The observable universe is 93 billion light years across, but this number represents only a tiny fraction of the total cosmos, which is likely hundreds of times larger or even infinite. The observable universe is not a container holding everything that exists, but rather a sphere of access defined by the distance light has traveled since the Big Bang. The true scale of the universe is measured not just in light years, but in everything we can never touch and everything we can never know. The universe is expanding at an accelerating rate due to dark energy, meaning that 94% of the galaxies we can see are already permanently beyond our reach, and the evidence of the universe's origin is gradually being erased over cosmic time.
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The True Scale of the Observable Universe Is Worse Than You Think
Added:You've probably heard how big the universe is. 93 billion lightyear across, 2 trillion galaxies, more stars than every grain of sand on Earth. It's the kind of thing that sounds amazing for a second and then you move on. But those numbers don't tell the real story.
[music] In fact, they hide it. The true scale of the universe isn't about how big it is. It's about something most people never stop to think about. And once you see it, the night sky never quite looks the same again. Tonight, I'll show you why the scale of the universe is stranger and worse than you think. Now, get yourself comfortable.
Hit subscribe. We begin.
Start here. Not with the stars, with you. Right now, wherever you are, your body is holding itself together against the dark. And inside that body sits a number almost too large to say out loud.
Roughly seven octilian atoms, a seven followed by 27 zeros are arranged this very second into the shape of a person listening. Written out that number would run right off the edge of the page. You are in the most literal sense more crowded on the inside than the night sky is on the outside. [music] Yet all of it fits inside a single chest, rising and falling in a quiet room. Here is the first strange thing. Almost every one of those atoms is older than the ground beneath you, older than the sun. Most of them were forged in the hearts of dead stars, scattered across space when those stars exploded and slowly gathered over billions of years into the temporary arrangement you call yourself. You are not separate from the cosmos out there.
You are a piece of it that briefly woke up. Hold on to that feeling because we are about to travel outward [music] and it only gets stranger from here. That body of yours is sitting on a rock. A warm wet rock spinning through the dark.
And that rock circles a single ordinary star. The sun. Nothing about it is special. It is one middle-aged star among hundreds of billions in our neighborhood. If you lined up every star in the Milky Way and looked for hours, you would not be able to find it. It doesn't stand out. It never did. And our sun does not sit at the center of anything. It drifts far out on a quiet edge about 26,000 light years from the heart of the galaxy. Let that distance breathe for a second. A lightyear is a distance. It's how far light travels in a year, moving at 300,000 km, about 186,000 m every single second. In one year, that adds up to roughly 9 12 trillion km, nearly 6 trillion miles.
And we are 26,000 of those from the middle of our own galaxy, parked out in a quiet spiral arm, far from the bright, crowded downtown.
26,000 years ago, on this rock, the last ice age still gripped the Earth, and our ancestors were painting animals onto cave walls by fire light. A beam of light leaving the galaxy's core back then would only now be arriving at your eyes. That is one journey inside one galaxy. Now widen it. The Milky Way spans about 100,000 lighty years from edge to edge. To cross it, light, again, the fastest thing in existence, needs a thousand centuries. Think of what that means. If you could somehow ride a beam of light straight across our galaxy, you would step on at one end as a certain kind of creature. And by the time you reach the far side, entire species could rise and vanish on the world you left behind. The galaxy is so vast that its fastest possible messenger still takes longer to cross it than modern humans have existed. And here is where your senses quietly lie to you. Step outside on the clearest, darkest night of your life. No city glow, no moon, perfect air. Look up [music] and try to take in every single star you can see. Guess how many that is. Most people imagine millions. The real answer is heartbreakingly small. Under ideal conditions, the human eye can make out only about 6,000 stars in the entire sky. And since half the sky is always beneath your feet, you are really seeing around 3,000 at any one moment. 6,000.
That is it. That is the whole glittering ceiling that has inspired every myth, every poem, every religion in human history. 6,000 stars out of a galaxy of several hundred billion. Do the arithmetic and it lands like a stone.
Everything you have ever seen with your naked eyes, every star that ever guided a ship or fell across a wish, adds up to less than 1 10,000th of 1% of just our own galaxy. The night sky is not the universe. It is barely the corner of one room in a mansion you will never finish walking. And the mansion has more rooms than that. Far more. Our galaxy holds somewhere between 100 billion and 400 billion stars.
Notice the size of that uncertainty. We cannot even count the stars in our own home to better than a factor of four because we live inside it. Our view blocked by dust and distance. Like trying to count the trees in a forest while standing in the middle of it at dusk. If we cannot be sure of the number in our own galaxy, sit with what that implies about everything beyond it.
Because now we leave home entirely. Look toward the constellation Andromeda. And on a good night, you can find a faint smudge of light. That smudge is another galaxy, the Andromeda galaxy, and it is the most distant thing your unaded eye can ever [music] see. Its light has been traveling toward you for 2 and a half million years. When that light left Andromeda, there were no humans here to catch it. Our earliest ancestors were just beginning to shape the first crude stone tools on the plains of Africa.
That beam crossed the darkness for the entire span of the human story. And tonight it finally ends its journey on your retina. You are quite literally looking 2 and a half million years into the past. The Milky Way and Andromeda are not alone. They travel together, bound by gravity, inside a small cluster we call the local group. A collection of more than 50 galaxies, most of them tiny dwarfs orbiting the two giants like moths around a pair of street lights. 50 galaxies. And already we have left behind everything a human eye can perceive. But the local group is nothing, a speck. Zoom out again and our entire local group turns out to be a single droplet caught in a vast cosmic watershed. A supercluster the discoverers named Lania, a Hawaiian word meaning immeasurable heaven. In 2014, a team led by astronomer Brent Tully mapped it for the first time by tracing how a 100,000 galaxies all drift like water toward a common gravitational valley. Lania stretches more than 500 million lightyears across. Try to hold that. Our whole galaxy is 100,000 lighty years wide and you could line up 5,000 Milky Ways end to end and still not reach across Lania. Everything we have discussed so far, the sun, the Milky Way, Andromeda, all 50 galaxies of the local group fits into one small unremarkable corner of this single supercluster. [snorts] And now the vertigo begins in earnest because Lania is just one supercluster among millions scattered across the cosmos. Keep pulling back and something eerie happens to the picture. The galaxies stop looking like scattered points and start to line up. They thin into glowing threads and the threads knot together into a vast branching structure that astronomers call the cosmic web. the largest pattern that exists anywhere. Picture the gray filaments of a spider's web strung across a dark attic or the delicate branching of neurons in a brain. That is the shape of the universe at the largest scale. Luminous strands of galaxies wrapped around enormous hollow spaces.
And those hollow spaces, the voids between the threads, are emptier [music] than anything your mind can hold. Some stretch hundreds of millions of light years across with almost nothing inside them. Not empty like a quiet room. Empty like a single grain of sand. A drift in a cathedral with the next grain hundreds of miles away. Most of the universe, it turns out, is void. We have been climbing outward this whole time. Now we reach the edge of what we can see. And this is where the numbers stop being large and start becoming something else entirely. The observable universe, the entire sphere of everything whose light has had time to reach us, measures about 93 billion lightyears across, 93 billion. There is no honest way to picture that directly. So let us not pretend. Instead, feel it through what it contains. Inside that sphere, by one leading estimate, there are as many as 2 trillion galaxies. Not two trillion stars, 2 trillion galaxies, each one holding its own 100 billion suns. And when you multiply it out, you arrive at a number that breaks the mind. Somewhere around 200 sexillion stars. That is a two followed by 23 zeros. Here is the comparison that has haunted astronomers since Carl Sean first spoke it. Walk down to a beach, scoop up a handful of sand, and let it run through your fingers. Now imagine every grain on that beach and every other beach on every shoreline on Earth. Scientists have actually tried to count them. The figure comes out to roughly 7 and 12 quintilion grains. A seven followed by 18 zeros. It sounds unbeatable. And yet the stars win. And it isn't even close. For every single grain of sand on every beach on this entire planet, the universe holds tens of thousands of stars. The night sky doesn't just rival the sand of the world. It drowns it. And if even that fails to land, go smaller still. Count not the stars, but the atoms, every particle of ordinary matter in the entire cosmos, in every star and planet and cloud of gas across all 2 trillion galaxies. The best estimate lands near 10 to the 80th power, a one followed by 80 zeros. That is the full material inventory of everything that exists and can be seen. There is no object, no crowd, no ocean on earth that comes within a hair of it. It is simply the largest meaningful number in physical reality. And now after all of that comes the twist that turns wonder into something colder. Everything we have just counted, every star, every galaxy, every filament of the cosmic web, every one of those 10 to the 80th atoms, all of it, the entire glowing inventory of the visible universe adds up to barely 5% of what is actually there. Read that again. 5%. The other 95% is completely invisible. About 27% of it is dark matter. A substance we cannot see, cannot touch, and have never held in a laboratory known only because its gravity bends and shapes the galaxies we can see. And the remaining 68% is something stranger still. Dark energy, a force woven into empty space itself, pushing the entire universe apart. We cannot see it. We cannot explain it. And yet it rules everything. It will decide the fate of all of it. We built our entire picture of the cosmos. And only when it was finished did we realize we had drawn just 20th of the page. By now something may have quietly happened inside you. The numbers have grown so large, trillions, sexilillions, 93 billion, that they have begun to blur into a kind of noise. They stop meaning anything. You feel the awe drain away and leave behind a strange numbness.
That is not [music] a failure of your mind. That is the first trap of scale and everyone falls into it. But here is what I need you to understand. The scale is not the disturbing part. The scale is just the surface. What scale hides, that is where the real unease begins. So look closely at two facts side by side and let the contradictions sit before I say a word about it. The universe is 13.8 billion years old. Nothing in it can travel faster than light. And yet the observable universe is 93 billion lightyears wide. Stop there. Those two things should not be able to coexist. If nothing outruns light and the universe has only had 13.8 8 billion years to exist. Then how can it possibly be 93 billion lightyears across? How does a cosmos reach almost seven times wider than its own age should allow? Something here does not add up. And the answer is more unsettling than the puzzle. The objects at that far edge are not sitting still. They are racing away from us. And the most distant of them are receding faster than the speed of light itself.
