pulled away from us faster than the speed of light until they disappear. And in that time, a new kind of star will emerge in the Milky Way. One that we do not see in the present epoch simply because the universe is not yet old enough. The largest stars will be long gone, as will almost all sunlike stars. Instead, our galaxy will be dominated by the red dwarfs. But as they evolve, they will gain temperature, becoming blue dwarfs, small, dim, but bright blue, like jewels set within the galaxy. After a trillion years, Mecomda, the nickname given to the future galaxy, composed of the mergers of the Milky Way, Andromeda, and all the other local group members, will begin to dissolve. Random chance encounters will scatter individual stars, planets, and remnants out of the galaxy where they will immediately get caught up in the accelerated expansion and fly away. At the same time, the cosmic microwave background, our backbone observation of the birth of the universe will become so redshifted and cold that it will be impossible to observe. And it is in this strange new reality that another kind of star would appear for the first time. In the present day, stars have a fundamental lower mass limit set by the need to achieve fusion reactions in their cores. But far into the future, the galaxy will be so enriched with heavy metals that it can lower the temperatures and masses of possible stars. Indeed, the smallest stars will be so dimminative and cool that their outer surfaces could have temperatures below zero. Encased in clouds of ice, these frozen stars will barely be detectable, but they will exist. Sometime between 10 and 100 trillion years from now, the last star will be born. By that time, the universe will be more than 10 to the power of 2,000 times larger than it is today. In what remains of the galaxy, brown dwarfs will occasionally collide, reaching the thresholds necessary to convert into red dwarfs. But that will be the end of stars as we know it. Some 10 to 20 trillion years after that, the last star will go out. In the ensuing eons, the galaxy will continue to dissolve. By 10 the power of 15 years, our sun, long since cooled to become a black dwarf, will have a temperature of only 5° above absolute zero. Through gravitational interactions, all planets would have

detached from their parent stars and likely been flung into the deep abyss. And by 10 the^ of 30 years, all macroscopic objects would have either completely detached from the galaxy or fallen into the central super massive black hole. By this time, all macroscopic objects will be solitary, living in pockets of the universe all of their own. In 10 the^ of 100 years, all black holes will disappear through Hawking radiation. And starting around 10 the thousand years from now, even the black dwarfs will die, exploding in supernova detonations as they lose their electrons, lowering the limit needed for catastrophic collapse. And finally, in 10 to the power of 1,500 years, perhaps the last kind of star appears on the cosmic scene. Although due to the accelerated expansion of dark energy, there will be no observers to witness it. A form of cold fusion moderated by muons will convert all planetary mass objects into solid spheres of pure iron. These are the iron stars. They will not glow except through the feeble heat due to their temperature a shred above absolute zero. But they will persist. For how long is anybody's guess? Indeed, one estimate suggests that some 10 to the^ of 76 years from now, all objects will dissolve through quantum tunneling, even the iron stars. And beyond this point, all molecules, atoms, and nucleons will disappear. The universe will be composed of nothing more than a thin soup of subatomic particles and radiation separated from each other by vast expanses of nothingness. And the decay that will lead to this cold empty cosmos has already begun. You've been watching the entire history of the universe. Don't forget to like and subscribe and leave us a comment to tell us what you think. Thanks for watching and we'll see you next time.