Not because they are flying through space at impossible speeds, but because space itself, the emptiness between the galaxies, is stretching. And when enough space stretches between us and a distant galaxy, the total gap can grow faster than any light beam could ever close, which leads to the quiet horror underneath all this beauty. Many of the galaxies you could see tonight are already at this very moment beyond our reach forever. Not hard to reach, not far away, forever unreachable. If we launched a beam of light toward them right now at the fastest speed the universe permits, it would never, not in a trillion years, not ever, arrive. The space in between is opening too fast, and it is happening continuously.
Every year, more of the universe silently crosses that line and slips out of our grasp. In the few minutes you have been listening, millions of stars have quietly passed beyond the horizon of anything we could ever touch. We are not looking at a finished stable cosmos.
We are watching a universe in the slow act of disappearing. A room whose walls are drifting outward faster than we could ever walk to reach them. And this is the moment the whole story turns over because everything I have just described, the 93 billion light years, the racing edge, the sense of a wall at the end of everything rests on a single word we have been using without questioning it. Observable. And that word is quietly deceiving you. That 93 billion lightyear number, the one that felt like the outer wall of reality. It was never the horror at all. Strange as it sounds, it is the reassurance. It is the small, safe, knowable part. Because the edge you've been picturing this whole time, that bright boundary at the end of the observable universe is not an edge. There is no wall out there. There is no rim where the cosmos stops. What you have been imagining as the border of everything is something else entirely.
And once you see what it really is, the universe becomes far larger, far stranger, and far lonelier than any number so far has suggested. The edge isn't there. And what that means is where this truly begins. And to understand why, we have to go back and catch the one word that has been quietly lying to you this entire time. That word is observable. When we say the observable universe is 93 billion lightyears across, it sounds like a measurement of everything. The full extent of reality, the outer shell of all that exists. But that is not what the word means at all. And here's the thing most people don't realize.
Observable is not a statement about the universe. It is a statement about us.
Let me be precise about what we mean.
The observable universe is simply the region from which light has had enough time to reach us since the beginning. A sphere defined not by where the universe stops, but by how far its light has managed to travel in 13.8 billion years.
Think of standing in a thick fog at night with a lantern. You can see a certain distance in every direction, a glowing circle around you where the light fades into gray. That circle is not the edge of the world. It is the edge of your seeing. The forest continues on, unbroken, past the last thing your lantern can touch. You simply have no way to know what's out there yet. That is the observable universe. A circle of light around a single observer. Nothing more.
And here is the part that trips almost everyone. This limit has nothing to do with the power of our instruments. It is tempting to assume that if we just built a bigger telescope, something 10 times sharper than the James Web, something monstrous in orbit around the sun, we could finally see past that edge. But we cannot, not ever. No telescope that could possibly be built will change it by a single foot. Because the barrier isn't weak technology. The barrier is time itself married to the speed of light. Light travels fast, but it is not infinitely fast. It crawls across the cosmos at a fixed, unbreakable speed, and the universe has only existed for 13.8 billion years. Any light from farther out simply has not arrived yet.
Its journey isn't finished. The photons are still on their way. [music] And for the most distant regions, they will never finish the trip. Scientists have a name for this true boundary. They call it the particle horizon. The maximum distance from which any signal, any light, any information could have reached us since the big bang. It is the real wall. And it is not made of matter.
It is made of the two most fundamental limits in physics. the age of the universe and how fast light can move through it. So the honest question becomes the one that opens the floor beneath your feet. What lies beyond it?
More universe. That much we can say with confidence. Past our horizon. The cosmos does not simply stop. More galaxies exist out there. More stars. More of the same cosmic web stretching on into regions whose light will never touch us.
The universe is bigger than the part we can see. On that there is no serious doubt. But how much bigger? That is where certainty ends and the vertigo begins. To feel why we have to go back all the way back to the universe's very first moment. Because in that first fraction of a second, something happened that no ordinary intuition can hold. In the standard picture, cosmologists describe an event called inflation. A burst of expansion so violent so fast that in less than a heartbeat, the universe ballooned in size by an unimaginable factor. We are not talking about doubling or a thousandfold or even a millionfold.
In the tiniest sliver of time after the beginning, a fraction of a second so small it barely qualifies as time at all. A region smaller than a single atom was stretched to something enormous and it happened faster than light. Now hold on because that should sound like a violation. Nothing outruns light, but space itself plays by different rules.
Light has a speed limit. The stretching of space does not. Space can expand at any rate it likes. And in that first instant, it did, carrying regions of the infant universe away from each other faster than any beam of light could ever chase them down. And here is the consequence that lands like a cold hand on the back of the neck. Inflation didn't just make the universe big.
[music] It took regions that were once right beside us, practically touching, and flung them so far, so fast that light from them can never reach us. Not because they are hostile or hidden or strange, but because the space between us and them was stretched beyond the point of any possible contact forever in the universe's opening moment. Those regions are real. They almost certainly contain galaxies, stars, perhaps worlds, and we will never receive a single photon from any of them. They were sealed away before the first star ever ignited. So, how large is the whole thing, the entire universe, seen and unseen together? We genuinely don't know. That is the honest answer, and it deserves to be said plainly rather than dressed up. But we can set a flaw. Using one careful statistical approach weighing which cosmic models best fit the real data, researchers concluded that the whole universe must be at least 250 times larger than the part we can observe, spanning something on the order of 7 trillion lightyear at the very minimum. Sit with the phrase at minimum.
93 billion light years. The number that felt infinite, the number that broke your mind a moment ago, turns out to be at the very least a tiny bubble inside something hundreds of times wider. And that is only the smallest the whole universe is allowed to be. The true size could be vastly unimaginably greater, possibly in fact without limit at all.
Which brings us to one of the deepest open questions in all of science. And here the honest tension has to stay unresolved because the evidence itself does not resolve it. To find out whether the universe is finite or infinite, cosmologists measure something called its curvature. The geometry of space on the largest scales. Picture the difference between the surface of a ball and the surface of an endless flat sheet. On a ball, if you travel far enough in a straight line, you eventually come back to where you started. The surface is finite and curves back on itself. On a perfectly flat sheet, a straight line just goes on forever, never returning, never ending.
Space can behave either way. So scientists went looking for the curve.
Using the faint afterglow of the Big Bang, [music] NASA's W map satellite and later the European Space Ay's Plank mission measured the shape of the cosmos with extraordinary precision. And what they found was this. The universe is flat. Not roughly flat. Flat to within a fraction of a percent. Balu map pinned the curvature to within about 4/10 of 1% of perfectly flat. And Planck's measurements pushed it even closer to zero. So close that no curve has ever been detected at all. And a perfectly flat universe carries a staggering implication. If space truly has no curve, if it never bends back on itself, then it may have no edge. and no end. It could stretch on the same in every direction forever, infinite. Now, honesty demands a caution here, and it matters. A flat measurement does not prove the universe is infinite. Space could still be flat and yet quietly wrap around on itself in some subtle way we cannot detect from inside. The way the floor of a vast room can look flat even if the whole building is secretly a loop. The data we have cannot rule that out. So the question remains genuinely stubbornly open. But the simplest reading of the best measurements we possess, the flatness we actually observe points toward a universe with no boundary at all. And if that reading is right, follow where it leads because it goes somewhere deeply strange. In an infinite universe filled everywhere with roughly the same kind of matter, every arrangement of particles that can happen does happen somewhere. Not once, but endlessly. There would be [music] out there in the boundless dark another region identical to the one you are sitting in right now. Another rock, another room, another version of you.
breathing at this exact moment, hearing these exact words, not as a metaphor, as a statistical near certainty. If space really does go on without end, infinity is not just large. Infinity means everything possible is realized and then realized again an infinite number of times. We cannot confirm it. We may never be able to. But it is where the mathematics quietly points and it should unsettle you. So here is where we have arrived and it's worth stopping to feel it fully. Everything from the opening, the sun, the galaxy, Andromeda, Lania, the two trillion galaxies, the 93 billion lightyear. All of it may be nothing more than one infinite decimal bubble suspended inside a hole so large it makes our entire observable cosmos look like a single grain of sand on an endless lightless beach. We spent all that time climbing outward, feeling smaller and smaller. And it turns out we hadn't even begun to leave home. And that bubble has one more property that quietly dismantles our last comfortable assumption. We tend to imagine ourselves at the center of it all. The observable universe as a sphere with earth sitting neatly in the middle. And in a sense we are in the middle. But so is everyone because that horizon is not a fixed wall standing somewhere out in space. It moves. It belongs to the observer. Every point in the universe has its own observable sphere drawn around itself.
its own lantern circle in the fog.
Picture an alien astronomer on a world a billion galaxies away from us. Right now, [music] tonight, that being looks up and sees its own observable universe. A sphere of light centered on its own planet. Its horizon reaches out to regions we cannot see. And our horizon reaches out to regions it cannot see. The two bubbles overlap, but they are not the same bubble. There is no shared single edge to the cosmos. There is no true center anywhere. Each of us, every possible observer sits at the middle of our own small private sphere of what light has had time to deliver. We are not at the center of the universe. We are at the center of our view of it. Those are very different things. And to be fully precise about what that sphere really is, we have to untangle two distances that get quietly confused. And the confusion hides the strangest truth of all. When light from the edge of the observable universe finally reaches us, it has been traveling for about 13.8 billion years. So you might think that edge sits 13.8 billion lightyears away.
But it doesn't. While that light crawled toward us, space itself kept stretching, carrying its source farther and farther away the whole time. By now, the region that emitted that ancient light has been swept out to roughly 46 12 billion lightyear from us. The number that doubled gives the 93 billion. So, the observable universe is not a container holding a fixed set of contents. It is better understood as a sphere of access, a record of everywhere we could have received a signal from, not a snapshot of where those things sit now or even whether they still exist. It maps what has been able to reach us. It is a boundary of contact, not a boundary of stuff. And this reframes the horizon into something far heavier than a line on a map. beyond it like galaxies, real ones burning with real stars whose existence we will never confirm. We will never know they were there. We will never count them, never name [music] them, never receive the faintest whisper of their light. This is not a temporary gap in our knowledge, the kind of future discovery might close. It is a permanent wall built into the structure of reality itself. A boundary past which some knowledge is sealed away from us forever. Not because we aren't clever enough, not because our tools are too weak, but because the universe itself forbids the information from ever arriving. There are truths out there with our names on them that we are structurally permanently incapable of ever learning. Scientists hate walls like this, and this is one they can never climb. And now the ground shifts one final time because everything we've just said, every number we've quoted since the very beginning rests on a single assumption that [music] we cannot fully verify. We have been quietly assuming that the unseen universe looks like the seen one. That the regions past our horizon are filled with the same kind of galaxies obeying the same laws, spread out in the same way as everything we can measure. Cosmologists call this the cosmological principle. The idea that the universe is on the largest scales roughly the same everywhere and in every direction. No special places, no privileged corners, the same cosmos wherever you stand. It is a reasonable assumption. Everything we can see supports it. But notice what it is. An assumption. We are guessing about the character of a place we can never visit based entirely on our own small bubble.
Every distance we've calculated, every galaxy count, every estimate of the whole universe's size quietly depends on this principle holding true out there in the dark where we cannot check. And what if it doesn't? Here the tension is real and unsettled.
Suppose the universe is not uniform on the larger scales. Suppose there are regions denser or emptier than ours or structures far bigger than our models should allow. Then the foundation cracks. Our distance estimates would shift. Our size figures would wobble.
The whole carefully built scaffold of numbers rests on the belief that our corner is a fair sample of the whole.
And we have no way, even in principle, to fully confirm that it is. Some observations have hinted at oddities, at structures that seem too large, at patterns that shouldn't be there. We'll return to those cracks later because they matter. For now, hold on to the discomfort. The numbers are only as trustworthy as an assumption we can never fully test. So look again at that word we started with, observable.
The edge of the observable universe was never the edge of the universe. It is the edge of us, the edge of our reach, the edge of our seeing, the edge of our knowledge. The cosmos does not end at 46 1/2 billion light years. We do. That boundary is not a rim where reality stops. It is the exact line where our ability to know it runs out. The universe doesn't have an edge sitting out in space waiting for us. What we called an edge is really the outer shell of our own ignorance drawn in light.
[music] And that would be enough to reframe everything. But we are not done because there is a second deception hiding inside the first. And this one lives not out at the distant horizon [music] but in every point of light you have ever seen. Because even inside our own bubble, even in the small sphere we can observe, the universe is not showing us what we think it is showing us. Every star, every galaxy, every distant smudge of light in the night sky is telling you something. And that something is not the truth about now. It is a lie about time.
And once you see how deep that deception runs, the night sky will never look the same again. A lie about time. That is what every star in the night sky is telling you. And to see why, we have to sit with one plain fact that sounds harmless, almost boring until you follow it all the way down. We never see the universe as it is now. Not any part of it. Not the sun, not the nearest star, not a single galaxy. What we call looking at the universe is something far stranger than looking. It is remembering. Here is the root of it.
Light is fast, staggeringly fast. But it is not infinitely fast. It travels at a fixed speed about 186,000 m or 300,000 km every second. Fast enough to circle the entire Earth 7 12 times in the time it takes you to say the word second. for anything happening in this room. That speed is so quick it may as well be instant. But space is not a room. Space is monstrous. And the moment you leave Earth behind, that instant begins to stretch because light for all its speed has to cross those distances one mile at a time. Which means the light arriving in your eyes right now left its source in the past. Always without exception to look out into space is to look back in time. Start close. Start with the sun.
It sits about 93 million miles or 150 million kilometers away. And its light, even at that blistering speed, takes roughly 8 minutes to cross the gap and reach you. 8 minutes. Think about what that actually means. The sun you see when you step outside is not [music] the sun as it is. It is the sun as it was 8 minutes ago. A portrait already out of date by the time it lands on your skin.
If the sun were to simply vanish this instant, cease to exist. You would feel nothing, see nothing, suspect nothing.
For a full 8 minutes you would go on living in the warm light of a star that was already gone. The sky would still be blue. The afternoon would still be [music] bright. And only after those 8 minutes had passed would the darkness arrive and the cold begin. Every glance at the sun is a glance at a ghost 8 minutes old. And the sun is our next door neighbor. Now push the distance out and the past deepens with it. Andromeda, that faint smudge we spoke of, the farthest thing your unaded eye can find, sits two and a half million light years away. Its light has been crossing the dark for 2 and a half million years to reach you tonight. Let that number breathe. When the light entering your eye right now first left Andromeda, there were no modern humans on Earth to see it. Our species did not exist on the plains of Africa. Our distant ancestors were just beginning to chip the first crude stone tools. Creatures who would not recognize us and whom we would barely recognize. That beam of light has been traveling since before humanity itself. It crossed 2 and a half million years of empty space, outlasting the entire human story, only to end its journey on the back of your eye on an ordinary night. You're not seeing Andromeda. You're seeing a message sent before you, before your language, before your kind, a photograph developed across the lifespan of a species. And this is only the beginning of how far back the sky reaches. Point a powerful telescope at the deep dark between the stars. And you begin collecting light that is not millions of years old, but billions.
Galaxies so remote that their light left them before the Earth had even formed, before the sun existed. We are seeing them as they were in the universe's distant youth. small, wild, misshapen infant galaxies, still forming, still colliding, glowing in a cosmos less than a fraction of its current age. We are quite literally looking at the universe's childhood photographs. Baby pictures scattered across the sky. And here's what most people never quite absorb. We never ever see its present face. There is no photograph of the cosmos as it is today. There cannot be.
Because in the billions of years it took that ancient light to reach us, everything it shows has already changed beyond recognition. Many of the galaxies we observe in that early light are not out there anymore. Not in the form we see. They have merged with others. They have been torn apart. They have transformed, aged, burned through their gas. and in some cases died entirely. We are watching in perfect detail events that finished happening billions of years ago. Picture receiving a letter from someone written in beautiful handwriting [music] describing their life and slowly realizing the letter took so long to arrive that the person who wrote it has long since lived out their entire life and passed away. You are reading a living voice that is already gone. That is what the deep sky is. Every distant galaxy is a letter from a sender who has already moved on. And there now what those galaxies are actually doing at this very moment is not just unknown to us. It is permanently fundamentally unknowable. We would have to wait billions of years for the news to arrive. And by then it too would be ancient history. And this finally makes sense of that strange split we touched on before. The two different distances to the edge of everything. When we say the oldest light has traveled for 13.8 billion years, we are describing time.
How long the journey took. That is the light travel distance. But the place that emitted it has not sat still.
[music] While the light crawled toward us across all those eons, space itself kept stretching, carrying the source farther and farther away. Until today, it rests some 46 and 12 billion light years off.
That is the koving distance where the thing is now long after its ancient light left. One number is a measure of how long we have been receiving the message. The other is how far away the sender has drifted while we waited. The gap between them is not an error. It is the fingerprint of an expanding universe. And it hides the strangest effect of all. Because as that light crosses billions of years of stretching space, the space does something to the light itself, it stretches it, too.
Picture a wave drawn on a rubber sheet.
And now imagine the sheet being pulled steadily at both ends. The wave doesn't vanish, it lengthens. Its peaks [music] spread farther apart. Light works the same way. As a beam travels across an expanding universe, the expansion drags its waves out longer and longer. And for light, a longer wave means a shift toward the red end of the spectrum.
Astronomers call it red shift. And the farther the light has come, the more stretched, the more reddened it arrives.
The bluest, most energetic light can leave a young galaxy and reach us shifted all the way down into deep red.
[music] And the very oldest light, light that has been traveling almost the entire age of the universe, gets stretched further still, past red, past what any eye can see, all the way down into microwaves. And that light has a name and a meaning that should stop you cold. We call it the cosmic microwave background. And it is the single oldest thing anyone will ever detect. Picture this. Everywhere you look in the sky, [music] in every direction beyond the last galaxy, there is a faint uniform glow invisible to your eyes but unmistakable to a radio telescope. It is not starlight. It is the light of the Big Bang itself, cooled and stretched by nearly 14 billion years of expansion into a whisper of microwave radiation, sitting today at just under 3° above absolute zero. The coldest temperature that can exist. The afterglow of creation still arriving from every corner of the sky at once. turn an old untuned television to static. And a tiny fraction of that snow, that hiss, is this the fading heat of the universe's first light falling on your antenna. But this glow comes from a very specific moment. And understanding that moment reveals a wall we can never cross. The cosmic microwave background was released about 380,000 years after the Big Bang.
On a cosmic time scale that is astonishingly early, the universe was less than a thousandth of 1% of its current age. A newborn only hours old in the span of a human life. Before that moment, atoms could not hold together.
The universe was too hot. A dense roing plasma of bare particles of free electrons swarming everywhere. And free electrons do something specific to light. They scatter it endlessly in every direction. So picture the infant universe as a thick glowing fog. Not dark, the opposite. Blindingly bright, but blinding in the way a thick bank of fog is blinding or the inside of a cloud. Light existed, but it could not travel in a straight line. Every photon was knocked sideways almost the instant it moved, bouncing between particles like a scream lost in a crowd. You could not have seen through it. Nothing [music] could. Then, 380,000 years in, the universe cooled just enough. The bare particles finally caught their electrons and settled into the first true atoms. A moment scientists call recombination.
The fog lifted. In an instant, on cosmic terms, space became transparent, and the light that had been trapped and scattering for all those years was suddenly set free to travel in straight lines. The same light we catch today as that microwave glow. And that moment forms a barrier, a literal wall in our vision. Scientists call it the surface of last scattering. the point where that ancient light scattered for the very last time before flying free. When we look out at the cosmic microwave background, we are looking directly at that wall, the oldest thing light can show us, and we cannot see past it. Not because our telescopes are too weak, but because beyond that wall there was no free light to see, only the impenetrable fog. Look as hard as we like with any instrument ever built or ever conceivable, and we will always hit the same barrier. A glowing curtain drawn across the first 380,000 years, hiding the Big Bang itself from every eye that will ever exist. No telescope ever will see the Big Bang in visible light. The universe sealed that moment away behind a wall of fog before the first star was even a possibility.
So is the beginning simply lost to us?
Not entirely. But the only way through is to abandon light altogether. Two things can pass through that primordial fog where light cannot. The first is the neutrino, a ghostly particle so unwilling to interact with matter that trillions are streaming through your body every second right now without touching a single atom of you. Nutrinos broke free from the cosmic soup not 380,000 years in, but in the first few seconds. If we could ever build detectors sensitive enough to catch that ancient neutrino background, we would be looking back past the wall of light [music] into the first breaths of the universe.
The second messenger is the gravitational wave, a ripple in the fabric of space itself, set off by the most violent events in existence. These ripples pass straight through the fog untouched, carrying information from before recombination, from the earliest instance. Photons cannot reach us from that era. But these two whispers, the neutrino and the tremor in space, just might. They are the only keys we may ever have to the room behind the wall.
And so we arrive at the truth that reorganizes everything we thought the night sky was. The observable universe is not a snapshot of space. It is a map of time. When you look out into it, distance and age become the same thing.
[music] The farther out you look, the deeper into the past you fall. The closer you get to the beginning. Look a little way and you see millions of years ago. Look far and you see billions. Look to the very limit and you reach the oldest light there is, the beginning itself. redshifted into a microwave hush. The edge of the observable universe is not a place. It is a time.
It is the past stretched into a glowing shell around us. And this carries a consequence that is quietly staggering.
One that no future technology will ever undo. We can never assemble a true present- day image of the universe.
Never. Every direction we look, we are seeing a different depth of the past.
The near things as they were recently, the far things as they were long ago, all layered together into a single false picture that corresponds to no single moment in time. There is no cosmic now that we can photograph, no way to freeze the entire universe as it truly is at this instant and look at it whole. The concept of a universal present.
Everything everywhere right now is something we can imagine but never see, never verify, never capture. The now of the distant cosmos is forever hidden behind the time it takes its light to reach us. So look up again and see the night sky for what it truly is. Not a window onto the present, a museum. Every point of light hanging above you is a ghost. A delayed echo of something that happened long ago. Some of it recently.
Some of it before the Earth was born.
The star you wish upon may have died a thousand years past. Its light still faithfully arriving. A candle burning in a room whose owner left long ago. The sky is a great hall of exhibits. Each one a portrait of a cosmos that has already moved on, already changed, already become something we will never be allowed to see. We are standing in a gallery of the dead and the departed, mistaking it for the living world. But here is where even that unsettling image is not the whole story. Because these ghosts are not simply old. They are not standing still on the walls of the museum waiting to be admired. They are moving. Every one of them is drifting away from us and not gently. Each one is receding and receding faster with every second that passes. The exhibits are fleeing the building. The whole museum is flying apart, the walls rushing outward, the portrait sliding away into the dark faster and faster the longer we watch. And once we understand why they are running and how quickly the quiet sorrow of a sky full of ghosts becomes something far more urgent and far more final. Because this running has a limit.
And beyond that limit, the ghosts don't just fade. They vanish forever. And to understand that limit, where it is, why it exists, and why it is closing on us right now, [music] we have to confront the single fact that undoes the last comfortable assumption you have left about the cosmos. You have probably been picturing the universe as a vast still room. Enormous, yes, ancient, yes, but fundamentally stable. A fixed stage on which the stars slowly turn. Set that picture down now because it is wrong.
The universe is not still. It is not stable. It is expanding, growing larger every second, and everything in it is being carried apart. We have known this for about a hundred years, and the discovery arrived as a genuine shock. In the 1920s, astronomers studying distant galaxies noticed something that made no sense at first. Nearly every galaxy they looked at was moving away from us. Not toward us, not drifting randomly, away.
[music] In every direction they pointed their telescopes. The galaxies were fleeing. And then came the pattern that turned a curiosity into a law. The astronomer Edwin Hubble, building on the earlier insight of the Belgian priest and physicist Gor Lamemetra, found a clean relationship hiding in the data.
The farther away a galaxy was, the faster it was rushing away. Double the distance and you double the speed.
Triple it and the galaxy flees three times as fast. We now call this the Hubble Latrore and it is one of the most important discoveries in the history of science because it meant the entire universe was growing. But here is where almost everyone forms the wrong mental image. And the wrong image will ruin everything that follows. So slow down with me for a moment. The galaxies are not flying through space like debris hurled outward from an explosion. That is the natural assumption and it is false. The galaxies are for the most part sitting still. What is moving is the space between them. space itself, the emptiness, the very fabric of distance is stretching, swelling, [music] growing in every direction at once. And as it grows, it carries the galaxies along with it like passengers on a surface that is expanding beneath their feet. Picture this. Take a deflated balloon and dab dots of ink across its surface. Each dot a galaxy.
Now begin to inflate it. As the rubber stretches, watch what happens to the dots. Every single one moves away from every other one. No dot is pushing itself across the balloon. None of them is the source of the expansion. They are simply riding the surface as it swells.
And notice the crucial detail. From the point of view of any one dot, all the others appear to be fleeing away from it. And the dots farthest away appear to move fastest because there is more stretching rubber piled up between them.
That is our universe. The galaxies are the dots. Space is the rubber. And the stretching is happening everywhere all at once, which quietly dismantles a very old piece of human vanity. When you see every galaxy racing away from us in every direction, it is tempting to conclude that we must be at the center of it all, the still point from which everything flees. But look back at the balloon, every dot sees exactly the same thing. Stand on any galaxy anywhere in the universe and you would see all the others rushing away from you with the distant ones moving fastest precisely as we do. There is no center to the expansion. Everyone is at the center of their own view and no one is at the center of the whole. We are not special.
We are just one dot on an endless swelling surface. For a while that was the settled picture, a universe expanding steadily, coasting outward from the force of its own beginning. But the universe was hiding something. And in 1998, [music] it broke the story wide open. Here is what scientists believed at the time. The universe was expanding.
Yes, but gravity should be slowly putting on the brakes. Every galaxy, every star, every atom pulls on every other with gravity. And all that mutual pull should be gently reigning in the expansion, slowing it down year by year.
The only real question was how much it was slowing, whether it would eventually stop or coast forever. That was the expectation. That was what the data was supposed to show. Two independent teams set out to measure it. One was the supernova cosmology project led by Saul Pearlmutter at the Lawrence Berkeley National Laboratory. The other was the Heisy supernova search team led by Brian Schmidt with Adam Ree leading the crucial analysis. Both teams were hunting the same rare beacons, a particular kind of stellar explosion called a type 1A supernova. Let me be precise about why those explosions mattered. A type 1A supernova detonates with almost exactly the same true brightness every single time, like a lighthouse bulb of a known fixed wattage. And when you know how bright something truly is, you can tell how far away it is simply by how dim it appears to you. A candle across a room looks bright. [music] The same candle across a stadium looks faint. Measure the faintness and you know the distance.
Astronomers call these explosions standard candles, and they let us measure distances across billions of light years with real precision. So, both teams measured dozens of these dying stars, expecting the light to reveal a universe gently slowing down.
And then the data came in. The supernovi were too faint, fainter than they should have been, farther away than a slowing universe could explain. The expansion was not slowing down at all. It was speeding up, accelerating, getting faster with every passing eon. As the physicist announcing the prize would later put it, the discovery came as a complete surprise, even to the scientists who made it. They had gone looking for the rate of the brakes and instead found a foot pressing harder and harder on the accelerator. No one predicted this. Nothing in the models called for it. The universe was not merely expanding. It was expanding faster and faster and something was driving it. The finding was so profound, so thoroughly confirmed that it earned Pearl Mutter, Schmidt, and Ree the Nobel Prize in physics in 2011. And it forced into existence one of the deepest mysteries in all of science. Because something has to be doing the pushing.
Something is forcing space apart harder every year, overpowering the collective gravity of every galaxy in existence. We do not know what it is. We can measure its effects with precision, but its nature remains a near total blank. So we gave the mystery a name, dark energy.
And when you tally up everything the universe is made of, dark energy is not some minor addition. It is the overwhelming majority. Roughly 68% of everything of all the energy and matter in the cosmos is this dark energy, this force we cannot explain. Add in dark matter, the invisible mass we met earlier at around 27%.
And what is left for ordinary matter?
Every star, every planet, every atom in your body, everything humanity has ever seen or touched is a poultry 5%.
Think about that for a moment. The entire visible universe, all of it, is a 5% sliver floating in an ocean of two things we cannot see and barely comprehend. And the larger of those two is actively tearing the cosmos apart.
And here is the property of dark energy that makes it truly relentless. The detail that turns a strange discovery into a cosmic sentence. Think about what happens to ordinary matter as space expands.
Spread the same amount of gas across a larger and larger volume and it thins out, grows less dense, weaker, more dilute. That is how everything normal behaves. Pour a drop of ink into a glass of water and it is dark. Pour it into a swimming pool and it all but vanishes.
But dark energy does not behave this way. As space expands, as the volume grows enormously, the density of dark energy appears to stay exactly the same.
It never thins. It never weakens. Every new cubic meter of space that comes into being arrives, filled with its own fresh supply of this repulsive energy. So the more the universe expands, the more dark energy there is and the harder it pushes, which expands the universe faster, which creates more dark energy still. It is a feedback loop with no break and no ceiling. The expansion doesn't just continue, it compounds. And that runaway acceleration leads somewhere genuinely disturbing. If the recession speeds keep climbing without limit, if the farther things always flee faster and the whole thing keeps speeding up, then beyond a certain distance, space is expanding so fast that it carries galaxies away from us faster than the speed of light. Now [music] I can feel the objection forming because it is the right one to have. Nothing can travel faster than light. That is Einstein's iron rule. So how can this be? Here is the resolution and it is the same key from before. Einstein's speed limit applies to objects moving through space. It says nothing about space itself. The galaxies are not racing through the cosmos faster than light.
They are barely moving at all. It is the space between us and them [music] that is stretching. And the stretching of space has no speed limit whatsoever.
Pile up enough expanding space between two points and the gap can grow faster than any light beam could ever cross it.
No law is broken. The rule simply never applied. And this creates a set of invisible boundaries around us.
Boundaries so easily confused that even careful people tangle them together. Let me separate three of them cleanly because the difference between them is the difference between far away and gone forever. The first is the Hubble sphere.
Draw an imaginary sphere around us at the distance where space is expanding so fast that galaxies there are receding at exactly the speed of light. That distance sits at [music] about 14 billion lightyear. But here is the subtlety that surprises everyone. This is not a wall. We can actually still see galaxies beyond it, [music] receiving ancient light they sent long ago. The Hubble sphere marks where things cross into faster than light recession. But it does not by itself cut them off from us.
The second boundary we have already met, the particle horizon at 46 1/2 billion light years. That is the edge of everything we can see. the farthest [music] distance any light could have crossed to reach us since the beginning.
And the third is the one that matters most for what is coming. And it is the most merciless of them all. Scientists call it the cosmic event horizon. And it sits at roughly 16 to 18 billion lightyears from us. Here is what makes it so final. If a galaxy sends out a beam of light today [music] and that galaxy lies within the event horizon, the light can eventually after a long journey reach us. But if the galaxy [music] lies even a single step beyond that horizon, its light launched right now at [music] the fastest speed the universe allows will never arrive. Not in a billion years, not in a trillion, not ever. The expanding space between us and it will always outrun the beam. The event horizon is the true point of no return. The line past which the present of a galaxy is severed from our future completely. And galaxies are crossing that line constantly. Right now as you breathe, picture it out at the edge of the reachable universe. Galaxy after galaxy is drifting across the event horizon, one after another, silently passing the point beyond which we can never affect them. And they can never affect us again. As each one crosses, its light stretches redder and redder, fading, dimming, sliding down the spectrum toward darkness until it winks out of any possible contact for the rest of time. They do not explode. They do not die. They simply slip over the edge of the world and are gone. [music] Carried off by the expansion into a place we can never follow. Once beyond it, a galaxy falls out of causal contact with us forever. Meaning nothing we could ever do could reach it. And nothing it does could ever reach us. Two regions of the same universe permanently sealed off from one another. Not by distance alone, by the expansion of space itself, opening between them faster than light can close it. [music] And this is the moment the whole story turns from grand to grim. Because the expansion is not just making the universe bigger. It is drawing a hard permanent boundary around what can ever touch us, around what we can ever touch.
Everything beyond the event horizon is not merely out of sight. It is out of reach, out of contact, [music] out of our future entirely for all time. The cosmos itself is enforcing an absolute limit and no discovery, no ship, no signal will ever cross it. This is where vast quietly becomes worse. All those staggering numbers from the beginning, the two trillion galaxies, the 200 seextillion stars described how much is out there. But this horizon describes something cruer. How little of it we can ever hold. The scale was never the real story. The isolation is because it means something that sounds almost impossible when you first hear it. Most of what you can see in the universe you can never reach. You can gaze at it, study it, photograph its ancient light, and still be as permanently cut off from it as if it had never existed at all. Seeing and reaching have come apart. The night sky is full of places we are allowed to look at and forbidden to ever visit. And when scientists sat down and actually counted how much of the visible universe has already crossed that line, how much is already lost to us right now, tonight?
The number they found was so large it reframes our entire place in the cosmos.
It is not a small fraction that has slipped away. It is nearly all of it.
And to understand how staggering that is, we have to sit with a distinction so easy to miss that most people go their whole lives never noticing it. A distinction that quietly separates everything we can see from everything we could ever touch. Observable does not mean reachable. Say it slowly because the entire weight of what follows hangs on it. To observe something is to receive its light. To reach something is to arrive there. And in an expanding universe, those two things, seeing and arriving, have come apart completely. We can see vastly more of the cosmos than we could ever in principle travel to.
The night sky is crowded with places we are permitted to look at and forbidden forever to visit.
>> [music] >> And here is the number that turns that abstract distinction into something that should make your breath catch. Of all the galaxies we can currently see, every smudge of light in every deep field image ever taken, about 94% are already right now beyond our reach. Sit with the phrasing because it matters enormously.
Not 94% are hard to reach. Not far away, not a long journey, not a challenge for some future generation with better engines. 94% are permanently physically impossible to reach ever by anyone.
There is no ship fast enough, no technology advanced enough, no span of time long enough to close the gap. The door to them is not merely locked. The door has been removed and the wall sealed over. And to feel how absolute that is, consider the fastest thing that exists. Imagine we fired a beam of light toward one of those galaxies today.
Light, the ultimate speed limit of the universe, 300,000 km or 186,000 mi every second. [music] You would think that surely given enough time that beam would get there. It never will because the expansion of space is carrying those galaxies away faster than the beam can chase them. The space between us and them is opening up more quickly than light can cross it. So the gap grows faster than the light can close it forever. The beam runs and the finish line runs faster and the distance between them only widens. That is not a failure of the light. It is the geometry of an expanding universe. The light is doing everything it can at the fastest speed reality permits and it still falls behind. Turn that around and it becomes even more personal. Suppose humanity did something profound tonight. Suppose we gathered every antenna on Earth and broadcast a single message to the cosmos. A signal announcing that we were here. A message moving outward at the speed of light in [music] every direction carrying everything we are.
How much of the observable universe would ever receive it? Only about 6%.
Just 6% of the galaxies we can see could ever across all of future time catch that signal. The other 94% are already beyond the reach of anything we could say or send. Our message would chase them across the widening dark and never ever arrive. We could shout into the universe with everything we have. And 94% of it would never hear a whisper. It is already too far gone. Where exactly does that frontier sit? The last line beyond which nothing we send can follow.
It sits at the event horizon we found a moment ago at roughly 18 billion light years. Everything inside that boundary we could in principle still reach if we left today at the speed of light.
Everything past it is a place we can see, study, [snorts] and photograph in its ancient light and never ever touch.
Picture standing at a train station watching a train pull away. For a moment, you could still run and catch it. Then it crosses a point where no matter how fast you sprint, the platform itself is stretching beneath you faster than your legs can carry you. And the train is gone. Not because it outran you, but because the ground between you grew. That crossing point is the event horizon. And almost the entire universe is already on the far side of it. Now let us put a truly human weight on what has been lost. Remember the number from the very beginning. The roughly 200 sexillion stars that fill the observable universe. A two followed by 23 zeros.
More than every grain of sand on every beach on Earth. Of all those stars, how many remain reachable? About 6%.
Only 6% of the stars we can see could ever be visited even in principle. Even at light speed, even given all of eternity to try, the other 94%, the overwhelming majority of every sun in the visible universe, are already lost to us. Not destroyed, not invisible, simply carried so far by the expansion of space that they have passed beyond any possible contact. We can see their light. We will never stand in it. And here is the detail that turns this from a static tragedy into an unfolding one.
That reachable fraction is not holding steady. It is shrinking right now. Every single second. With every tick of the clock, more stars cross the line. On average, the equivalent of 20,000 stars slip from reachable to unreachable every second. 20,000.
In the time it has taken to speak this sentence, tens of thousands of suns have quietly passed the point of no return.
Their light from a moment ago will someday still reach us. But the light they emit now never will. They have fallen out of our future while you are listening. Think of it as a tide going out that never comes back in. Grain by grain, the reachable universe is draining away quietly, relentlessly, and completely without mercy. The cosmos we could touch is not a fixed inheritance.
It is a closing door, and the gap is narrowing every moment we exist. And I want to be precise about the cause because it is easy to mistake. This is not happening because those stars are simply far away. Distance alone in a static universe could always be crossed with enough patience. What is doing this is the expansion of space itself, outrunning light. It is not that the destinations are distant. It is that the road between us keeps lengthening faster than anyone could ever walk [music] it.
And that means no clever engineering will ever save it. No warp drive, no wormhole in any confirmed physics, no breakthrough waiting in a future laboratory. This is not a problem of speed or of will. It is written into the behavior of space and time themselves.
What has crossed the horizon is gone [music] under every law of physics we actually possess. But here is where the loneliness reaches even closer to home, closer than the distant galaxies, closer than we might want. Remember Lania, our vast home supercluster, the immeasurable heaven, 500 million lighty years across, holding a 100,000 galaxies. Here is what most people never learn about it. Lania is not actually held together. It is not a gravitationally bound structure at all. For a long time, astronomers pictured superclusters as real permanent islands of matter bound by their own gravity.
But the discovery of dark energy shattered that. We now understand there is a maximum size for any structure that gravity can permanently hold together.
And it is far smaller than a supercluster beyond about 1 and a half billion lightyear. Nothing stays bound.
Lania is well past that limit. Which means the grand supercluster we call home was never truly a home at all. It is a pattern in the act of dissolving.
Dark energy will tear Lania apart galaxy by galaxy. The Virgo cluster, the great attractor, all the vast rivers of galaxies flowing through it. Every piece will be carried away from every other piece as space expands between them.
What looks today like a single mighty structure is really a crowd of strangers, briefly near one another, already drifting towards separate and permanent exile. Given enough time, every one of those 100,000 galaxies will recede beyond our horizon and vanish from any possible contact. So, what survives? What is actually truly bound to us? Held tight enough by gravity to resist the expansion forever. only the local group, just our small neighborhood, the Milky Way, Andromeda, the Triangulum Galaxy, and the 50some smaller galaxies gravitationally locked to us. That is the whole of it. That is everything we will ever keep. Everything beyond those 50 odd galaxies is [music] destined to drift away and disappear one by one until nothing remains outside our little island. And the galaxies within that island will not stay separate.
Bound together, they will slowly fall toward one another over billions of years, merging and coalescing. Andromeda and the Milky Way colliding and fusing, [music] drawing in the smaller galaxies until in the deep future, the entire local group becomes a single vast galaxy. One island of stars alone in an ocean of black empty expanding space. Everything else gone. And this carries an implication that reaches far beyond us. If this is true for the Milky Way, it is true everywhere. Any civilization on any world anywhere in the cosmos is confined to its own gravitationally bound island.
Its own small cluster of galaxies surrounded by an expanding gulf it can never cross. No matter how advanced, no matter how ancient, no matter how desperate to reach out, every intelligence in the universe is sealed inside its own local pocket, cut off from all the others by the widening dark. The cosmos itself has drawn the boundary. Not distance, not danger, not the limits of any one species cleverness, but space and time enforcing a permanent horizon on exploration and on contact. We were born into a universe that is for the overwhelming most part a locked door, a sky full of lights we can see and study and yearn toward and never once touch. And here honesty compels a pause because there is one number in all of this that scientists genuinely cannot agree on and I will not pretend otherwise. How many galaxies are actually out there? For years the answer was around 100 to 200 billion. Then in 2016, a team led by Christopher Consulus at the University of Nottingham used deep Hubble images and mathematical modeling to argue the true number was around 2 trillion, 10 times higher, with the vast majority too faint for any telescope to see directly. That is the figure that has echoed everywhere since.
But then came the challenge. Astronomers using NASA's New Horizon spacecraft far out past Pluto in the darkest sky ever measured looked at the faint background glow of the cosmos and found less light than two trillion galaxies should produce. Their reading pointed back toward the hundreds of billions. and other researchers pushing the models the other way argue the number could be even higher than 2 trillion, perhaps 6 trillion, perhaps 20. The honest truth is that the count climbs and falls with every new instrument. And the debate is not settled. Each number says as much about the reach of our telescopes as about the universe itself. But notice something crucial. Every side of that argument, 200 billion, 2 trillion, 20 trillion, agrees on the thing that matters here. Whatever the true count, the overwhelming majority of those galaxies lie beyond our reach. The number of galaxies is disputed. The isolation is not. Whether the cosmos holds hundreds of billions or tens of trillions, we can touch only the same tiny sliver of it and lose more with every passing second. The uncertainty is in the size of the ocean. It is not in the size of our island. And so the true scale of the universe reveals its second colder face. At the start, scale meant abundance. How staggeringly much is out there. How many stars? How many galaxies? How vast the whole. But the real measure of our place in the cosmos was never how much exists. It is how little of it we can ever hold. All that immensity and almost none of it ours. A treasure house the size of the universe.
And we are locked in a single room watching the rest recede through a window that is slowly quietly closing.
And if you think that is the end of it, a universe vast, visible, and forever out of reach, then hold on. Because the isolation does not simply stay as it is.
It deepens. The window does not just close. Eventually, it goes dark. Because everything we have described so far is the universe as it is now in [music] what may be a uniquely rich and crowded moment. Run the clock forward, far, far forward. And the galaxies we can still see tonight begin one by one to redshift away and vanish from the sky entirely.
The isolation we have been describing is only the beginning of the emptiness.
What comes is a universe that doesn't just lock its doors. It erases the evidence that the rooms were ever there at all. And to understand what that means, to truly feel it, we have to do something the human mind was never built to do. We have to take the cosmic clock and turn it forward. Not by years, not by centuries, by billions of years. Come with me into the deep future. Fast forward and watch what happens to the sky. That expansion we have been tracing, the relentless [music] stretching of space, the galaxies sliding past the event horizon, it does not stop. It continues patiently, mercilessly, for eon after eon. And one by one, the galaxies beyond our local group redshift out of view. Picture it happening in slow motion across unimaginable time. A distant galaxy, its light stretching redder and redder as the space between us swells, dimming, cooling, sliding down the spectrum toward the infrared. Then the microwave, then nothing at all. Like a ship's lantern receding into fog until the fog simply swallows it. Then another. Then a thousand more. then a million. The great cosmic web we mapped at the very beginning, the filaments, the superclusters, the two trillion galaxies, all of it slowly draining out of the observable sky, [music] galaxy by galaxy, light by light. And there is a moment when it finishes in roughly 100 to 150 billion years, a span so vast that our universe today at 13.8 8 billion years old, has lived less than a tenth of it. The last external galaxy will slip beyond reach. And no galaxy outside our own will be detectable at all. Not dim, not faint, gone. Every one of the hundreds of billions or trillions of galaxies that fill our sky tonight will have vanished past the horizon.
Their light stretched so far it can never be caught. By then, our own local group will have finished its slow collapse. Andromeda and the Milky Way, drawn together by gravity, will have collided and fused. The smaller galaxies will have fallen in, and the whole neighborhood will have merged into a single enormous island of stars, a drift in a darkness with no visible bottom.
Now, place yourself in that future.
Imagine you are a curious being born on some world within that merged galaxy 100 billion years from now looking up at your night sky for the first time and wondering what lies beyond. You would see stars, your own galaxy full of suns overhead exactly as ours does. But point your finest telescope past the edge of that galaxy into the dark between and you would find nothing. No Andromeda, no distant smudges of other galaxies, no cosmic web, [music] just an empty black silent void stretching out in every direction, apparently forever. As far as any instrument could reach, your galaxy would appear to be a single lonely island of light [music] floating in an infinite featureless ocean of nothing.
And here is the quiet horror of it. That future astronomer would not be looking at a broken sky or a damaged one. They would be looking at the honest, complete result of everything their instruments could detect. The emptiness would be all the evidence they had. Two physicists worked out exactly what such a being would conclude and their answer is one of the most unsettling papers in modern science. In 2007, Lawrence Krauss, then at Case Western Reserve University and Robert Sherer of Vanderbilt University, sat down and did the calculation. They took our best model of the universe and simply ran it forward, asking a deceptively simple question. What will future scientists actually be able to see and what will they be able to figure out from it? Their conclusion was so stark that they gave it a name. They called it the end of cosmology because they realized that everything we know about the origin of the universe, every piece of hard evidence that tells us where it all came from is temporary. It is being erased. And there will come a time when it is simply gone. To understand what they meant, you have to understand how we know the universe had a beginning at all. Our entire picture of the Big Bang rests on three great pillars of evidence. Three independent clues, each pointing to the same story.
And Krauss and Sharer showed that all three will vanish. Let me walk you through each one and watch it disappear.
The first pillar is the red shift of galaxies. The very discovery we traced earlier. When we look out and see distant galaxies fleeing from us, their light stretched toward the red. That flight is our single most direct proof that the universe is expanding. It is the fingerprint of the big bang written across the sky. Run the expansion backward and everything converges to a single hot dense beginning. But now remember what the future holds. In that far epoch, there will be no distant galaxies left to measure. [music] They will all have fled beyond the horizon.
The future astronomer will look out and see only their own single galaxy. Its stars held together by gravity, not receding at all. With nothing external to measure, the red shift is not just hard to detect. It is undetectable.
The first pillar crumbles into dust. The evidence for expansion erased. The second pillar is the cosmic microwave background. That faint afterglow of the big bang we met before. The oldest light in existence glowing softly from every direction. Today it bathes the whole sky. And it is one of our most powerful proofs that the universe was once hot, dense, and young. But that glow is being stretched too. As space expands, the microwave background stretches with it, its waves growing longer and its temperature falling. Today, it sits at about 2.7° above absolute zero. By around 150 billion years from now, the calculations show it will have cooled and diluted to a fraction of a degree, stretched so far, so thin that it will fall below the level any conceivable instrument could detect. The afterlow of creation will not just fade. It will drop entirely beneath the floor of what can be measured. The second pillar gone.
And the third pillar is the most subtle and the most quietly devastating. In the first few minutes after the big bang, the universe forged its first atoms. And it made them in a very specific recipe, roughly 3/4 hydrogen, 1/4 helium with faint traces of a few other light elements. That exact ratio is a fingerprint of those first minutes. And when we measure the primordial gas in the cosmos today, we find precisely that ratio, exactly as the big bang predicts.
It is a stunning confirmation. But stars are chemical factories. Over billions of years, generation after generation of stars fuse hydrogen and helium into heavier elements and spew them back into space, and they forge fresh helium of their own. Given enough time, hundreds of billions of years, thousands of generations of stars, that pristine primordial ratio gets buried. The helium made by stars will so overwhelm the helium made by the big bang that the original signal is lost, drowned beneath the output of countless dead suns. The future astronomer measuring the gas in their galaxy would find no trace of the primordial recipe. The third pillar buried. So picture that future scientist again with every one of these clues erased. No fleeing galaxies, no afterglow, no primordial fingerprint.
What would they conclude? They would look at their single isolated galaxy sitting quietly in an infinite black void showing no sign of expansion, no sign of a beginning. And they would conclude quite reasonably that they lived in a static eternal unchanging universe. One island of stars alone in an infinite emptiness that had always existed and always would. No big bang, no expansion, no creation, just an eternal, motionless cosmos. And here is what makes it a genuine tragedy. Their science would not be sloppy. They would not be foolish or careless. Working with every scrap of evidence available to them, reasoning carefully and rigorously, they would arrive at a confident, wellsupported, and completely wrong picture of reality. Their conclusion would be flawless. It would also be false. The truth would exist, but every trace of it would have been carried beyond their reach. Sit with what that means because it changes something fundamental. Cosmic truth has an expiration date. The deepest facts about our universe, where it came from, how it began, what it is made of are not permanently available. They are written in evidence that fades, dilutes, [music] and disappears. There is a window during which the universe is knowable and then that window closes and the truth becomes unreoverable.
Not through any failure of intelligence but because the physical evidence itself has been erased by the very expansion it once revealed. And that means we are living in something extraordinary, a privileged epoch. Right now tonight the big bang is still knowable. The galaxies still flee. The afterglow still glows. The primordial fingerprint still survives. We happen to exist in the narrow window of cosmic history when the universe's origin can still be read. And future minds, no matter how brilliant, will be locked out of it forever. We are not just observers of the cosmos. We are witnesses at the one moment its story can still be told. But now turn that same logic around and feel the floor shift beneath you. If evidence vanishes over time, if the universe is steadily erasing the record of its own past, then here is the question that should keep you awake. What has already vanished?
What clues about the universe existed once in some earlier epoch that have already slipped beyond our horizon, already faded below detection, already been buried before we ever arrived to read them. We assume we are the lucky ones, the privileged witnesses. But we are not at the beginning. We are 13.8 [music] billion years in. And if the future will lose the red shift, the afterglow, the primordial ratio, then what did the deep past hold that we are already right now [music] too late to ever recover? What truths about the origin and nature of reality were written in evidence that vanished before humanity opened its eyes? Some knowledge almost certainly is already gone. Whole categories of information about the early universe may have already been carried beyond our horizon or diluted past detection, sealed away before we could ever measure them. We tend to think of ourselves as standing at the dawn of understanding with the whole cosmic story laid out before us.
But we may instead be standing in the middle of a slow erasure that began long before us. Reading a book from which pages have already been silently torn out, never knowing exactly what they said or even how many are missing. That is the deeper meaning of the end of cosmology.
It is not only a warning about the future. It is a shadow cast over the present. And if we push even further forward, far past the disappearance of the galaxies, the universe does not simply grow empty. It begins to die in stages, each one stranger and colder than the last. Astrophysicists Fred Adams and Gregory Laughlin map these stages, dividing the life of the cosmos into great ages. We live now in what they call the Stelliferous era. The age of stars, the age of light, the brief brilliant chapter when the universe is full of shining suns. But it will not last. Eventually, the gas that makes new stars runs out. Around 100 trillion years from now, a number so vast that the entire age of the universe so far is less than a rounding error against it.
Star formation ends. No new stars will ever be born. And then one by one, the existing stars burn through the last of their fuel and die. The bright ones go first in mere millions of years. The small, frugal red dwarfs linger for trillions, but they all fade in the end.
The last star flickers out, and the age of starlight is over. What remains is the degenerate era. A universe lit by nothing but the cooling embers of dead stars. White dwarfs slowly darkening.
Neutron stars. Cold dead remnants drifting in blackness, radiating away the last of their heat. The lights quite simply go out. And then over spans of time that shatter every intuition, even those remnants dissolve and the universe enters the black hole era, an age when black holes are the last great structures left, the final sentinels of a dying cosmos. But even black holes are not eternal. Steven Hawking showed that they slowly evaporate, leaking away their mass as the faintest possible radiation over time scales so long they defy any comparison the human mind can hold. A one followed by dozens, even a 100 zeros in years. Numbers so large that calling them astronomical is an insult to how small astronomical really is. But eventually even the largest black hole flickers away into nothing.
And what is left at the very end is the dark era. No stars, no galaxies, no black holes, no structure of any kind.
Just a thin, cold, everthinning mist of stray particles and dying radiation drifting through an endlessly expanding void, [music] growing colder and emptier and more silent with every passing eon.
approaching a final state of maximum entropy. A universe in which nothing happens, nothing changes, and nothing ever will again. The stillness after the last event, the dark after the last light. And this tells us something profound about our own existence. The window for life, for stars, for warmth, for chemistry, for minds capable of wondering about any of this is finite.
It is not the natural state of the universe. It is a brief bright anomaly, a short flowering between an unimaginably hot beginning and an unimaginably cold, dark, endless end.
The capacity to know anything at all, to look up and ask what the universe is, exists only during this narrow, luminous chapter, and the chapter is closing. So the universe is not merely isolating us as we saw before, walling us off inside our shrinking island. It is doing something worse. It is erasing the record. It is carrying away the galaxies, diluting the afterglow, burying the fingerprints, and eventually snuffing out the very light by which any of it could be read. The Great Museum of the Cosmos, the sky full of ancient ghosts we stood in earlier, is not just closing its doors. It is quietly removing the exhibits one by one until the halls stand empty. And no one will ever know they were full. And that would be the final word. A universe destined for isolation, erasia, and silence with us as its last lucky witnesses. except for one possibility we have not yet allowed ourselves to consider. All of this, the accelerating expansion, the vanishing galaxies, the end of cosmology, the long slide into the dark, [music] every bit of it rests on one thing. It rests on our current map of the universe being correct, on our understanding of dark energy holding true, on the numbers we have measured being the right numbers. But what if they are not? What if the map itself is flawed? Because when scientists look closely at that map today, at the very foundations everything we have said depends upon, they are beginning to find cracks. And some of those cracks are widening. Because here is the thing we have been quietly leaning on this entire journey. Every number, every distance, every prediction, the expanding universe, the vanishing galaxies, the end of cosmology, the long slide into the dark, all of it rests on a single model. One shared picture of how the universe works, how fast it grows, what it is made of. Cosmologists call it the standard model of cosmology. And for decades, it has been extraordinarily successful. But a model is only as trustworthy as the measurements it rests on. And when scientists look closely today, closer than ever before, the foundation is not as solid as we thought. Let me show you three cracks and watch how each one runs deeper than the last. The first crack is the most embarrassing of all because it is about the most basic number in cosmology. We cannot agree on how fast the universe is even expanding. Think about what that means. The expansion is the beating heart of everything we have discussed.
The vanishing galaxies, the horizon, the fate of the cosmos. And we have two different ways of measuring its speed.
Both are careful. Both are rigorous. And they give two different answers. Here is the first method. Look out at the nearby universe. Measure the distances to relatively close galaxies using stars of known brightness and exploding stars as mile markers and watch how fast they recede. Do this and you get an expansion rate of about 73. What that number means in plain terms is this. For every 3 and a4 million lighty years you move out into space, the expansion carries things apart about 73 km or 45 m faster every second. That is the local measurement led by teams like the one under Nobel laurate Adam Ree. Now here is the second method. Instead of looking at nearby galaxies, look at the afterglow of the big bang, the cosmic microwave background, that ancient light we met earlier, and use the physics of the infant universe to calculate how fast it should be expanding today. Do this using the exquisite data from the European Space Ay's Plank satellite, and you get a different number, about 67. 73 from one method, 67 from the other. That is roughly a 9% difference and it will not go away. You might think [music] 9% surely that is just measurement error.
The kind of small gap that gets ironed out with better instruments. That is exactly what scientists hoped for years.
But the gap has held firm. And as both measurements grew more precise, the error bars shrank while the disagreement stayed.
>> [music] >> which is the opposite of what should happen if it were just noise. The tension now sits at what physicists call 5 sigma. In plain language, that means the odds of this gap [music] being a random fluke are less than 1 in 3 1/2 million. That is not a coincidence. That is the threshold at which scientists stop saying probably a mistake and start saying crisis. Cosmologists gave it a name, the Hubble Tension. And at a major astronomy meeting in early 2025, one leading researcher stood up and declared it exactly that, a crisis. There was one great hope for resolving it. The James Web Space Telescope, humanity's most powerful eye on the cosmos, launched to see the universe in unprecedented detail. Surely with its extraordinary vision, it could settle the matter. The suspicion was that the nearby measurements might be subtly flawed.
That cosmic dust or the crowding of stars was throwing off the local distance measurements and inflating that 73. So the James Web was turned toward those same stars to check. And here is what most people expected. A clean resolution. The tension dissolved. The gap explained away as a measurement quirk. But that is not what they found.
The James Web data matched the earlier measurements almost exactly. The dust was ruled out. The crowding was ruled out. The local measurement of 73 stood firm. Rather than closing the crack, the James Web deepened it. The most powerful telescope ever built looked straight at the problem and confirmed that the problem is real. So, we are left with something genuinely unsettling. two rigorous, independent, carefully checked ways of measuring the universe's expansion. And they flatly, [snorts] stubbornly disagree. Both look sound.
Both cannot be right. And that means almost certainly that something is missing from our understanding. Some piece of physics we have not yet discovered, hiding in the gap between 67 and 73. To be fair, a few researchers argue the tension may still soften with better calibration. The debate is not entirely one-sided, but the mainstream view has hardened. This crack is not closing, and it is only the first. The second crack cuts even deeper because it strikes at the very thing we said would decide the [music] fate of the universe.
Dark energy. Remember what we assumed about it. Dark energy, that mysterious force pushing space apart, was thought to be constant, a fixed property of empty space, never changing, never diluting, driving the expansion faster and faster forever. That assumption is the entire basis for the grim future we traced, the vanishing galaxies, the isolation, the end of cosmology. It all depends on dark energy staying exactly as strong as it is now for all time. But what if it doesn't? In March of 2025, a collaboration running an instrument called Desi, the dark energy spectroscopic instrument perched on a telescope at Kit Peak in Arizona, released a startling result. Using the largest three-dimensional map of the universe ever made, tracing some 15 million galaxies and quazars across 11 billion years of cosmic history, they looked at how dark energy has behaved over time. And the data whispered something no one wanted to hear. Dark energy may not be constant after all. It may be weakening, fading, growing gradually weaker as the eons pass rather than holding steady forever. Now I have to be careful and honest here because this is exactly the kind of claim that demands caution and the scientists themselves are cautious. The signal is not certain. Its statistical strength sits somewhere around 2.8 to just over 4 sigma depending on which supernova data sets are folded in. That is suggestive.
It is intriguing, [music] but it is below the five sigma gold standard that physicists require before declaring a genuine discovery. And the result is contested. It depends on carefully combining different data sets and some analyses weaken the signal considerably when other measurements are included.
The Desi team is not claiming victory.
They are raising a flag. But think about what that flag would mean if it holds.
If dark energy is fading, if it weakens over cosmic time instead of staying constant, then the entire dark isolating future we traced may not be guaranteed after all. The runaway expansion might slow. The galaxies might not all vanish.
The end of cosmology might not arrive on schedule or might not arrive at all.
Some models of weakening dark energy even end in a completely different fate for the universe. In other words, this one uncertain signal still years from confirmation reaches out and touches the ultimate destiny of everything. The grim isolation we described rests on an assumption that is now for the first time genuinely in question. The crack in the map is not just about a number. It is about the ending of the whole story.
And the third crack may be the deepest of all because it strikes at the foundation beneath all the others. The very assumption we flagged at the beginning. The idea that the universe is smooth, uniform, the same everywhere.
Remember the cosmological principle, the assumption that on the largest scales, the cosmos looks roughly the same in every direction with no special places and no giant irregularities.
Every distance we calculate, every number we have quoted quietly depends on it. And there is a hard limit that comes with it. In our standard model, no single connected structure should be able to grow larger than about 1.2 2 billion light years because there simply hasn't been enough time since the big bang for gravity to assemble anything bigger. That is the rule and the universe may be breaking it. In 2013, a team of astronomers led by Istvan Horvath mapping the locations of gammaray bursts, the most violent explosions in the cosmos bright enough to be seen across the universe, notice something that should not be there. an enormous concentration of them clustered together into a single colossal structure. They called it the Hercules Corona Borealis Great Wall. And its size defies belief. Original estimates put it at around 10 billion lightyears across and more recent analyses published in 2025 suggest it may be even larger. Set that against the limit. The model says nothing should exceed 1.2 2 billion light years. This structure may be eight times that size or more. As one of the researchers involved put it, he would have thought a structure this big was simply too big to exist and admitted that even as a co-author, he still had his doubts. And those doubts matter. So, let me be fair about them. The Great Wall was traced using gammaray bursts, which are an awkward tool. They are not spread evenly across the sky, and the way different satellites observe them can introduce subtle biases that might mimic a structure that isn't really there. Some researchers have argued exactly that, that the wall may be partly an artifact of how we sample the sky. The debate is genuinely open, but the burst clustering has stubbornly refused to fully explain itself away, even when those effects are accounted for. And the Great Wall is not alone.
Other giant formations like the Sloan Great Wall and enormous groupings of quazars also seem to strain the limit.
Alongside them, astronomers have found odd unexpected patterns in the cosmic afterglow itself. Hints, some argue, of a broken symmetry in the universe on the larger scales. And if the universe is not uniform, if the cosmological principle fails on the larger scales, then the ground shifts beneath everything. Because that assumption is loadbearing. Every distance, every estimate of the universe's size, [music] every number in this entire journey was built on the belief that our corner is a fair sample of the whole. Pull that assumption out and the whole framework wobbles. The measurements would need to be redone. the scale itself would come into question. And here we arrive at the most humbling truth of all, the one that ties these three cracks together into something larger than any one of them.
Remember where we stand. We measure the entire cosmos from a single point, one small planet in a single instant of cosmic time. We have never stepped outside it. We cannot. We are forever trapped inside the very thing we are trying to measure. like someone trying to draw an accurate map of a house they can never leave from a single window in a single afternoon. That may mean something profound and unsettling. We may be structurally incapable of ever knowing the true whole. Not because we aren't clever, not because our instruments aren't magnificent, but because our vantage point is fixed, our sample is partial, and the universe keeps hiding the rest behind horizons of time and distance and expansion. Every crack we have found may be a symptom of that deeper limitation, the possibility that the view from inside can never be the whole picture. So the real dread at the end of all this is not what we know.
It is how much we might have wrong. We have built a magnificent map of the cosmos, and it is cracking in at least three places at once along its most fundamental lines. The expansion rate refuses to agree with itself. The force shaping the universe's fate may be quietly changing, and the smoothness we assumed may be an illusion hiding structures that should not exist. Any one of these could be a small correction or any one of them could be the loose thread that unravels the entire tapestry and that leaves us in a strange and vertigenous place. We began this journey with a number that felt like the edge of everything 93 billion light years and we have watched it dissolve again and again into something larger stranger lonelier and less certain than we ever imagined.
We have seen that the edge isn't an edge, that the sky is a museum of ghosts, that almost everything is unreachable, that the evidence is being erased, and now that even our map of it all may be flawed. So after all of this, after the vast, the vanishing, the unreachable, and the uncertain, one question remains, [music] and it is the one that matters most. What does the true scale of the universe actually mean? Not for the numbers, not for the models, for us, the brief, curious, conscious things who happen to be here, awake, looking up in the one moment when any of this can still be seen at all.
Hold there. Slow down. We have been descending for a long time now, deeper and deeper into something vast and strange and at times genuinely unsettling. So, let the camera pull all the way back. Take a breath and let us look together at how far down we have come. Because the journey we just made has a shape and only now from the bottom can you finally see it whole. We began with a number 93 billion light years.
The width of the observable universe. It arrived like the edge of everything. A figure so enormous it seemed to hold all of reality inside it. And for most people that is where the story of cosmic scale begins and ends. The universe is unimaginably big. Full stop. But watch what happened to that number as we went.
It did not hold. One by one, every comfortable assumption we carried into this dissolved beneath our feet. That famous width, [music] 93 billion lightyear, turned out to be the reassurance, not the horror. Because the edge it described was never the edge of the universe. It was the edge of us.
The whole cosmos is almost certainly hundreds of times larger, perhaps infinite. [music] And our bright, staggering 93 billion light-year sphere is only a finite bubble suspended inside a hole so large it may have no end at all. The number that felt like everything was in truth a measure of our own smallalness. Then we learned that we never see that bubble as it is. We see only its past. Every star, every galaxy, every point of light above you tonight is a delayed echo. Light that left its source thousands, millions, billions of years ago. We do not observe the universe. We remember it. The night sky is a sky of ghosts and its true present is something we can never witness, never photograph, never verify. What we call the cosmos is a portrait of a thing that has already moved on and those ghosts are fleeing. We found that most of what we can see we can never reach. That the expansion of space carries the overwhelming majority of galaxies away [music] faster than any ship or signal could ever follow. 94% of what we can see is already permanently beyond our grasp. And that reachable fraction shrinks with every second that passes.
Tens of thousands of stars slipping over the horizon of possibility even as we speak. We are watching a universe quietly close around us. Then it deepened again. We saw that the evidence itself is fading. That in the far future, the [music] galaxies will vanish. The afterlow of creation will dilute below detection. and the very proof of the Big Bang will be erased, leaving future minds to reach confident, rigorous, and utterly false conclusions about a universe that appears eternal and still. And finally, we found that even our map of all this may be flawed.
That the expansion rate refuses to agree with itself. That the force shaping the cosmos may be changing. That the smoothness we assumed may hide structures that should not exist. That [music] is the descent. That is how far down we came. So, here is the thing most people misunderstand from the very first moment they hear a number like 93 billion lightyear. They think the unsettling part is the size. They think worse than you think means bigger than you think. It never did. The true weight of it was never about how much is out there. It was about isolation. That we are sealed inside a shrinking island, cut off from almost everything. It was about impermanence. that the cosmos is disappearing, erasing itself, closing its own doors. And it was about the hard limits of knowing that we are trapped inside the thing we are trying to measure, able to see only a sliver and never certain even of that. The real scale of the universe is not measured only in light years. It is measured in distance we can never cross and truth we can never reach. We are observers with a horizon in every possible sense of that word. And this is where the descent turns gently into something I did not expect when I first began to sit with all of this. Something that feels against all odds like grace. Because look again at everything we just said.
The galaxies are fleeing, but they have not fled yet. The evidence is fading, but it has not faded yet. The doors are closing, but they are not closed. And that changes the meaning of everything.
We exist in the one narrow window when all of this is still visible. Think about what that means truly. Of all the vast stretch of cosmic time, from the hot beginning to the cold, dark, endless end, we happen to be awake in the brief luminous epoch when the universe can still be read. The galaxies are still there in our sky, glowing, [music] waiting to be counted. The afterglow of creation is still arriving, still whispering its ancient light onto our instruments from every direction. The fingerprints of the first minutes still survive in the primordial gas. A future astronomer 100 billion years from now will have none of this. They will look up into an empty black void and never know the universe had a beginning at all. But we can see it [music] right now. We are not too late. We are astonishingly exactly on time. And here is the turn within the turn. The part that took me longest to feel. The very expansion that isolates us, that carries the galaxies away and closes the doors and erases the evidence. That same force is the reason any of this exists at all. Consider it.
That gentle, relentless stretching of space in the early universe helped matter spread and cool and clump into the first galaxies. The same cosmic dynamics that will one day carry everything beyond our reach are the dynamics that gave galaxies room to form. That let stars ignite. That seeded the heavy elements. That made planets.
That made oceans. That made the long chain of chemistry that ends here tonight in you. The force that dooms the view is the force that built the viewer.
We do not get one without the other. The isolation and the existence are the same coin. Which means we were never separate from this scale, standing [snorts] outside it, measuring it from some safe distance. We are what it briefly produced. We are made of it. The seven octilian atoms in your body. More than the stars in this entire galaxy. every one of them forged in the hearts of ancient stars or the first three minutes of the universe itself. You are not looking at the cosmos from the outside.
You are a piece of the cosmos arranged for a little while into something that can look. The same primordial hydrogen we spoke of at the very beginning is inside you right now in the water in your cells in the breath moving through you as you listen. So let me say the whole thing plainly the way I wish someone had said it to me at the start.
The observable universe is not the universe. It never was. It is not a container holding everything that exists. It is our small temporary window, a bright circle of access drawn around one point in one instant of cosmic time onto something far larger, far stranger, and far more unreachable than any number can hold. Its true scale is measured not only in light years, but in everything we can never touch and everything we can never know. That is the real answer to how big the universe is. It is bigger than we can see, older than we can prove, and mostly permanently beyond us. And yet, from inside this small closen bubble, trapped at a single point in a single moment, seeing only ghosts able to reach almost nothing. A species of atoms woke up and measured the whole thing. Anyway, think about how extraordinary that is. We could never leave our galaxy. We will never cross the horizon. [music] We cannot even see the present or prove the beginning or reach 94% of what hangs in our own sky. [music] And still, we worked out the width of the observable universe to 11 digits. We charted horizons we can never cross. We named a beginning we can never see. We calculated the fate of galaxies that vanished before the Earth was born. Carl Sean said it best decades ago that we are a way for the cosmos to know itself.
Star stuff contemplating the stars and that is not poetry. It is literally physically true. The universe in this one small corner briefly arranged itself into something that could look back and understand it. The atoms learn to measure the whole. And you are doing it right now. Wherever you are, whatever chair holds you, whatever quiet surrounds you in this moment as you listen, you are a small, warm cluster of ancient atoms, briefly awake, contemplating a cosmos 93 billion lightyear wide. That is not a small thing. That may be the least small thing there is. So look up tonight, tomorrow, whenever the sky is clear. The light is still arriving for now. Yes, the night sky is a museum of a cosmos already gone. We know that now. Every exhibit is a ghost. Every point of light a portrait of something that has moved on or merged or died, its [music] glow still faithfully crossing the dark to reach your eye. But we are the only visitors this museum will ever have who arrived while the lights are still on. Who can still walk the halls and read the plaques and understand what they are seeing. Future minds will inherit only the empty building. We inherited the exhibits briefly luminously right now and we are here awake to witness it before the doors close. Perhaps that is the real scale of it in the end. Not how vast the universe is, though it is vast beyond any word we have. Not how much of it we will lose, though we will lose nearly all of it. But how vanishingly rare it is in all that immensity and all that time. To be a part of the universe that is looking at all to exist in the one epoch when the story can still be told. To be made of the oldest things there are and to have woken up for one brief moment inside the only stretch of cosmic history when waking up was even possible. The universe is disappearing.
That is true. But you are here now while it is still here to be seen. And in a cosmos this large, this old, this indifferent, and this fleeting, that is not a small consolation. It may be the whole point. So look up. The light is still arriving. Hold on the stars and fade to black. Think for a moment about the light [music] landing in your eyes right now. Not the light in your room.
The light from up there. Every faint point in the night sky is something that survived a journey almost beyond belief.
light that left its source thousands, millions, sometimes billions of years ago and crossed an ocean of cold, expanding dark through a universe already carrying its own galaxies away just to end its long journey here on you. And here is the quiet miracle of it. Out of all that vast and closing cosmos, that ancient light found the one rare thing that could catch it and understand what it was. A set of eyes, a curious mind, a warm and improbable arrangement of ancient atoms awake in the one brief epoch when any of this can still be seen. You are not a small thing beneath an indifferent sky. You are what the light was traveling toward. the rare, brief, improbable place where the universe finally becomes something that can look back. Thank you for crossing this long dark with me, for staying to the end. If it meant something, a like and a subscribe are how you tell me to keep going. Quieter journeys are still coming. I hope you'll be there. Sleep well tonight. [music] The light is still on its way. And tonight it arrived at you.
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